Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Traffic Detector Handbook:

Third Edition—Volume II

Publication No. FHWA-HRT-06-139 October 2006
FOREWORD 

The objective of the third edition of the Traffic Detector Handbook is to provide a comprehensive 

reference document to aid the practicing traffic engineer, planner, or technician in selecting, 

designing, installing, and maintaining traffic sensors for signalized intersections and freeways. 

Judicious application of the concepts and procedures set forth in the Handbook should result in 

improved installations and operations of traffic sensors and a long-term savings of public funds. 

Sensor types include both in-roadway and over-roadway sensors. Topics covered include sensor 

technology, sensor applications, in-roadway sensor design, sensor installation techniques and sensor 

maintenance. The sensor technology chapter discusses the operation and uses of inductive loop 

detectors, magnetic sensors and detectors, video image processors, microwave radar sensors, laser 

radars, passive infrared and passive acoustic array sensors, and ultrasonic sensors, plus 

combinations of sensor technologies. Sensor application topics include safety, operation, multimodal 

issues, and physical and economic factors that affect installation and performance. The appendixes 

include a variety of research, background papers, and implementation guidance. The information 

contained in this Handbook is based on the latest research on available treatments and best 

practices in use by jurisdictions across the United States and elsewhere. References are provided for 

the student, practitioner, researcher, or decisionmaker who wishes to learn more about a particular 

subject. 

The third edition is published in two volumes, of which this is the second, Volume II (FHWA-HRT06-139), containing Chapters 5 and 6 and all Appendixes. Volume I (FHWA-HRT-06-108) contains 

Chapters 1 through 4. 

Antoinette Wilbur, Director 

Office of Operations 

 Research and Development 

NOTICE 

This document is disseminated under the sponsorship of the U.S. Department of Transportation in 

the interest of information exchange. The U.S. Government assumes no liability for the use of the 

information contained in this document. This report does not constitute a standard, specification, or 

regulation. 

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers' 

names appear in this report only because they are considered essential to the objective of the 

document. 

QUALITY ASSURANCE STATEMENT 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.
Technical Report Documentation Page 

1. Report No. 

FHWA-HRT-06-139 

2. Government Accession No. 3. Recipient's Catalog No. 

5. Report Date 

October 2006 

4. Title and Subtitle 

Traffic Detector Handbook: Third Edition—Volume II 

6. Performing Organization Code 

7. Author(s) 

Principal Author: Lawrence A. Klein 

Coauthors: Milton K. Mills, David R.P. Gibson 

8. Performing Organization Report No. 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Lawrence A. Klein 

3 Via San Remo 

Rancho Palos Verdes, CA 90275 

11. Contract or Grant No. 

Contract No. DTFH61-03-P00317 

13. Type of Report and Period Covered 

Implementation Package 

12. Sponsoring Agency Name and Address 

Federal Highway Administration 

Turner-Fairbank Highway Research Center 

6300 Georgetown Pike, HRDO-04, Room No. T204 

McLean, VA 22101-2296 14. Sponsoring Agency Code 

15. Supplementary Notes 

David R.P. Gibson (David.Gibson@fhwa.dot.gov) at the Turner-Fairbank Highway Research Center 

(http://www.tfhrc.gov) was the Technical Representative for the Federal Highway Administration (FHWA). Milton K. 

Mills, Advanced Research Team FHWA, contributed many technical papers. Traffic sensor researchers and 

practitioners contributed significantly to document organization, content, and exhibits. The peer review panel, 

consisting of Darcy Bullock of Purdue University, Dan Middleton of Texas Transportation Institute, and Tom Urbanik 

of the University of Tennessee, provided peer review and advice based on their use and testing of sensors. Tom Potter 

of Reno A&E gave technical advice on the electrical functioning of loop detectors. Sensor vendors provided technical 

information and exhibits on their technologies. In addition, many FHWA staff members participated as sensor group 

members and/or provided comments throughout the project, including Pamela Crenshaw, Ralph Gillman, Peter 

Huang, David Jones, and Raj Ghaman. 

16. Abstract 

The objective of this Handbook is to provide a comprehensive resource for selecting, designing, installing, and 

maintaining traffic sensors for signalized intersections and freeways. It is intended for use by traffic engineers and 

technicians having responsibility for traffic sensors, whether in-roadway or over-roadway sensors. These two families 

of sensors have different characteristics and thus corresponding advantages and disadvantages that are discussed 

throughout the Handbook. Topics covered include sensor technology, applications, in-roadway sensor design, 

installation techniques, and maintenance. The sensor technology chapter discusses the operation and uses of inductive 

loop detectors, magnetic sensors and detectors, video image processors, microwave radar sensors, laser radars, passive 

infrared and passive acoustic array sensors, and ultrasonic sensors, plus combinations of sensor technologies. The 

sensor application topics addresses safety, operational performance, multimodal issues, and physical and economic 

factors that the practitioner should consider. Appendixes include research, background papers, and implementation 

guidance. The information contained in this Handbook is based on the latest research available on treatments and best 

practices in use by the surveyed jurisdictions. References are provided for the student, practitioner, researcher, or 

decisionmaker who wishes to learn more about a particular subject. 

The third edition is published in two volumes, of which this is the second, Volume II (FHWA-HRT-06-139), containing 

Chapters 5 and 6 and all Appendixes. Volume I (FHWA-HRT-06-108) contains Chapters 1 through 4. 

17. Key Words 

Traffic detectors, sensors, detector installation, detector maintenance, 

signalized intersections, intersection safety, intersection treatments, 

infrared sensor, inductive loop, magnetometer, video image processor, 

microwave radar sensor, laser radar sensor, acoustic sensor, ultrasonic 

sensor, magnetic sensor. 

18. Distribution Statement 

No restrictions. This document is available 

to the public through the National 

Information Technical Service, Springfield, 

VA, 22161 and Research and Technology 

Product Distribution Center, 9701 

Philadelphia Court, Unit Q, Lanham, MD 

20706; telephone: 301–577–0818; fax: 301–

577–1421. 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

394 

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
ii

SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH 

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha 

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS 

oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius oC 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx 

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH

mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2

 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac 

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for th International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E e 380. 

(Revised March 2003)
iii 

CONTENTS OF VOLUME I 

CHAPTER 1. INTRODUCTION..........................................................................................................1-1 

SCOPE AND OBJECTIVES OF THE HANDBOOK ..................................................................1-1 

THE NEED FOR SENSORS IN MODERN TRAFFIC MANAGEMENT SYSTEMS...............1-2 

EVOLUTION OF TRAFFIC FLOW SENSOR TECHNOLOGY ................................................1-4 

NEED FOR SENSOR ALTERNATIVES .....................................................................................1-5 

SENSOR TECHNOLOGY CHARACTERISTICS.......................................................................1-7 

MODERN VEHICLE SENSORS ................................................................................................1-11 

INDUCTIVE LOOP DETECTORS.............................................................................................1-11 

MAGNETIC SENSORS.................................................................................................................1-13 

VIDEO IMAGE PROCESSORS ..................................................................................................1-14 

MICROWAVE RADAR SENSORS..............................................................................................1-15 

INFRARED SENSORS .................................................................................................................1-17 

LASER RADAR SENSORS..........................................................................................................1-18 

OTHER SENSOR TECHNOLOGIES.........................................................................................1-19 

DEFINITION OF TERMS ..........................................................................................................1-19 

ORGANIZATION OF HANDBOOK...........................................................................................1-21 

REFERENCES .............................................................................................................................1-21 

CHAPTER 2. SENSOR TECHNOLOGY ............................................................................................2-1 

INDUCTIVE LOOP DETECTORS...............................................................................................2-1 

THEORY OF OPERATION ..........................................................................................................2-2 

LOOP WIRE AND CABLE RESISTANCE..................................................................................2-2 

LOOP INDUCTANCE.....................................................................................................................2-2 

FERROMAGNETIC EFFECT AND VEHICLE DETECTION ................................................2-4 

LOOP CHARACTERISTICS ........................................................................................................2-5

LOOP CAPACITANCE...................................................................................................................2-5 

LOOP QUALITY FACTOR Q ........................................................................................................2-7 

LOOP LEAD-IN WIRE .................................................................................................................2-10 

LEAD-IN CABLE ...........................................................................................................................2-11 

CALCULATING INDUCTANCE................................................................................................2-11 

LOOP SYSTEM INDUCTANCE CALCULATIONS ................................................................2-12 

NUMBER OF TURNS REQUIRED............................................................................................2-13 

LOOP SENSITIVITY TO AN ELECTRICALLY CONDUCTIVE OBJECT ........................2-13 

BICYCLE AND MOTORIZED VEHICLE DETECTION MODELS......................................2-14 

MUTUAL INDUCTANCE.............................................................................................................2-15 

LOOP SENSITIVITY ....................................................................................................................2-15 

EFFECT OF REINFORCING STEEL .......................................................................................2-17 

LOOP SYSTEM SENSITIVITY...................................................................................................2-19 

SENSITIVITY OF TWO SERIES INDUCTORS ......................................................................2-20 

SENSITIVITY OF TWO PARALLEL INDUCTORS ...............................................................2-21 

SINGLE LOOP EXAMPLE ..........................................................................................................2-21 

TWO LOOPS IN SERIES EXAMPLE ........................................................................................2-23 

TWO LOOPS IN PARALLEL EXAMPLE .................................................................................2-24 

RESONANT CIRCUIT..................................................................................................................2-26 

TEMPORARY LOOPS.................................................................................................................2-27 

MAT-TYPE LOOPS .......................................................................................................................2-27 

OPEN LOOP CONFIGURATION...............................................................................................2-27 

ELECTRONICS UNITS ..............................................................................................................2-30 

ANALOG PHASE-SHIFT ELECTRONICS UNIT ...................................................................2-30 

OVERVIEW OF DIGITAL ELECTRONICS UNITS ...............................................................2-31 
iv 

DIGITAL FREQUENCY SHIFT ELECTRONICS UNIT........................................................2-32 

DIGITAL RATIOED FREQUENCY SHIFT ELECTRONICS UNIT....................................2-33 

DIGITAL PERIOD SHIFT ELECTRONICS UNIT .................................................................2-35 

DIGITAL RATIOED PERIOD SHIFT ELECTRONICS UNIT .............................................2-38 

COMPARISON OF DIGITAL ELECTRONICS UNITS..........................................................2-38 

MULTICHANNEL DIGITAL MODELS ....................................................................................2-39 

ADDED FEATURES FOR DIGITAL ELECTRONICS UNITS .............................................2-40 

Open Loop Test..........................................................................................................................2-40 

Automatic Reset ........................................................................................................................2-40 

Remote Reset..............................................................................................................................2-40 

INDEPENDENT LOOP FAIL OUTPUT ...................................................................................2-41 

VEHICLE CLASSIFICATION.....................................................................................................2-41

NEMA STANDARDS.....................................................................................................................2-43 

NEMA ELECTRONICS UNIT CONFIGURATIONS ..............................................................2-43 

OUTPUT TYPES............................................................................................................................2-44 

PRESENCE AND PULSE MODES OF OPERATION ............................................................2-46 

CROSSTALK ..................................................................................................................................2-46 

TIMING FEATURES ....................................................................................................................2-46 

TYPE 170 CONTROLLER SPECIFICATION..........................................................................2-48 

2070 ADVANCED TRANSPORTATION CONTROLLER SPECIFICATION ....................2-48 

MODEL 222/224 INDUCTIVE-LOOP DETECTOR ELECTRONICS UNITS ....................2-53 

SELECTION FACTORS...............................................................................................................2-53 

Tuning Range.............................................................................................................................2-53 

Response Time ...........................................................................................................................2-54 

Recovery from Sustained Occupancy ..................................................................................2-54 

Loss of Detection During Saturated Flow ..........................................................................2-54 

Sensitivity with Pavement Overlay......................................................................................2-54 

Pulse-Mode Reset ......................................................................................................................2-55 

OPERATION WITH GROUNDED OR OPEN LOOPS...........................................................2-55 

Lightning Damage and Electrical Interference ................................................................2-55 

MAGNETIC SENSORS ...............................................................................................................2-56 

THEORY OF OPERATION ........................................................................................................2-57

TWO-AXIS FLUXGATE MAGNETOMETERS..........................................................................2-59 

MAGNETOMETER SENSOR AND COMPANION ELECTRONICS UNIT DESIGN.......2-60 

MAGNETOMETER DETECTION SENSITIVITY...................................................................2-61 

ELECTRONICS UNIT ..................................................................................................................2-62 

MODEL 227/228 MAGNETOMETER SYSTEM .......................................................................2-62 

SPVD TWO-AXIS FLUXGATE MAGNETOMETER ...............................................................2-63 

GROUNDHOG MAGNETOMETER ...........................................................................................2-64 

MAGNETIC DETECTORS..........................................................................................................2-65 

THEORY OF OPERATION ........................................................................................................2-65

MODEL 231/232 MAGNETIC DETECTOR SYSTEM.............................................................2-66 

FLUSH-MOUNTED MAGNETIC DETECTOR........................................................................2-67 

MICROLOOP PROBES................................................................................................................2-67 

VIDEO IMAGE PROCESSORS..................................................................................................2-68 

VIDEO IMAGE PROCESSING VEHICLE DETECTION CONCEPTS...............................2-68 

SIGNAL PROCESSING................................................................................................................2-70 

PERFORMANCE...........................................................................................................................2-72 

MOUNTING AND TRAFFIC VIEWING CONSIDERATIONS..............................................2-75 

VIDEO SURVEILLANCE ON FREEWAYS AND ARTERIALS............................................2-79 

MICROWAVE RADAR SENSORS .............................................................................................2-79 

CW DOPPLER RADAR ................................................................................................................2-79 

FMCW RADAR...............................................................................................................................2-80 
v 

LASER RADARS..........................................................................................................................2-83 

PASSIVE INFRARED SENSORS..............................................................................................2-85 

THEORY OF OPERATION .........................................................................................................2-87 

ULTRASONIC SENSORS ..........................................................................................................2-89 

PASSIVE ACOUSTIC ARRAY SENSORS ................................................................................2-92 

SINGLE LANE MODEL OPERATION......................................................................................2-92 

MULTIPLE LANE MODEL OPERATION ...............................................................................2-94 

SENSOR COMBINATIONS .......................................................................................................2-94 

REFERENCES.............................................................................................................................2-95 

CHAPTER 3. SENSOR APPLICATIONS..........................................................................................3-1 

PRESENCE AND PASSAGE SENSORS.....................................................................................3-1 

SPEED MONITORING WITH INDUCTIVE LOOP AND OTHER POINT SENSORS .........3-2 

TRAFFIC MANAGEMENT CONCEPTS FOR FREEWAYS......................................................3-3 

SIGNAL CONTROL CONCEPTS FOR CITY STREETS ..........................................................3-4 

STRATEGIES FOR LOCAL INTERSECTION SIGNAL CONTROL ....................................3-6 

Pretimed Control.........................................................................................................................3-6 

Actuated Control .........................................................................................................................3-6 

STRATEGIES FOR CONTROL OF GROUPS OF INTERCONNECTED 

INTERSECTIONS........................................................................................................................3-8 

Sensor Data...................................................................................................................................3-9 

Arterial Systems ..........................................................................................................................3-9 

Interconnected Control and Classic Pretimed Network Systems ................................3-10 

Control of Groups of Intersections Using On-Line Generated Timing Plans............3-10 

Review of Strategies for Control of Groups of Intersections.........................................3-13 

SYSTEMS ENGINEERING PROCESS IN DESIGN OF TRAFFIC SIGNAL SYSTEMS ....3-15 

FREEWAY SURVEILLANCE AND CONTROL........................................................................3-16 

FREEWAY INCIDENT DETECTION........................................................................................3-16 

Traffic Flow Characteristics During an Incident .............................................................3-17 

FREEWAY METERING ...............................................................................................................3-18 

Ramp Closure.............................................................................................................................3-19 

Ramp Metering ..........................................................................................................................3-19 

Pretimed Ramp Metering........................................................................................................3-21 

Local Traffic Responsive Metering.......................................................................................3-22 

Coordinated Traffic Responsive Control............................................................................3-25 

Mainline Metering.....................................................................................................................3-27 

Freeway-to-Freeway Metering ..............................................................................................3-29 

SPEED MONITORING AND DRIVER NOTIFICATION AT CRITICAL 

FREEWAY SEGMENTS............................................................................................................3-30 

COORDINATED OPERATION OF FREEWAYS AND SURFACE ARTERIALS .................3-31 

TRAFFIC DATA COLLECTION ................................................................................................3-32 

DATA FOR REAL-TIME TRAFFIC OPERATIONS ...............................................................3-33 

DATA FOR OFFLINE TRAFFIC PLANNING AND ADMINISTRATION .........................3-35 

DATA FOR EVALUATING MEASURES OF EFFECTIVENESS.........................................3-36 

System-Wide MOEs ...................................................................................................................3-36 

DETECTION OF PRIORITY VEHICLES.................................................................................3-37 

HIGH-INTENSITY LIGHT BEACON ........................................................................................3-38 

RADIO BEACON ...........................................................................................................................3-38 

INFRARED BEACON...................................................................................................................3-39 

INDUCTIVE LOOP WITH VEHICLE-MOUNTED TRANSMITTER ..................................3-39 

INDUCTIVE LOOP WITH VEHICLE CLASSIFICATION ALGORITHM .........................3-40 

LASER RADAR..............................................................................................................................3-41 

SOUND DETECTION SYSTEMS ...............................................................................................3-41 
vi 

PEDESTRIAN DETECTION AND SIGNAL ACTUATION ....................................................3-42 

DRIVER WARNING FOR RED SIGNAL AHEAD ...................................................................3-45 

TRAFFIC COUNTING AND VEHICLE CLASSIFICATION..................................................3-46 

COUNTS BY LOOP INDUCTIVE DETECTORS.....................................................................3-47 

COUNTS USING LONG LOOPS ................................................................................................3-47 

DIRECTIONAL DETECTION USING INDUCTIVE LOOPS ...............................................3-48 

VEHICLE CLASSIFICATION SENSORS.................................................................................3-48 

OVERHEIGHT SENSORS..........................................................................................................3-48 

WEATHER SENSORS.................................................................................................................3-49 

VEHICLE-MOUNTED SENSORS THAT ENHANCE SAFE OPERATION ..........................3-51 

IN-VEHICLE SENSORS FOR DISTANCE WARNING, CRUISE CONTROL, 

AUTOMATIC HIGHWAY SYSTEM FUNCTIONS, AND PRECRASH DETECTION .......3-52 

IN-VEHICLE OPTICAL SENSORS FOR PASSENGER DETECTION, VISION 

ENHANCEMENT, AND LANE CONTROL ...........................................................................3-53 

REFERENCES.............................................................................................................................3-55 

CHAPTER 4. IN-ROADWAY SENSOR DESIGN .............................................................................4-1 

SELECTION CRITERIA...............................................................................................................4-1 

SELECTION BASED ON SENSOR TECHNOLOGY................................................................4-1 

SELECTION BASED ON APPLICATION..................................................................................4-2 

SELECTION BASED ON EASE OF INSTALLATION .............................................................4-2 

SELECTION BASED ON EASE OF MAINTENANCE .............................................................4-2 

DESIGN CONSIDERATIONS......................................................................................................4-2 

TIMING PARAMETERS ................................................................................................................4-3 

Minimum Green Interval ..........................................................................................................4-3 

Passage Time Interval................................................................................................................4-4 

Maximum Green Interval..........................................................................................................4-5 

Volume-Density Mode ................................................................................................................4-5 

LOW-SPEED APPROACHES........................................................................................................4-6

Locking Memory with Point Detection .................................................................................4-7 

Nonlocking Memory with Larger-Area Presence Detection............................................4-7 

High-Speed Approaches ............................................................................................................4-9 

Rural High-Speed Roadways..................................................................................................4-11

REST-IN-RED SPEED CONTROL ............................................................................................4-13 

INDUCTIVE LOOP DETECTOR DESIGN ALTERNATIVES................................................4-14 

SMALL-AREA DETECTION .......................................................................................................4-14

LARGER-AREA DETECTION ....................................................................................................4-16 

Long Loops..................................................................................................................................4-17 

Sequential Short Loops ...........................................................................................................4-19 

Wide Loops..................................................................................................................................4-20 

LEFT-TURN LANE DETECTION..............................................................................................4-21 

THROUGH LANE DETECTION ................................................................................................4-23 

Single-Point Detection.............................................................................................................4-23 

Long-Loop-Occupancy Detection..........................................................................................4-24 

High-Speed Point Detection...................................................................................................4-24 

DETECTION FOR DILEMMA ZONES .....................................................................................4-25 

Definition of the Dilemma Zone Problem ...........................................................................4-25 

Multiple-Point Detection.........................................................................................................4-32 

Dilemma Zone Design Options...............................................................................................4-37 

SPECIAL FUNCTION APPLICATIONS ..................................................................................4-38 

DETECTION OF SMALL VEHICLES.......................................................................................4-38 

Multiple, Interconnected Small Loops.................................................................................4-40 

Quadrupole-Loop Configuration...........................................................................................4-40 
vii 

Chevron-Loop Configuration.................................................................................................4-43

Long Loop with Powerhead....................................................................................................4-43 

Inductive-Loop Bicycle Detectors.........................................................................................4-44 

DETECTION OF LONG, HIGH-BED VEHICLES ..................................................................4-46 

QUEUE DETECTION...................................................................................................................4-47 

INDUCTIVE-LOOP DETECTOR DESIGN FOR TRAFFIC SIGNAL 

CONTROL SYSTEMS................................................................................................................4-48 

Accuracy Goals ..........................................................................................................................4-49 

Link Selection for Critical Intersection Control (CIC) ...................................................4-49 

Lane Selection............................................................................................................................4-50 

Longitudinal Placement..........................................................................................................4-51 

Summary of System Sensor Location Considerations.....................................................4-51 

INDUCTIVE-LOOP DETECTOR ELECTRONICS UNITS ....................................................4-52 

MAGNETOMETER CONFIGURATIONS.................................................................................4-52 

SITE SELECTION.........................................................................................................................4-53 

SENSITIVITY.................................................................................................................................4-54 

PROBE PLACEMENT..................................................................................................................4-55 

PROBE DEPTH .............................................................................................................................4-56 

SUGGESTED CONFIGURATIONS ...........................................................................................4-57 

WIRE SIZE AND CABLE SELECTION ....................................................................................4-57 

MAGNETIC-DETECTOR CONFIGURATION .........................................................................4-58 

MAGNETIC-DETECTOR PROBE PLACEMENT...................................................................4-59 

Distance from the Stopline .....................................................................................................4-59 

Lateral Placement of Probe....................................................................................................4-59 

SENSITIVITY.................................................................................................................................4-60 

REFERENCES.............................................................................................................................4-61 
viii 

CONTENTS OF VOLUME II 

CHAPTER 5. SENSOR-INSTALLATION TECHNIQUES..............................................................5-1 

TYPICAL PREINSTALLATION ACTIVITIES...........................................................................5-1 

SCALE DRAWING OF SENSOR INSTALLATION SITE........................................................5-2 

FIELD VISITS..................................................................................................................................5-2 

INSTALLATION CREW ESTIMATES ........................................................................................5-2 

EQUIPMENT REQUIREMENTS .................................................................................................5-3 

MATERIAL REQUIREMENTS.....................................................................................................5-4 

INDUCTIVE-LOOP DETECTOR INSTALLATION ..................................................................5-4 

INSTALLATION TECHNIQUES..................................................................................................5-5 

INDUCTIVE-LOOP DETECTOR FAILURES..........................................................................5-10 

Causes of Inductive-Loop Detector Failures......................................................................5-10 

Failure Frequency ....................................................................................................................5-11 

Failure Mechanisms .................................................................................................................5-11 

LOOP LAYOUT AND SAWCUTS ...............................................................................................5-12 

Corner Treatment .....................................................................................................................5-12 

Saw-Cutting Operations ..........................................................................................................5-13 

INSTALLING LOOP WIRE .........................................................................................................5-18 

Wire Type ....................................................................................................................................5-18 

Wire Insertion ............................................................................................................................5-24 

CROSSING PAVEMENT JOINTS .............................................................................................5-24 

TWISTING THE LOOP WIRE LEAD-IN ..................................................................................5-27 

CROSSING CURBS.......................................................................................................................5-29 

INSTALLATION OF PULL BOX AND CONDUIT..................................................................5-31 

TESTING THE LOOP...................................................................................................................5-32 

SEALING THE SAWCUT.............................................................................................................5-34 

Types of Sealants.......................................................................................................................5-34 

Sealant Application Techniques ...........................................................................................5-37 

SPLICING THE WIRE .................................................................................................................5-41 

Connecting the Wires ...............................................................................................................5-41 

Sealing the Splice from the Operating Environment ......................................................5-42 

LEAD-IN CABLE INSTALLATION ...........................................................................................5-45 

FINAL TESTS AND RECORD KEEPING................................................................................5-47 

GENERAL INSTALLATION GUIDELINES ............................................................................5-49 

INDUCTIVE-LOOP DETECTOR INSTALLATION ALTERNATIVES .................................5-51 

SLAB LOOPS .................................................................................................................................5-51 

ROUND LOOPS.............................................................................................................................5-53 

Round-Loop Installation in Roadways ................................................................................5-54 

Round-Loop Installation on Sidewalks ...............................................................................5-58 

INSTALLATION OF TWO-AXIS FLUXGATE MAGNETOMETERS.....................................5-63 

INSTALLATION OF SENSING PROBE...................................................................................5-63 

SPLICING THE CABLES ............................................................................................................5-65 

TESTING THE SYSTEM..............................................................................................................5-65 

SEALING THE HOLES AND CUTS ..........................................................................................5-66 

INSTALLATION OF MAGNETIC DETECTORS.....................................................................5-66 

INSTALLATION PROCEDURE .................................................................................................5-66 

TRENCHING..................................................................................................................................5-67 

CONNECTING THE SYSTEM....................................................................................................5-67 

TESTING THE SYSTEM..............................................................................................................5-68 

OVER-ROADWAY SENSOR INSTALLATION ........................................................................5-69 

INITIAL SENSOR EVALUATIONS...........................................................................................5-69 
ix 

VIDEO IMAGE PROCESSORS..................................................................................................5-69 

SITE SURVEYS .............................................................................................................................5-69 

LENS SELECTION .......................................................................................................................5-72 

CALIBRATION ISSUES...............................................................................................................5-76 

SERIAL DATA INTERPRETATION SOFTWARE..................................................................5-79 

AUTOSCOPE 2004 VIDEO IMAGE PROCESSOR DETECTION ZONES .........................5-79 

TRAFICON VIP2 VIDEO IMAGE PROCESSOR DETECTION ZONES............................5-80 

MICROWAVE RADAR SENSORS .............................................................................................5-82 

WHELEN TDN-30 CW DOPPLER SENSOR............................................................................5-83 

MICROWAVE SENSORS TC-20 CW DOPPLER SENSOR ...................................................5-84 

EIS RTMS FMCW RADAR ..........................................................................................................5-84 

ACCUWAVE 150LX FMCW RADAR..........................................................................................5-86 

LASER RADAR SENSORS.........................................................................................................5-87 

PASSIVE INFRARED SENSORS..............................................................................................5-89 

ULTRASONIC SENSORS ..........................................................................................................5-89 

PASSIVE ACOUSTIC SENSORS ..............................................................................................5-90 

IRD SMARTSONIC SINGLE-LANE ACOUSTIC SENSOR ..................................................5-90 

SMARTEK SAS-1 MULTIPLE-LANE ACOUSTIC SENSOR ................................................5-92 

SENSOR COMBINATIONS .......................................................................................................5-92 

REFERENCES .............................................................................................................................5-93 

CHAPTER 6. SENSOR MAINTENANCE ..........................................................................................6-1 

NATURE OF THE PROBLEM .....................................................................................................6-1 

FAILURE MODES IN INDUCTIVE-LOOP DETECTOR SYSTEMS.......................................6-2 

FAILURE MECHANISMS .............................................................................................................6-2 

Omitted Phase..............................................................................................................................6-2 

Stuck Signal..................................................................................................................................6-2 

Phase Extending to Maximum.................................................................................................6-2 

Intermittent Problems ...............................................................................................................6-3 

Crosstalk........................................................................................................................................6-3 

Splashover.....................................................................................................................................6-4 

CAUSES OF LOOP SYSTEM FAILURE.....................................................................................6-4 

FACTORS AFFECTING REQUIRED MAINTENANCE..........................................................6-4 

TROUBLESHOOTING PROCEDURES FOR INDUCTIVE-LOOP DETECTORS ................6-5 

SAWCUT MAINTENANCE............................................................................................................6-5 

REARRANGEMENT OF LOOP CONNECTIONS.....................................................................6-6 

ELIMINATING CROSSTALK .......................................................................................................6-6

SUBSTITUTION OF ELECTRONICS UNITS ...........................................................................6-8 

OPERATIONAL CHECK OF MALFUNCTIONING INDUCTIVE-LOOP DETECTOR.....6-9 

Adjacent Lane Detection...........................................................................................................6-9 

Intermittent Operation..............................................................................................................6-9 

System Sensitivity.......................................................................................................................6-9 

ELECTRICAL TESTING OF AN INDUCTIVE-LOOP DETECTOR SYSTEM..................6-10 

TEST PROCEDURES...................................................................................................................6-11 

Sequential Test Procedure .....................................................................................................6-14 

MAGNETOMETER SYSTEM MAINTENANCE .......................................................................6-16 

CAUSES OF MAGNETOMETER FAILURES..........................................................................6-17 

Probe Burial Depth...................................................................................................................6-17 

Probe Movement........................................................................................................................6-17 

Probe Cable.................................................................................................................................6-17 

Sawcut Maintenance ................................................................................................................6-18 

TROUBLESHOOTING PROCEDURES....................................................................................6-18 
x 

MAGNETIC DETECTOR MAINTENANCE..............................................................................6-19 

OVER-ROADWAY SENSOR MAINTENANCE.........................................................................6-21 

VIDEO IMAGE PROCESSORS ..................................................................................................6-21 

MICROWAVE RADAR SENSORS..............................................................................................6-21 

MAINTENANCE COST COMPARISONS AMONG SENSOR TECHNOLOGIES................6-21 

REFERENCES .............................................................................................................................6-22 

APPENDIX A. INDUCTIVE-LOOP SYSTEM EQUIVALENT CIRCUIT MODEL ................... A-1 

ABSTRACT.................................................................................................................................... A-1 

INTRODUCTION ......................................................................................................................... A-1 

LOOP CAPACITANCE THEORY ............................................................................................... A-2 

INTERNAL-LOOP CAPACITANCE ........................................................................................... A-2 

EXTERNAL-LOOP CAPACITANCE........................................................................................... A-4 

LOOP RESISTANCE THEORY .................................................................................................. A-6 

LOOP RESISTANCE ..................................................................................................................... A-6 

INTERNAL INDUCTANCE AND RESISTANCE PER UNIT LENGTH FOR A 

CYLINDRICAL CONDUCTOR................................................................................................. A-6 

LOOP-INDUCTANCE THEORY................................................................................................. A-7 

SELF-INDUCTANCE OF SINGLE-TURN CIRCULAR LOOP.............................................. A-7 

SELF-INDUCTANCE OF MULTITURN CIRCULAR LOOP................................................. A-8 

EXTERNAL INDUCTANCE OF SINGLE-TURN RECTANGULAR LOOP ........................ A-8 

SELF-INDUCTANCE OF SINGLE-TURN RECTANGULAR LOOP.................................... A-9 

SELF-INDUCTANCE OF MULTITURN RECTANGULAR LOOP ....................................... A-9 

MUTUAL INDUCTANCE OF PARALLEL FILAMENTARY CIRCUITS ............................ A-9 

MUTUAL INDUCTANCE OF TWO COAXIAL AND PARALLEL 

RECTANGULAR LOOPS ........................................................................................................A-10 

SELF-INDUCTANCE OF MULTITURN QUADRUPOLE LOOP .......................................A-10 

GENERAL FORMULA FOR MUTUAL INDUCTANCE OF 

PARALLEL FILAMENTS........................................................................................................A-11 

INDUCTIVE-LOOP CIRCUIT MODEL ...................................................................................A-12 

LOOP TRANSMISSION LINE THEORY ................................................................................ A-13 

LOOP TRANSMISSION LINE MODEL...................................................................................A-13 

FREQUENCY SHIFT ELECTRONICS UNIT SENSITIVITY..............................................A-14 

LOOP TRANSFORMER THEORY ........................................................................................... A-15 

INDUCTIVE-LOOP TRANSFORMER MODEL .....................................................................A-15 

LOOP-DETECTOR ANALYSIS SYSTEM PROGRAM ........................................................... A-17 

COMPARISON OF CALCULATED AND MEASURED LOOP SELF-INDUCTANCE 

AND QUALITY FACTOR........................................................................................................ A-17 

CONCLUSIONS ............................................................................................................................A-19 

APPENDIX A-1 ........................................................................................................................... A-20 

LOOP GROUND-RESISTANCE DERIVATION.....................................................................A-20 

APPENDIX A-2 ........................................................................................................................... A-20 

REAL PART OF COMPLEX BESSEL FUNCTION OF FIRST KIND ...............................A-20 

DERIVATIVE OF REAL PART .................................................................................................A-21 

IMAGINARY PART OF COMPLEX BESSEL FUNCTION OF FIRST KIND...................A-21 

DERIVATIVE OF IMAGINARY PART ....................................................................................A-21 

APPENDIX A-3 ........................................................................................................................... A-21 

COMPLETE ELLIPTIC INTEGRAL OF FIRST KIND ........................................................A-21 

COMPLETE ELLIPTIC INTEGRAL OF SECOND KIND....................................................A-21 

APPENDIX A-4 ........................................................................................................................... A-22 

SERIES TO PARALLEL CIRCUIT TRANSFORMATION...................................................A-22 
xi 

APPENDIX A-5 ........................................................................................................................... A-23 

TRANSFORMER MODEL INPUT IMPEDANCE..................................................................A-23 

REFERENCES ............................................................................................................................ A-24 

APPENDIX B. CURRENT SHEET FORMULA FOR CALCULATION OF 

LOOP INDUCTANCE .......................................................................................................................B-1 

SAMPLE INDUCTANCE CALCULATION................................................................................ B-2 

REFERENCES .............................................................................................................................. B-3 

APPENDIX C. LOOP INDUCTANCE AND QUALITY FACTOR TABLES...............................C-1 

ABSTRACT.................................................................................................................................... C-1 

APPENDIX D. ELECTRICAL CHARACTERISTICS OF WIRE AND CABLE .........................D-1 

CAPACITANCE OF PARALLEL CONDUCTORS ....................................................................D-1 

INDUCTANCE OF PARALLEL CONDUCTORS......................................................................D-3 

APPENDIX E. VEHICLE DETECTION SENSITIVITY FORMULAS FOR 

RECTANGULAR LOOPS................................................................................................................. E-1 

ABSTRACT.................................................................................................................................... E-1 

INTRODUCTION ......................................................................................................................... E-1 

VEHICLE DETECTION SENSITIVITY THEORY ................................................................... E-2 

VEHICLE MODEL ......................................................................................................................... E-2 

REINFORCING STEEL MODEL ................................................................................................ E-2 

CIRCUIT MODEL FOR ONE-TURN LOOP.............................................................................. E-3 

Circuit Equations for One-Turn Loop...................................................................................E-3 

Inductive-Loop Driving-Point Impedance........................................................................... E-4 

Inductive-Loop Sensitivity ...................................................................................................... E-4 

CIRCUIT MODEL FOR TWO-TURN LOOP ............................................................................. E-5 

Circuit Equations for Two-Turn Loop..................................................................................E-6 

CIRCUIT EQUATIONS FOR MULTITURN LOOP................................................................. E-6 

VEHICLE DETECTION SENSITIVITY FOR A MULTITURN LOOP................................. E-7 

INDUCTIVE-LOOP DETECTOR SENSITIVITY .....................................................................E-8 

LOOP-DETECTOR SENSITIVITY MEASURED DATA AND 

CALCULATED RESULTS........................................................................................................ E-9 

LOOP-DETECTOR SENSITIVITY COMPUTER PROGRAM............................................... E-9 

MEASURED-LOOP DETECTOR SENSITIVITY DATA.........................................................E-9 

COMPARISON BETWEEN MEASURED AND CALCULATED 

SENSITIVITY DATA .................................................................................................................. E-9 

EFFECT OF LOOP TURNS ON SENSITIVITY .....................................................................E-10 

EFFECT OF LOOP VOLUME ON SENSITIVITY .................................................................E-10 

EFFECT OF LEAD-IN CABLE INDUCTANCE ON SENSITIVITY ...................................E-11 

EFFECT OF MESH ON SENSITIVITY....................................................................................E-12 

EFFECT OF LEAD-IN CABLE AND MESH ON SENSITIVITY .........................................E-12 

RESULTS AND CONCLUSIONS ............................................................................................. E-13 

REFERENCES ............................................................................................................................ E-14 

APPENDIX F. DIGITAL FREQUENCY-SHIFT ELECTRONICS UNIT ANALYSIS............... F-1 

ABSTRACT.................................................................................................................................... F-1 

ANALYSIS..................................................................................................................................... F-1 
xii 

APPENDIX G. DIGITAL RATIOED FREQUENCY-SHIFT ELECTRONICS 

UNIT ANALYSIS................................................................................................................................G-1 

ABSTRACT.................................................................................................................................... G-1 

ANALYSIS..................................................................................................................................... G-1 

APPENDIX H. DIGITAL PERIOD-SHIFT ELECTRONICS UNIT ANALYSIS........................H-1 

ABSTRACT....................................................................................................................................H-1 

ANALYSIS.....................................................................................................................................H-1 

APPENDIX I. DIGITAL RATIOED PERIOD-SHIFT ELECTRONICS UNIT ANALYSIS ...... I-1 

ABSTRACT..................................................................................................................................... I-1 

ANALYSIS...................................................................................................................................... I-1 

APPENDIX J. NEMA DETECTOR STANDARDS EXCERPTS....................................................J-1 

INTRODUCTION ..........................................................................................................................J-1 

NEMA TS 1 AND TS 2 TRAFFIC CONTROL SYSTEMS..........................................................J-2 

DETECTOR TERMS AND DEFINITIONS.................................................................................J-2 

1.2.4 DETECTION ...........................................................................................................................J-2 

1.2.4.1 Advisory Detection.........................................................................................................J-2 

1.2.4.2 Passage Detection...........................................................................................................J-2 

1.2.4.3 Presence Detection.........................................................................................................J-3 

1.2.5 DETECTOR.............................................................................................................................J-3 

1.2.5.1 Bidirectional Detector...................................................................................................J-3 

1.2.5.2 Calling Detector ..............................................................................................................J-3 

1.2.5.3 Classification Detector..................................................................................................J-3 

1.2.5.4 Directional Detector ......................................................................................................J-3 

1.2.5.5 Extension Detector.........................................................................................................J-3 

1.2.5.6 Infrared Detector............................................................................................................J-3 

1.2.5.7 Light-Sensitive Detector...............................................................................................J-3 

1.2.5.8 Loop Detector ..................................................................................................................J-3 

1.2.5.9 Magnetic Detector ..........................................................................................................J-3 

1.2.5.10 Magnetometer Detector ..............................................................................................J-3

1.2.5.11 Nondirectional Detector .............................................................................................J-4 

1.2.5.12 Pedestrian Detector .....................................................................................................J-4 

1.2.5.13 Pneumatic Detector .....................................................................................................J-4 

1.2.5.14 Pressure-Sensitive Detector ......................................................................................J-4 

1.2.5.15 Radar Detector..............................................................................................................J-4 

1.2.5.16 System Detector ............................................................................................................J-4 

1.2.5.17 Side-Fire Detector ........................................................................................................J-4 

1.2.5.18 Sound-Sensitive Vehicle Detector ............................................................................J-4 

1.2.5.19 Ultrasonic Detector......................................................................................................J-4 

1.2.6 DETECTOR MODE................................................................................................................J-4 

1.2.7 INDUCTIVE LOOP DETECTOR SYSTEM.......................................................................J-4 

1.2.8 INDUCTIVE LOOP DETECTOR UNIT.............................................................................J-4 

1.2.9 LEAD-IN CABLE....................................................................................................................J-5 

1.2.10 OUTPUT ................................................................................................................................J-5 

1.2.10.1 Extension Output..........................................................................................................J-5 

1.2.10.2 Delayed Output .............................................................................................................J-5 

1.2.11 PROBE ...................................................................................................................................J-5 

1.2.12 SENSOR.................................................................................................................................J-5 

1.2.13 VEHICLE DETECTOR SYSTEM......................................................................................J-5 

1.2.14 ZONE OF DETECTION (SENSING ZONE)....................................................................J-5 
xiii 

ENVIRONMENTAL TESTING OF DETECTORS .....................................................................J-5 

2.8 LOOP DETECTOR UNIT TESTS ..........................................................................................J-5 

2.8.1 Environmental Requirements ........................................................................................J-5 

MINIMUM REQUIREMENTS FOR DETECTORS....................................................................J-8 

6.5 INDUCTIVE LOOP DETECTOR UNITS..............................................................................J-8 

6.5.1 Loop Detector Unit Definitions......................................................................................J-8 

6.5.2 Functional Standards .................................................................................................... J-10 

REFERENCES .............................................................................................................................J-25 

APPENDIX K. CALTRANS TRANSPORTATION ELECTRICAL 

EQUIPMENT SPECIFICATIONS (TEES) FOR ELECTRONICS 

UNITS AND MAGNETIC DETECTORS .......................................................................................K-1 

SECTION 1. GENERAL REQUIREMENTS .............................................................................. K-1 

5.1.1 CHANNEL INDEPENDENCE AND POWER REQUIREMENTS................................K-1 

5.1.2 SENSOR UNIT FEATURES................................................................................................K-1 

5.1.3 OUTPUT DEVICE TYPE.....................................................................................................K-1 

5.1.4 OUTPUT SIGNAL CHARACTERISTICS.........................................................................K-1 

5.1.5 INTERFACE REQUIREMENTS ........................................................................................K-2 

5.1.6 OUTPUT DEVICE SWITCHING TIME ............................................................................K-2 

5.1.7 SERIAL OUTPUT DEFINITION .......................................................................................K-2 

5.1.8 ELECTRICAL SURGE PROTECTION.............................................................................K-2 

SECTION 2. MODEL 222E AND 224E LOOP DETECTOR SENSOR UNIT 

REQUIREMENTS ..................................................................................................................... K-2 

5.2.1 OUTPUT SIGNAL DEFINITION.......................................................................................K-2 

5.2.2 MALFUNCTIONING LOOP SIGNAL DEFINITION .....................................................K-2 

5.2.3 SUPPORTED LOOP CONFIGURATIONS ......................................................................K-3 

5.2.4 INDUCTANCE AND Q RANGE OF SUPPORTED LOOP AND LEAD-IN 

CABLE ASSEMBLIES................................................................................................................K-3 

5.2.5 TRANSFORMER ISOLATION OF LOOP INPUTS .......................................................K-3 

5.2.6 NUMBER OF SUPPORTED OPERATING FREQUENCIES.......................................K-3 

5.2.7 DRIFT COMPENSATION OF TUNING CIRCUITS ......................................................K-3 

5.2.8 PULSE AND PRESENCE MODE SUPPORT..................................................................K-3 

5.2.8.1: Pulse Mode Description ..............................................................................................K-3 

5.2.8.2: Presence Mode Description .......................................................................................K-4 

5.2.9 SENSITIVITY.........................................................................................................................K-4 

5.2.10 RESPONSE TIME...............................................................................................................K-5 

5.2.11 NORMAL OPERATION TIME .........................................................................................K-5 

5.2.12 TRACKING RATE...............................................................................................................K-5 

5.2.13 TRACKING RANGE ...........................................................................................................K-5 

5.2.14 TEMPERATURE CHANGE ......................................................................................................... K-5 

SECTION 3. MAGNETIC DETECTOR REQUIREMENTS...................................................... K-6 

5.3.1 MODEL 231E MAGNETIC DETECTOR SENSING ELEMENT ..................................K-6 

5.3.1.1 Design Requirements....................................................................................................K-6 

5.3.1.2 DC Resistance.................................................................................................................K-6 

5.3.2 MODEL 232E TWO CHANNEL MAGNETIC DETECTOR SENSING UNIT.............K-6 

5.3.2.1 Output Signal Definition .............................................................................................K-6 

REFERENCES .............................................................................................................................. K-6 

APPENDIX L. CLASSIFICATION OF SENSOR SYSTEMS BY SENSOR 

DENSITY LEVEL .............................................................................................................................. L-1 

ABSTRACT.................................................................................................................................... L-1 

ANALYSIS..................................................................................................................................... L-1 
xiv 

TERMINOLOGY ........................................................................................................................... L-1 

UNCOORDINATED SIGNALS or UNCOORDINATED CONTROL .................................... L-1 

TIME-BASED COORDINATION or TIME-BASED COORDINATED CONTROL ............ L-1 

INTERCONNECTED CONTROL................................................................................................ L-2 

TRAFFIC ADJUSTED CONTROL.............................................................................................. L-2 

TRAFFIC RESPONSIVE CONTROL ......................................................................................... L-2 

TRAFFIC ADAPTIVE CONTROL............................................................................................... L-2 

SENSOR WEB DENSITY LEVELS ............................................................................................ L-3 

SENSOR WEB DENSITY LEVEL 0.0 ......................................................................................... L-3 

SENSOR WEB DENSITY LEVEL 0.5 ......................................................................................... L-4 

SENSOR WEB DENSITY LEVEL 1.0 ......................................................................................... L-4 

SENSOR WEB DENSITY LEVEL 1.5 ......................................................................................... L-5 

SENSOR WEB DENSITY LEVEL 2.0 ......................................................................................... L-6 

SENSOR WEB DENSITY LEVEL 2.5 ......................................................................................... L-7 

SENSOR WEB DENSITY LEVEL 3.0 ......................................................................................... L-8 

SENSOR WEB DENSITY LEVEL 3.5 ....................................................................................... L-10 

SENSOR WEB DENSITY LEVEL 4.0 ....................................................................................... L-10 

GENERAL OBSERVATIONS.................................................................................................... L-10 

REFERENCES ............................................................................................................................ L-11 

APPENDIX M. EXTENT AND CAUSES OF INDUCTIVE-LOOP FAILURES ........................ M-1 

STUDIES SUMMARIZED IN THIS APPENDIX ......................................................................M-1 

OREGON STUDY .........................................................................................................................M-1 

OREGON SURVEY RESULTS.................................................................................................... M-2 

OREGON CONCLUSIONS AND RECOMMENDATIONS .................................................... M-3 

WASHINGTON STUDY ...............................................................................................................M-4 

WASHINGTON SURVEY RESULTS ......................................................................................... M-4 

WASHINGTON CONCLUSIONS AND RECOMMENDATIONS.......................................... M-6 

MINNESOTA STUDY ..................................................................................................................M-7 

MINNESOTA SURVEY RESULTS............................................................................................. M-7 

MINNESOTA CONCLUSIONS AND RECOMMENDATIONS ............................................. M-9 

NEW YORK STUDY....................................................................................................................M-11 

NEW YORK SURVEY RESULTS ............................................................................................. M-11 

NEW YORK CONCLUSIONS AND RECOMMENDATIONS .............................................. M-13 

APPENDIX N. GROUNDING (DESIGN GUIDELINES)...............................................................N-1 

SECTION I—REASONS FOR GROUNDING ...........................................................................N-1 

1. SAFETY GROUNDING .............................................................................................................N-1 

2. SYSTEM GROUNDING.............................................................................................................N-1 

3. LIGHTNING DISCHARGE ......................................................................................................N-1

SECTION II—CALCULATION OF RESISTANCE TO GROUND..........................................N-1 

1. GENERAL....................................................................................................................................N-1 

2. SOIL RESISTIVITY ...................................................................................................................N-2 

3. GROUND ELECTRODE RESISTANCE TO GROUND ......................................................N-3 

3.1 Ground Rods .........................................................................................................................N-3 

3.2 Pedestals ................................................................................................................................N-4 

3.3 Plate Electrodes ...................................................................................................................N-4 

3.4 Wire Grids..............................................................................................................................N-5 

3.5 Multiple Rods........................................................................................................................N-5 

3.6 Combination Rod and Wire Grids....................................................................................N-6 

3.7 Single Wire.............................................................................................................................N-7 

3.8 Summary of Calculations...................................................................................................N-8 
xv 

3.9 Application ............................................................................................................................N-9 

3.10 Problem Areas ..................................................................................................................N-10 

3.11 Application Guidelines...................................................................................................N-14 

SECTION III—EFFECTS OF LIGHTNING............................................................................N-14 

1. GENERAL..................................................................................................................................N-14 

2. DESIGN CRITERIA.................................................................................................................N-15 

3. POWER SURGES .....................................................................................................................N-16 

4. OTHER SOURCES OF POSSIBLE DAMAGE....................................................................N-18 

SECTION IV—SUMMARY OF DESIGN GUIDELINES ........................................................N-21 

1. TRAFFIC SIGNAL SYSTEMS ...............................................................................................N-21 

SECTION V—REFERENCES ...................................................................................................N-22 

APPENDIX O. GROUNDING (MAINTENANCE GUIDELINES)................................................O-1 

SECTION I—TRAFFIC SIGNAL GROUNDING ......................................................................O-1 

1. HYDRO GROUNDS ...................................................................................................................O-1 

2. SERVICE GROUNDS ................................................................................................................O-1 

3. CABINET GROUNDS................................................................................................................O-2 

4. SPECIAL SOIL CONDITIONS................................................................................................O-2 

5. SYSTEM GROUND ....................................................................................................................O-2 

6. ROUTINE INSPECTION..........................................................................................................O-2 

7. EMERGENCY INSPECTION...................................................................................................O-2 

8. MEASUREMENTS .....................................................................................................................O-3 

9. STEEL FOOTINGS ....................................................................................................................O-4 

10. GROUND RODS .......................................................................................................................O-4 

11. GOOD PRACTICE ...................................................................................................................O-7 

SECTION II—REFERENCES.....................................................................................................O-7 

APPENDIX P. GLOSSARY ................................................................................................................. P-1 

APPENDIX Q. REGISTERED TRADEMARKS ..............................................................................Q-1 
xvi 

FIGURES IN VOLUME I 

CHAPTER 1. INTRODUCTION..........................................................................................................1-1

Figure 1-1. Growth in highway miles traveled in the U.S. ...............................................................1-2 

Figure 1-2. Freeway-to-freeway HOV bypass lane structure under construction at 

intersection of CA-57 and CA-91 freeways in Anaheim, CA. .........................................1-3 

Figure 1-3. Inductive-loop detector system. ......................................................................................1-12 

Figure 1-4. Inductive-loop installation example...............................................................................1-12 

Figure 1-5. Magnetic anomaly in the Earth’s magnetic field induced by magnetic dipoles 

in a ferrous metal vehicle................................................................................................1-13 

Figure 1-6. Overhead camera mounting on a mast arm as typically used to provide imagery 

to a VIP for arterial signal control.. ...............................................................................1-15 

Figure 1-7. Microwave radar operation.............................................................................................1-16 

Figure 1-8. Mounting of presence-detecting microwave radar sensors for multilane vehicle 

detection and signal actuation at an intersection. ........................................................1-17 

Figure 1-9. Scanning infrared laser radar two-beam pattern across a traffic lane........................1-18 

Figure 1-10. 3-D laser radar range image of a van pulling a boat. ..................................................1-18 

CHAPTER 2. SENSOR TECHNOLOGY ............................................................................................2-1 

Figure 2-1. Inductive-loop detector system (notional)........................................................................2-1 

Figure 2-2. Magnetic flux around loop. ...............................................................................................2-3 

Figure 2-3. Magnetic flux for solenoid (coil)........................................................................................2-3 

Figure 2-4. Capacitive coupling between the loop wires themselves and the sawcut 

slot sidewalls......................................................................................................................2-5 

Figure 2-5. Equivalent electrical circuit for an inductive loop with capacitive coupling to the 

sidewalls of a sawcut slot..................................................................................................2-5

Figure 2-6. Average values of loop inductance vs. measuring frequency for series, parallel, 

and series-parallel connections of 6- x 6-ft (1.8- x 1.8-m) inductive loops. ...................2-6 

Figure 2-7. Loop system quality factor sample calculation................................................................2-9 

Figure 2-8. Four 6- x 6-ft (1.8- x 1.8-m) three-turn loops connected in series, parallel, and 

series-parallel. .................................................................................................................2-13 

Figure 2-9. Bicycle detection showing induced eddy currents. .......................................................2-14 

Figure 2-10. Vehicle undercarriage model. .........................................................................................2-15 

Figure 2-11. Calculated sensitivity of three-turn inductive loops as a function of vehicle 

undercarriage height.......................................................................................................2-17

Figure 2-12. Calculated sensitivity of three-turn inductive loops with 200 ft (60 m) 

of lead-in cable as a function of vehicle undercarriage height. ....................................2-18 

Figure 2-13. Calculated sensitivity of two-turn long inductive loops as a function of vehicle 

undercarriage height.......................................................................................................2-18

Figure 2-14. Calculated sensitivity of 6- x 6-ft (1.8- x 1.8-m) inductive loop over 

reinforcing steel. ..............................................................................................................2-19 

Figure 2-15. Equivalent total inductance from two inductive loops in series. .................................2-20 

Figure 2-16. Equivalent total inductance from two inductive loops in parallel. ..............................2-21 

Figure 2-17. Single inductive loop connected to a pull box and electronics unit..............................2-22 

Figure 2-18. Equivalent single loop electrical circuit.........................................................................2-22 

Figure 2-19. Two inductive loops connected in series to a pull box and electronics unit.................2-23 

Figure 2-20. Equivalent electrical circuit for two loops connected in series to a pull box and 

electronics unit.................................................................................................................2-24 

Figure 2-21. Two inductive loops connected in parallel to a pull box and electronics unit. ............2-25 

Figure 2-22. Equivalent electrical circuit for two loops connected in parallel to a pull box 

and electronics unit. ........................................................................................................2-25 

Figure 2-23. Typical installation of mat-type temporary inductive loop detector............................2-27 
xvii 

Figure 2-24. Five-layer temporary open loop detector configuration................................................2-28 

Figure 2-25. Nevada portable open loop installation. ........................................................................2-29 

Figure 2-26. Inductance change produced by a small motorcycle as a function of lead-in cable 

length for series, parallel, and series-parallel connections of four 6- x 6-ft loops.......2-35 

Figure 2-27. Measurement of inductive loop oscillation period by a reference clock. .....................2-36 

Figure 2-28. Model S-1500 inductive-loop vehicle classifier and speed sensor. ...............................2-42 

Figure 2-29. Classes available from inductive-loop classifying sensor. ..........................................2-42 

Figure 2-30. Axle location and vehicle classification using an array of inductive loops..................2-43 

Figure 2-31. Shelf-mounted NEMA electronics unit. .........................................................................2-44

Figure 2-32. Two-channel card rack mounted electronics unit. ........................................................2-45 

Figure 2-33. Four-channel card rack mounted electronics unit. .......................................................2-45 

Figure 2-34. Delay operation................................................................................................................2-47 

Figure 2-35. Extension operation.........................................................................................................2-47 

Figure 2-36. Model 170 controller........................................................................................................2-48 

Figure 2-37. Type 170 cabinet layout (California)..............................................................................2-49 

Figure 2-38. Model 2070 controller......................................................................................................2-50 

Figure 2-39. Magnetic sensor installation...........................................................................................2-56 

Figure 2-40. Earth’s magnetic flux lines. ............................................................................................2-57 

Figure 2-41. Equatorial belt showing where Earth’s magnetic field is too small for deployment 

of simple vertical axis magnetometers between 20 degrees north and 20 degrees 

south of the Equator........................................................................................................2-58 

Figure 2-42. Distortion of Earth’s quiescent magnetic field by a ferrous metal vehicle..................2-59 

Figure 2-43. Magnetometer sensor electrical circuit (notional).........................................................2-60 

Figure 2-44. Magnetometer sensor probe and electronics unit equivalent electrical circuit...........2-61 

Figure 2-45. SPVD-2 magnetometer system.......................................................................................2-63 

Figure 2-46. Groundhog magnetometer sensors.................................................................................2-65 

Figure 2-47. Magnetic probe detector..................................................................................................2-67 

Figure 2-48. Microloop probes..............................................................................................................2-68 

Figure 2-49. Tripline video image processors. ....................................................................................2-69 

Figure 2-50. Closed-loop tracking video image processor. .................................................................2-69 

Figure 2-51. Video image processor installed in a roadside cabinet. ................................................2-70 

Figure 2-52. Conceptual image processing for vehicle detection, classification, and tracking........2-71 

Figure 2-53. Vehicle count comparison from four VIPs and inductive-loop detectors.. ...................2-73 

Figure 2-54. VIP vehicle count errors under varying illumination conditions.................................2-74 

Figure 2-55. Distinguishing between two closely spaced vehicles.....................................................2-76 

Figure 2-56. Distance d along the roadway at which a VIP can distinguish vehicles. ....................2-77 

Figure 2-57. Constant frequency waveform. .......................................................................................2-80 

Figure 2-58. CW Doppler microwave radars.......................................................................................2-80 

Figure 2-59. FMCW signal and radar processing as utilized to measure vehicle 

presence and speed..........................................................................................................2-81 

Figure 2-60. FMCW microwave radars. ..............................................................................................2-83 

Figure 2-61. Laser radar beam geometry............................................................................................2-84 

Figure 2-62. Autosense II laser radar sensor......................................................................................2-86 

Figure 2-63. EFKON Traffic Observation Module (TOM) laser radar sensor..................................2-84 

Figure 2-64. Active infrared sensor installed for transmitting traffic conditions to motorists.. .....2-85 

Figure 2-65. Visible spectrum CCD camera imagery of approaching traffic typical of dawn 

and dusk lighting.............................................................................................................2-86 

Figure 2-66. Passive infrared sensor. ..................................................................................................2-87 

Figure 2-67. Emission and reflection of energy by vehicle and road surface....................................2-87 

Figure 2-68. Multiple detection zone configuration in a passive infrared sensor. ...........................2-89 

Figure 2-69. TC-30C ultrasonic range-measuring sensor. .................................................................2-90 

Figure 2-70. Mounting of ultrasonic range-measuring sensors.........................................................2-90 

Figure 2-71. Speed-measuring RDU-101 Doppler ultrasonic sensor. ...............................................2-91 
xviii 

Figure 2-72. Operation of range-measuring ultrasonic sensor..........................................................2-92 

Figure 2-73. Acoustic array sensors.....................................................................................................2-93 

Figure 2-74. Passive infrared combination sensors............................................................................2-95 

CHAPTER 3. SENSOR APPLICATIONS..........................................................................................3-1 

Figure 3-1. Vehicle speed measurement using two inductive-loop detectors placed a known 

distance apart. ...................................................................................................................3-3 

Figure 3-2. Isolated intersection control. ............................................................................................3-7 

Figure 3-3. Data processing at an intersection with isolated intersection control...........................3-7 

Figure 3-4. Placement of sensors for fully actuated intersection control..........................................3-8 

Figure 3-5. Arterial open network and surface street closed network traffic signal 

configurations typical of those found in interconnected intersection control................3-9 

Figure 3-6. UTCS timing plan selection procedure. .........................................................................3-11 

Figure 3-7. Traffic flow characteristics during an incident. ............................................................3-17 

Figure 3-8. Freeway incident detection scenario..............................................................................3-18 

Figure 3-9. Conceptual ramp metering sensor and roadway configurations..................................3-21 

Figure 3-10. Conceptual coordinated traffic responsive ramp control system. ................................3-25 

Figure 3-11. Principles of SWARM 1 ramp metering algorithm. ......................................................3-26 

Figure 3-12. Mainline metering configuration on I-80 westbound crossing the San FranciscoOakland Bay Bridge. .......................................................................................................3-28 

Figure 3-13. Freeway mainline metering at El Cajon, CA.................................................................3-29 

Figure 3-14. Freeway-to-freeway metering at the junction of the I-105 and I-605 freeways 

in Norwalk, CA. ...............................................................................................................3-30 

Figure 3-15. Warning sign alerting drivers to unsafe speed..............................................................3-31 

Figure 3-16. Infrared beacon priority system.. ...................................................................................3-39 

Figure 3-17. Transmitter mounted under a vehicle identifies it to a subsurface inductiveloop detector.....................................................................................................................3-40 

Figure 3-18. Inductive loop detection system for transit vehicles.....................................................3-40 

Figure 3-19. Representative vehicle signatures obtained from loops excited by high-frequency 

signals from specialized electronics units......................................................................3-41 

Figure 3-20. Sound detection priority system for 4-way intersection control...................................3-41 

Figure 3-21. Manually operated pedestrian push button. .................................................................3-42 

Figure 3-22. Interactive pedestrian push button................................................................................3-43 

Figure 3-23. Countdown timer showing time remaining for pedestrian crossing............................3-43 

Figure 3-24. Audible pedestrian signal device....................................................................................3-44 

Figure 3-25. Pedestrian crossing safety devices. ................................................................................3-45 

Figure 3-26. PREPARE TO STOP lighted sign...................................................................................3-46 

Figure 3-27. Three-loop layout for counts. ..........................................................................................3-47 

Figure 3-28. Directional detection using point sensors......................................................................3-48 

Figure 3-29. Overheight sensor configuration. ...................................................................................3-49 

Figure 3-30. Integrated weather information system. .......................................................................3-50 

Figure 3-31. Weather imagery and data display. ...............................................................................3-50 

Figure 3-32. Autovue lane tracking sensor. ........................................................................................3-54 

Figure 3 33. Output voltage generated by passive lane positioning sensor. ....................................3-54 

CHAPTER 4. IN-ROADWAY SENSOR DESIGN .............................................................................4-1 

Figure 4-1. Actuated controller green phase intervals.......................................................................4-3

Figure 4-2. Variable initial NEMA timing for green signal phase....................................................4-6 

Figure 4-3. NEMA gap reduction procedure. ......................................................................................4-6 

Figure 4-4. Intersection configured for loop occupancy control. ........................................................4-8 

Figure 4-5. Inductive loop detector length for loop occupancy control..............................................4-9 

Figure 4-6. Dilemma zone for vehicle approaching an intersection at 40 mi/h (64 km/h).............4-11 
xix 

Figure 4-7. PREPARE TO STOP sign on an arterial in nonilluminated and 

illuminated states............................................................................................................4-12 

Figure 4-8. PREPARE TO STOP inductive-loop detector system. ..................................................4-13 

Figure 4-9. Small loop shapes. ...........................................................................................................4-15 

Figure 4-10. Caltrans-specified loop shapes. ......................................................................................4-16 

Figure 4-11. Long loop shapes..............................................................................................................4-17 

Figure 4-12. Caltrans standard sequential inductive loop configurations. ......................................4-19 

Figure 4-13. PennDOT short inductive loop configurations. .............................................................4-20 

Figure 4-14. Wide inductive-loop detector layout...............................................................................4-21 

Figure 4-15. Left turn detection inductive loop configuration used by IDOT. .................................4-23 

Figure 4-16. “Cannot Stop” and “Cannot Go” regions. .......................................................................4-26 

Figure 4-17. Dilemma zone (Xs > Xc)....................................................................................................4-28 

Figure 4-18. Dilemma zone removal (Xs = Xc). ....................................................................................4-28 

Figure 4-19. Optional zone creation (Xs < Xc)......................................................................................4-29 

Figure 4-20. Green extension system using two inductive-loop detectors........................................4-33 

Figure 4-21. Extended call inductive-loop detector system. ..............................................................4-34 

Figure 4-22. Multiple inductive-loop detector placement (TxDOT) for alleviating the effects 

of dilemma zones. ............................................................................................................4-35 

Figure 4-23. Quadrupole loop configuration. ......................................................................................4-40 

Figure 4-24. Bicycle sensor sign and markings used in Clarke County, GA. ...................................4-42 

Figure 4-25. Special bicycle pavement marking used in San Luis Obispo, CA. ...............................4-42 

Figure 4-26. Chevron loop configuration.............................................................................................4-43 

Figure 4-27. Long loop with powerhead. .............................................................................................4-44 

Figure 4-28. Type D loop configuration (Caltrans).............................................................................4-44 

Figure 4-29. Type D loop installation. .................................................................................................4-45 

Figure 4-30. Wide coverage bicycle loop..............................................................................................4-46 

Figure 4-31. Bicycle lane loop layout with side-by-side quadrupoles................................................4-46 

Figure 4-32. Queue discharge system..................................................................................................4-48 

Figure 4-33. Conceptual magnetometer sensor installation..............................................................4-53 

Figure 4-34. Typical installation of magnetometers on a bridge deck. .............................................4-55 

Figure 4-35. Magnetic field analyzer...................................................................................................4-56 

Figure 4-36. Magnetometer probe placement in support of several data gathering functions. ......4-58 

Figure 4-37. Setback of magnetic probe from stopline as a function of vehicle speed and 

allowable gap. ..................................................................................................................4-60 

Figure 4-38. Magnetic probe placement for a single-lane and two-lane approach to a stopline. ....4-60 
xx 

FIGURES IN VOLUME II 

CHAPTER 5. SENSOR INSTALLATION TECHNIQUES..............................................................5-1 

Figure 5-1. Inductive-loop detector system configuration..................................................................5-5 

Figure 5-2. Loop sawcut process. .........................................................................................................5-6 

Figure 5-3. Pavement washing.............................................................................................................5-6 

Figure 5-4. Pavement blow drying.......................................................................................................5-7 

Figure 5-5. Pressing in wire and backer rod. ......................................................................................5-7 

Figure 5-6. Inserting loop sealant........................................................................................................5-8 

Figure 5-7. Detail of final inductive-loop installation. .......................................................................5-8 

Figure 5-8. Chamfer cut corner treatment........................................................................................5-13 

Figure 5-9. Core drilled corner treatment.........................................................................................5-14 

Figure 5-10. Prewound loop installation. ............................................................................................5-20 

Figure 5-11. Preformed loop assembly. ...............................................................................................5-21 

Figure 5-12. InstaLoopTM Paveover Sensor.........................................................................................5-22 

Figure 5-13. Metal-sheathed loop cable installation. .........................................................................5-24 

Figure 5-14. IDOT specification and installation procedures for metal sheathed loop cable..........5-25 

Figure 5-15. Crossing pavement joints using rigid tubing.................................................................5-26 

Figure 5-16. Pavement joint crossing details......................................................................................5-26 

Figure 5-17. Crossing pavement joints using diamond cut................................................................5-27 

Figure 5-18. Method of twisting lead-in wires. ...................................................................................5-28 

Figure 5-19. Loop wire winding diagrams for multiloop installations..............................................5-28 

Figure 5-20. Entry hole for curb crossing installations......................................................................5-29 

Figure 5-21. Loop lead-in wire placement in conduit at roadway-curb interface.............................5-29 

Figure 5-22. Detail of lead-in wire placement for shallow curb section............................................5-30 

Figure 5-23. Loop lead-in wires at pavement edge when no curb is present....................................5-30 

Figure 5-24. Standard pull box. ...........................................................................................................5-31 

Figure 5-25. Pull box installation detail..............................................................................................5-31 

Figure 5-26. Loop test devices..............................................................................................................5-33 

Figure 5-27. Methods of applying sealant...........................................................................................5-38 

Figure 5-28. Poor sealant application..................................................................................................5-40 

Figure 5-29. Crew applying sealant by hand. .....................................................................................5-40 

Figure 5-30. Finishing sealant application. ........................................................................................5-41 

Figure 5-31. Splicing loop wire at pull box..........................................................................................5-42 

Figure 5-32. Pill bottle splice. ..............................................................................................................5-44 

Figure 5-33. Pill bottle splice details. ..................................................................................................5-44 

Figure 5-34. Lead-in cable grounding at the field terminal strip......................................................5-46 

Figure 5-35. Representative inductive-loop installation data sheet. ................................................5-48 

Figure 5-36. Plan details of slab-loop installation..............................................................................5-52 

Figure 5-37. Precast loop slab configurations. ....................................................................................5-53 

Figure 5-38. Slab-loop installations.....................................................................................................5-54 

Figure 5-39. Roadway installation of round loops—cutting loop circle.............................................5-56 

Figure 5-40. Roadway installation of round loops—retraction of saw. .............................................5-57 

Figure 5-41. Roadway installation of round loops—completed installation. ....................................5-58 

Figure 5-42. Round loop saw at beginning of installation process. ...................................................5-59 

Figure 5-43. Round sawcut in process. ................................................................................................5-59 

Figure 5-44. Completed round sawcut.................................................................................................5-60 

Figure 5-45. Blowing out debris from sawcut. ....................................................................................5-60 

Figure 5-46. Firmly seating loop wire in sawcut with rolling-blade tool. .........................................5-60 

Figure 5-47. Inserting backer rod material over loop wire. ...............................................................5-61 

Figure 5-48. Firmly seating backer rod material over loop wire.......................................................5-61 

Figure 5-49. Tape placed around both sides of sawcut to prevent sealant from adhering 

to concrete. .......................................................................................................................5-61 
xxi 

Figure 5-50. Placing sealant in sawcut. ..............................................................................................5-62 

Figure 5-51. Removing excess sealant.................................................................................................5-62 

Figure 5-52. Isolation of affected area to allow sealant to dry...........................................................5-62

Figure 5-53. Fluxgate magnetometer installations for various lane configurations........................5-63 

Figure 5-54. Standard plan for fluxgate magnetometer installation. ...............................................5-64 

Figure 5-55. Magnetic detector probe installation from the side or median of a road.....................5-66 

Figure 5-56. Field of view of a converging lens...................................................................................5-73 

Figure 5-57. Image area dimensions for 30- to 70-ft (9.1- to 21.3-m) camera mounting heights 

assuming an 8-mm focal length lens..............................................................................5-75 

Figure 5-58. Up-lane distance measure aided by traffic cones placed at 25-ft (7.6-m) intervals 

(temporary markings). ....................................................................................................5-78 

Figure 5-59. Down-lane road surface dimensions marked with paint as used for sensor 

evaluation and performance comparison. ......................................................................5-78 

Figure 5-60. Mainline count and speed detection zones for an Autoscope 2004 VIP using a sidemounted camera. .............................................................................................................5-79 

Figure 5-61. Ramp demand zones (H configuration) for an Autoscope 2004 VIP with 

a side-mounted camera. ..................................................................................................5-80 

Figure 5-62. Ramp detection zone placement for an Autoscope 2004 VIP when the camera 

is neither close to the monitored roadway nor at a sufficient height. .........................5-81 

Figure 5-63. Image and data displays for Traficon VIP2 traffic data detector.................................5-81 

Figure 5-64. Microwave sensor detection area for upstream and downstream viewing..................5-83 

Figure 5-65. Approximate detection pattern for Microwave Sensors TC-20 Doppler radar............5-84 

Figure 5-66. RTMS microwave sensor detection zone setup and calibration screen.. .....................5-85 

Figure 5-67. Accuwave 150LX microwave sensor interface and setup options screen.. ..................5-86 

Figure 5-68. Accuwave 150LX microwave sensor road surface detection area.. ..............................5-87 

Figure 5-69. Autosense II laser radar mounting. ...............................................................................5-88 

Figure 5-70. TC-30C ultrasonic sensor overhead mounting pattern.................................................5-91 

Figure 5-71. SmartSonic acoustic sensor installation geometry. .....................................................5-91 

Figure 5-72. SAS-1 acoustic sensor data display................................................................................5-92 

Figure 5-73. ASIM DT 281 dual technology passive infrared and CW Doppler microwave 

sensor ground footprints. ................................................................................................5-93 

CHAPTER 6. SENSOR MAINTENANCE ..........................................................................................6-1 

Figure 6-1. Pavement failure near an inductive-loop installation. ...................................................6-5 

Figure 6-2. Model ILA-550 inductive-loop system analyzer.. ..........................................................6-11 

Figure 6-3. Quality factor Q data form..............................................................................................6-12 

Figure 6-4. Sensitivity S data form....................................................................................................6-13 

Figure 6-5. Loop finder. ......................................................................................................................6-14 

APPENDIX A. INDUCTIVE-LOOP SYSTEM EQUIVALENT CIRCUIT MODEL .................. A-1 

Figure A-1. Capacitance between adjacent isolated loop turns ........................................................ A-2 

Figure A-2. Connection of adjacent loop turns. .................................................................................. A-3 

Figure A-3. Multiturn loop equivalent circuit model. ........................................................................ A-3 

Figure A-4. Capacitance between loop conductors and slot............................................................... A-4 

Figure A-5. Capacitance between loop conductors and infinitely conducting planes...................... A-5 

Figure A-6. Balanced inductive loop. .................................................................................................. A-5 

Figure A-7. Pair of parallel current elements................................................................................... A-10 

Figure A-8. Geometry for calculating mutual inductance of coaxial and parallel 

rectangular loops. ........................................................................................................... A-10 

Figure A-9. Two-turn quadrupole loop.............................................................................................. A-11 

Figure A-10. Multiturn quadrupole loop............................................................................................. A-11 

Figure A-11. Mutual inductance of two parallel segments i and k. .................................................. A-12 
xxii 

Figure A-12. Inductive loop system circuit model.............................................................................. A-12 

Figure A-13. Equivalent loop circuit model. ....................................................................................... A-13 

Figure A-14. Loop transformer model................................................................................................. A-15 

Figure A-15. Equivalent transformer model....................................................................................... A-16 

Figure A-16. Series-circuit to parallel-circuit equivalency (general)................................................ A-22 

Figure A-17. Loop-series to parallel-circuit equivalency. .................................................................. A-22

Figure A-18. Transformer nodal analysis diagram............................................................................ A-23

APPENDIX B. CURRENT SHEET FORMULA FOR CALCULATION OF LOOP 

INDUCTANCE....................................................................................................................................B-1 

There are no Figures in Appendix B. 

APPENDIX C. LOOP INDUCTANCE AND QUALITY FACTOR TABLES...............................C-1 

There are no Figures in Appendix C. 

APPENDIX D. ELECTRICAL CHARACTERISTICS OF WIRE AND CABLE .........................D-1 

Figure D-1. Wire spacing and size definitions used in wire inductance and 

capacitance calculations................................................................................................... D-1 

Figure D-2. Dielectric field surrounding a pair of wires. ................................................................... D-2 

Figure D-3. Simplified geometry that includes most of the electromagnetic field surrounding 

a twisted pair of wires. ..................................................................................................... D-2 

APPENDIX E. VEHICLE DETECTION SENSITIVITY FORMULAS FOR 

RECTANGULAR LOOPS................................................................................................................. E-1 

Figure E-1. Two-turn loop in roadway. ............................................................................................... E-3 

Figure E-2. Single-turn inductive loop circuit model......................................................................... E-3 

Figure E-3. Two-turn inductive loop circuit model. ........................................................................... E-6 

APPENDIX F. DIGITAL FREQUENCY-SHIFT ELECTRONICS UNIT ANALYSIS............... F-1 

Figure F-1. Digital frequency-shift electronics unit block diagram...................................................F-1 

Figure F-2. Loop-inductive and capacitive elements at electronics unit terminals..........................F-4 

APPENDIX G. DIGITAL RATIOED FREQUENCY-SHIFT ELECTRONICS 

UNIT ANALYSIS................................................................................................................................G-1 

Figure G-1. Digital ratioed frequency-shift electronics unit block diagram..................................... G-1 

APPENDIX H. DIGITAL PERIOD SHIFT ELECTRONICS UNIT ANALYSIS........................H-1 

Figure H-1. Digital period-shift electronics unit block diagram........................................................H-1 

APPENDIX I. DIGITAL RATIOED PERIOD-SHIFT ELECTRONICS UNIT ANALYSIS ...... I-1 

Figure I-1. Digital ratioed period-shift electronics unit block diagram............................................ I-1 

APPENDIX J. NEMA STANDARDS..................................................................................................H-1

Figure 2-4 Test Configurations...........................................................................................................J-6 

Figure 2-5 Loop Input Terminal Transient Tests..............................................................................J-7 

Figure 6-4 Two Channel Card Rack Unit.........................................................................................J-11 

Figure 6-5 Four Channel Card Rack Unit........................................................................................J-12 

Figure 6-6 Test Loop Configurations................................................................................................J-16 
xxiii 

Figure 6-7 Delay Operation...............................................................................................................J-19 

Figure 6-8 Extension Operation........................................................................................................J-19 

APPENDIX K. CALTRANS TRANSPORTATION ELECTRICAL EQUIPMENT 

SPECIFICATIONS (TEES) FOR ELECTRONICS UNITS AND 

MAGNETIC DETECTORS...............................................................................................................K-1 

There are no Figures in Appendix K. 

APPENDIX L. CLASSIFICATION OF SENSOR SYSTEMS BY SENSOR 

DENSITY LEVEL .............................................................................................................................. L-1 

Figure L-1. Network sensor web density level 0.0—no sensors.........................................................L-3 

Figure L-2. Network sensor web density level 0.5—occasional minor movement stop 

bar detection ......................................................................................................................L-4 

Figure L-3. Network sensor web density level 1.0 ..............................................................................L-5 

Figure L-4. Network sensor web density level 1.5. .............................................................................L-6 

Figure L-5. Network sensor web density level 2.0. .............................................................................L-6 

Figure L-6. Network sensor web intensity level 2.5. (SCATS)...........................................................L-8 

Figure L-7. Network sensor web density level 2.5. (SCOOT).............................................................L-8 

Figure L-8. Network sensor web density level 3.0. .............................................................................L-9 

Figure L-9. Network sensor web density level 3.0a. ........................................................................ L-10

APPENDIX M. EXTENT AND CAUSES OF INDUCTIVE-LOOP FAILURES ........................ M-1 

Figure M-1. Oregon presence loop configuration. .............................................................................. M-3 

APPENDIX N. GROUNDING (DESIGN GUIDELINES)...............................................................N-1 

Figure N-1. Voltage and current waveforms.....................................................................................N-15 

Figure N-2. Voltage in earth due to discharging lightning current at service pole. ......................N-17 

Figure N-3. Recommended improvement to system ground connections. ......................................N-18 

Figure N-4. Controller cabinet location for best lightning protection.............................................N-20 

Figure N-5. Signal grounding system (with or without lighting)....................................................N-21 

APPENDIX O. GROUNDING (MAINTENANCE GUIDELINES)................................................O-1 

Figure O-1. Recommended improvement to system ground connection........................................... O-1 

Figure O-2. Signal grounding system (with or without lighting)...................................................... O-3 

APPENDIX P. GLOSSARY ................................................................................................................. P-1 

There are no Figures in Appendix P. 

APPENDIX Q. REGISTERED TRADEMARKS ..............................................................................Q-1 

There are no Figures in Appendix Q. 
xxiv 

TABLES IN VOLUME I 

CHAPTER 1. INTRODUCTION..........................................................................................................1-1 

Table 1-1. Strengths and weaknesses of commercially available sensor technologies...................1-8 

Table 1-2. Traffic output data (typical), communications bandwidth, and cost of 

commercially available sensors. .....................................................................................1-10 

CHAPTER 2. SENSOR TECHNOLOGY ............................................................................................2-1 

Table 2-1. Resistance of cables commonly found in inductive-loop detector systems. ...................2-2 

Table 2-2. Rectangular loop inductance and quality factor parameters at f = 20 kHz. ...............2-10 

Table 2-3. Quadrupole loop inductance and quality factor parameters at f = 20 kHz. ................2-10 

Table 2-4. Circular loop inductance and quality factor parameters at f = 20 kHz. ......................2-10 

Table 2-5. Twisted loop lead-in wire specifications. .......................................................................2-11 

Table 2-6. Commercial lead-in cable specifications. .......................................................................2-11

Table 2-7. Influence of lead-in cable type and length on Q. ...........................................................2-12 

Table 2-8. Influence of reinforcing steel on loop inductance (μH)..................................................2-19 

Table 2-9. Comparison of sensitivities and response times of digital electronics units...............2-39 

Table 2-10. 2070 ATC module options. ..............................................................................................2-50 

Table 2-11. 2070 ATC ITS cabinet options. .......................................................................................2-52 

Table 2-12. 2070 ATC models and cabinet options. ..........................................................................2-52

Table 2-13. 2070 ATC modules corresponding to model and cabinet options.................................2-53 

Table 2-14. Performance comparison of a visible spectrum video image processor using 

upstream and downstream viewing. ..............................................................................2-75 

CHAPTER 3. SENSOR APPLICATIONS..........................................................................................3-1 

Table 3-1. Categories of traffic signal systems and their characteristics or requirements. ........3-14 

Table 3-2. Local responsive ramp metering rates based on mainline occupancy. ........................3-23 

Table 3-3. Ramp metering rates by metering strategy...................................................................3-27 

Table 3-4. Strategies supporting real-time traffic management. ..................................................3-33 

Table 3-5. Data supporting offline traffic planning and administration.......................................3-35 

Table 3-6. Collision avoidance strategies and preventable crash types that are supported 

by vehicle-mounted sensors. ...........................................................................................3-52 

Table 3-7. Automotive radar requirements for distance warning systems...................................3-53 

CHAPTER 4. IN-ROADWAY SENSOR DESIGN .............................................................................4-1 

Table 4-1. Minimum green interval. ..................................................................................................4-4 

Table 4-2. Inductive-loop detector location and timing parameters................................................4-7 

Table 4-3. Dilemma zone boundaries...............................................................................................4-10 

Table 4-4. Wide inductive-loop detector dimensions. .....................................................................4-21 

Table 4-5. Loop lengths for long-loop-occupancy detection. ...........................................................4-24 

Table 4-6. Stopping and clearance distances for intersection width of 48 feet.............................4-30 

Table 4-7. Stopping and clearance distances for intersection width of 15 meters........................4-30 

Table 4-8. Stopping and clearance distances for intersection width of 76 feet.............................4-31 

Table 4-9. Stopping and clearance distances for intersection width of 23 meters........................4-31 

Table 4-10. Inductive-loop detector placement in extension or extended call systems used to 

ameliorate effects of dilemma zones...............................................................................4-36 

Table 4-11. Inductive-loop detector placement in multiple-point detection systems used to 

ameliorate effects of dilemma zones...............................................................................4-37 
xxv 

TABLES IN VOLUME II 

CHAPTER 5. SENSOR-INSTALLATION TECHNIQUES..............................................................5-1 

Table 5-1. Equipment for typical inductive-loop detector installation............................................5-3 

Table 5-2. List of materials for typical inductive-loop detector installation...................................5-4 

Table 5-3. Installation practices in western States. .........................................................................5-9 

Table 5-4. Summary of inductive-loop detector failures.................................................................5-12 

Table 5-5. Saw-blade issue resolution..............................................................................................5-15 

Table 5-6. Average saw-cutting speeds (inch-foot) as a function of surface type and 

slot depth..........................................................................................................................5-16 

Table 5-7. Sawcut slot depth guide. .................................................................................................5-17 

Table 5-8. Wire standard designations............................................................................................5-18 

Table 5-9. Conduit size......................................................................................................................5-32 

Table 5-10. Asphalt concrete crack treatment materials. ................................................................5-35 

Table 5-11. Properties of asphalt concrete filler and sealant materials..........................................5-36 

Table 5-12. Portland cement concrete sealant materials. ................................................................5-36 

Table 5-13. Backer rod materials.......................................................................................................5-38 

Table 5-14. General installation guidelines for installing inductive-loop detectors.......................5-49 

Table 5-15. Horizontal and vertical fields of view (FOV) of standard lenses..................................5-73 

Table 5-16. Calculation of image area dimensions for an 8-mm focal length lens. ........................5-73 

Table 5-17. Image area dimensions for 30-, 40-, and 50-ft camera mounting heights using 

an 8-mm focal length lens. ..............................................................................................5-74 

Table 5-18. Image area dimensions for 60- and 70-ft camera mounting heights using an 

8-mm focal length lens. ...................................................................................................5-74 

Table 5-19. Typical camera specifications for VIP applications. .....................................................5-76 

Table 5-20. Image area and data calibration information................................................................5-77 

Table 5-21. Alternate image area and data calibration information...............................................5-77 

Table 5-22. Detection zone of Whelen TDN-30 CW Doppler microwave sensor.............................5-83 

Table 5-23. Eltec 842 infrared sensor footprint dimensions. ...........................................................5-90 

Table 5-24. ASIM DT 281 dual-technology passive infrared and CW Doppler radar sensor 

footprint dimensions........................................................................................................5-93 

CHAPTER 6. SENSOR MAINTENANCE ..........................................................................................6-1 

Table 6-1. Inductive-loop detector malfunctions and potential causes. ........................................6-10 

Table 6-2. Inductive-loop detector troubleshooting guide. .............................................................6-18 

Table 6-3. Annualized per-lane sensor cost comparison to instrument a six-lane freeway 

sensor station...................................................................................................................6-22 

APPENDIX A. INDUCTIVE-LOOP SYSTEM EQUIVALENT CIRCUIT MODEL ................... A-1 

Table A-1. Measured inductive-loop parameters of 3-turn 6- x 6-ft (1.8- x 1.8-m) loop with 

no vehicle present. .......................................................................................................... A-17 

Table A-2. Comparison of measured and computed inductive-loop parameters of 3-turn 

6- x 6-ft (1.8- x 1.8-m) loop with no vehicle present. .................................................... A-18 

Table A-3. Rectangular-loop parameters. ....................................................................................... A-18 

Table A-4. Quadrupole-loop parameters. ........................................................................................ A-19 

Table A-5. Circular-loop parameters. .............................................................................................. A-19 

APPENDIX B. CURRENT SHEET FORMULA FOR CALCULATION OF LOOP 

INDUCTANCE....................................................................................................................................B-1 

Table B-1. Values of β coefficients for short rectangular solenoids................................................. B-1 
xxvi 

APPENDIX C. LOOP INDUCTANCE AND QUALITY FACTOR TABLES...............................C-1 

Table C-1. Rectangular-loop inductance in μH at f = 1 kHz. ........................................................... C-2 

Table C-2. Rectangular-loop quality factor at f = 1 kHz. ................................................................. C-3 

Table C-3. Rectangular-loop inductance in μH at f = 20 kHz. ......................................................... C-4 

Table C-4. Rectangular-loop quality factor at f = 20 kHz. ............................................................... C-5 

Table C-5. Rectangular-loop inductance in μH at f = 40 kHz. ......................................................... C-6 

Table C-6. Rectangular-loop quality factor at f = 40 kHz. ............................................................... C-7 

Table C-7. Rectangular-loop inductance in μH at f = 60 kHz. ......................................................... C-8 

Table C-8. Rectangular-loop quality factor at f = 60 kHz. ............................................................... C-9 

Table C-9. Quadrupole-loop inductance in μH at f = 1 kHz........................................................... C-10 

Table C-10. Quadrupole-loop quality factor at f = 1 kHz. ................................................................ C-11 

Table C-11. Quadrupole-loop inductance in μH at f = 20 kHz......................................................... C-12 

Table C-12. Quadrupole-loop quality factor at f = 20 kHz. .............................................................. C-13 

Table C-13. Quadrupole-loop inductance in μH at f = 40 kHz......................................................... C-14 

Table C-14. Quadrupole-loop quality factor at f = 40 kHz. .............................................................. C-15 

Table C-15. Quadrupole-loop inductance in μH at f = 60 kHz......................................................... C-16 

Table C-16. Quadrupole-loop quality factor at f = 60 kHz. .............................................................. C-17 

Table C-17. Circular-loop inductance in μH at f = 1 kHz................................................................. C-18 

Table C-18. Circular-loop quality factor at f = 1 kHz....................................................................... C-18 

Table C-19. Circular-loop inductance in μH at f = 20 kHz............................................................... C-18 

Table C-20. Circular-loop quality factor at f = 20 kHz..................................................................... C-18 

Table C-21. Circular-loop inductance in μH at f = 40 kHz............................................................... C-19 

Table C-22. Circular-loop quality factor at f = 40 kHz..................................................................... C-19 

Table C-23. Circular-loop inductance in mμ at f = 60 kHz............................................................... C-19 

Table C-24. Circular-loop quality factor at f = 60 kHz..................................................................... C-19 

APPENDIX D. ELECTRICAL CHARACTERISTICS OF WIRE AND CABLE .........................D-1 

Table D-1. Measured lead-in cable electrical characteristics of #14 AWG twisted pair. ............... D-4 

Table D-2. Measured lead-in cable electrical characteristics: Shielded cable lead-in.................... D-5 

Table D-3. Measured inductive-loop electrical characteristics: Belden 8718 shielded lead-in...... D-6 

Table D-4. Measured inductive-loop electrical characteristics: Belden 8720 shielded lead-in...... D-7 

Table D-5. Measured inductive-loop electrical characteristics: Belden 9438 twisted 

pair lead-in........................................................................................................................ D-8 

Table D-6. Measured inductive-loop electrical characteristics: Loop without lead-in cable.......... D-9 

APPENDIX E. VEHICLE DETECTION SENSITIVITY FORMULAS FOR 

RECTANGULAR LOOPS................................................................................................................. E-1 

Table E-1. Comparison of measured and calculated loop detector sensitivity data..................... E-10 

Table E-2. Vehicle undercarriage detection height versus number of loop turns and 

spacing (calculated results)............................................................................................ E-11 

Table E-3. Vehicle undercarriage detection height versus loop-turn spacing for 3-turn loop 

in inch-pound and metric units (calculated results). ................................................... E-11 

Table E-4. Vehicle undercarriage detection height versus number of loop turns and 

spacing with lead-in cable and reinforcing steel mesh for 5- x 5-ft loop 

(calculated results). ........................................................................................................ E-12 

Table E-5. Vehicle undercarriage detection height versus number of loop turns and 

spacing with lead-in cable and reinforcing steel mesh for 1.52- x 1.52-m loop 

(calculated results). ........................................................................................................ E-13 

APPENDIX F. DIGITAL FREQUENCY-SHIFT ELECTRONICS UNIT ANALYSIS............... F-1 

There are no Tables in Appendix F. 
xxvii 

APPENDIX G. DIGITAL RATIOED FREQUENCY-SHIFT ELECTRONICS 

UNIT ANALYSIS................................................................................................................................G-1 

There are no Tables in Appendix G. 

APPENDIX H. DIGITAL PERIOD-SHIFT ELECTRONICS UNIT ANALYSIS........................H-1 

There are no Tables in Appendix H. 

APPENDIX I. DIGITAL RATIOED FREQUENCY-SHIFT 

ELECTRONICS UNIT ANALYSIS.................................................................................................G-1 

There are no Tables in Appendix I. 

APPENDIX J. NEMA DETECTOR STANDARDS EXCERPTS....................................................J-1 

Transient Test Configurations ................................................................................................................J-8 

Table 6-1. Detector Unit Types ........................................................................................................J-11 

Table 6-2. Connector Terminations..................................................................................................J-24 

APPENDIX K. CALTRANS TRANSPORTATION ELECTRICAL 

EQUIPMENT SPECIFICATIONS (TEES) FOR ELECTRONICS 

UNITS AND MAGNETIC DETECTORS ......................................................................................K-1 

Table K-1. Presence detection time for detector sensitivity settings of 2 and 6............................. K-4 

Table K-2. Sensitivity settings for presence and pulse modes......................................................... K-4 

APPENDIX L. CLASSIFICATION OF SENSOR SYSTEMS BY SENSOR 

DENSITY LEVEL .............................................................................................................................. L-1 

There are no Tables in Appendix L. 

APPENDIX M. EXTENT AND CAUSES OF INDUCTIVE LOOP FAILURES......................... M-1 

Table M-1. Oregon inductive-loop failure survey results. ............................................................... M-2 

Table M-2. Washington State inductive-loop failure rate survey results....................................... M-5 

Table M-3. New York State distribution of inductive loops by pavement type in failure 

survey results................................................................................................................. M-11 

Table M-4. Inductive loop failure percentage by cause of failure and pavement type. ............... M-12 

APPENDIX N. GROUNDING (DESIGN GUIDELINES)...............................................................N-1 

Table N-1. Representative values of soil resistivity. ........................................................................N-2 

Table N-2. Variation of resistivity with ground temperature..........................................................N-2 

Table N-3. Resistance to ground for various ground system configurations and soils.................N-12 

APPENDIX O. GROUNDING (MAINTENANCE GUIDELINES)................................................O-1 

Table O-1. Resistance to ground for various ground system configurations and soils................... O-5 

APPENDIX P. GLOSSARY ................................................................................................................. P-1 

There are no Tables in Appendix P. 

APPENDIX Q. REGISTERED TRADEMARKS ..............................................................................Q-1 

Table Q-1. Registered Trademarks.................................................................................................... Q-1

 Traffic Detector Handbook—2006 

October 2006 Page 5-i Federal Highway Administration 

TABLE OF CONTENTS: CHAPTER 5. 

CHAPTER 5. SENSOR-INSTALLATION TECHNIQUES .....................................5-1

TYPICAL PREINSTALLATION ACTIVITIES.......................................................5-1

SCALE DRAWING OF SENSOR INSTALLATION SITE............................5-2

FIELD VISITS .....................................................................................................5-2

INSTALLATION CREW ESTIMATES ...........................................................5-2

EQUIPMENT REQUIREMENTS .....................................................................5-3

MATERIAL REQUIREMENTS........................................................................5-4

INDUCTIVE-LOOP DETECTOR INSTALLATION..............................................5-4

INSTALLATION TECHNIQUES .....................................................................5-5

INDUCTIVE-LOOP DETECTOR FAILURES..............................................5-10

Causes of Inductive-Loop Detector Failures...............................................5-10 

Failure Frequency .........................................................................................5-11 

Failure Mechanisms ......................................................................................5-11 

LOOP LAYOUT AND SAWCUTS ..................................................................5-12

Corner Treatment .........................................................................................5-12 

Saw-Cutting Operations ...............................................................................5-13 

Overview of Saw-Cutting Equipment........................................................5-13

Diamond-Blade Design .............................................................................5-14

Saw-Blade Troubleshooting......................................................................5-14

Saw Horsepower ........................................................................................5-15

Wet Versus Dry Cutting.............................................................................5-16

Sawcut Depth .............................................................................................5-17

Finishing the Sawcut.................................................................................5-17

INSTALLING LOOP WIRE ............................................................................5-18

Wire Type ......................................................................................................5-18 

Ducted Wire ...............................................................................................5-19

Prewound Loops ........................................................................................5-19

Preformed Loops .......................................................................................5-20

Metal-Sheathed Loop Cable......................................................................5-23

Wire Insertion................................................................................................5-24 

CROSSING PAVEMENT JOINTS .................................................................5-24

TWISTING THE LOOP WIRE LEAD-IN .....................................................5-27

CROSSING CURBS ..........................................................................................5-29

INSTALLATION OF PULL BOX AND CONDUIT......................................5-31

TESTING THE LOOP ......................................................................................5-32

SEALING THE SAWCUT................................................................................5-34

Types of Sealants...........................................................................................5-34

Sealant Application Techniques...................................................................5-37

SPLICING THE WIRE.....................................................................................5-41

Connecting the Wires....................................................................................5-41

Sealing the Splice from the Operating Environment .................................5-43

LEAD-IN CABLE INSTALLATION ..............................................................5-45

FINAL TESTS AND RECORD KEEPING ....................................................5-47

GENERAL INSTALLATION GUIDELINES ................................................5-49

INDUCTIVE-LOOP DETECTOR INSTALLATION ALTERNATIVES .............5-51

SLAB LOOPS.....................................................................................................5-51

ROUND LOOPS ................................................................................................5-53

Round-Loop Installation in Roadways........................................................5-54

Round-Loop Installation on Sidewalks .......................................................5-58

INSTALLATION OF TWO-AXIS FLUXGATE MAGNETOMETERS ...............5-63

INSTALLATION OF SENSING PROBE .......................................................5-63

SPLICING THE CABLES................................................................................5-65

TESTING THE SYSTEM .................................................................................5-65
Chapter 5—Sensor-Installation Techniques

October 2006 Page 5-ii Federal Highway Administration

SEALING THE HOLES AND CUTS ............................................................. 5-66

INSTALLATION OF MAGNETIC DETECTORS ............................................... 5-66

INSTALLATION PROCEDURE.................................................................... 5-66

TRENCHING .................................................................................................... 5-67

CONNECTING THE SYSTEM ...................................................................... 5-67

TESTING THE SYSTEM ................................................................................ 5-68

OVER-ROADWAY SENSOR INSTALLATION ................................................... 5-69

INITIAL SENSOR EVALUATIONS.............................................................. 5-69

VIDEO IMAGE PROCESSORS ............................................................................ 5-69

SITE SURVEYS................................................................................................ 5-69

LENS SELECTION.......................................................................................... 5-72

CALIBRATION ISSUES ................................................................................. 5-76

SERIAL DATA INTERPRETATION SOFTWARE .................................... 5-79

AUTOSCOPE 2004 VIDEO IMAGE PROCESSOR DETECTION 

ZONES .......................................................................................................... 5-79

TRAFICON VIP9 VIDEO IMAGE PROCESSOR DETECTION 

ZONES .......................................................................................................... 5-80

MICROWAVE RADAR SENSORS ....................................................................... 5-82

WHELEN TDN-30 CW DOPPLER SENSOR ............................................... 5-83

MICROWAVE SENSORS TC-20 CW DOPPLER SENSOR ...................... 5-84

EIS RTMS FMCW RADAR ............................................................................ 5-84

ACCUWAVE 150LX FMCW RADAR........................................................... 5-86

LASER RADAR SENSORS.................................................................................... 5-87

PASSIVE INFRARED SENSORS......................................................................... 5-89

ULTRASONIC SENSORS ..................................................................................... 5-89

PASSIVE ACOUSTIC SENSORS ......................................................................... 5-90

IRD SMARTSONIC SINGLE-LANE ACOUSTIC SENSOR...................... 5-90

SMARTEK SAS-1 MULTIPLE-LANE ACOUSTIC SENSOR ................... 5-92

SENSOR COMBINATIONS .................................................................................. 5-92

REFERENCES ....................................................................................................... 5-93
 Traffic Detector Handbook—2006 

October 2006 Page 5-iii Federal Highway Administration 

LIST OF FIGURES: CHAPTER 5. 

Figure 5-1. Inductive-loop detector system configuration. ........................... 5-5

Figure 5-2. Loop sawcut process..................................................................... 5-6

Figure 5-3. Pavement washing. ...................................................................... 5-6

Figure 5-4. Pavement blow drying. ................................................................ 5-7

Figure 5-5. Pressing in wire and backer rod.................................................. 5-7

Figure 5-6. Inserting loop sealant. ................................................................. 5-8

Figure 5-7. Detail of final inductive-loop installation................................... 5-8

Figure 5-8. Chamfer-cut corner treatment. ................................................. 5-13

Figure 5-9. Core-drilled corner treatment. .................................................. 5-14

Figure 5-10. Prewound loop installation...................................................... 5-20

Figure 5-11. Preformed loop assembly......................................................... 5-21

Figure 5-12. InstaLoopTM Paveover Sensor.................................................. 5-22

Figure 5-13. Metal-sheathed loop cable installation................................... 5-24

Figure 5-14. IDOT specification and installation procedures for metalsheathed loop cable................................................................. 5-25

Figure 5-15. Crossing pavement joints using rigid tubing. ........................ 5-26

Figure 5-16. Pavement joint crossing details. ............................................. 5-26

Figure 5-17. Crossing pavement joints using diamond cut. ....................... 5-27

Figure 5-18. Method of twisting lead-in wires............................................. 5-28

Figure 5-19. Loop wire winding diagrams for multiloop installations. ..... 5-28

Figure 5-20. Entry hole for curb crossing installations. ............................. 5-29

Figure 5-21. Loop lead-in wire placement in conduit at roadway-curb 

interface................................................................................... 5-29

Figure 5-22. Detail of lead-in wire placement for shallow curb section. ... 5-30

Figure 5-23. Loop lead-in wires at pavement edge when no curb is 

present..................................................................................... 5-30

Figure 5-24. Standard pull box..................................................................... 5-31

Figure 5-25. Pull box installation detail. ..................................................... 5-31

Figure 5-26. Loop test devices. ..................................................................... 5-33

Figure 5-27. Methods of applying sealant. .................................................. 5-38

Figure 5-28. Poor sealant application. ......................................................... 5-40

Figure 5-29. Crew applying sealant by hand............................................... 5-40

Figure 5-30. Finishing sealant application.................................................. 5-41

Figure 5-31. Splicing loop wire at pull box. ................................................. 5-42

Figure 5-32. Pill bottle splice........................................................................ 5-44

Figure 5-33. Pill bottle splice details............................................................ 5-44

Figure 5-34. Lead-in cable grounding at the field terminal strip. ............. 5-46

Figure 5-35. Representative inductive-loop installation data sheet.......... 5-48

Figure 5-36. Plan details of slab-loop installation. ..................................... 5-52

Figure 5-37. Precast loop slab configurations.............................................. 5-53

Figure 5-38. Slab-loop installations. ............................................................ 5-54

Figure 5-39. Roadway installation of round loops—cutting loop circle. .... 5-56

Figure 5-40. Roadway installation of round loops—retraction of saw....... 5-57

Figure 5-41. Roadway installation of round loops—completed 

installation. ............................................................................. 5-58

Figure 5-42. Round loop saw at beginning of installation process............. 5-59

Figure 5-43. Round sawcut in process.......................................................... 5-59

Figure 5-44. Completed round sawcut. ........................................................ 5-60

Figure 5-45. Blowing out debris from sawcut.............................................. 5-60

Figure 5-46. Firmly seating loop wire in sawcut with rolling 

blade tool. ................................................................................ 5-60

Figure 5-47. Inserting backer rod material over loop wire......................... 5-61
Chapter 5—Sensor-Installation Techniques

October 2006 Page 5-iv Federal Highway Administration

Figure 5-48. Firmly seating backer rod material over loop wire. ...............5-61

Figure 5-49. Tape placed around both sides of sawcut to prevent 

sealant from adhering to concrete..........................................5-61

Figure 5-50. Placing sealant in sawcut.........................................................5-62

Figure 5-51. Removing excess sealant. .........................................................5-62

Figure 5-52. Isolation of affected area to allow sealant to dry....................5-62

Figure 5-53. Fluxgate magnetometer installations for various lane 

configurations. .........................................................................5-63

Figure 5-54. Standard plan for fluxgate magnetometer installation. ........5-64

Figure 5-55. Magnetic detector probe installation from the side or 

median of a road. .....................................................................5-67

Figure 5-56. Field of view of a converging lens. ...........................................5-73

Figure 5-57. Image area dimensions for 30- to 70-ft (9.1- to 21.3-m) 

camera mounting heights, assuming an 8-mm focal 

length lens................................................................................5-75

Figure 5-58. Up-lane distance measure aided by traffic cones placed 

at 25-ft (7.6-m) intervals (temporary markings). .................5-78

Figure 5-59. Down-lane road surface dimensions marked with paint as 

used for sensor evaluation and performance comparison.....5-78

Figure 5-60. Mainline count and speed detection zones for an Autoscope 

2004 VIP using a side-mounted camera. ...............................5-79

Figure 5-61. Ramp demand zones (× configuration) for an Autoscope 

2004 VIP with a side-mounted camera. .................................5-80

Figure 5-62. Ramp detection zone placement for an Autoscope 2004 

VIP when the camera is neither close to the monitored 

roadway nor at a sufficient height. ........................................5-81

Figure 5-63. Image and data displays for Traficon VIP2 traffic data 

detector.....................................................................................5-81

Figure 5-64. Microwave sensor detection area for upstream and 

downstream viewing................................................................5-83

Figure 5-65. Approximate detection pattern for Microwave Sensors 

TC-20 Doppler radar. ..............................................................5-84

Figure 5-66. RTMS microwave sensor detection zone setup and 

calibration screen. ...................................................................5-85

Figure 5-67. Accuwave 150LX microwave sensor interface and setup 

options screen. .........................................................................5-86

Figure 5-68. Accuwave 150LX microwave sensor road surface 

detection area. .........................................................................5-87

Figure 5-69. Autosense II laser radar mounting..........................................5-88

Figure 5-70. TC-30C ultrasonic sensor overhead mounting pattern..........5-91

Figure 5-71. SmartSonic acoustic sensor installation geometry.................5-91

Figure 5-72. SAS-1 acoustic sensor data display. ........................................5-92

Figure 5-73. ASIM DT 281 dual technology passive infrared and CW 

Doppler microwave sensor ground footprints........................5-93

 
 Traffic Detector Handbook—2006 

October 2006 Page 5-v Federal Highway Administration 

LIST OF TABLES: CHAPTER 5. 

Table 5-1. Equipment for typical inductive-loop detector installation. ....... 5-3

Table 5-2. List of materials for typical inductive-loop detector 

installation. ............................................................................... 5-4

Table 5-3. Installation practices in western States....................................... 5-9

Table 5-4. Summary of inductive-loop detector failures............................. 5-12

Table 5-5. Saw blade issue resolution. ......................................................... 5-15

Table 5-6. Average saw cutting speeds (inch-foot) as a function of surface 

type and slot depth. ................................................................ 5-16

Table 5-7. Sawcut slot depth guide. ............................................................. 5-17

Table 5-8. Wire standard designations. ....................................................... 5-18

Table 5-9. Conduit size.................................................................................. 5-32

Table 5-10. Asphalt concrete crack treatment materials............................ 5-35

Table 5-11. Properties of asphalt concrete filler and sealant materials.... 5-36

Table 5-12. Portland cement concrete sealant materials............................ 5-36

Table 5-13. Backer rod materials. ................................................................ 5-38

Table 5-14. General installation guidelines for installing inductive-loop 

detectors. ................................................................................. 5-49

Table 5-15. Horizontal and vertical fields of view (FOV) of standard 

lenses. ...................................................................................... 5-73

Table 5-16. Calculation of image area dimensions for an 8-mm focal 

length lens.a

............................................................................. 5-73

Table 5-17. Image area dimensions for 30-, 40-, and 50-ft (9.1- 12.2-, 

and 15.2-m) camera mounting heights, using an 8-mm 

focal length lens...................................................................... 5-74

Table 5-18. Image area dimensions for 60- and 70-ft (18.3- and 

21.3-m) camera mounting heights, using an 8-mm 

focal length lens...................................................................... 5-74

Table 5-19. Typical camera specifications for VIP applications................. 5-76

Table 5-20. Image area and data calibration information.......................... 5-77

Table 5-21. Alternate image area and data calibration information......... 5-77

Table 5-22. Detection zone of Whelen TDN-30 CW Doppler microwave 

sensor....................................................................................... 5-83

Table 5-23. Eltec 842 infrared sensor footprint dimensions....................... 5-90

Table 5-24. ASIM DT 281 dual-technology passive infrared and CW 

Doppler radar sensor footprint dimensions. ......................... 5-93

 Traffic Detector Handbook—2006 

October 2006 Page 5-1 Federal Highway Administration

CHAPTER 5. SENSORINSTALLATION TECHNIQUES 

Sensor installation may appear to be a relatively easy task once the 

operation of candidate sensor technologies is understood, the suitability of a 

particular technology to an application is reviewed, and the design, 

specifications, and plans are developed. However, the mechanical operations 

associated with sensor installation often present challenges. For example, 

inductive-loop detector installation requires approved procedures for cutting 

a slot in the pavement, laying turns of wire in the slot, covering them with 

sealant, removing excess sealant, splicing the wire to the cable, and 

connecting the cable to the electronics unit in the controller cabinet. For overroadway sensors, analogous tasks and related issues appear, such as 

installation of mounting structures, power and data cables, sensor alignment, 

and calibration verification. 

In reality, the seemingly uncomplicated activity of sensor installation is 

composed of several critical processes. Improper or sloppy installation causes 

many of the sensor failures and signal malfunctions that are observed. For 

example, consider that the detection element represents about 4 to 10 

percent of the cost of implementing intersection-traffic control, yet accounts 

for a significant portion of maintenance dollars. Moreover, dependence on 

properly functioning sensors increases as traffic congestion and delay become 

a more urgent problem and traffic-control equipment becomes more 

sophisticated. 

This chapter addresses the needs of project engineers, contractors, 

inspectors, field crew supervisors, and traffic technicians by reviewing the 

proper techniques for installing traffic-flow sensors; and it discusses 

preinstallation activities that apply to all types of sensors. To underscore the 

criticality of the installation process, the magnitude and causes of inductiveloop detector failures are addressed next. Inductive-loop detector installation 

procedures for existing roadways are then described, followed by a discussion 

of loop-installation procedures for new and repaved roadways. Installation of 

magnetometers, magnetic detectors, and over-roadway sensors (e.g., video 

image processors, microwave and laser radars, passive infrared sensors, 

ultrasonic sensors, passive acoustic sensors, and sensor combinations) are 

discussed in sequence in the latter sections of the chapter. 

TYPICAL PREINSTALLATION ACTIVITIES 

The design of the sensor system involves a number of decisions that are made 

previous to installation. Examples of these decisions for in-roadway sensors 

utilized for signal control were discussed in Chapter 4. Included there are 

specifications for the type and configuration of the sensor hardware 

necessary to meet previously identified operational and data requirements, 

and a preliminary plan showing the sensor layout at the selected location. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-2 Federal Highway Administration 

Preinstallation activities should include a thorough review of the design 

documents, preparation of scale drawings, and field visits to the location. 

Upon completion of these activities, the engineer is ready to hand the job 

over to the installation crew foreman or the contractor, who must then 

develop manpower estimates and material and equipment requirements. All 

information should be clearly defined and complete so that installation can 

proceed in an orderly and effective manner. The implication of these 

activities is discussed further in the following paragraphs. 

SCALE DRAWING OF SENSOR INSTALLATION SITE 

A scale drawing of the location showing the geometry of the roadway and the 

location of the in-roadway or over-roadway sensor components in relation to 

the pavement markings or mounting structure is needed. The drawing 

should depict the location and content of conduit, manholes, power sources, 

pavement materials, and electrical equipment that interface with the 

installation. Because it provides fundamental guidance for the installation 

crew and becomes part of the procurement package needed for a contractor 

installation, the accuracy of this drawing directly affects sensor installation. 

The completed drawing should be reviewed with the design engineer to 

insure that the sensors are located as specified in the configuration design 

and in areas free of underground or under pavement hardware or overhead 

obstructions (in the case of over-roadway sensors) that could interfere with 

proper operation. 

FIELD VISITS 

Field visits to the installation site are required before and after the scale 

drawing is made. The field visit made prior to drawing preparation is for 

inventorying the existing conditions and identifying potential problem areas. 

These include pavement joints, which may affect the design of an inductiveloop detector system, and lines of sight and identification or lack of support 

structures, which affect installation of over-roadway sensors. Other factors to 

be noted during the site visit are listed in the “Video Image Processors” 

section under “Site Surveys.” 

As a result of this visit, the project engineer should determine the method for 

burying cables, installing in-roadway and over-roadway sensors, and the 

types of equipment needed for the installation strategy. Methods for traffic 

routing and control during installation should also be determined, including 

the position and quantity of barricades or cones that may be needed for 

traffic control. Finally, the permits and licenses required for installation 

should be identified. 

The information obtained from the initial visit should be incorporated into 

the plan drawings. After the drawings are completed, a second field visit is 

made to verify their accuracy. 

INSTALLATION CREW ESTIMATES 

The size and skills required of the installation crew should be estimated prior 

to installation. Plan drawings should provide sufficient information to 

determine the scope of the installation tasks, the amount of time required, 

and the size of the crew necessary for the installation. Depending upon the 

size of the crew, many of the tasks may be performed simultaneously, 

thereby reducing the time traffic is disrupted. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-3 Federal Highway Administration

EQUIPMENT REQUIREMENTS 

The sensor types and layout dictate the equipment required for the 

installation. Table 5-1 lists equipment commonly needed for a typical 

inductive-loop detector installation. Barricades, signs, cones, safety vests, 

and other devices are also necessary to control traffic safely during the 

installation process. 

In addition, magnetometer installation requires a drill and bit to bore the 

vertical hole in the roadway for the sensor probe. Magnetic detector 

installation usually requires a backhoe, a horizontal boring machine, a level, 

and a tamping machine. 

Over-roadway sensor installation usually requires a bucket truck. 

Table 5-1. Equipment for typical inductive-loop detector installation. 

Item Description and use 

Power saw Self-propelled 18–65 horsepower (hp) saw equipped with 1 / 4 to 3 / 8 inch (6.4 

to 9.5 mm) thick blade (abrasive or diamond), water valve, depth gauge, and 

horizontal guide 

Water supply Used with diamond blade for cooling and cleaning out of sawcuts 

Jackhammer Bores holes through concrete curb 

Air compressor Used with jackhammer to clean and dry sawed cuts 

Chisel and hammer Removes sharp edges at corners of sawcuts 

Blunt tool For example, a wooden stirring stick for seating wire in saw slot 

Twister Provides uniform twists of the lead-in wires 

Template or straight 

edge 

Marks outlines of loops on pavement 

Trenching machine Used to bury cable in dirt 

Megohmmeter and 

volt/ohm meter 

Used to inspect and test wire continuity and resistance 

Loop analyzer Tests continuity and inductance of the wire loop 

Soldering iron Either a butane torch with a soldering tip or an electric soldering iron for making 

solder connections at the splice 

Measuring tape 100 ft (30 m) minimum-length tape for making exact measurements for loop 

placement 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-4 Federal Highway Administration 

MATERIAL REQUIREMENTS 

As with manpower and equipment, the type and amount of installation 

materials should be determined. Material quantities should be ample to 

avoid any interruptions of work. A typical material list for inductive-loop 

detector installations is provided in Table 5-2. Other types of power and data 

cables and suitable mounting brackets are required when installing overroadway sensors. 

Table 5-2. List of materials for typical inductive-loop detector installation. 

Material Function 

Detector wire Forms the loop and lead-in wires 

Lead-in cable Spliced to the loop lead-in wire in the pull box, it connects the lead-in 

wires to the electronics unit in the controller cabinet 

Pull box Contains the connection (splice) between the lead-in wire and lead-in 

cable and provides access for maintenance 

Sealant Seals the loop wire in the sawcut and protects the loop from 

environmental damage 

Cement, sand, or talc Dusted on sawcut after sealant is applied to prevent tracking 

Concrete Holds the pull box in place 

Surge voltage protector Provides protection for the electronics in the controller cabinet 

Solder Needed to make splices 

Splice kits (or equivalent) Environmentally seals the splices 

Spray paint or chalk and line Outlines loop dimensions prior to sawcutting 

INDUCTIVE-LOOP DETECTOR INSTALLATION 

The inductive-loop detector system is composed of one or more wire loops 

embedded in the pavement (the sensing element), a splice between the leadin wire and the lead-in cable in the pull box, lead-in cable (usually in a 

conduit) connecting to the terminal strip in the controller cabinet, cable from 

the terminal strip to the inductive-loop electronics unit, and finally, the 

electronics unit, itself. The interconnections among these various components 

are illustrated in Figure 5-1. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-5 Federal Highway Administration

Detector 

Electronics Unit

Chassis Ground

Loop Input

Loop Input

Green

Red

Orange

Shield Drain Wire Insulate shield to prevent 

accidental grounding

Shielded and

Twisted Pair

Controller

Cabinet

Field

Installation

Lead-In Cable

(Shielded & Twisted Pair) Pull Box

Buried Wire 

Loops

•

Ground

Rod

Field

Terminals

Figure 5-1. Inductive-loop detector system configuration. 

The paragraphs below describe the installation of inductive-loop detectors, 

beginning with an overview and followed by detailed discussions of failure 

mechanisms associated with poor installation, common installation 

techniques and materials, and alternative installation techniques. 

INSTALLATION TECHNIQUES 

Inductive-loop detectors are installed in asphalt or portland cement concrete 

pavements using the process illustrated in Figures 5-2 through 5-7 and 

described by the steps below: 

• Cutting a slot. 

• Cleaning and drying the slot. 

• Laying in the inductive-loop detector wire. 

• Sealing the sawcut. 

• Connecting the wire to the lead-in cable. 

• Connecting the cable to the terminal strip in the cabinet. 

• Ensuring the harness connects the terminal strip to the electronics 

unit. 

Installation techniques differ in the treatment of the corners where two saw 

cuts intersect, splicing techniques, sealant types, and sealant application. 

Guidelines and recommendations to assist highway maintenance agencies 

and other related organizations in planning and installing inductive-loop 

detectors are found in this chapter and other FHWA reports.(1) 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-6 Federal Highway Administration 

Figure 5-2. Loop sawcut process. 

Figure 5-3. Pavement washing. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-7 Federal Highway Administration

Figure 5-4. Pavement blow drying. 

Figure 5-5. Pressing in wire and backer rod. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-8 Federal Highway Administration 

Figure 5-6. Inserting loop sealant. 

Figure 5-7. Detail of final inductive-loop installation. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-9 Federal Highway Administration

Installation techniques and theories vary widely among traffic agencies as 

shown, for example, in the survey of western State practices exhibited in 

Table 5-3.(2,3) Procedures developed over time frequently become outdated or 

are no longer effective; yet there is often great resistance to change. In many 

cases, contractors perform sensor installation using proprietary shortcuts 

(and shortcomings). The importance of appropriate installation procedures 

for long-term operational effectiveness of sensors cannot be overstressed. 

Since contractors primarily install inductive-loop detectors, it follows that 

construction supervision and inspection by the responsible agency is critical 

to their proper operation and reduced maintenance. 

Table 5-3. Installation practices in western States. 

State Loop type Sawcut 

dimensions Cleaning 

method 

Loop wire Lead-in cable Electronics 

unit 

mounting 

Alaska 6 x 6 ft square (3 

turns) 

6 x 20 ft (2 turns) 

– – #14 THNN 

stranded 

IMSA 50-2 or #12 

twisted pair w/ 

shield and drain 

Shelf 

California 6 x 6 ft square or 

diamond (3 turns) 

6 x 6–50ft 

quadrupole (2-4-2 

or 1-2-1) 

W = 1 / 4 to 

1 / 2 inches 

(6.4-12.7 mm); 

D = 1-3/4 

inches (44.5 

mm) 

Flush with 

water and 

blow out 

and dry 

with air 

#12 RHW 

Use 

neoprene or 

x-link 

polyethylene 

stranded 

2 #12 solid 

unshielded 

2 #14 or #16 

stranded, 

shielded; 

4 #18 stranded, 

shielded 

Rack 

Idaho 6 x 6 ft square (2 

turns) 

W =3/8 inch 

(9.5mm); 

D = 1-1/2 

inches (38.1 

mm) min. 

Blow out 

with air 

Model 20002 

vehicle 

homerun 

cable (4 #18 

conductors) 

Belden 8227 Rack 

Montana 6 x 6 ft square 

6 x 20 ft 

quadrupole 

(various numbers 

of turns) 

Asphalt: W = 

1/4 inch (6.4 

mm); D = 2 

inches (50.8 

mm); Concrete: 

W = 1/4 inch 

(6.4 mm); 

D = 11/4 

inches (31.8 

mm) 

Blow out 

with air 

#12 XHHN 

stranded 

Belden 8720 Shelf 

Nevada 6 x 6 ft square (3 

turns) 

6 x 25 ft rectangle 

(2 turns) 

6 x 70 ft rectangle 

(1 turn) 

6 x 75 ft 

quadrupole (1-2-1) 

W = 1/4 inch 

(6.4 mm); 

D = 2 inches 

(50.8 mm) 

Blow out 

with air 

#12 RHW 

and XHHW 

N. Nevada 

solid 

S. Nevada 

stranded 

IMSA 19-2 (pair 

communication 

cable with 

shield) 

Mostly shelf 

Oregon Single loop – 4 x 

4ft diamond 

Series of loops – 3 

x 3ft diamonds 

(both 4 turns) 

W = 1/4 inch 

(6.4 mm); 

D = 2 inches 

(50.8 mm) 

Flush with 

water and 

blow out 

with air 

#14 THWN 

stranded 

IMSA 50-2, 

Belden 8720, or 

equivalent 

Rack 

Utah 6 x 6 ft square (3 

turns) 

6 x 16 ft rectangle 

(3 turns) 

6 x >16 ft rectangle 

(2 turns) 

W = 1/4 inch 

(6.4 mm); 

D = 2 inches 

(50.8 mm) 

Blow out 

with air 

#14 THHN or 

THNN 

stranded 

Belden 8720 or 

equivalent 

Shelf 

Washington 6 x 6 ft square (4 

turns) 

6 x 50 ft 

quadrupole 

(2-4-2) 

W = 1/4 inch 

(6.4 mm); 

D = 2 1/2 to 3 

inches (38.1 

to 63.5 mm) 

inches 

Blow out 

with air 

#14 XLP, 

RHH, or 

RHW 

or #12 XLP 

stranded 

AIW 7311 or 

Belden 8718 

Shelf and 

rack 

1 ft = 0.3 m; W = width; D = depth 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-10 Federal Highway Administration 

Newer inductive-loop detector installations are even more varied. Because of 

the high failure rates attributed to moisture or breaks in wire, the trend is to 

encase and seal the loop wires in a protective covering prior to sealing the 

sawcut. Some agencies choose to prewind and bundle the loops in the shop to 

ensure the proper number of turns and to reduce street installation time. 

Other agencies preform the loops (i.e., the loop consists of 1/2- or 3/4-inch (12- 

or 19-mm) polyvinyl chloride (PVC) pipe with the wire enclosed). These loops 

require at least a 1-inch (25-mm) wide slot in the pavement rather than the 

narrow sawcut used for conventional loops. Another technique utilizes wider 

slots of up to 4 inches (100 mm) to facilitate placement of the preformed loop. 

Because of harsh weather, Alaska enhances loop protection with their 

preformed loop. The loop is enclosed in 1-inch (25-mm) Schedule 40 PVC pipe 

(instead of Schedule 80). The loop wire is #12 AWG crosslinked polyethylene 

with backer rod fill to prevent damage caused by water encroachment and 

eventual freezing. 

In addition, many inductive-loop detectors are now built into the pavement 

during construction of a new roadway or during repaving. In Puerto Rico, 

concrete slabs with the inductive-loop detectors in place are being installed 

(see later discussion under “Inductive-Loop Detector Installation 

Alternatives”). This technique replaces the sawcut that usually contains the 

loop wires with a slab that includes a built-in loop. The slab, which replaces a 

section of roadway, can also be installed in new road construction. 

INDUCTIVE-LOOP DETECTOR FAILURES 

The relatively large number of inductive-loop detector failures nationwide in 

the 1980s resulted in an aggressive effort to determine their major causes 

and seek ways to eliminate or minimize them. FHWA, in cooperation with 

various State agencies, funded a number of studies to quantify the extent of 

inductive-loop detector failures, identify the causes, and evaluate the various 

installation procedures (e.g., sawcutting and cleaning slots) and materials 

(e.g., sealants, conduits, wires, and cables). The results of these studies are 

summarized below and are presented in more detail in Appendix M. 

Causes of Inductive-Loop Detector Failures 

Inductive-loop detector system failures are frequently traced to the in-road 

loop wire or to the splice between the loop wire lead-in and the lead-in cable. 

Since the introduction of the digital self-tuning electronics units, failure 

attributed to the amplifier/oscillator unit has all but disappeared. Failures 

continue to plague agencies using older electronics units that do not adjust to 

changes in temperature, moisture, and number of turns or type of loop wire 

and lead-in cable type and length. Loop failure literature is difficult to 

synthesize because of the different terminology used to define failures. For 

example, one report may categorize a failure as a “break in loop wire.” This 

may be caused by crumbling pavements, failure of the sealant, a foreign 

substance in the slot, or any number of other reasons. A report from another 

agency may report this failure as caused by “deteriorated pavement.” 

No matter how failures are categorized, the inescapable conclusion is that 

the predominant causes for failures in the inductive-loop detector system can 

be ameliorated by improved installation techniques and vigilant supervision 

and inspection. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-11 Federal Highway Administration

Failure Frequency 

Inductive-loop detector failure rates differ from agency to agency because of 

the large number of variables that contribute to the failures. In addition, 

until recently, very few agencies maintained comprehensive records. If a loop 

in a traffic-signal control system failed, it was repaired or replaced as a 

signal maintenance activity. The cause of the failure, the age of the loop, the 

condition of the pavement, etc., were not recorded. Consequently, many of 

the surveys reported in the literature were based on subjective, after-the-fact 

judgments. 

Perhaps the largest of the FHWA studies was conducted by the State of New 

York.(4,5) It was found that, of the 15,000 existing inductive-loop detectors 

maintained by the State, 25 percent were not operating at any given time. It 

was also found that, on the average, loop installations generally operated 

maintenance free for only 2 years.(4,5) This high failure rate encouraged New 

York State to develop the improved installation methods described later in 

the chapter. 

The failure rate reported by New York is consistent with other failure rate 

literature. For example, one district in Minnesota reported an annual failure 

rate of 24 percent and Cincinnati, OH, reported 29 percent failures per year. 

Although these areas experience cold weather climates, failure rates in the 

sun-belt States are about the same, but the causal factors differ. 

Failure Mechanisms 

Although most failures originate in the loop wire, the wire itself is not 

necessarily the precipitating cause of the failure. The failure is usually 

caused by one of several breakdown mechanisms, such as poor pavement 

condition or improper installation of sealant, which allows the wire to float to 

the top and become vulnerable to deterioration caused by traffic. 

Table 5-4 summarizes the results of an inductive-loop detector failure survey 

of eight western States.(2,3) Further discussion of loop failures is found in 

Appendix M. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-12 Federal Highway Administration 

Table 5-4. Summary of inductive-loop detector failures. 

 Percent installed by 

State State Contractor Major failures Remarks 

Alaska 10 90 No loop failures reported Preformed loops used 

exclusively 

California 5 95 Improper sealing and foreign 

material in saw slot 

Preformed loops used in 

poor pavement and dirt 

detours 

Idaho 10 90 Improper sealing No failure for loops made 

of #20002 cable 

Montana 10 90 Improper sealing ―

Nevada 5 95 Improper sealing and pavement 

deterioration 

―

Oregon 10 90 Improper sealing ―

Utah 70 30 Improper sealing and pavement 

deterioration 

Some preformed loops 

used with no failures 

reported 

Washington 10 90 Improper sealing and foreign 

material in saw slot 

Need better inspection to 

improve loop performance

LOOP LAYOUT AND SAWCUTS 

After securing the work zone with appropriate barricades, cones, etc., to 

divert traffic from the work area, carefully mark the pavement to depict the 

size and shape of the loop to be installed. A lumber crayon, chalk, or spray 

paint is typically used for this purpose. If available, a template of the proper 

size and shape is recommended. However, a straight edge or a tightened 

string can be used as a marking guide. It is critical that the marking reflect 

the exact location as shown on the construction plans. 

Corner Treatment 

Corner treatments vary among agencies. Traditionally, a chamfer cut such as 

that shown in Figure 5-8 is made to ease the stress on the wire that would 

otherwise occur from a sharp 90-degree bend. These diagonal cuts are 

overlapped so that the slot is at full depth at the turn points. The diagonal 

cut should be far enough back from the corner to prevent pavement breakout 

at the corners. 

A diagonal cut of approximately 12 inches (30 cm) or more is recommended. 

Some agencies use a hexagonal, octagonal, or round loop design in order to 

eliminate the sharp turn and reduce splashover. Another technique for 

installing round loops is presented later in this chapter under “Loop 

Installation Alternatives.” 

Many agencies find that diagonal cuts across the corners of the sawcut cause 

the triangular portion of the pavement to break up. Instead, 1.25-inch (3.1-cm) 

holes are drilled at the loop corners before the slots are sawcut. Core drilling 

the corners is also faster (15 seconds per drilled hole), and the integrity of the 

pavement is preserved. This technique is illustrated in Figure 5-9. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-13 Federal Highway Administration

The State of New York uses an alternative to core-drilled corners.(4) Their 

technique overlaps straight sawcuts at the corners (i.e., no diagonal sawcuts) 

and then chips out the inside corner using a small hand chisel and hammer 

or small air-powered impact chisel to cut a smooth curve for the wire to 

follow. 

Saw-Cutting Operations 

Sawing the slot for the loop wire is one of the most time-consuming parts of 

the installation process. The cost effectiveness of the saw-cutting operation is 

dependent on selecting the most appropriate equipment and ensuring it is in 

good operating condition. 

Overview of Saw-Cutting Equipment 

Many saw types and sizes are available for cutting slots. In the past, 

equipment for saw-cutting specified at least an 8- or 9-hp gasoline enginepowered saw with a 1/4- or 3/8-inch (6- or 9-mm) abrasive or diamond blade. 

Because it was perceived to be more economical, the abrasive blade was 

favored by some agencies. It could also be used for dry cutting and, therefore, 

did not require a water supply or produce a wet slurry that had to be cleaned 

out with compressed air. Most agencies, however, argue that the dust created 

by the dry-cutting method with an abrasive blade is irritating and dangerous 

to workers, motorists, and pedestrians. Furthermore, loop installers report 

that the abrasive blade wears out quickly, requires too much time for the 

cutting operation, and is difficult to maintain at a constant depth in the slot. 

Lead-in

Cut diagonals to prevent

sharp bends of wire

Overlap cut so that slot

at corner has full depth

Wire

Figure 5-8. Chamfer-cut corner treatment. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-14 Federal Highway Administration 

1-1/4”

To pull box

1 inch = 2.5 cm 

Figure 5-9. Core-drilled corner treatment. 

Although abrasive blades are less expensive than diamond blades on a perblade basis, the service life of abrasive blades is very short and is, therefore, 

not cost effective when compared with the diamond blade. After a 

comprehensive evaluation of saw blades by the State of New York, they 

concluded that a higher horsepower saw using a water-cooled diamond blade 

reduced cutting time by two-thirds (when compared to a dry-cutting system) 

with only a slight increase in initial saw costs. Increased material cost could 

be reclaimed in a savings of time, labor, and equipment longevity. 

Diamond-Blade Design 

The design of diamond blades is a specialized area of expertise. A number of 

manufacturers have their own proprietary designs. Generalization about the 

best design is difficult because of the differences in aggregate used in 

pavement surfaces. The best blade for a specific site is one that matches the 

cutting requirements (speed and pavement hardness) to the requirements 

and condition of the particular saw being used. The objectives guiding 

diamond-blade design include maximizing blade life and exposing new 

diamond cutting edges only after the surface chips are rounded to the point 

where the blade no longer cuts efficiently. 

The diamond chips are set in a matrix, which is laser-welded to the blade 

blank. Faster cutting implies exposing new diamond chips more quickly. 

This, in turn, means a reduction in the blade life. The matrix used to hold the 

diamond chips must be matched to the speed of the saw. The centrifugal force 

throws the diamond chips out when the edges become rounded. By making 

the matrix softer, cutting diamonds are released sooner and a faster cutting 

rate is possible. Increasing or decreasing the power of the saw requires a 

change to a blade suited for the particular saw and the speed at which it 

operates. 

Saw-Blade Troubleshooting 

Table 5-5 lists several problems encountered when using saw blades to cut 

pavements for loops and magnetometers.(6) The probable cause of the problem 

and potential solutions are also shown. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-15 Federal Highway Administration

Table 5-5. Saw-blade issue resolution. 

Problem Possible cause Solution 

Loss of one or more cutting 

elements 

Blade temperature too high and/or 

inadequate water supply 

Return to manufacturer for repair 

Blade stops cutting, but cutting 

elements appear OK 

Blade too hard for application Periodically redress blade on 

Haydite block or silica brick 

Vertical hairline cracks appear in 

cutting elements 

Blade matrix metal too hard for 

application or saw RPM too high 

Purchase new blade to match job 

requirements and saw being used 

Vertical hairline cracks appear in 

core below air gap 

Cutting element bond too hard or 

improper blade tension for RPMs 

being used 

Purchase blade with softer bond 

or drill a hole at base of crack to 

stop its growth 

Horizontal hairline crack appears 

at base of air gap 

Blade wobbling due to improper 

blade tension, or cutting element 

bond too hard, or bent arbor not 

tracking blade path 

Replace and/or reblank blade 

Arbor hole out of round Saw mandrel scored or too small 

for blade 

Replace or reblank blade; for 

minor damage, ream and insert a 

reducer bushing 

Undercutting of core below the 

cutting element 

Material being cut is excessively 

abrasive 

Remove blade before cutting 

elements are lost; replace or 

reblank blade if cutting is to 

continue; a wear guard may be 

added during reblanking 

Core dished—blade tension lost Bond may be too hard for RPMs 

or incompatible blade dimension 

Reset tension on blade; if problem 

persists, replace with blade better 

suited to job and saw 

Inconsistent blade life Blade used for various 

applications or blade not suited for 

application 

Insure proper blade for job 

requirements 

Short blade life Improper RPM selection, 

inadequate water, blade not suited 

to job, or excessive friction 

Insure adequate water supply that 

strikes blade near each side of 

arbor; check saw RPM against 

blade requirements 

Saw Horsepower 

Saw horsepower is a critical factor affecting the cost of the sawcut. Cutting 

speed is directly proportional to saw horsepower and is influenced by the 

hardness of the surface to be cut. Accordingly, a 65-hp saw will cut almost 

twice as fast as a 35-hp saw and four times as fast as an 18-hp saw. While 

there is slightly more wear on the blade, higher cutting speeds result in a 

decrease in the time a lane must be closed to traffic. 

The saw must be large enough to provide the power needed to keep the blade 

from bogging down.(6) Other indications of an underpowered saw or the 

improper match of the blade to the saw are described below: 

• White or blue smoke emitted from the front of the saw indicates that 

the blade is excessively hot. Blue to black discoloration of the blade 

core just below the cutting edges is another indicator of excessive heat. 

Water can be used to reduce blade temperature. A more appropriate 

blade may be necessary. Failure to correct this condition results in 

excessive diamond chip loss and short blade life. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-16 Federal Highway Administration 

• Breakup of part or complete loss of the cutting edge indicates 

excessive pressure on the blade during cutting. Reduction in cutting 

speed or selection of a better-suited blade is required to avoid this 

difficulty. 

On the basis of its l983 study, the State of New York revised its 

recommendation for its saw from 9 hp to l8 hp.(4) However, most contractors 

and a number of State agencies favor a more powerful saw. Wet-cut tests 

conducted in Texas with a 65-hp saw with a diamond blade reached cutting 

speeds ranging from 37- to 82-inch-ft (28- to 62-cm-m) per minute in 1-inch- 

(2.5-cm) deep cut in asphalt concrete. The average speeds are given in Table 

5-6. Ten tests were run on asphalt concrete (AC) surfaces and four on 

portland cement concrete (PCC). All tests cut to a 1-inch (2.5-cm) depth. The 

table also lists the average projected cutting speeds for other slot depths. 

During the tests with the 65-hp saw, the blade tended to ride up out of the 

slot, resulting in a substantial variation in the depth of the cut. A difference 

of up to 0.5 inch (1.3 cm) was common. Adding a 50-pound (lb) (23-kilogram 

(kg)) dead weight directly over the blade driveshaft reduced this problem 

when the cutting speed was between 25 and 30 inch-ft (19 to 23 cm-m) per 

minute. 

Table 5-6. Average saw-cutting speeds (inch-ft) as a function of surface type and slot depth. 

Type of 

surface 

Number of 

test cuts 

Actual slot 

depth 

Projected slot depth

 1.0 inch 1.5 inches 2 inches 2.5 inches 

AC 10 55.7 inch-ft 37.1 inch-ft 27.9 inch-ft 22.3 inch-ft 

PCC 4 9.0 inch-ft 6.1 inch-ft 4.5 inch-ft 3.5 inch-ft 

1 inch = 2.54 cm; 1 inch-ft * 0.77 = 1 cm-meter 

Wet Versus Dry Cutting

The preponderance of experience supports the need for wet cutting in 

portland cement concrete and its desirability in asphalt surfaces, particularly 

with higher-speed saws. While asphalt concrete can be successfully dry cut 

at high speed, such speeds reduce blade life. For example, it is estimated that 

dry cutting reduces the life of a diamond blade by one-third. 

Simply adding water to the saw cutting operation does not guarantee that 

the blade will not overheat and cut inefficiently. A poor water distribution 

system can simulate a dry cutting situation and should be avoided. The 

water jets must direct the water so that it strikes the blade near the center of 

the core. The centrifugal force will then throw the water out to the edge of 

the blade. The water should be directed to arrive at the cutting edge of the 

blade just as the edge point goes down into the pavement surface. This 

maximizes cooling and lubrication. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-17 Federal Highway Administration

Sawcut Depth 

Saws should be equipped with a depth gage to assist in maintaining proper 

depth and a horizontal guide to assure alignment. The appropriate depth is 

dependent on the type of pavement and the number of turns of wire. Table 5-

7 contains a general guide for slot depth, which suggests a minimum of 1-1/8 

inches (29 mm) when one turn of wire is used and a maximum of 2 inches (51 

mm) when 5 or 6 turns of wire are used.(6) However, some agencies such as 

the Virginia DOT use 3- and 4-inch (76- and 102-mm) slot depths for multiple 

wire-turn loops to prevent overlays from destroying the loop. The depth of the 

sawcut should be verified frequently during cutting to assure a constant 

value. Several agencies specify additional depth to allow for a 0.5-inch (13-

mm) sealant cover over the wires, while some manufacturers recommend 

planning for at least 1-inch (25-mm) of sealant cover. 

Table 5-7. Sawcut slot depth guide. 

Saw slot depth 

Turns of wire Inches Millimeters 

1 1-1/8 29 

2 1-3/8 35 

3 1-9/16 40 

4 1-3/4 44 

5 2 51 

6 2 51 

Finishing the Sawcut 

Sawcuts should be clean and well defined. All jagged edges and protrusions 

should be removed with a small chisel and hammer. It is crucial that the 

sawcut be clean and dry. Cleaning should take place immediately after the 

cutting is complete. Cutting dust, grit, oil, and contaminants must be kept 

out of the cut, which should be flushed clean with pressurized water and then 

dried with compressed air. 

Many agencies only require that the sawcut be cleaned with compressed air. 

If an inordinate rate of wire failure occurs, however, these agencies should 

consider flushing the cut with pressurized water as well as with compressed 

air. Cleaning procedures should be designed to avoid blowing the debris in 

the direction of passing pedestrians or vehicles. 

Only one sawcut along the marking should be made. If the sawcut is out of 

alignment, another cut should not be made near the first cut as this 

significantly weakens the pavement. If another cut is necessary, at least 4 to 

6 inches (10 to 15 cm) should be allowed between the old and new cuts. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-18 Federal Highway Administration 

INSTALLING LOOP WIRE 

The loop wire can be installed after the slot is cut and cleaned. The following 

discussion describes common practices utilized by a number of loop installers. 

Many contractors specializing in installing inductive-loop detectors have 

developed individual methods and short cuts, some of which are highly 

effective. Others are merely expedient and, in the long run, prove costly to 

the agency. This underlies the need for careful inspection procedures during 

the installation process. 

Field crew personnel are not always as vigilant or careful in handling loop 

wire as would be desired. Accordingly, the installation supervisor and/or 

inspector should ensure that the wire is not damaged during the installation 

process as damage could cause malfunction of the inductive loop and its 

eventual replacement. Should any damage occur to the wire during handling, 

it should be pulled up immediately and replaced. 

Wire Type 

Wire sizes are classified in accordance with the American Wire Gauge 

(AWG), originally called Brown & SharpeTM Gauge. AWG numerically 

represents wire size, where the lowest AWG numbers correspond to the 

largest wire cross sections. The gauge designations differ by 20.6 percent in 

cross-sectional area. Thus, #12 AWG is 20.6 percent larger than #14 AWG, 

etc. 

Wire is further defined by insulation type. Designations for some of the more 

commonly used wire for inductive-loop detector applications are shown in 

Table 5-8. The most common wire sizes range from #12 AWG to #16 AWG. 

Agencies that use the larger wire are about equally divided between #12 

AWG and #14 AWG. 

Table 5-8. Wire standard designations. 

Designation Description 

TFF Stranded copper conductor insulated with thermoplastic lead wire 

THHN Building wire, plastic insulated, 90 °C, 600-volt, nylon jacketed 

THW Building wire; plastic insulated; heat-, flame-, and moisture-resistant, 75 °C 

THWN Same as THW with overall nylon jacket 

TW The UL designation for thermoplastic insulated wire for use in conduit, underground, 

and wet locations; it is a common building wire having a bare, soft-copper solid or 

stranded conductora 

XHHN Cross-linked, polyethylene-insulated wire rated at 90 °C in dry locations and at 75 °C 

in wet locations 

a Building wire is defined as a commercial wire used to supply energy to light sources and power outlets in 

permanent installations utilizing 600 volts or less. Typically used in an enclosure not exposed to outdoor 

environments. 

Wire gauge designation is 

based on cross sectional 

area, where gauge numbers 

are separated by 20.6 

percent in wire cross 

sectional area. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-19 Federal Highway Administration

However, some manufacturers recommend #16 AWG, claiming that the 

difference in wire size is not as critical as the quality and thickness of the 

wire insulation. The insulation used on the wire may be rubber, 

thermoplastic, or synthetic polymer. Cross-linked polyethylene appears to be 

the most popular insulation and is strongly recommended by inductive-loop 

detector manufacturers. The insulation must withstand wear and abrasion 

from shifting streets, moisture, attack by solvents and oils, and the heat of 

high-temperature sealants. Stranded-loop wire is preferred over solid wire 

because stranded wire is more likely to survive bending and stretching due to 

its mechanical characteristics. 

Ducted Wire 

A growing tendency in inductive-loop design is to use insulated loop wire 

encased in continuous cross-linked polyethylene tubing. A number of 

manufacturers make these loop materials. Brand names include DetectaDuct, Power Loop, Signal Duct, and Electroloop. 

A typical product is composed of flexible ducting encasing THHN type #14 

AWG stranded wire conductors. Ducted-loop wire has the following 

advantages: 

• Conductor wires are able to move freely within the duct, 

compensating for the shearing stress caused by pavement 

displacement. 

• The duct protects against moisture penetration and temperature 

extremes. 

• The duct maintains its integrity when in contact with hightemperature (400 °F (149 °C) or more) sealing compounds. 

• Ducted-loop wire retains its flexibility over a wide range of 

temperatures. 

Prewound Loops 

Some agencies prewind and bind loop wire (or ducted wire) in their shops to 

meet length and size specifications, thus making a prefabricated loop for 

transport to the site. This is accomplished by winding the loop wire around 

pegs mounted on a wall or table. The spacing between pegs is measured to 

accurately reflect the required loop dimensions. Prewinding minimizes the 

tendency of the wire to spring into a coil after removing it from the supply 

reel. The completed loop consists of the proper number of turns along with 

the required length of lead-in to the pull box. Although prewinding appears 

to be a labor-intensive operation, it can cut installation time significantly. 

This procedure seems to be best suited for smaller loops. A prewound loop 

ready for installation in the sawcut is shown in Figure 5-10. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-20 Federal Highway Administration 

Figure 5-10. Prewound loop installation. 

Preformed Loops 

A preformed inductive loop is an inductive coil that is manufactured before 

installation in the road surface. Preformed loops are usually encased in a 

protective jacket or conduit to mechanically and electrically isolate the coil 

and lead-in wires. Preformed loops are installed using two different methods: 

paveover and sawcut. Primarily, they are paved over during the construction 

of new road surfaces. Secondarily, they are installed in sawcut grooves 

similar to field-wound sawcut loops. Preformed loops are an important 

alternative to field-wound sawcut loops, which suffer frequent failure as a 

result of the factors below: 

• Wire damage during installation or from sharp debris left in the 

sawcut groove. 

• Incorrectly formed sensors resulting from an incorrect number of 

turns or incorrect sawcut shape. 

• Eventual exposure of the loop wires to vehicle-caused abrasion from 

sealant or road material failure. 

Preformed loops address these problems by mechanically protecting the wires 

from damage and by fixing the number of turns and perimeter of the loop. 

Paveover preformed loops avoid the problems associated with sealant 

because there is no sawcut to compromise road surface integrity. However, 

the low profile of some sawcut preformed loops means they are less likely to 

migrate out of the sawcut. Preformed loops have demonstrated improved 

reliability, stable electrical isolation from ground, and a long physical life. 

Preformed loops are available in two different designs: conduit-type 

preformed loops and the InstaLoopTM sensor. Conduit-type loops are 

constructed of a continuous length of insulated wire encased in a heavy wall 

plastic pipe or conduit. The InstaLoopTM is manufactured from heavy-duty 

multiconductor cable. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-21 Federal Highway Administration

Figure 5-11 illustrates a conduit-type preformed loop commercially 

manufactured by several companies. In addition to the square shape, 

preformed loops are also available in rectangular and round configurations. 

Local signal maintenance shops frequently construct these loops from readily 

available materials for installation in their own municipalities. Homemade 

preformed loops can easily be customized to suit the requirements of the local 

agency, and are less expensive than the commercially available preformed 

loops. Commercial preformed loops come in several conduit sizes and 

materials suitable for different methods of installation. The outside 

diameters of the conduits range from a minimum of 3/8 inch (9 mm) to 

greater than 1 inch (25 mm). Typical conduit materials are common PVC 

irrigation pipe and fiber-reinforced hydraulic hose. Although the narrowest 

versions of this style of preformed loop are advertised for installation in a 

sawcut, a perfectly formed 0.5-inch- (12-mm-) wide sawcut is required to 

accommodate pipe fittings, causing a concomitant lack of adjustability. 

Therefore, conduit-type preformed loops are limited to paveover installations. 

Figure 5-12 shows the InstaLoopTM sensor, which is made of heavy-duty 

multiconductor cable instead of wire wound inside conduit. This more 

efficient design enables the use of narrow, 0.25-inch- (6-mm-) wide sawcuts, 

which minimize road damage. Also, the lower profile InstaLoopTM paveover 

sensor reduces reflective road surface cracking. Sensitivity, signal quality, 

and noise rejection are all improved due to optimal geometry and twisted 

pair lead-in cable. InstaLoopsTM are far more flexible than conduit-type loops, 

simplifying shipment, storage, transportation, and installation at the job site. 

Most importantly, they are field- and laboratory-tested to be highly resistant 

to cuts, scrapes, chemicals, and water damage. 

Figure 5-11. Preformed loop assembly. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-22 Federal Highway Administration 

Figure 5-12. InstaLoopTM Paveover Sensor. 

The InstaLoopTM has two unique features that improve performance: 

installation in a 0.25-inch- (6-mm-) wide sawcut groove and adjustable loop 

size. Sensor width is minimized and sensitivity maximized through use of 

multiconductor flat cable that stacks the wires in the slot. A 0.25-inch- (6- 

mm-) wide linear junction transitions the loop cable into the lead-in cable 

and thereby allows adjustment of the loop head size through movement of the 

junction located in the lead-in sawcut. The junction is fully sealed and rated 

for direct burial. 

Preformed Loop Sensor Installation 

Preformed loops can be installed in new road surfaces under concrete or 

asphalt and can be installed in sawcut slots. 

Concrete Paveover Installation 

To install paveover loop sensors under concrete, the loop must be secured in 

the desired position while the cement is poured. On bridge decks and roads 

where reinforcement bar is exposed before the pour, the preformed loops can 

be tied directly to the bars to hold them in place. Otherwise, they can be laid 

on the base course in position, and small piles of concrete placed on points 

along the loop head and lead-in to hold it in place. Once the loops are in 

place, concrete is poured as usual while making sure not to move the loops 

after they are covered. 

Asphalt Paveover Installation 

Installing preformed loops under asphalt is more challenging because of the 

requirement to ensure that the loops are not damaged or moved during the 

paving operation. Installation problems may occur due to the following three 

factors: 

1. High-temperature asphalt can melt loop insulation and conduit. 

2. Low-riding, hard-wheeled asphalt spreaders can move or 

destroy loops. 

3. Rough, unpaved surfaces and sharp aggregate can cut loop 

insulation and wires during compression. 

Other manufacturers of 

preformed paveover 

inductive loops for asphalt 

and concrete are Patriot 

Technologies, International 

Road Dynamics, Lead-In 

Systems and Debron Inc. 

These products have a 

maximum outside diameter 

of 5/8 inch (16 mm). 
 Traffic Detector Handbook—2006 

October 2006 Page 5-23 Federal Highway Administration

Metal-Sheathed Loop Cable 

A metal-sheathed loop known as mineral-insulated cable was first included 

in National Electrical Code (NEC) in 1953. It finds use in hazardous locations 

or difficult environments. Mineral-insulated cable is an assembly of one or 

more conductors insulated with a highly compressed refractory mineral 

insulation and enclosed in a liquid-tight seamless metallic sheath with a 

polyethylene jacket. The mineral insulation is magnesium oxide and the 

seamless sheath is phosphorus deoxidized copper. 

Mineral-insulated installations are found in many industries, including 

mining, aerospace, marine, petrochemical, and cryogenic, and in blast 

furnaces. Its application to inductive-loop detector installations is made 

possible by the observation that loop wires operate efficiently while encased 

in metal. These installations appear to be best suited to loops installed on the 

base course, previous to their being covered with the asphalt or concrete 

surface course. The loop is factory-assembled. The conductors are installed 

in the tubing parallel to each other, permitting the formation of a loop of the 

desired number of turns using one cable. 

Among the inherent advantages of this type of loop cable is the excellent 

shielding system. Shielding starts at the electronics unit and continues 

through the electronics unit wiring harness to the controller cabinet field 

terminals. The shielding proceeds through the lead-in cable to the corner of 

the loop and finally terminates at the opposite corner of the loop. To realize 

the advantage of the shielding, the metal shield must be opened at the far 

corner of the loop. It is recommended that 1/2 to 1 inch (1.27–2.54 cm) of the 

metal shield be removed and the exposed material and sheath waterproofed 

at this corner. The lead-in cable shield needs to be connected to both sides of 

the exposed sheath at the connection corner of the loop in the pull box. All 

loop and sheath connections must be waterproofed to ensure that the 

shielding system is insulated from ground except at the electronics unit end. 

IDOT developed a regulated procedure for installation of loop wire to combat 

haphazard construction procedures that seriously degraded performance. 

IDOT uses mineral-insulated cable loops primarily in new pavements and for 

replacing loops that are destroyed by widening and resurfacing projects. 

Figure 5-13 shows the installation detail suggested by the IDOT. Their 

specification is reproduced in Figure 5-14 for those agencies interested in 

using this type of product.(7) 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-24 Federal Highway Administration 

1

2

3

4

1

2

3

4

Lead-in cable

to controller 

1

234 1

23 4

Spread wires of both ends of loop cable

and test for continuity. Make numbered 

connections to form loop.

Figure 5-13. Metal-sheathed loop cable installation. 

Wire Insertion 

One continuous wire must be laid from the pull box through the curb, around 

the loop the designated number of turns, and back to the pull box. The supply 

reel should be checked before installation to ensure that there is a sufficient 

amount of wire on the reel for the particular installation. Careful count must 

be made of the turns of wire installed in the slot. This is a common error; the 

workmen simply lose count, particularly when four or more turns are 

required. Prewound and preformed loops, created in the shop, help ensure 

the proper number of turns in the loop. 

The wire should be laid in the slot so that there are no kinks or curls. A blunt 

tool such as a paint stirrer may be used to push the wires to the bottom of the 

slot. Some agencies use a narrow blunt wheel rolled along the slot. A sharp 

instrument (e.g., a screw driver) should never be used as it easily penetrates 

the wire insulation. 

CROSSING PAVEMENT JOINTS 

A special installation problem is encountered when the wire has to cross a 

pavement joint. When the loop is placed in a concrete roadway and crosses a 

pavement joint, particularly an expansion joint, the movement of one slab 

relative to another will cause the wire to break if some special treatment is 

not used to protect the wire. Additionally, one of the most common failure 

points is where the road surface meets the curb line. This area is subject to 

both vertical and horizontal shifting. 

There are two basic treatments for crossing the pavement joint. One method 

is to encase the portion of wire crossing the joint in some type of conduit, as 

shown in Figure 5-15. Many agencies use rigid plastic conduit or flexible 

tubing. The State of Illinois has used common rubber garden hose in some of 

their installations. The material should be approximately 0.75 inch (1.9 cm) 

in diameter and at least 1 ft (30 cm) long. The conduit is placed in a cut, 

which measures 1.25 inches (31 mm) wide and 16 inches (40 cm) long, as 

shown in Figure 5-16. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-25 Federal Highway Administration

SPECIFICATION FOR METAL SHEATHED LOOP CABLE 

The cable shall be #16 AWG with two, three, or four conductors as shown on the plans rated for 300 

Volts with outside diameters of 0.306, 0.337, and 0.353 inches, respectively. The cable shall be 

furnished with a terminal with subassembly kit composed of a pot, cap, sealer, and sleeves. 

Installation shall consist of furnishing and installing Mineral-Insulated Cable Detector Loop on a 

bituminous or PCC base course and covering it with bituminous surface course as described and 

detailed herein and in the plans. 

To install the cable, slanted holes of 1-inch diameter shall be drilled through the base course. Where 

curb and gutter are present, the hole shall begin where the base course and gutter meet. Where 

curbs are not present, the hole shall begin about 1 foot from the edge of the pavement. The cable 

shall not be installed until the loop area is ready to be covered (by the surface course) to minimize the 

traffic or asphalt truck running over the cable. 

The installation of the cable requires the forming of the loop size including the leads to terminal end 

with some spare, cutting the cable, and immediately sealing the terminal ends to prevent absorption 

of moisture by the mineral. The leads shall be bound together and inserted through the hole and 

positioned in place to make splices in the proposed junction box. 

The cable shall be secured to an asphaltic base course including the leads. The corner radius for the 

loop shall not be less than 6 inches. The leads shall be bound together with straps or fish tape rope 

(do not use wire) to prevent cutting or damaging the polyethylene cover. Shovels of asphalt can be 

used to hold the cable in place. 

The termination of the cable involves the stripping of the cable ends, installation of fittings, 

application, or insulating compound and installing the sleeve assembly according to manufacturer's 

instructions. When the stripping is completed, the exposed mineral-insulated material should be 

sprayed with an insulating spray. The termination procedure must be completed to avoid leaving the 

magnesium oxide exposed and the conductor sleeves open that allows moisture to enter them. 

The sleeved conductors shall be spliced together and tested with a loop test meter to ensure a proper 

connection. The conductors must be cleaned of the material coating to ensure a good connection 

while metering. The conductors must be soldered together and each conductor wrapped with two 

layers of rubber or vinyl electrical tape. The wrapping shall completely cover the soldered connection 

and the conductor sleeves for 1/2 inch. 

A loop test meter is used to ensure the loop will perform for detecting vehicles and after obtaining an 

acceptable reading, the spliced conductors are then sealed by centering them in a bottle mold and 

filling the mold with epoxy-type resin. The resin must completely cover the tapes on the sleeves. 

Any exposed copper sheath and the end seal pot shall be taped and coated with a silicone spray to 

prevent any moisture contact with their surfaces. Any electrical contact between the copper sheaves 

and the ends of the cables will destroy the inductance reading. 

No splice shall be permitted in the loop wire beyond the lead-in cable splice or controller terminal 

when the loop wire is connected directly to the controller terminal. 

The inductance and resistance of the loop as metered shall be within 10% of the calculated values for 

that loop as shown on the loop detail sheet. 

1 inch = 2.5 cm 

Figure 5-14. IDOT specification and installation procedures for metal-sheathed loop cable. 

The alternative installation technique provides an excess of wire at the 

pavement joint. The most common method for securing additional space for 

the extra wire is to make the cut in the shape of a diamond. The cut should 

be small, 2 inches (5 cm) from edge to edge, excavated to the full depth of the 

sawed slot. This allows sufficient space for an “S” shape of excess wire to be 

placed in the cut out diamond as shown on Figure 5-17. The diamond (and 

the adjacent joint, if necessary) is filled with an appropriate sealant. 

 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-26 Federal Highway Administration 

Figure 5-15. Crossing pavement joints using rigid tubing. 

Loop Wire

Side View

Loop Sealant Pavement Joint or Crack

3/4" Flexible Tubing

1-1/4" Sawcut

Loop Wire

Top View

1/4" Sawcut

6" 6"

8" 8"

1 inch = 2.5 cm 

Figure 5-16. Pavement joint crossing details. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-27 Federal Highway Administration

Pavement Joint 

or Crack

Saw Cut

Loop Wire

Diamond Cut

Extra Loop 

Wire

 

Figure 5-17. Crossing pavement joints using diamond cut. 

TWISTING THE LOOP WIRE LEAD-IN 

The two lead-in wires from the start and end of the loop turns should be 

twisted together to form a symmetrically twisted pair from the loop to the 

pull box. Depending on agency policy, an additional 3 to 5 ft (0.9 to 1.5 m) of 

lead-in pair slack should be provided in the pull box. 

Lead-in wire must be twisted to avoid crosstalk. Agencies vary on the 

specified number of twists per foot, ranging from two twists per foot to five 

twists per foot. Manufacturers feel strongly that wires should be twisted a 

minimum of five to six twists per foot. Some agencies or contractors do not 

twist the wires because the twisted wires require a wider slot for the lead-in 

wires, necessitating a larger saw blade than that required for the loop 

sawcut. Agencies that do not twist the lead-in wires usually allow only one 

pair of lead-ins per lead-in slot. 

The correct and incorrect ways of twisting lead-in wires are illustrated in the 

top portion of Figure 5-18. Although wires may be twisted by hand, more 

effective methods, shown at the bottom of the figure, employ (1) an anchor 

clamp and stick and (2) electrical tape and a mechanical twister to speed up 

the process. 

A multiple-loop configuration is frequently used to emulate a long loop. For 

this type of loop system, the loops may be wired in series, in parallel, or in 

series-parallel, as discussed in Chapter 2. The applicable wiring alternative 

is determined by the inductive-loop system designer through consideration of 

factors such as system configuration, system requirements, and the 

recommendations of the selected electronics unit manufacturer. The proper 

connections for each of the wiring alternatives are shown in Figure 5-19. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-28 Federal Highway Administration 

Figure 5-18. Method of twisting lead-in wires. 

Figure 5-19. Loop wire winding diagrams for multiloop installations.
 Traffic Detector Handbook—2006 

October 2006 Page 5-29 Federal Highway Administration

CROSSING CURBS 

Typically, the twisted lead-in wire must extend from its lane location in the 

roadway across the curb or shoulder to the pull box. If a curb is present, 

passage through the curb is usually accomplished by using a jackhammer 

drill or punch-type tool to make the entry as shown in Figure 5-20. Liquidtight flexible conduit is then installed in the hole for insertion of the twisted 

loop lead-in wire from the sawcut. The conduit should terminate in the pull 

box above the drainage to prevent moisture from entering the conduit. Figure 

5-21 shows a cross-section view of the conduit through which wire from the 

sawcut enters the pull box. The conduit should be installed at the same time 

as the pull box. 

Figure 5-20. Entry hole for curb crossing installations. 

1 inch = 2.5 cm 

Figure 5-21. Loop lead-in wire placement in conduit at roadway-curb interface. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-30 Federal Highway Administration 

Where the curb and gutter section is relatively shallow, the lead-in wire is 

placed in rigid conduit near the edge of the roadway. The conduit is inserted 

under the curb and gutter as shown in Figure 5-22. The roadway end of the 

conduit is normally 2 inches (5 cm) below the roadway surface. 

In some cases, the crew will simply cut the curb and pass the lead-in wire 

through the curb. Unless the cut is made to a minimum depth of 18 inches 

(45 cm) below the surface, this practice is not recommended. When the wires 

remain too close to the surface, they could be severed by grass trimming or 

other maintenance activities. 

Figure 5-22. Detail of lead-in wire placement for shallow curb section. 

Some agencies will drill through pavement before reaching the curb and 

gutter and will install a conduit beneath the curb section en route to the pull 

box. These agencies believe this is a simpler installation process than drilling 

through the concrete curb. Other agencies claim it to be more disruptive to 

the pavement integrity. It does, however, eliminate problems with the joint 

between the pavement slab and the curb-gutter section. 

When a curb is not present, a hole is drilled through the edge of the 

pavement to the pull box at a 45-degree angle, as depicted in Figure 5-23. 

The top of the hole should be at least 6 inches (15 cm) from the edge of the 

pavement. The hole should be aligned with the pull box and be of sufficient 

size to accept an appropriately sized conduit. 

1 inch = 2.5 cm 

Figure 5-23. Loop lead-in wires at pavement edge when no curb is present. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-31 Federal Highway Administration

INSTALLATION OF PULL BOX AND CONDUIT 

The purpose of the pull box (also referred to as a splice box, handhole, or 

junction box) is to house the splices between the lead-in wires from the loop 

and the lead-in cable to the controller cabinet. Additional pull boxes may be 

required at specified intervals on long runs to the electronics units in the 

controller housing. Pull boxes can be made of concrete, plastic, metal, or 

fiberglass. 

Figure 5-24 illustrates a standard pull box. Typically, this is a no. 3 or no. 5 

pull box in most installations. The box type and location should be specified 

and shown on the construction plans. Many agencies specify that the pull 

boxes, conduits, and curb cuts must be completed before beginning the loop 

wire installation. Typical installation details are shown in Figure 5-25. 

Figure 5-24. Standard pull box. 

Precast 

reinforced 

concrete box

Bushing

6" min 12" min

Clean crushed rock sump

Extension

Seal around 

conduit with 

mortar

1 inch = 2.5 cm 

Figure 5-25. Pull box installation detail. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-32 Federal Highway Administration 

The lead-in wires are protected by conduit inserted into the hole through the 

curb or pavement and connected to the pull box. Although a 3/4-inch (1.9-cm) 

conduit is adequate for a single pair of lead-in wires (one loop), it is common 

to use a 1-inch (2.5-cm) minimum conduit. Table 5-9 gives the size of conduit 

as a function of the number of loop conductors. Normally, no more than two 

twisted pairs are installed in one sawcut slot; however, sometimes loop leadin wires are collected at an in-street pull box (i.e., a pull box located in the 

travel way) and then routed to the terminal pull box in a conduit under the 

pavement. 

At the roadway end, the conduit should be terminated 2 inches (5 cm) below 

the pavement surface and a nonmetallic bushing attached to protect the leadin wires. For further protection, the lead-in wires should be taped for several 

inches on each side of the bushing. Finally, each loop lead-in wire should be 

tagged for identification with the loop number and the start (“S”) and finish 

(“F”) of each individual loop. 

Table 5-9. Conduit size. 

Conduit size 

inches (cm) 

Number of pairs of loop 

conductors 

1 (2.5) 1–2 

1.5 (3.7) 3–4 

2 (5) 5 or more 

TESTING THE LOOP 

Continuity, inductance, and resistance of the loop and lead-in wire should be 

verified before the loop wires are sealed in the pavement. Tests should be 

conducted with one or more loop tester devices capable of measuring the 

induced ac voltage, inductance in microhenrys (μH), integrity of the wire 

insulation, and loop wire resistance in ohms. A loop insulation tester, such as 

the one shown on the left in Figure 5-26, measures loop insulation resistance 

at 500 volts in the range of 5 megohms to 500 megohms and loop wire 

resistance in the range of 0 to 10 ohms. The frequency tester, shown on the 

right, measures loop frequency to determine inductance. An inductive-loop 

system analyzer, such as the model illustrated in Figure 6-2, may also be 

used to measure these parameters. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-33 Federal Highway Administration

 Insulation tester Frequency tester 

Figure 5-26. Loop test devices. 

The integrity of the wire insulation can also be measured with a 

megohmmeter (popularly called a “megger”). The loop insulation resistance 

should be in excess of 100 megohms between each end of the lead-in and the 

nearest reliable electrical ground (e.g., street light or fire hydrant) under any 

operational condition. 

The wiring diagram of the plan set or the inspection report should include a 

table of calculated values of the inductance in microhenrys and resistance in 

ohms for each loop. Two values should be shown: one at the pull box without 

the lead-in cable, and the second at the controller cabinet with the lead-in 

cable connected. The loop installation is acceptable under the following 

conditions: 

• Induced voltage: There is no deflection of the pointer on a voltmeter. 

• Inductance: The inductance reading on the loop tester is within 10 

percent of calculated value. 

• Leakage to ground: The resistance to ground of a newly installed loop 

exceeds 100 megohms as measured with a 500 volt (V) megger. 

• Loop resistance: The reading on an ohmmeter is within 10 percent of 

the calculated value. 

The measured values obtained from the test should be recorded on the wiring 

plan (or on an inspection report described later under “Final Tests and 

Record Keeping”). This information is used for future testing and 

maintenance. Reference 8 describes procedures utilized by Caltrans to verify 

the operation and measure the sensitivity of a new loop installation. These 

procedures involve the following steps: 

1. Measurement of the resonant frequency of the wire loop and lead-in 

cable combination. 

2. Calculation of the inductance corresponding to the measured 

resonant frequency. This computation includes the effect of any 

external capacitance that may have been added to the wire loop 

terminals. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-34 Federal Highway Administration 

3. Comparison of the calculated inductance (from step 2) with the 

nominal value found from curves such as those in Figure 2-6 and data 

from tables such as Tables 2-6 and 2-7. Caltrans recommends that 

the calculated and nominal values should not differ by more than 

approximately 20 percent. 

4. Measurement of the change in frequency created when a vehicle 

model (such as that for a small motorcycle) is placed in the center of 

the loop under test. The small motorcycle (100 cc Honda) model 

consists of a 2- x 2-ft (0.6- x 0.6-m), 1/2-inch- (1.3-cm-) thick plywood 

board, containing one turn of #14 AWG solid wire laid into a 1/8-inch- 

(4-mm-) wide by 1/4-inch- (7-mm-) deep groove cut into its perimeter. 

The wire is spliced and soldered to form a shorted turn. 

5. Comparison of the measured frequency change (from step 4) with the 

expected frequency change for the wire loop and lead-in cable 

combination. The expected frequency change is calculated using 

equations similar to those described in the “Loop System Sensitivity” 

and “Single-Loop Example” sections of Chapter 2. Caltrans 

recommends that the measured frequency change should not be less 

than the expected value by more than 20 percent. 

SEALING THE SAWCUT 

One of the major challenges in inductive-loop detector technology is 

protecting the loop wires from breakage, moisture, and from floating to the 

top of the sawcut and becoming vulnerable to the ravages of traffic. The 

critical factors are sealant selection and application technique. Sealant types, 

application methods, and wire protection within the slots are discussed 

below. 

Types of Sealants 

Matching the proper sealant with the type and condition of the roadway, 

together with appropriate, well-supervised installation techniques, are the 

key factors to effective, maintenance-free inductive-loop detector operation. 

During the 1970s, sealant types were about equally divided between asphaltbased and epoxy-based sealants. Early asphalt-based sealants were heated 

before application. These were soon replaced with gun-grade asphalts 

marketed as caulking compounds. These, in turn, were superseded by epoxy 

sealants. The early epoxies were too hard and brittle to adapt well to shifts in 

pavements and were expensive and difficult to apply. 

Available sealants possess a variety of desirable characteristics. Hardness 

allows the sealant to resist the penetration of foreign materials and street 

debris such as nails or metal fragments, which might pierce or break the 

wire. Flexibility allows deformation without cracking during thermal 

expansion and contraction. Corrosive resistance protects against road salts, 

gasoline, antifreeze, transmission fluids, brake fluid, and engine oils 

commonly found on roadway surfaces. Other advantageous sealant properties 

are good adhesion, and contraction and expansion characteristics similar to 

those of the highway material in which it is installed. 

A rapid curing rate and the ability to be applied to damp surfaces are other 

beneficial sealant traits. A rapid curing rate is desirable because it minimizes 

lane-closure time to traffic. During the saw-cutting operation, a water-cooled 
 Traffic Detector Handbook—2006 

October 2006 Page 5-35 Federal Highway Administration

blade is generally used, and even though the slot may be cleaned with 

compressed air, some residual dampness may remain. In such cases, it is 

desirable to apply the sealant directly to damp surfaces rather than first 

drying them with butane torches, which is time consuming and may damage 

the asphalt road surface. 

Three material families grouped by composition and manufacturing process 

are recommended for sealing and filling asphalt-surfaced pavements.(1) These 

are: 

• Cold-applied thermoplastic bituminous materials— 

o Liquid asphalt (emulsion). 

o Polymer-modified liquid asphalt. 

• Hot-asphalt thermoplastic bituminous materials— 

o Asphalt cement. 

o Fiberized asphalt. 

o Asphalt rubber. 

o Rubberized asphalt. 

o Low-modulus rubberized asphalt. 

• Chemically cured thermosetting materials— 

o Self-leveling silicone. 

A summary of asphalt concrete crack treatment material types, test methods, 

applications, and cost is given in Table 5-10. Table 5-11 describes the 

properties of recommended asphalt concrete sealant materials. Sealants 

recommended for use with portland cement concrete are shown in Table 5-

12.(9) A synthesis of the state of the technology in overlays for hot-mix asphalt 

and portland cement concrete can be found in Reference 10. 

Table 5-10. Asphalt concrete crack treatment materials.(1) 

Material Applicable specification Recommended 

application 

Cost range based 

on 1998 costs 

($/kg) 

Asphalt emulsion ASTM D 977, ASTM D 

2397, AASHTO M 140, 

AASHTO M 208 

Filling 0.15 to 0.30 

Asphalt cement ASTM D 3381, AASHTO 

M 20, AASHTO M 226 

Filling 0.15 to 0.30 

Fiberized asphalt Manufacturer’s 

recommended 

specifications 

Filling 0.35 to 0.60 

Polymer-modified 

emulsion 

ASTM D 977, ASTM D 

2397, AASHTO M 140, 

AASHTO M 208 

Filling 

(possibly sealing) 

0.80 to 1.20 

Asphalt rubber State specifications, 

ASTM 5078 

Sealing 

(possibly filling) 

0.45 to 0.65 

Rubberized asphalt ASTM D 1190, AASHTO 

M 173, Fed SS-S-164 

ASTM D 3405, AASHTO 

M 301, Fed SS-S-1401 

Sealing 

Sealing 

0.55 to 0.85 

0.65 to 1.10 

Low-modulus rubberized 

asphalt 

State-modified ASTM D 

3405 specifications 

Sealing 0.75 to 1.40 

Self-leveling silicone ASTM D 5893 Sealing 5.75 to 6.75 

$1 /kg = $2.17 /lb 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-36 Federal Highway Administration 

Sealant applications are characterized as either cold-pour or hot-pour types. 

Cold-pour types include polyester resin, epoxies, polysulfide bases, and 

rubberized asphalt. The hot-pour varieties include hot pitch, asphalt, and 

rubberized asphalt. Both cold- and hot-pour sealants must not revert to their 

liquid state during hot weather as this would allow the loop wire to float to 

the surface. Sealant should always be fluid enough during application to 

level itself on a horizontal surface, but should not run when applied on an 

inclined surface. The sealant must always be applied in strict adherence with 

the manufacturer’s instructions. 

Table 5-11. Properties of asphalt concrete filler and sealant materials.(1) 

Property Material type 

EmulSion Asphalt

cement 

Fiberized 

asphalt 

Polymermodified

emulsion 

Asphalt

rubber 

Rubberized 

asphalt 

Lowmodulus 

rubberized 

asphalt 

Selfleveling

silicone 

Short 

preparation 

z z zz

Quick 

and easy to 

place 

z zz zz z zz zz zz

Short 

cure time 

zz zz zz zz zz z

Adhesiveness 

zz zz z z z z z z

Cohesiveness 

z z zz z

Resistance 

to softening 

and flow in 

cured state 

z z z zz zz

Flexibility z z z zz zz

Elasticity z z z z zz

Resistance 

to aging and 

weathering 

z z zz

Resistance 

to abrasion 

z zz z

z = applicable, zz = very applicable. 

Table 5-12. Portland cement concrete sealant materials.(9) 

Material Applicable 

specification 

Design extension* 

(%) 

Cost range based 

on 1998 estimated 

costs ($/L) 

PVC coal tar ASTM D 3406 10 to 20 1.75 to 2.75 

Rubberized asphalt ASTM D 1190, ASTM D 

3405, AASHTO M 173, 

AASHTO M 301 

15 to 30 0.60 to 1.00 

Low-modulus 

rubberized asphalt 

Modified ASTM D 3405 30 to 50 0.70 to 1.20 

Polysulfide 

(1 and 2 part) 

Fed SS-S-200E 10 to 20 Not available 

Polyurethane Fed SS-S-200E 10 to 20 5.20 to 7.20 

Silicone (nonsag) ASTM D 5893 30 to 50 6.50 to 9.00 

Silicone (self-leveling) ASTM D 5893 30 to 50 6.50 to 9.50 

* Consult manufacturers’ specifications for specific design extensions. 

$1 /G = $3.8 /L 
 Traffic Detector Handbook—2006 

October 2006 Page 5-37 Federal Highway Administration

Hot tar continues to be used because of its low initial cost. Several States, 

however, have prohibited its use due to the high percentage of failures and 

the danger and inconvenience to workers during application. The heat 

involved (sometimes exceeding 500 °F (260 °C)) in the process frequently 

breaks down or deforms the insulation of the loop wire, diminishing its 

insulating integrity. In addition, hot tar sealant becomes soft in hot weather 

and allows vehicles to track the tar from the sawcut. Rocks and other debris 

can penetrate the soft surface and eventually damage the loop wire 

insulation. 

Rubberized asphalt appears to be the sealant of choice, particularly for 

asphalt pavements. Its use requires the specified loop wire to have insulation 

that can withstand the 400 °F (205 °C) application temperature of the 

rubberized asphalt sealant. Modern epoxy formulations have overcome some 

of their early drawbacks and are now formulated to provide a greater degree 

of flexibility. 

A number of States have conducted extensive tests of various, commercially 

available sealants prior to listing the acceptable sealant products in their 

specifications.(11) In reviewing the documentation of these State tests, a 

product that scored highest in one State was sometimes considered 

unacceptable in another State. The disparities in test results are probably 

caused by differences in geographic and climatic conditions, as well as 

methods of testing the products. However, most agencies are consistent in 

product approval criteria. This, in turn, suggests that agencies should 

periodically validate their tests and test procedures to ensure that their 

specifications are appropriate. 

Sealant Application Techniques 

Common practices for sealing the loop wire are depicted in Figure 5-27. One 

procedure (shown on the left of Figure 5-27) consists of applying a layer of 

sealant to the floor of the sawcut after thoroughly cleaning and drying the 

slot. The loop wires are then laid in the slot and covered with a second, final 

layer of sealant. This method tends to fix the position of the loop wires in the 

middle of the sawcut, protecting them on the top and bottom. Some agencies 

believe that this procedure, although more costly, protects the loop wires 

from water intrusion. 

In the technique illustrated in the middle of Figure 5-27, the wire is simply 

laid in the slot and covered with sealant. There is no way to control the 

positioning of the wire in the slot. In a three-wire installation, the three 

layers of wire may form a triangle on the bottom of the slot or may stack over 

each other. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-38 Federal Highway Administration 

The backer rod/sealant combination shown on the right of Figure 5-27 is 

based on the theory that stresses on sealant during elongation are reduced if 

the sealant has less depth. With this method, the wires are placed in the slot 

and then a backer rod (generally a closed-cell polyethylene rope) is forced 

into the slot over the wires. The remainder of the slot is then filled with 

sealant. The backer rod assures a shallow layer of sealant, reducing tensile 

stresses and leaving the wires free to adapt to shifting of the pavement. An 

alternative method that is becoming increasingly popular is to insert short 

pieces of the backer rod of approximately 1 inch (2.5 cm) in length every foot 

or two (30 cm to 61 cm) to anchor the wire in the slot before applying the 

sealant. 

1-9/16"

Loop Wires

1st Layer

of sealant 

Sealant

2nd Layer

of sealant

Backer Rod

Volume unfilled

by sealant

1 inch = 2.5 cm 

Figure 5-27. Methods of applying sealant. 

Table 5-13 lists commonly used backer rod materials, testing method 

standards, backer rod properties, and compatibility. 

Table 5-13. Backer rod materials.(9) 

Material Applicable standard Properties* Compatibility 

Extruded closed-cell 

polyethylene 

ASTM D 5249, Type 3 NMA, ECI, NS Most cold-applied 

sealants 

Cross-linked extruded 

closed-cell polyethylene 

ASTM D 5249, Type 1 HR, NMA, ECI, NS Most hot- and coldapplied sealants 

Extruded polyolefin ASTM D 5249, Type 3 NMA, NS, NG, CI, IJ Most cold-applied 

sealants 

* CI = chemically inert, ECI = essentially chemically inert, HR = heat resistant, IJ = fills irregular joint 

well, NG = nongassing, NMA = nonmoisture-absorbing, NS = nonstaining. 

No published evidence of the superiority of one sealant application method 

over another has appeared. Most inductive-loop detector installers agree that 

the neat arrangement of wires as shown in many published illustrations is 

simply not indicative of actual installations; rather, the lay of the wires is 

random in the slot. They agree that complete encapsulation by the sealant is 

seldom achievable. Some installers also argue that placing sealant in the 

bottom of the saw cut (as depicted on the left in Figure 5-27) before laying the 

wire is time-consuming and requires more road-closure time. Installers 

indicated that even when this method of installation is specified, it is unlikely 

to be followed unless an agency inspector was actually overseeing the 

installation. This is another illustration of the need for careful installation 

inspection. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-39 Federal Highway Administration

On the other hand, proponents of placing sealant on the bottom of the sawcut 

report that the extra protection afforded by the sealant bed prevents the 

intrusion of water through small pavement cracks. It also avoids the 

possibility of sharp edges or rocks becoming dislodged and piercing the 

installation. Others feel that this is a remote possibility, particularly if the 

sawcut is well cleaned of debris. Alternatively, some agencies specify the 

placement of a layer of sand rather than sealant at the bottom of the sawcut. 

This provides a smooth bed but does not prevent the intrusion of water 

through pavement cracks. 

The amount of sealant to be applied should be sufficient to completely fill the 

sawcut, but not overfill. A trowel or other tool should be used to ensure that 

the sealant is slightly below the pavement surface and to remove any excess 

sealant. Figure 5-28 shows the result of poor installation procedures. It 

shows overfilling, underfilling, and air bubbles in the sealant. All three 

conditions can lead to inductive-loop detector failure and should have been 

corrected during the installation process. 

Sealant may be applied with a special applicator as shown earlier in Figure 

5-6 or by hand directly from a container as depicted in Figure 5-29. In this 

illustration, one member of the crew is placing the lead-in wires in the slot 

while another is pouring the sealant into the sawcut. A paint stirrer has been 

inserted into the slot to hold the wire down while the sealant is being 

applied. Other techniques can be used (e.g., backer rod strips, and nylon 

rope) to hold the wire securely in place while sealant is added. Figure 5-30 

shows the crew properly completing the sealant application procedure by 

removing any excess material from the pavement and dusting talc on the 

fresh sealant. 

A number of agencies and loop installers coat the newly applied sealant with 

sand or talc after the sealant has been applied and before opening the lane to 

traffic. This prevents tracking of the sealant during its curing process and 

allows earlier opening of the traffic lane. 

Some sunbelt agencies use sand as the sealant by tamping it into the slot 

after the wire is placed in the sawcut. However, the sand is easily tracked out 

of the slot and the wires may become dislodged. Therefore, this practice is not 

recommended. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-40 Federal Highway Administration 

Figure 5-28. Poor sealant application. 

Figure 5-29. Crew applying sealant by hand. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-41 Federal Highway Administration

Figure 5-30. Finishing sealant application. 

SPLICING THE WIRE 

Another critical step in the loop installation process is splicing the loop leadin wire to the lead-in cable that connects to the electronics unit in the 

controller cabinet. This splice, located in the pull box, should be the only 

splice in the loop system. The splice is frequently the cause of inductive-loop 

detector system failure; however, if proper splicing procedures are used, the 

splice should not pose a problem. There are two steps to creating a splice: the 

physical connection of the wires and the environmental sealing of the 

connection. 

Connecting the Wires 

Methods for physically connecting the lead-in wires from the loop with the 

lead-in cable vary among agencies. The two preferred methods are twisting 

and soldering or crimping and soldering. Most electronics unit manufacturers 

specify a solder connection in their installation procedures. The argument for 

soldering is that it provides a connection with lower resistance and has less 

susceptibility to corrosive degradation. The soldered connection will, 

therefore, require less maintenance in the long run. 

While pressure connectors (crimping) without soldering may have been 

generally acceptable in the past, the use of solid-state electronic assemblies 

now makes soldered connections preferable. These assemblies operate at low 

voltage levels and minimum current loads. Because of this, they are 

susceptible to even slight voltage drops, which occur where poor electrical 

connections cause high resistance in a circuit. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-42 Federal Highway Administration 

The procedures for making a twisting and soldering or crimping and 

soldering connection begin in the same manner as shown in Figure 5-31. 

First, about 8 in (20 cm) of the outer covering of the lead-in cable is stripped 

away. One wire is clipped about 3 in (7.5 cm) shorter than the other. About 

1.5 inch (4 cm) of the wire insulation must be stripped away from the cable 

wires and the loop lead-in wires. The appropriate wires are then twisted 

together (end to end) or crimped with a noninsulated butt connector. Care 

must be exercised to assure that the correct wires are connected. 

High-quality crimp connections can be made using a pressure crimping tool 

that provides a uniform 360-degree crimp. Only bare wire, and not the 

adjacent insulation, should be inserted into the crimp connector. The 

individual wire splices should be staggered so that the entire cable spice does 

not become too bulky. Soldered connections should be made with 60/40 

(tin/lead) resin core solder. The Ontario Ministry of Transport specifies the 

use of 60/40 (lead/zinc) resin core solder as an alternative.(6) 

Step 1. Strip loop wires and lead-in cable

Lead-In Cable

Ground

Wire 1-1/2”

2” 3” 1-1/2”

Shield

Loop Wire

3”

1-1/2”

Step 2. Connect and Solder

Twist bare conductors

together

and solder with 60/40

(tin/lead) resin solder

OR

Crimp bare conductors 

together with an 

uninsulated butt connector

and solder with 60/40

(tin/lead) resin solder

Step 3.Insulate each solder joint separately

Electrical Tape Shrink Tube

Step 4.Environmentally seal total splice

1 inch = 2.5 cm 

Figure 5-31. Splicing loop wire at pull box. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-43 Federal Highway Administration

Sealing the Splice from the Operating Environment 

Once the wires are firmly spliced, it is essential that the splice be 

environmentally sealed against weather, moisture, abrasion, etc. A variety 

of methods are used, including heat-shrinkable tubing, special sealant kits, 

special forms to be filled by sealant, pill bottles with slot sealant, tape and 

coating, etc. Any method is acceptable as long as it provides a reliable 

environmental seal. 

When using heat shrinkable tubing, the wires must be inserted in the tubes 

before they are joined. Two small tubes are used for the individual wires and 

one large tube to cover the entire splice area. The wires are connected and 

soldered as described earlier, and the small tubes are centered over the 

connections and are gradually heated by an electric heat gun or butane torch 

until the tubes have shrunk uniformly around the wires. The larger tube is 

now positioned over the splice and heated in the same manner, taking care to 

avoid burning the wire insulation or the cable jacket. Once the tubes have 

been shrunk over the splice area including the cable end, a secure 

environmental seal has been established. 

Special sealant kits are commercially available. These kits may have onepart or two-part sealants. One-part sealants are ready to apply. Two-part 

sealant kits usually contain the resin and hardener in separate 

compartments of a plastic package. The seal between the two parts is broken 

and the two components mixed. The package is then cut open and the sealant 

is formed over the splice area. It is important to ensure that the mixture 

penetrates between the wires. 

An alternative process places a form around the splice area. A liquid sealant 

is poured into the form; care must be taken to totally fill it with sealant and 

ensure that no voids appear within the splice area. 

A simple encapsulation process consists of using a common drug store pill 

bottle as shown in Figures 5-32 and 5-33. The wires are spliced side by side 

rather than end to end. After the wires are twisted and soldered (some 

agencies add a wire nut or tape the joint), the completed splices are inserted 

into a pill bottle that has been filled with sealant or electrical epoxy. 

Some agencies still use electrical tape to seal the splices. This procedure is 

not recommended. However, if tape is used, it should be high-quality 

electrical tape. Each wire connection should be taped separately and then the 

entire splice area covered by several layers of overlapped tape. A waterproof 

sealant should then be applied to the entire taped area. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-44 Federal Highway Administration 

Figure 5-32. Pill bottle splice. 

1 inch = 2.5 cm 

Figure 5-33. Pill bottle splice details. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-45 Federal Highway Administration

LEAD-IN CABLE INSTALLATION 

In general practice, the lead-in cable from the pull box to the controller 

cabinet is buried bare or placed in conduit below the surface of the ground. In 

either case, the cable should be buried in a trench at least 18 inches (45 cm) 

below the surface. If conduit is used, it should be waterproofed. 

Trenching is the most common method for installing underground cable. The 

construction plans should specify the depth of the trench. The trench is 

backfilled in layers not to exceed 6 inches (15 cm) after the cable is placed in 

the trench. It is recommended that each layer of backfill be compacted with 

mechanical tampers to the approximate density of the surrounding ground. 

No extra material should remain when the backfill procedure is complete. 

The lead-in cable terminates inside the controller cabinet at the field 

terminal strip as illustrated in Figure 5-34. An excess of 18 inches (45 cm) of 

cable should be provided in the cabinet housing. 

Some equipment manufacturers and agencies recommend that the shield of 

the cable not be connected to the third or ground terminal. One manufacturer 

describes this technique by stating that the cable shield should be insulated 

from ground and float at both ends (namely at the cabinet and in the pull 

box). This recommendation is the same as that from the Ontario Ministry of 

Transportation, which does not recommend grounding the lead-in cable 

shield at either end. (See Appendix N, Section IV, bullet 3.) The justification 

given for this procedure is that the detection system operates at extra-low 

voltage (e.g., typically, output voltage for an inductive-loop electronics card is 

5 V, with a current draw measured in milliamperes) and the cabinet and card 

rack are firmly connected to ground. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-46 Federal Highway Administration 

Figure 5-34. Lead-in cable grounding at the field terminal strip. 

Other manufacturers recommend grounding at the cabinet (i.e., connecting 

the shield to the terminal as in Figure 5-34) and insulating the end of the 

cable in the pull box. This allows any electrical disturbance or interference to 

be safely grounded without affecting the loop lead-in cable. 

NEMA avoids the issue by stating that “Field installation practices or 

inductive-loop detector electronics unit design may require grounding the 

shield of the loop lead-in cable. Such grounding should be in accordance with 

the electronics unit manufacturer’s recommendation.” 

The loops are capacitively coupled to the earth and receive a voltage surge 

whenever lightning currents enter the ground nearby and cause an earth 

voltage rise. These surges may need to be suppressed where the cable enters 

the controller cabinet or where these circuits enter the electronics units. 

Back-to-back zener diodes or neon lamps usually provide adequate 

protection. 

Most modern electronics units have built-in lightning protection features. 

Even so, some agencies in lightning prone areas (specifically Ontario, 

Canada(6) and Florida) recommend that additional protection be added, such 

as two neon lamps or two gas-filled surge-voltage protectors connected to the 

terminal strip, as shown in Figure 5-34. Other grounding and lightning 

protection approaches are described in Appendixes N and O. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-47 Federal Highway Administration

FINAL TESTS AND RECORD KEEPING 

Final tests are recommended after all loops and cables are installed. The 

final tests repeat the initial tests, described previously in this section, to 

ensure the inductive-loop detector installation is performing as expected and 

is fully operational after all cables have been buried. 

The test results should be recorded on an installation data sheet such as the 

one depicted in Figure 5-35. Any modifications made to the original plan 

drawings should be noted, dated, and retained as “As Built” plans. 

The inspector should verify that the correct readings from the tests are 

recorded and that the modified plans reflect the as-built conditions. These 

records comprise the history of the particular inductive-loop detector system 

and serve as the baseline for any future maintenance or repair activity. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-48 Federal Highway Administration 

LOOP INSTALLATION DATA

Date: ________________________ 

Weather: ________________________________________________________ 

Approximate Temperature: ____________ 

Pavement Condition: Wet (___) Dry (___) 

Location: _______________________________________________________ 

Drawing Number: _________________________________________________ 

Contractor: ______________________________________________________ 

Material: 

 Loop Wire: __________________________________ 

Lead-In Cable: _______________________________ 

Pull Box: ____________________________________ 

 Splice Kit: ___________________________________ 

Conduit: 

 Curbside to Pull Box: ___________________________________ 

 Pull Box to Cabinet: ____________________________________ 

 Sealant: ____________________________________ 

Loop System: 

Position: _________________________________________________ 

 Configuration: ______________________________________________ 

 Estimated Measured 

 Twisted Lead-In Length: _________ _________ 

 Lead-In Cable Length: _________ _________ 

 Inductance: _________ _________ 

 DC Resistance: _________ _________ 

 Leakage Resistance: _________ _________ 

Comments on Workmanship: _______________________________________________ 

 ____________________________________________________________________ 

Remarks: _______________________________________________________________ 

 ____________________________________________________________________ 

Recommendation: ________________________________________________________ 

Inspected By: _______________________________________________ 

Figure 5-35. Representative inductive-loop installation data sheet. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-49 Federal Highway Administration

GENERAL INSTALLATION GUIDELINES 

General guidelines for installing inductive-loop detectors at signalized 

intersections are presented in Table 5-14.(12,13) These general guidelines apply 

under a wide variety of circumstances. The design engineer is cautioned that 

installations at specific locations may require that other design, installation 

procedures, testing, etc., be followed. 

Table 5-14. General installation guidelines for installing inductive-loop detectors. 

Design 

• The width of the loop should be tailored to the width of the lane 

• Loops should not be over 6 ft (1.8 m) wide in a 12 ft (3.7 m) lane. 

• Loops should not be less than 5 ft (1.5 m) wide (height of detection is 

approximately one-half to two-thirds of the loop width). 

• All loops should have a minimum of two turns of wire in any sawcut except 

in a quadrupole. 

• One additional turn of wire may be specified for loops installed in reinforced 

concrete or over 2 inches (5 cm) deep. 

Installation of Loop Wire and Lead-in Cables 

• The corner of loop sawcuts should be cored, chiseled, beveled, or 

diagonally cut to eliminate sharp turns. 

• Sawcut should be deep enough to provide for a minimum of 1 inch (2.5 cm) 

of sealant over uppermost wire. 

• Sawcut should be cleaned out with high-pressure water after cutting and 

then dried with compressed air. 

• If 1/4-inch (0.64 cm) sawcut is used, select wire size to allow encapsulation 

of the wires (AWG #14 or #16). 

• Loop wires should have a high-quality insulation such as cross-linked 

polyethylene or polypropylene. 

• Wire should be laid in sawcuts using the same rotation (clockwise or 

counter-clockwise) in each loop. 

• Loop wires should be tagged to indicate start (S) and finish (F) and should 

indicate the loop number in the pull box to facilitate series splicing with 

alternate polarity connections. 

• Sawcuts for the loop lead-in wire should be at least 12 inches (2.5 cm) from 

adjacent loop edges. 

• The loop lead-in wire from the loop to the pull box should be twisted a 

minimum of 3 to 5 turns per foot. 

• Splices of loop lead-in wire to lead-in (home-run) cable must be soldered, 

insulated, and waterproofed to ensure environmental protection and proper 

operation. 

• The lead-in cable should be twisted, shielded, and waterproofed. 

• The cable selected should have a polyethylene jacket. 

• For most installations, the lead-in cable should not be connected to earth 

ground at the pull box, only insulated and floating. Manufacturer’s 

recommendations should be followed concerning whether the cabinet end is 

grounded or not (per NEMA). 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-50 Federal Highway Administration 

Table 5-14. General installation guidelines for installing inductive-loop detectors—Continued 

Testing

• Prior to filling sawcuts with sealant, loops should be tested with an ohmmeter for continuity, and loop and lead-ins in pull boxes should be tested 

with a 500 V DC megger to confirm insulation resistance >100 megohms. 

• Loops should be tested with a direct reading inductance meter at the pull 

box to confirm the number of turns of wire in any loop. The following 

formula provides a simple method to calculate the approximate inductance 

of any loop configuration and/or confirm the number of turns in the loop: 

• Inductance (L) = K x feet of sawcut, where K is found from the table 

below: 

No. of Turns K (μH/ft) 

 1 0.5 

 2 1.5 

 3 3.0 

 4 5.0 

 5 7.5 

• The electrical splice configuration of multiple loops should be confirmed 

with the inductance meter to assist in the selection of the correct sensitivity 

setting on the electronics unit. 

Sealant

• Loop sealant should be applied to insulate and protect wires. 

• Loop sealant should encapsulate loop wires to the fullest extent possible. 

• At least 1 inch (25 mm) of sealant should cover the loop wires (i.e., do not 

allow wires to float). 

• Sealant should adhere to asphalt or concrete, but not necessarily to both. 

• For installation in an existing roadway, loop sealant should be poured to 

within 1/8 inch (3 mm) of surface. For installation prior to overlay, the 

sawcut should be filled completely with sealant before paving. 

Connections

• All spade lug connections in the loop circuit should be soldered. 

• Multiple loops connected to the same channel of an electronics unit should 

be connected in series or series/parallel. (Some manufacturers dispute the 

use of series/parallel.) 

• Series splice should be verified with inductance measurement prior to 

connecting to lead-in cable. 

• Multiple loops connected to the same channel of an electronics unit should 

be connected with alternate polarity (clockwise—counter clockwise) to 

improve noise immunity and stability. 

• Loops in adjacent lanes should be connected to the same multiple channel 

electronics unit. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-51 Federal Highway Administration

INDUCTIVE-LOOP DETECTOR INSTALLATION 

ALTERNATIVES 

Agencies and vendors continue to search for better ways to manufacture and 

install inductive-loop detectors. The following sections describe two 

alternatives to conventional loops that have been developed, tested, and are 

currently operational. The conventional loop installation processes such as 

splicing, testing, etc., described earlier also apply to these alternative 

designs. 

SLAB LOOPS 

In the early 1980s, the Puerto Rico Highway Authority and the Department 

of Transportation and Public Works began to experiment with placing 

inductive-loop detectors in heavily reinforced precast concrete slabs for 

installation in bituminous pavements.(14) Earlier experimentation with loop 

detectors placed in cast-in-place concrete slabs 12 ft (3.7 m) in width and 6 ft 

(1.8 m) in length were unsuccessful due to the difficulties in maintaining 

traffic during the relatively long curing period. These slabs could not be 

precast because of their size and weight. 

Puerto Rico selected smaller unitized slabs as an effective sensor design. 

They are used extensively for left-turn lane detection, off-ramp detection, 

arterial detection, and system detection. 

The final design of the precast or cast-in-place loop slabs (both types are 

used) is 4 ft (1.2 m) square and 8 inch (20.3 cm) deep. A circular slot 40 

inches (102 cm) in diameter, 3 inches (7.6 cm) deep, and 0.5–0.75 inches (13–

19 mm) wide is impressed in the concrete to house the conductors forming 

the loop. 

Five turns of wire are standard in these installations, with three turns 

considered to be the minimum acceptable. Adequate vehicle detection has 

been obtained with up to 1,000 ft (300 m) of lead-in cable between the pull 

box and the electronics unit. After the wire is wound in the slot, a 5/8- to 7/8-

inch (16- to 22-mm) nylon rope is forced into the slot on top of the loop wires 

and then the slot is filled with an asphalt sealant. A 1-inch (25-mm) flexible 

metallic conduct encloses the lead-in wires leading to the pull box. 

As shown in the plan details in Figure 5-36, the slabs are provided with four 

lift point devices for handling during the original installation and for 

resetting in the case of overlays.(14) These slabs can be easily handled by two 

men and a small loader-backhoe combination. 

Hot-poured asphaltic sealant and silicone sealant have been used 

successfully. An emulsified asphalt sealant is also being tested because it is 

less expensive and does not need to be heated. Epoxy sealant has not been 

used for this purpose because it may make reentry into the slot for repairs 

more difficult. 

Although these slabs were originally intended for use on bituminous 

pavements, their performance and ease of maintenance have led to the use of 

slabs on new and existing PCC pavements. On existing PCC pavements, an 

oversized square is cut out of the pavement and the precast slab is installed 

with concrete backfill. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-52 Federal Highway Administration 

Puerto Rico uses nonreinforced concrete pavements. Therefore, on new PCC 

pavements, a grid of reinforcing bars is placed at the bottom of the new slab 

where the loop is to be placed prior to the pouring of the concrete. After the 

pour, a form for the slot is impressed into the fresh concrete. 

1 inch = 2.5 cm 

Figure 5-36. Plan details of slab-loop installation. 

Precast loop slabs have been used successfully in four basic configurations, as 

shown in Figure 5-37: actuation (LC-1), directional (LC-2), presence (LC-4), 

and high-speed detection (not shown on drawings). The loops have been 

connected in series and in parallel and have been used with standard and 

self-tuning electronics units. 

Precast loop slabs have been installed at isolated intersections and in 

coordinated systems. In coordinated systems, they have been used for both 

local detection and as strategic system sensors to feed information to the 

master controller for traffic-responsive operation. 

By 1988, there were more than 1,800 slab-loop detectors installed in Puerto 

Rico. Over a 5-year period there had been no traffic- or pavement-induced 

failures. On three occasions, excavations in the sidewalk or adjacent 

pavement areas had damaged the lead-in wires. In addition, 13 incidences of 

failure in the wire insulation resulted from a manufacturing error. 

Although Puerto Rico experiences heavy rainfall, the annual temperature 

falls between a low normally about 60 °F (16 °C) and a high normally below 

97 °F (36 °C). Consequently, there is no freeze-thaw problem. Freeze-thaw 

could possibly affect the use of these slabs in colder climates. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-53 Federal Highway Administration

Those failures provided opportunities to demonstrate the ease with which 

loops can be repaired. With a screwdriver or similar tool, the nylon rope is 

pried from the slot and, by pulling on the rope, the rope and sealant are 

removed. The conductor wires are then removed from the slot. New 

conductors, rope, and sealant are then installed and an epoxy-encapsulated 

splice is made at the nearest pull box. This completes the repair with little 

disruption to traffic. 

Figure 5-37. Precast loop slab configurations. 

Loop slabs have been very successful in Puerto Rico. Typical installations on 

an off-ramp and in a left-turn lane are shown in Figure 5-38. In the mid1980s, the cost in Puerto Rico was approximately $350 per loop, including the 

slab, installation, conduit, wiring to the nearest pull box, and the splice at 

that pull box. 

Older signal projects with sawed slots in the pavement are being retrofitted 

with precast slabs to prevent the deterioration and interruption of service 

due to loop failures. Although a formal benefit/cost comparison has not been 

conducted between the precast and the sawed loop detectors, the zero failure 

rate of the precast system during a 5-year operating period is considered 

sufficient justification for Puerto Rico to adopt the precast loop as standard 

practice. 

ROUND LOOPS 

Although square or rectangular loops are the most commonly used shapes, 

many loop designers have theorized that circular loops would provide 

optimum detection characteristics. In theory, the round loop will produce a 

uniform magnetic field without dead spots. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-54 Federal Highway Administration 

Proponents of the round loop argue that the circular design maximizes loop 

sensitivity for detection of motorcycles as well as high-bed trucks while 

eliminating splashover from adjacent lanes. Other advantages cited include 

the elimination of sharp corners and the reduction in wire stress. Modern 

cutting techniques appear to have eliminated the difficulty associated with 

cutting the circular shape in the pavement. 

Figure 5-38. Slab-loop installations. 

Round-Loop Installation in Roadways 

One installer has addressed the cutting issue by developing a patented 

vehicle designed to saw a circular slot 6 ft (1.8 m) in diameter, 0.5 inch (1.25 

cm) wide, and 3 inches (7.6 cm) deep for circular loop installation in either 

asphalt or concrete road surfaces.(15) The self-contained vehicle, shown in 

Figure 5-39, is equipped with a 6-ft (1.8-m) diameter bit to cut the circle, a 

500-gallon (1.9-kiloliters) water supply, central vacuum system, a 6.5-kVA 

generator, arrow boards front and rear, portable hot sealant unit, and a 35-

hp flat saw for cutting the slot from the loop to the pull box. The loop wire 

used in this installation is encased in continuous PVC tubing and prewound 

for the specified number of turns, stacked, and taped at the factory. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-55 Federal Highway Administration

A typical roadway installation process is illustrated in Figures 5-39 through 

5-41. The truck arrives at the site and the two-man crew sets out cones to 

divert traffic. The lift gate is used to off-load the hot rubberized sealant 

machine and the flat saw. The sealant material is placed in the machine to 

melt to pumping consistency. A center mark for the drilling bit is marked on 

the roadway. The truck is positioned to center the bit over the mark and the 

platform holding the bit is lowered. The drive button on the control panel is 

depressed to activate the bit. The depth gauge on the bit allows the operator 

to monitor the accuracy of the depth of the cut. When the proper depth is 

reached, the drive button is released and the bit comes to a stop. The 

platform is raised and the truck can be repositioned for the next cut. The 

average cutting time is 5 minutes. 

The flat saw is used to cut the slot from the loop to the pull box. A 3-inch (7.6-

cm) core is drilled at the point where the lead-in slot joins the circle. After all 

cuts have been made, the slots are washed out to remove debris and 

vacuumed dry. 

The prewound loop and its lead-in wires are then installed. All loops are 

wound in a clockwise pattern and are connected in series. The top wire is 

wound around the 3-inch (7.6-cm) core to reduce bending stress. The 

installation is now ready to be sealed. The sealant is pumped into the slot 

after it reaches its melting point. The installation is complete when the 

sealant is applied, and the lane may be opened to traffic immediately as the 

sealant hardens as soon as it is exposed to air. 

The circular loop installer reports that since the truck and cutting tool are a 

totally self-contained unit, the loop cutting operation can be confined to a 

single lane at a time. The body of the truck faces oncoming traffic, which 

provides an added measure of protection for the operator and others working 

in the area. Thus the loop cutting can be accomplished safely, in minimal 

time, and with minimal disruption to traffic flow in the area. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-56 Federal Highway Administration 

Figure 5-39. Roadway installation of round loops—cutting loop circle. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-57 Federal Highway Administration

Figure 5-40. Roadway installation of round loops—retraction of saw. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-58 Federal Highway Administration 

Figure 5-41. Roadway installation of round loops—completed installation. 

Round-Loop Installation on Sidewalks 

Other cutting equipment is used to cut round loops in more confined areas 

such as sidewalks. The series of photographs showing the installation 

procedure using this equipment is found in Figures 5-42 through 5-52 and 

was provided by the Traffic Signal Laboratory at Purdue University. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-59 Federal Highway Administration

There is debate regarding placing backer rod material over the entire wire or 

simply using short pieces of material every foot or so. The first method 

supports easier installation from the contractor’s perspective; the second 

provides better encapsulation by the sealant and less wire movement. 

Figure 5-42. Round loop saw at beginning of installation process. 

Figure 5-43. Round sawcut in process. 
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October 2006 Page 5-60 Federal Highway Administration 

Figure 5-44. Completed round sawcut. 

Figure 5-45. Blowing out debris from sawcut. 

Figure 5-46. Firmly seating loop wire in sawcut with rolling-blade tool. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-61 Federal Highway Administration

Figure 5-47. Inserting backer rod material over loop wire. 

Figure 5-48. Firmly seating backer rod material over loop wire. 

Figure 5-49. Tape placed around both sides of sawcut to prevent 

sealant from adhering to concrete. 
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October 2006 Page 5-62 Federal Highway Administration 

Figure 5-50. Placing sealant in sawcut. 

Figure 5-51. Removing excess sealant. 

Figure 5-52. Isolation of affected area to allow sealant to dry. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-63 Federal Highway Administration

INSTALLATION OF TWO-AXIS FLUXGATE 

MAGNETOMETERS 

Representative fluxgate magnetometer installations for various lane 

configurations are illustrated in Figure 5-53. The probes are buried beneath 

the roadway surface in cored holes. The optimum depth and placement of the 

probes depends on the type of detection required (see Chapter 4 and 

Appendix L) and the size of the probes. 

After the preinstallation activities (described earlier in this chapter) have 

been completed, installation usually follows the step-by-step procedures 

discussed below. 

4' 4'

TYPE 3 TYPE 4

Lane Lines

E.P. E.P.

TYPE 1 TYPE 2 4'

Lane 

Width

Direction 

of 

travel

E.P. E.P.

Pull Box

Pull Box

4' 4'

4' 4' 4' 4'

4'

1 ft = 0.3 m 

Figure 5-53. Fluxgate magnetometer installations for various lane configurations. 

INSTALLATION OF SENSING PROBE 

After securing the work zone with appropriate barricades, cones, etc., to 

divert traffic from the area, the first step is to layout and mark the detection 

area with spray paint or chalk markers to match the construction plans. A 

magnetic field analyzer should be placed over each marked probe location to 

verify that the area is free of ferrous metal objects that would degrade 

performance as discussed in Chapter 4. 

Holes for the probes are subsequently drilled through the roadway to the 

proper depth as defined in the construction plans. The general rule for the 

diameter of the hole is to use the diameter of the sensing probe plus 1/8-inch 

(3.2 mm). 
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October 2006 Page 5-64 Federal Highway Administration 

Slots for the connecting cable are then sawed in the pavement to the adjacent 

pull box and blown clean and dry with compressed air. The pull box is 

installed using the same procedures as previously described for loop 

detectors. 

The probe should be installed with the long dimension vertical and with the 

cable ends at the top. The probe should be firmly supported in the hole so it 

will not shift from its vertical position. 

There are several approaches for placement of the probe in the hole. 

Manufacturers indicate that the probe is made so that it can be placed in the 

hole without any protective housing and a number of agencies follow this 

procedure. 

Other agencies prefer to provide some type of housing (e.g., PVC conduit) for 

placing the probe, while others choose to secure the probe by tightly packing 

sand or using sealant around it. A representative standard plan for 

magnetometer installation is shown in Figure 5-54. 

3"

Loop Sealant

Concrete

 PVC Tube

ROADWAY

15"

Sand

Probe

Sand 2"

1 inch = 2.5 cm 

Figure 5-54. Standard plan for fluxgate magnetometer installation. 

The cables from the probe to the pull box are installed in the sawed slot. A 

3/16- to 1/4-in (4.8- to 6.4-mm) wood paddle is used to seat the cable in the 

sawed slot. A 5-ft (1.5-m) length of extra cable should be left at the pull box. 

The cables should be identified by lane or by probe designation. 

A volt–ohmmeter should be used to verify the series resistance (continuity) of 

the probe and cable. This value should be the resistance of each probe (4 to 6 

ohms per probe) plus the resistance of the cable. The value should be within 

10 percent of the calculated value. 

Resistance to ground should also be checked. The value should be high, 

indicating that there are no breaks in the probe or wire insulation. A megger 

should not be used to check resistance to ground, as the large value of 

induced voltage will destroy the probe. 
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October 2006 Page 5-65 Federal Highway Administration

SPLICING THE CABLES 

Splicing of the sensing element cable to the lead-in cables to the controller 

cabinet should be soldered using resin core solder. The techniques for splicing 

are the same as those used for inductive-loop detectors. The lead-in cable 

should not be spliced between the pull box and the controller cabinet. 

The same precautions identified for loop detectors should be addressed when 

installing magnetometers. Two types of heat-shrinkable polyolefin tubing are 

used for splicing probe cables to the lead-in (home-run) cable—small 

diameter tubes insulate the individual conductors and a single large tube 

seals and protects the splice. An electric heat gun and butane hand torch or 

electric soldering iron are required to complete the splice. The procedure 

recommended for splicing probe cables to the lead-in (home-run) cable 

consists of the steps below: 

• Strip both cable jackets back 8 inches (27 cm). 

• Clip 3 inches (7.6 cm) from one colored wire of one cable and clip 3 

inches (7.6 cm) from the other colored wire of the second cable. 

• Strip individual conductors back 1-1/2 inches (38 mm). 

• Install large- and small-diameter heat shrink tubing over either of 

the cables to be joined. 

• Matching color for color, twist together each of the individual 

conductors. 

• Using an electric or butane solder iron, solder the twisted joints 

(avoid excessive heat which could melt or burn the wire insulation). 

• Slide the small-diameter tubes over the soldered joints. 

• Using an electric heat gun or butane torch, gradually heat the small 

diameter tubes until they have shrunk uniformly and the inner wall 

melts and begins to ooze. 

• Slide the large diameter tube over the entire cable splice. 

• Heat the larger tube in the same manner. Proper heat exposure is 

indicated by a color change. Stop heating when the blue strip changes 

to brown and, as before, avoid burning the cable jacket. 

TESTING THE SYSTEM 

Each probe element circuit should be tested at the controller cabinet before 

filling holes and slots. The same measurements described for inductive loop 

installation should be made using a low range ohmmeter. In this test, the 

series resistance will be higher because of the added resistance of the lead-in 

cable. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-66 Federal Highway Administration 

An operational check should also be conducted at the controller cabinet. 

Connecting the probe cables to an electronics unit and applying power may 

accomplish this. The electronics unit should then be calibrated following the 

manufacturer’s guidelines. Check each probe using a bar magnet oriented in 

a direction which aids the horizontal or vertical components of the Earth’s 

magnetic field intensity. Dual-axis fluxgate magnetometers are responsive to 

horizontal and vertical components of the Earth’s magnetic field. The 

channel indicator should light up when the bar magnet is located over any 

probe in the set being tested. 

Record the measurements taken. One additional measurement should be 

made using a yard stick with 1/16-inch (1.6-mm) increment marks and the 

bar magnet. Stand the yardstick vertically over the probe, place the magnet 

at the top of the yard stick, and slowly move the magnet down toward the 

probe until the indicator light on the electronics unit turns on. If the system 

fails at a later time, the same measurement procedure can be repeated. If the 

magnet must be closer to the probe to light the indicator, this indicates that 

the probe has rotated. 

SEALING THE HOLES AND CUTS 

Once the system tests are competed, the probe holes and the sawcuts 

containing the cables can be sealed in the same manner used for inductiveloop detectors. This completes the installation of the magnetometer detector 

system. 

INSTALLATION OF MAGNETIC DETECTORS 

The preinstallation activities for magnetic detectors are much the same as 

those described for inductive loops. When the magnetic detector probe is 

installed 12 to 30 inches (30 to 76 cm) under the roadway, the long axis is 

generally horizontal and is perpendicular to the direction of travel. The 

conduit, housing the probe, can be installed by either drilling or trenching 

under the pavement surface as described below. 

INSTALLATION PROCEDURE 

The preferred method of installation is to tunnel a hole under the pavement 

from the side of the road and push the conduit into the hole as shown in 

Figure 5-55. For installation in a four-lane, divided roadway with a median, 

many agencies have found it desirable to install from the median. The 

median is preferable as these areas have fewer sign posts, guard rails, and 

utility activities. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-67 Federal Highway Administration

Detector probe under 

right side of average 

car

4” Fiber duct pitch down 

toward curb

Highway 

Centerline

Seal cover

Crushed stone 

for drain

Pull 

Box

Plug end

1 inch = 2.5 cm 

Figure 5-55. Magnetic detector probe installation from the side or median of a road. 

After locating and marking the desired area for the placement of the probe, 

measure the distance from the pull box to a point on the road where the 

conduit ends. Using a backhoe, dig a pit 3 ft (0.9 m) deep, 2 ft (0.6 m) wide, 

and 10 ft (3 m) long. A horizontal boring machine is then placed in the pit, 

leveled, squared, and secured. The hole is bored under the pavement, taking 

care that the drill does not stray from the desired path. 

When the drilling is complete, the machine is removed and the pit is 

backfilled to a level just below the bore hole. A 3- or 4-inch (76- or 102-mm) 

schedule 40 PVC is capped and placed in the hole, section by section. Each 

section of pipe must be fitted squarely and glued. Before the final section of 

PVC is fitted, place the pull box in the trench and set it so that the hole in 

the box lines up with the pipe. The pipe should fit flush with the wall of the 

pull box. 

TRENCHING 

Trenching is an alternative to the boring method of installing a conduit to 

hold the magnetic detector. The first step in this procedure is to mark the 

boundary of the trench. The pavement is then cut and a trench is excavated 

including the area for the pull box. A 3- to 4-inch (76- to 102-mm) plastic 

conduit or fiber duct is laid in the trench and sloped to drain into the pull box 

in case of water leakage or condensation. The outer end of the conduit should 

be capped before placing in the trench. 

To prevent damage to the probe by the pressure of traffic above it, the 

conduit should be covered with a minimum of 6 inches (15 cm) of concrete. 

When dirt or stone covers the conduit, as with macadam roads, the probe 

should be at least 1 ft (30 cm) below the surface. A sand cushion should be 

placed immediately below it and above it to prevent stones from being driven 

into the conduit by impact from the vehicles traveling on the road above. 

CONNECTING THE SYSTEM 

The probe sensor lead-in wire should be marked when measuring the correct 

position by laying the probe on the road in the desired position. The mark 

helps to locate the sensor in the correct position when the wire reaches the 

edge of the pull box. The sensor is inserted into the conduit and pushed into 

position using a steel 3/8-inch (9-mm) span wire. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-68 Federal Highway Administration 

After the probe is placed in the conduit, the next step is to install the pull box 

in the same way as described for loop detectors. A drain should be provided 

in the bottom of the box, and the cover should be sealed to prevent the 

entrance of surface water. 

The probe lead-in wire should be spliced to the lead-in cable running to the 

controller. Because the magnetic detector generates an extremely small 

current, splices should be soldered. Procedures were discussed previously in 

this chapter for splicing and environmentally sealing splices used with loopdetector and fluxgate magnetometer installations. 

Two conductors are required for each magnetic detector. Since magnetic 

detectors generate extremely small currents, any size copper wire in the 

cable is satisfactory. Magnetic-detector circuits should not be run in the same 

cable sheath with wires carrying traffic signal currents. Leakage between the 

traffic signal wires and detector wires will cause erratic operation of the 

detectors. Electromagnetic or electrostatic induction between traffic signal 

wires and detector wires could cause false calls. However, any number of 

detector wires can be housed in the same cable sheath without causing 

interference. 

TESTING THE SYSTEM 

Two tests should be conducted with a volt-ohm meter during installation to 

determine the series resistance (continuity) and ground resistance. For each 

probe in the series circuit, the series resistance should register 3,800 ohms, 

plus or minus 10 percent. If there are two probes, the reading should be 

approximately 7,600 ohms. If the meter reads several times the normal 

resistance, a check should be made of the wiring and splicing, and each probe 

should be tested individually. 

Resistance to ground should measure several megohms. Low resistance 

indicates leakage through the wire insulation or the probe. Such leakage will 

not prevent relay response unless it is sufficient to reduce the probe output. 

When the gain is set on the electronics unit amplifier, it should be at the 

lowest value possible to insure proper operation. This is done to avoid 

spurious detection of vehicles in other lanes. 

The test readings and gain settings should be recorded on the construction 

plans and on forms which are stored in the controller cabinet, as well as 

those filed in the maintenance office. This information should always be 

referred to when maintenance is required. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-69 Federal Highway Administration

OVER-ROADWAY SENSOR INSTALLATION 

Over-roadway sensors require structures such as sign bridges, mast arms, 

and poles for mounting. If these are not already in place, then appropriate 

overhead structures must be installed to support the sensors. The field of 

view or footprint of a sensor, i.e., the area of the roadway within which a 

vehicle is detected and data are collected by a sensor system, is a function of 

mounting height, offset of the mounting location from the lanes to be 

monitored, aperture size, elevation changes, curves, and objects that may 

block the view. The footprint for many sensor technologies is calculated in a 

similar manner, with only the aperture, i.e., the active transmitting and 

receiving area of the sensor, varying. In a microwave sensor, for example, the 

antenna beamwidth definition utilized to calculate the footprint is the angle 

in degrees or radians at which the radiated power is reduced by one half (i.e., 

3 dB). Ultrasonic sensors use the same beamwidth definition. By contrast, 

infrared sensors and other optical devices express the beamwidth as the 

angle at which the first null in the diffraction pattern occurs. In any case, the 

geometrical relations for the footprint are the same for all these sensor 

technologies.(16) 

The following sections describe representative installation and calibration 

procedures for video image processor, microwave radar, and laser radar 

sensors. Sensor manufacturers should be contacted to obtain detailed 

installation and operating instructions for their products. 

INITIAL SENSOR EVALUATIONS 

Before embarking on an initial large-scale deployment of over-roadway 

sensors, it is wise to evaluate the performance of the candidate sensors under 

real-time operating conditions where differences in flow rate, road 

configuration, obstructions, lighting, and weather can be experienced. Such 

evaluations have been performed for many of the sensors currently available, 

but the introduction of new models and the unique features of each 

deployment location make this requirement a recurring one. Guidelines for 

conducting such tests are found in the reports issued by several sensor 

evaluation projects. (See references 17–23.) 

VIDEO IMAGE PROCESSORS 

Since each site has unique features, distributors of the Autoscope VIP

recommend that potential users evaluate candidate sites to determine their 

suitability for video detection.(24) This site survey assists in developing 

requirements for lens selection, camera mounting, power and signal 

transmission, and image processor location. 

SITE SURVEYS 

Site surveys assist in determining whether geometric factors and lack of 

obstructions near each sensor station allow for the installation of overhead 

sensors and, if so, what additional mounting structures are required. When 

wireless transmission of data and video are part of the design, the site survey 

also determines whether the transmission path between the sensor and data 

reception locations permits unimpeded transmission. 

Questions posed during a site survey include: 

• What traffic flow parameters are needed? 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-70 Federal Highway Administration 

• Where are the locations of the detection areas? Are there obstructions 

such as power lines, signs, billboards, or trees? 

• Is occlusion of the detection area by moving vehicles a concern? 

• Besides utilizing video imagery to obtain traffic flow parameters, is 

the video imagery required for surveillance and will it be shared with 

other agencies? 

• Where will the video image processor be located, e.g., nearby 

controller cabinet, more distant controller cabinet, or traffic 

management center? What are video transmission options from the 

camera to the image processor? 

• Is remote retrieval of data required? 

• Where will cameras be mounted? Are special mounting facilities 

planned? If mounting structures are already present, are they stiff 

enough to prevent unwanted camera movement? Can a better view of 

traffic, and hence more accurate data, be obtained by mounting the 

camera over the center of the lanes of interest? Is lighting available 

to enhance nighttime performance at signalized intersections? 

• Can a camera be mounted temporarily in the same location as the 

permanent camera to obtain imagery that can be played through the 

image processor to determine performance? 

• Is power available at the site? 

• Have provisions for grounding and terminating the power, data, and 

video cables been made? Are other lightning protection protocols 

needed? 

• Are the video image processor outputs compatible with the controller 

that will be used? 

• Are special protection measures required to protect equipment from 

vandalism or theft? 

Rules of thumb are also available to assist in camera location, mounting, and 

setup. These are based on the following: 

• Optimal performance is obtained by maximizing camera height and 

centering the camera over the traffic flow as much as possible. 

Recommended minimum camera mounting height has been increased 

to 40 ft (12 m) above the detection area if the camera is centered over 

the roadway. Higher mounting heights (on the order of 50 ft (15 m) or 

greater) are generally required if the camera is located at the side of 

a roadway. The camera mounting height should increase as the 

camera is moved further from the road edge to achieve optimal 

performance. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-71 Federal Highway Administration

• Effective camera viewing distance is related to camera height. For 

each foot (0.3 m) of camera mounting height, viewing distance 

upstream and downstream of the camera location is estimated as 

about 10 ft (3 m) by some VIP manufacturers. Across traffic lanes, the 

camera can view about 2.5 ft (0.8 m) for every foot (0.3 m) of 

mounting height. However, conservative freeway design procedures 

reduce the upstream and downstream viewing distance estimate 

because of factors such as road configuration (e.g., elevation changes, 

curvature, and overhead or underpass structures), congestion level, 

vehicle mix, and inclement weather. Occlusion will probably generate 

volume errors at the furthest observation points at traffic levels of 

service D and E. Although a calibration factor or some other logic 

may assist in reducing the volume error, reduction of presence and 

occupancy errors may be more difficult. 

• Select the camera location to minimize occlusion of down-lane and 

cross-lane vehicles. Down-lane occlusion refers to vehicle blocked 

from view by a tall vehicle in front of it. Cross-lane occlusion refers to 

a vehicle blocked from view by a tall vehicle in a lane closer to the 

camera. 

• Select camera locations that minimize vibration and motion. 

• Camera mounting should prevent the horizon from being visible in 

the horizontal or vertical fields of view. This may require a downward 

tilt to the camera or the installation of a sunshield. 

• Minimize reflections from the pavement, for example by viewing 

traffic approaching the camera from uphill. However, uphill traffic 

detection will increase the probability that headlights will shine 

directly into the lens at night. 

• Approaching nighttime traffic will produce headlight reflections from 

the road surface that may cause false detections. This is more likely 

with low camera mounting heights, under bridges, in tunnels, and on 

curved roadway sections. 

• A longer focal length lens will minimize glare entering the camera. 

On shorter poles, the longer focal length assists in minimizing the 

horizon in the field of view. For example, an 8-mm lens (as contrasted 

with a 6-mm lens) allows greater adjustment of the sunshield to 

minimize glare and the horizon. With shorter poles, aiming the 

camera at the lower half of trucks minimizes the horizon. A longer 

focal length lens also improves performance by increasing the relative 

size of the detection zones that are placed on the pavement. 

Reports of poor VIP nighttime performance at unlighted intersections at 

several locations in Indiana led to an investigation into the probable causes 

of these events.(25,26) The following recommendations were developed by the 

Indiana DOT (INDOT) with the concurrence of several VIP manufacturers to 

address these issues:(27) 

• Height of camera shall be 40 to 41 ft (12.2 to 12.5 m) above the 

pavement surface directly under the camera. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-72 Federal Highway Administration 

• Maximum detection distance for full detection functionality of the 

VIP is 200 ft (61 m) from the spot on the pavement surface directly 

below camera (based on 5:1 aspect ratio at 40 ft (12.2 m) camera 

height). 

• Lateral camera placement: 

o Cameras mounted on the far side: Camera shall be laterally 

aligned on an extension of the lane-line separating the left and 

through lanes of the inbound approach. 

o Cameras mounted on the near side (over lanes to be detected): 

Camera shall be laterally aligned over the center of the approach. 

o Field inspection will occur with the inspector 150 ft (45.7 m) back 

from the stop bar and sighting the camera with a transit/plumb 

bob. Tolerance on camera placement is ± 1 ft (0.3 m). 

• Highway lighting shall be provided. 

• Highway lighting shall be 40 to 41 ft (12.2 to 12.5 m) above the 

pavement surface. Consider specifying an airport-type lighting head. 

• Design shall provide cross section within 5 ft (1.5 m) of pole location. 

This ensures that poles are not located in ditches below the roadway 

grade, which would adversely affect proper camera mounting height. 

• All utilities and facilities affecting pole locations and camera views, 

both overhead and underground, must be accurately located on plans 

and their height noted. 

• Camera sight triangles (elevation view) shall be included in the plans 

to show: 

o Location of detection zones. 

o Any occlusions such as, but not limited to, signal heads, mast 

arms, span catenary, tether, and overhead utilities. 

TXDOT has also developed a manual for deploying video detection at 

signalized intersections.(28) 

LENS SELECTION 

The focal length of the lens is dependent on the mounting height of the 

camera, the local topography, distance to the nearest detection area, and the 

width of the detection area.(24,29) Some vendors supply a program that 

calculates the horizontal and vertical fields of view and corresponding focal 

length of the lens from these input data. If the required focal length does not 

correspond to a standard lens, then the mounting height or one of the 

detection area parameters is varied in order to specify a standard lens having 

a horizontal focal length above or below that of the initial calculation. Table 

5-15 shows the horizontal and vertical fields of view for several standard 

lenses. Some VIP vendors supply variable focal length lenses with their 

cameras so that a specific lens does not have to be selected in advance. 

Another advantage of this approach is that the number of different focal 

length lenses required for maintenance operations is reduced. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-73 Federal Highway Administration

Table 5-15. Horizontal and vertical fields of view (FOV) of standard lenses. 

Lens (mm) 4.8 6.0 8.0 12.5 16.0 25.0 

Horizontal FOV (deg) 67.4 56.1 43.6 28.7 22.6 14.6 

Vertical FOV (deg) 53.1 43.6 33.4 21.7 17.1 11.0 

Table 5-16 illustrates how the dimensions of the image area are calculated 

once the focal length is chosen. A camera mounting height of 40 ft (12.2 m) is 

assumed. 

Table 5-16. Calculation of image area dimensions for an 8-mm focal length lens.a

Dimension Symbol How determined Value 

Camera height above 

detection area 

h Input parameter 40 ft (12.2 m) 

Half the vertical angular 

FOV of the lens 

αV Lens specification (e.g., from Table 

5-11) based on required FOV 

16.7 deg 

Half the horizontal angular 

FOV of the lens 

αH Lens specification (e.g., from Table 

5-11) based on required FOV 

21.8 deg 

Angle between camera lineof-sight and mounting 

structure 

θ θ = (90-5) – αV, where 5 is the number 

of degrees the FOV is below the 

horizon 

68.3 deg 

Distance to bottom of image d1 d1 = h tan (θ – α V) 50.5 ft (15.4 m) 

Width of bottom of image w1 w1 = 2h tan α H /cos (θ – α V) 51.5 ft (15.7 m) 

Distance to top of image d2 d2 = h tan (θ + α V) 457.2 ft (139.4m) 

Width of top of image w2 w2 = 2h tan α H /cos (θ + αV) 367.1 ft (111.9 m) 

a Camera mounting height of 40 ft (12.2 m) and level topography are assumed (from: Autoscope Consultant’s 

Designers Guide. Econolite Control Products, Anaheim, CA. 1998). 

Figure 5-56 defines the angular field of view of the camera lens, which is 

used in the image area calculations. The other dimensions are illustrated in 

Figure 5-57. The angle α, which the extreme rays make with the axis, is the 

half field of view angle or more simply the half-field angle of the lens. It 

limits the dimensions of the object seen by the lens. The chief rays SET and 

REU incident at the periphery of the lens that pass through the entrance 

pupil E are refracted through the conjugate plane E', in this case the location 

of the charge-coupled device (CCD) camera array. The focal length of the lens 

is the distance CF or CF'. The shaded cones ETU and ERS indicate the 

boundaries within which any object must lie in order to be observed in the 

image field.(30) 

U

T Entrance

Pupil

Object

Axis

Image Field

Exit

Pupil

P'

L'

Aperture

Stop

R

S

Field

P

C

E

L

E' (CCD Array) α

F F'

• • α'

Figure 5-56. Field of view of a converging lens. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-74 Federal Highway Administration 

Tables 5-17 and 5-18 display the image area dimensions for 30-, 40-, and 50-

ft (9.1-, 12.2-, and 15.2-m) camera mounting heights. These dimensions are 

plotted in Figure 5-57, which shows the increased sensitivity of the top image 

area dimensions to variations in camera height. 

Table 5-17. Image area dimensions for 30-, 40-, and 50-ft (9.1- 12.2-, and 15.2-m) camera 

mounting heights, using an 8-mm focal length lens.

Camera mounting height 30 ft (9.1 m) 40 ft (12.2 m) 50 ft (15.2 m) 

Distance to top of image 343 ft (104.5 m) 457 ft (139.3 m) 572 ft (174.3 m) 

Width of top of image 275 ft (83.8 m) 367 ft (111.9 m) 459 ft (139.9 m) 

Distance to bottom of image 38 ft (11.6 m) 50 ft (15.2 m) 63 ft (19.2 m) 

Width of bottom of image 39 ft (11.9 m) 52 ft (15.8 m) 64 ft (19.5 m) 

Table 5-18. Image area dimensions for 60- and 70-ft (18.3- and 21.3-m) camera 

mounting heights, using an 8-mm focal length lens.

Camera mounting height 60 ft (18.3 m) 70 ft (21.3 m) 

Distance to top of image 686 ft (209.1 m) 800 ft (243.8 m) 

Width of top of image 551 ft (167.9 m) 642 ft (195.7 m) 

Distance to bottom of image 76 ft (23.2 m) 88 ft (26.8 m) 

Width of bottom of image 77 ft (23.5 m) 90 ft (27.4 m) 

Camera specifications vary with the VIP manufacturer. Most VIP 

manufacturers will recommend one or more cameras that are compatible 

with the software and capabilities in the image processor. The specifications 

in Table 5-19 are typical of those required.(21,24,31) Camera horizontal 

resolution (pixel size) must be adequate to calibrate the system, especially at 

longer distances from the camera mounting location. Resolution also affects 

the accuracy of vehicle size measurement and the tracking of vehicles 

through the camera’s field of view. Vertical resolution is controlled by the 

Electronic Industry Association (EIA) or International Radio Consultative 

Committee (CCIR) specifications. Dynamic range indicates the range of light 

levels that are distinguished by the camera, while sensitivity describes the 

ability of the camera to detect objects under low illumination conditions. 

Tests at California Polytechnic Institute at San Luis Obispo suggest that the 

dynamic range specification is more critical than sensitivity in most traffic 

surveillance applications.(32) 
 Traffic Detector Handbook—2006 

October 2006 Page 5-75 Federal Highway Administration

0

50

100

150

200

250

300

350

400

450

500

550

600

650

700

750

800

30 40 50 60 70

Camera Mounting Height (ft)

Length (ft)

1 ft = 0.305 m

Distance to bottom of image

Distance to top of image

Width of top of image

Width of bottom of image

Figure 5-57. Image area dimensions for 30- to 70-ft (9.1- to 21.3-m) camera 

mounting heights, assuming an 8-mm focal length lens. 

After power, control, and data cables are in place and the camera mounted 

and transmitting video imagery to the image processor, the VIP operating 

parameters can be set up, the image area calibrated, and detection zones 

placed on the roadway. Most VIPs contain software for this purpose; some 

require separate purchase of the setup and calibration software. The 

software operates on a personal computer that connects to the VIP through 

an RS-232 interface. 

Calibration menu items used by one manufacturer include set lanes; set 

external communication rate; set video reference level to correspond to 

camera output voltage; set alarm parameters based on flow rate, lane 

occupancy, speed, headway, or vehicle length; and set clock. The detection 

zones typically gather vehicle presence, count, speed, lane occupancy, and 

vehicle length classification data. Some processors provide Boolean 

operators, such as AND and OR, for connecting detection zone outputs. These 

may be useful in obtaining more accurate counts and presence calls. The 

detection zones are drawn with a mouse or keyboard commands on the 

computer monitor screen displaying the field of view of the camera. Zones can 

be created for multiple lanes that are contained within the field of view of the 

camera. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-76 Federal Highway Administration 

Table 5-19. Typical camera specifications for VIP applications.(19,22,28) 

Parameter Value 

Minimum light level for usable video 0.1 lux for intersections, 0.04 lux for freewaysa 

Maximum light level for usable video 10,000 lux 

Dynamic range 56 decibels (dB) minimum from minimum to maximum 

usable video signal 

Automatic gain control (AGC) range 20 dB minimum with 1 s damping 

Minimum vertical resolutionb 350 to 383 lines 

Minimum horizontal resolutionb 550 to 580 lines 

Lens format .25- to 1-inch interline or frame transfer CCD 

Active pixel elements 768 horizontal, 494 vertical minimum 

Iris Automatic with damping controlled by a time constant of 

0.25 s or greater 

Wavelength filter 600 nm to 1400 nm wavelengths attenuated to 10% 

Operating temperature range -40 oC to +60 oC 

Sunshield Required 

Internal heater Required 

Maximum weight Compatible with pan and tilt camera mount 

Video signal interfacec

 RS 170, NTSC, CCIR, or PALd 

a Sensitivity is also specified with respect to video, AGC, and signal-to-noise ratio (S/N) levels. Thus, a minimum 

sensitivity specification may appear as 0.1 lux at 100 percent video with no AGC and S/N = 55 dB. 

b Distant vehicles at the upper edge of the field of view often span only two video resolution lines, making 

them barely detectable. A lower resolution camera will reduce the usable field of view to the lower half of 

the video image or less. c

 Not all VIP models support all video signal interfaces. 

d NTSC = National Television Standards Committee. PAL = Phase alternate lines. 

CALIBRATION ISSUES 

VIP models require several pieces of information to calibrate the image area 

dimensions and hence the data acquired by the sensor. Most calibration 

methods utilize combinations of along-lane and cross-lane dimensions, 

camera mounting height and angle (measured from the vertical or 

horizontal), lens focal length, and CCD array format (size). The image area 

calibration information required by one VIP system is given in Tables 5-20 

and 5-21.(33) 
 Traffic Detector Handbook—2006 

October 2006 Page 5-77 Federal Highway Administration

Lane dimensions can be measured and marked with traffic cones or paint 

stripes before beginning the calibration process, as shown in Figures 5-58 

and 5-59. These dimensions are usually needed at the near and far edges or 

at the corners that mark the viewing area boundaries. Image area markers 

can be positioned using two people, one to view a monitor containing the field 

of view and another to move the markers into place according to instructions 

received from the image viewer. Lane widths can be measured in advance to 

provide cross-lane calibration information. Existing roadway and lane 

markers can be used for image area calibration, but only if distances are 

measured and confirmed before the calibration procedure begins. Nominal 

values of marker separation and location should not be used, as discrepancies 

in their assumed placement cause VIP system calibration errors and poor 

data quality. 

Table 5-20. Image area and data calibration information. 

Parameter Interpretation 

Focal length The focal length of the camera lens. 

CCD array size Size of CCD array used in the camera. 

Image area Camera image area as defined by a rectangle drawn on the road surface 

shown on the monitor. 

The dimension of one side (either length or width) of the rectangle. 

An alternate dimension is the distance between dashed lines that separate 

lanes. 

Number of lanes Number of lanes in which data are collected. 

Detection zone location Positions of markers that define two detection zones in each lane. 

Length and width dimensions between markers. 

Traffic flow direction For each lane, define flow as toward top or bottom of video image or in both 

directions. 

Edit detection zones Modify location or size of detection zones. 

Table 5-21. Alternate image area and data calibration information. 

Parameter Interpretation 

Camera vertical angle Vertical angle between axis in plane of road surface and camera viewing 

direction. 

Camera horizontal angle Horizontal angle between camera viewing direction and traffic flow 

direction. 

Camera height Height of camera above road surface. 

Camera offset Ground distance between the camera and a calibration axis parallel to the 

traffic flow direction. 

Detection zone location Ground distance between camera and near and far detection zone 

markers in each lane. 

Detection zone length Ground distance between markers that define the near and far detection 

zones in each lane. 

Camera mounting height and angle can be measured, while focal length and 

CCD format are determined from lens and camera specifications. Some VIPs 

may require the camera offset angle with respect to the traffic flow direction 

and camera offset distance with respect to a calibration axis parallel to the 

traffic flow.(31) The effect of improper lens selection, unsuitable camera 

mounting location, poor image area calibration, and low camera sensitivity 

on VIP data quality were discussed with respect to the VIP performance 

shown in Figure 2-54.(19) 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-78 Federal Highway Administration 

Figure 5-58. Up-lane distance measure aided by traffic cones placed at 25-ft 

(7.6-m) intervals (temporary markings). 

Figure 5-59. Down-lane road surface dimensions marked with paint as used 

for sensor evaluation and performance comparison. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-79 Federal Highway Administration

SERIAL DATA INTERPRETATION SOFTWARE 

Some VIPs transmit data to the controller through a serial interface and 

make the data protocol available to the user. Others require interface or 

driver software to be purchased or written by the user agency so that their 

central computer system or controller can interpret the serial data. VIPs also 

provide data that emulate inductive loop outputs, i.e., optically isolated 

transitions, so that the VIP data appear to the controller as if they originated 

from inductive loops. Hence, the controller treats these VIP data in the same 

manner as inductive-loop data and the same traffic management software 

can be utilized. 

AUTOSCOPE 2004 VIDEO IMAGE PROCESSOR 

DETECTION ZONES 

Figures 5-60 and 5-61 contain examples of detection zones drawn for the 

Autoscope 2004 VIP on the I-5 Freeway in Orange County, CA, as part of the 

evaluation of a mobile surveillance and wireless communications system.(34) 

On the mainline (Figure 5-60), vehicle count sensors were connected by AND 

logical operators to reduce false counts caused by image projection from 

vehicles into more than one lane. This problem was caused by the relatively 

low 30-ft (9-m) camera height and side-viewing geometry. Speed sensors (the 

long rectangular detection zones), presence sensors, and demand sensors 

were created to demonstrate the different options available for vehicle 

detection. 

Figure 5-60. Mainline count and speed detection zones for an Autoscope 2004 VIP using 

a side-mounted camera. Count sensors are represented by the lines perpendicular 

to traffic flow; speed sensors by the long, rectangular box. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-80 Federal Highway Administration 

The detection zones on the ramp (Figure 5-61) consisted of demand and count 

sensors. Sometimes detection zones were placed parallel to the traffic flow 

direction, and other times sensors were configured in the shape of an × to 

increase vehicle detection probability. Depending on whether projection of 

tall vehicle images from one lane into another or occlusion was the more 

severe problem at a particular site, the × sensors were connected with AND 

or OR logical operators respectively. OR logic was also applied to increase the 

probability of vehicle detection by the VIP and the consequent change of 

signal from red to green. In this application, the primary goal was to prevent 

vehicles from being trapped at a red signal, rather than concern about a 

potential increase in false calls.(34) 

Figure 5-61. Ramp demand zones (× configuration) for an Autoscope 2004 VIP with 

a side-mounted camera. The zones beyond the stop line record vehicle passage. 

Figure 5-62 illustrates the viewing perspective in detection zones created on 

roadway surfaces, far from low-mounted cameras. The detection zones now 

enclose a smaller percentage of the pixels in the field of view, decreasing the 

probability of a detection. In addition, occlusion and projection of tall vehicle 

images from one lane into another are more prevalent because of the relative 

size of the detection zone.(34) 

TRAFICON VIP2 VIDEO IMAGE PROCESSOR 

DETECTION ZONES 

Figure 5-63 displays the image and data provided by the Traficon VIP2 

traffic data detector, designed for collection of traffic flow data on freeways 

and arterials.(35) The configuration and data acquisition software operate in a 

personal computer architecture using RS-232 or RS-485 serial interfaces to 

the VIP. Data are output to a controller through eight optically isolated 

semiconductors. The data provided for each lane and the three selectable 

length-based vehicle classes are volume, speed, gap times, and headway. In 

addition, the VIP gives the occupancy and density for each lane. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-81 Federal Highway Administration

Figure 5-62. Ramp detection zone placement for an Autoscope 2004 VIP when the camera is 

neither close to the monitored roadway nor at a sufficient height. 

Figure 5-63. Image and data displays for Traficon VIP2 traffic data detector 

(Source: CCATS® VIP2 Specification. VDS, Inc., San Diego, CA. 1997). 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-82 Federal Highway Administration 

MICROWAVE RADAR SENSORS 

The two types of microwave radars exploited for traffic management are CW 

Doppler and FMCW presence-detecting sensors. CW Doppler radar sensors 

detect vehicles traveling at speeds greater than 3 to 5 mi/h (4.8 to 8.0 km/h), 

but cannot detect stopped vehicles. CW Doppler sensors do not require 

programming to operate. Most do have jumpers or switches, however, that 

convey the direction of traffic flow, i.e., approaching or departing, to the 

sensor. This feature enables some models to detect traffic moving in the 

wrong direction, such as wrong-way traffic on an entry or exit ramp or on a 

reversible lane. Presence and passage of moving vehicles are provided from 

optically isolated semiconductor or relay outputs. Speed is supplied through a 

serial interface, which usually requires a software driver to be written by the 

user agency. Some CW Doppler radars allow the detection pattern to be 

expanded or contracted to cover the area of interest. The antenna design 

provides the proper beamwidth for measuring traffic parameters over single 

or multiple lanes, depending on the sensor application, when the sensor is 

mounted at the recommended height. 

Forward- and side-looking radar sensors form elliptical footprints on the road 

surface. Figure 5-64 defines the footprint and the parameters for a forwardlooking sensor. When the 3-dB antenna beamwidths are available from the 

manufacturer, the along-lane and cross-lane footprint dimensions can be 

estimated as(16) 

da = h[tan(θ + θa/2) – tan(θ – θa/2)] (5-1) 

dc = 2R sin θ tan(θc/2) = 2h tan θ tan(θc/2) (5-2) 

where 

 da, dc = along-lane and cross-lane sensor footprint diameter, 

respectively 

h = R cos θ = sensor mounting height 

R = slant range from sensor aperture to center of ground footprint 

R sin θ = distance along roadway measured from projection of sensor 

aperture onto the roadway directly under the aperture to 

the center of the ground footprint 

θa = along-lane two-way 3-dB antenna beamwidth 

θc = cross-lane two-way 3-dB antenna beamwidth 

θ = angle of incidence of the sensor measured from nadir 

(downward-looking direction). 

The footprint dimension equations assume that range binning is not used to 

increase the resolution of the footprint along the major axis. The range bin 

concept is depicted in Figure 2-59. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-83 Federal Highway Administration

Angle of

incidence θ

Sensor mounting

height h

Range R to center of 

footprint

•

da

dc

Along-lane antenna beamwidth θa

Traffic flow

direction

Cross-lane sensor 

footprint diameter dc

Along-lane sensor

footprint diameter da

θc

dc

Distance along roadway

to center of footprint

Figure 5-64. Microwave sensor detection area for upstream and downstream viewing. 

WHELEN TDN-30 CW DOPPLER SENSOR 

The Whelen TDN-30 narrow beam CW Doppler microwave sensor is mounted 

centered over the lane of interest (within ± 1.5 ft (0.5 m)) with its bottom 

surface parallel to the roadway (within ± 5 degrees). With this orientation, 

the angle of incidence of the antenna with respect to the road surface is 45 

deg. The antenna is located inside the sensor housing. The along-lane and 

cross-lane dimensions of the detection zone are established by the mounting 

height, as shown in Table 5-22.(36) 

Table 5-22. Detection zone of Whelen TDN-30 CW Doppler microwave sensor. 

Mounting height Along-lane beam diametera Cross-lane beam diametera

(ft) (m) (ft) (m) (ft) (m) 

15 4.6 4.2 1.3 2.9 0.9 

20 6.1 5.6 1.7 3.9 1.2 

25 7.6 7.0 2.1 4.9 1.5 

30 9.1 8.4 2.6 5.9 1.8 

a Calculated using Equations 5-1 and 5-2 and approximate antenna 3-dB beamwidths of 8 deg for each 

direction. 

In addition to a jumper for selecting travel direction, the TDN-30 has 

jumpers that specify the RS-232 data transmission rate (1200 or 2400 baud), 

transmission mode (modem or free running serial data), operating mode 

(freeway management, incident detection, or speed and direction detection), 

dwell count utilized with the incident detection mode, and optically isolated 

closure time used in the speed and direction detection mode. The freeway 

management mode permits vehicle speed and count data to be collected. The 

incident detection mode allows selection of an incident speed threshold and 

dwell count threshold. If the speed is below the speed threshold, the dwell 

count is increased by one. If consecutive vehicles, equal in number to the 

dwell count threshold, are detected below the speed threshold, then a 

normally open contact is pulsed to indicate the detection of an incident. 

However, if a vehicle is detected with a speed greater than the speed 

threshold, the dwell counter is reset. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-84 Federal Highway Administration 

Once an incident is detected, the sensor begins to measure vehicle speeds to 

determine if the incident has cleared. An automatic incident cleared speed 

threshold is calculated equal to the speed threshold plus 25 percent. As 

before, the dwell counter must increase on consecutive vehicles before a 

second normally open contact is pulsed, indicating that the incident has 

cleared. In the speed and direction detection mode, a speed threshold 

between 5 and 80 mi/h (8 and 129 km/h) is set on thumbwheel switches. As a 

vehicle is detected, its speed is compared to the threshold. If the speed 

exceeds the threshold, an optically isolated contact is activated for a userselectable time of 2, 4, 8, or 16 s.(36) 

MICROWAVE SENSORS TC-20 CW DOPPLER SENSOR 

Some CW Doppler radar manufacturers provide plots of the detection 

pattern. One such plot is given in Figure 5-65 for the Microwave Sensors TC20. The coverage is adjustable by turning a potentiometer clockwise to 

maximize the pattern size and counterclockwise to minimize it.(37) 

Range (ft)

0 10 20 30 40 50 60

TC-20

16-deg

beamwidth

4 ft 7 ft 10 ft 12.5 ft 15.5 ft Width = 

18.5 ft

1 ft = 0.305 m

Figure 5-65. Approximate detection pattern for Microwave Sensors TC-20 Doppler radar 

(Source:TC-20 installation instructions. Microwave Sensors, Ann Arbor, MI). 

EIS RTMS FMCW RADAR 

The RTMS presence-detecting microwave sensor combines FMCW and 

Doppler operation. In its side-fired configuration, it is a multilane sensor 

capable of detecting stopped or moving vehicles in up to eight lanes. The 

traffic flow parameters provided per lane are volume, occupancy, speed, and 

classification based on user-specified vehicle length. In its forward-looking 

mode, the sensor’s detection zones form a speed trap and measure speed and 

length of vehicles in a single lane. The speed and length measurements can 

each be placed in up to seven bins when the sensor is operated in the 

forward-looking mode. The sensor adds a Doppler speed measurement 

(maximum error = 2 percent) for speeds exceeding 10 mi/h (16 km/h) in 

forward-looking operation. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-85 Federal Highway Administration

Optically isolated contacts in the RTMS provide presence indication in each 

detection zone similarly to that of inductive-loop detectors. Thus, the sensor 

is compatible with controllers that are configured to accept inductive-loop 

data. Flow rate, occupancy, speed, and classification are available through 

RS-232 and RS-485 serial interfaces at rates up to 115,200 Baud or through a 

TCP/IP interface option. Real-time vehicle speed and length are also 

available for specialized applications, e.g., speed and red light enforcement or 

excessive speed warning systems. 

RTMS setup and calibration are performed using a personal computer and 

manufacturer-supplied software through an RS-232, RS-485, or TCP/IP 

interface located on the sensor.(38) The setup wizard can automatically 

determine the optimum calibration of the RTMS, including operating mode, 

required number of detection zones and locations, and verify correct 

operation of the sensor as shown on the setup utility image in Figure 5-66. 

Figure 5-66. RTMS microwave sensor detection zone setup and calibration screen 

(Image courtesy of EIS Electronic Integrated Systems Inc., Toronto, Canada). 

During setup, the lower portion of the figure displays every vehicle within 

the radar’s field-of-view at that moment as a dark rectangular blip at its 

corresponding range. A detection zone location is defined by surrounding the 

blip with a rectangular outline box, which is moved with the arrow keys on 

the keyboard. A detection zone can include one or more lanes. After a zone is 

defined, its corresponding optically isolated output contact pair closes every 

time a vehicle is detected. Once all zones are defined, a manual vehicle count 

is made and compared with the RTMS count to calculate the accuracy of the 

setup configuration. If the accuracy is not adequate, the detection zones are 

adjusted and the manual and RTMS count comparison is repeated. 

Early users of the RTMS have remarked about the time needed to accurately 

aim the radar and calibrate the multiple detection zones when the sensor is 

in a side-looking configuration to view five or more lanes of traffic. Others 

have not voiced this concern. Newer versions of the setup software claim to 

alleviate this issue. Nevertheless, once the viewing angle has been set, sensor 

performance generally falls within the manufacturer’s specifications. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-86 Federal Highway Administration 

ACCUWAVE 150LX FMCW RADAR 

The single detection zone Accuwave 150LX presence-detecting microwave 

radar by Naztec also is software programmable from a personal computer 

using the RS-232 interface on the sensor. Figure 5-67 displays the basic and 

advanced parameters that can be adjusted. 

Retrieve

File Communication Port Help

Default

Detect

Initially Tuned

Low Signal

Previous Low Signal

Cycle Time (sec)

Sensitivity

Presence Timeout (min)

30

8

20

0.

0.

Detect Delay (sec)

Delay Inhibit (sec)

Retune

Quick Retune

Clear

Normal Returning

Under Ideal Conditions Only

Previous Low Signal Indicator

Sensor ID

New Sensor ID

Response

Hysteresis

Profile

0

0

10

MEDIUM

24

Signal Sample

Signature 0

Deviation 0

Advanced

Current Unit Firmware Version:

Accuwave

STOP

Delay Option

Additional Functions

Figure 5-67. Accuwave 150LX microwave sensor interface and setup options screen 

(Source: Accuwave 150LX Instruction Manual. Naztec, Inc., Sugar Land, TX). 

Basic parameters are cycle time (longest expected intersection cycle time), 

sensitivity (smaller numbers require a larger signal for a detection), presence 

timeout (elapsed number of minutes while in a continuous detect state before 

sensor recycles), detect delay time (elapsed time after sensor detects a vehicle 

until the sensor becomes active again), and delay inhibit (elapsed time after a 

vehicle detection before the detect delay time is used again). Advance 

parameters are sensor identification (address to differentiate a sensor from 

others on the same communications line), response time (application 

dependent—intersection applications with slow moving and stationary 

vehicles require longer response times, hysteresis (low, medium, high), and a 

fine tuning parameter called profile.(39) 

After all entries are made, the download button on the toolbar is pressed to 

write the parameters into nonvolatile memory in the sensor. The 150LX 

provides vehicle presence through an optically isolated semiconductor output. 

The detection area of the 150LX along the road is shown in Figure 5-68. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-87 Federal Highway Administration

w

Accuwave 150LX

d

d1

d2

Aimpoint

Aimpoint d d1 d2 w

(ft) (ft) (ft) (ft)

30 20 44 14

45 30 65 24

50 35 75 25

80 40 90 30

1 ft = 0.305 m

For Mounting Height = 20 ft:

Figure 5-68. Accuwave 150LX microwave sensor road surface detection area 

(Source: Accuwave 150LX Instruction Manual. Naztec, Inc., Sugar Land, TX). 

LASER RADAR SENSORS 

Laser radar sensors operate in the near infrared spectrum by transmitting 

beams that fully scan one or two lanes to provide presence, count, speed, and 

classification data. Figure 5-69 illustrates the mounting configuration and 

beam geometry for the Autosense II sensor.(40) The first beam normally has 

an incidence angle of 10 deg and the second beam an incidence angle of 0 deg. 

The specified incidence angles are obtained by mounting the sensor with a 

5-deg forward tilt, although other mounting configurations can be 

accommodated. The operating temperature range is from –40 o

F to +158 oF 

(–40 o

C to +70 oC) plus sun loading. A 30-minute warmup time is needed at 

–40 o

F (–40 oC). 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-88 Federal Highway Administration 

10

deg

First beam

23 ft (7 m)

5-deg tilt

Autosense II

Figure 5-69. Autosense II laser radar mounting (Source: Autosense II User’s Manual and System 

Specification. Schwartz Electro-Optics, Inc, Orlando, FL, now OSI Laserscan, 1998). 

Autosense II communicates over the factory default RS-422 interface at 57.6 

kbaud, 8 data bits, 1 stop bit, and no parity. An optional RS-232 full-duplex 

serial interface is available. Other outputs include a logic-level vehicle detect 

line and a solid-state relay (optional). The manufacturer recommends that 

the data cable be made of low-capacitance polyethylene (such as Belden 9807) 

as the cable capacitance, which should not exceed 2500 pF, limits the 

maximum cable length for reliable RS-232 operation. 

The Autosense configuration software is executed on a 486 or Pentium 

personal computer. The data frame format provides frame synchronization, 

frame start, message, and checksum information. The software menu 

provides the option of saving all of the data to an ASCII text file. The file 

name field permits the data to be redirected to another drive. After the 

software is configured, a power-on message can be accessed to show self-test 

results, firmware version number, and the measured range to the road for 

each sample in both beams. A test data output command is available to verify 

the range and intensity data for each sample in both beams. 

Classification is performed by using software that categorizes the profiles of 

different vehicles. For each vehicle passing through the Autosense II field of 

view, the sensor outputs five messages based on the location of the vehicle. 

These messages are as follows: 
 Traffic Detector Handbook—2006 

October 2006 Page 5-89 Federal Highway Administration

• Message 1—First beam vehicle detection message. 

• Message 2—Second beam vehicle detection message. 

• Message 3—First beam end of vehicle message. 

• Message 4—Second beam end of vehicle message. 

• Message 5—Vehicle classification message. 

As the vehicle passes through the first laser beam, the sensor detects the 

vehicle, assigns a vehicle identification number and outputs Message 1 to the 

computer. When the front bumper of the vehicle passes the second laser 

beam, the sensor outputs left edge position, right edge position, and vehicle 

speed to the computer as Message 2. This message is used to estimate the 

position of the vehicle. When the rear bumper clears the first laser beam, the 

left edge position and right edge positions are transmitted from the sensor to 

the computer as Message 3. As the vehicle continues forward, its rear 

bumper passes through the second laser beam. Message 4 is transmitted at 

this time to indicate the end of the vehicle. The Autosense II compiles data 

accumulated for the vehicle, generates a classification code, a confidence level 

for the classification estimate, and transmits these data to the computer as 

Message 5. 

PASSIVE INFRARED SENSORS 

Passive infrared sensors contain fixed field of view optics. Some offer a 

variety of focal length lenses to suit various applications and mounting 

distances from the detection area of interest. Presence and passage are 

provided from optically isolated semiconductors or relays. Multiple detection 

zone passive infrared sensors that measure speed contain a serial interface, 

which usually requires a software driver to be written by the user agency. 

Manufacturers typically provide footprint dimensions for passage infrared 

sensors as illustrated in Table 5-23.(41) The mounting height h and the range 

R from the sensor to the center of the ground detection area define the 

incidence angle θ, as was shown in Figure 5-64. Multiple detection zone 

passive infrared sensors provide tables that give the dimensions of each zone 

as a function of mounting height and the distance between the edges of the 

detection zones and the base of the mounting structure. 

ULTRASONIC SENSORS 

The field of view of most ultrasonic pulsed sensors illuminates one traffic 

lane when they are mounted at the recommended height and pointed directly 

downward. The one exception is the two-lane Lane King model manufactured 

by Novax. 

Figure 5-70 shows the overhead mounting configuration for the Microwave 

Sensors TC-30C ultrasonic sensor. The range of the TC-30C is adjusted so 

that it detects objects at least 2 to 3 ft (0.61 to 0.91 m) above the road surface, 

e.g., tops of vehicles. The detection gate established by this adjustment 

differentiates between pulses reflected from the road surface and those 

reflected from vehicles. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-90 Federal Highway Administration 

Table 5-23. Eltec 842 infrared sensor footprint dimensions. 

Mounting 

height h

Range R Incidence 

angle θ

Ground 

distance d

Footprint 

width w

Footprint 

length l

15 ft 21 ft, 3 in 45 deg 15 ft 2 ft, 8 in 2 ft, 6 in 

15 ft 25 ft, 0 in 53 deg 20 ft 3 ft, 1 in 3 ft, 6 in 

15 ft 29 ft, 2 in 59 deg 25 ft 3 ft, 8 in 4 ft, 9 in 

15 ft 33 ft, 6 in 63 deg 30 ft 4 ft, 3 in 6 ft, 3 in 

20 ft 40 ft, 4 in 60 deg 35 ft 5 ft, 1 in 7 ft, 3 in 

20 ft 44 ft, 9 in 63 deg 40 ft 5 ft, 7 in 8 ft, 5 in 

20 ft 49 ft, 3 in 66 deg 45 ft 6 ft. 2 in 10 ft, 2 in 

20 ft 53 ft, 10 in 68 deg 50 ft 6 ft, 9 in 12 ft, 3 in 

1 ft = 0.3 m 

h = mounting height, d = ground distance between center of the detection area and the base of the sensor mounting 

structure, R = range from sensor aperture to center of detection area, w = ground footprint width, l = ground footprint 

length. θ is not selected independently, but is determined from the h and R values through cos θ = h/R. 

(From: Eltec Instruments, Model 842 Presence Sensor Data Sheet, Daytona Beach, FL.) 

When mounted horizontally, ultrasonic sensors view the side of any vehicles 

that pass by. In this configuration, the range is adjusted so that the detection 

zone extends approximately half way through the lane.(42) 

Presence and passage are provided from optically isolated semiconductors or 

relays. Ultrasonic sensors that measure speed contain a serial interface, 

which usually requires a software driver to be written by the user agency. 

PASSIVE ACOUSTIC SENSORS 

The guidelines for installing single-lane and multiple-lane passive acoustic 

sensor models are described below. 

IRD SMARTSONIC SINGLE-LANE ACOUSTIC SENSOR 

The SmartSonic sensor receives acoustic energy through one beam pointed at 

the center of the monitored lane. The sensor is aimed by sighting along the 

tube that serves as part of the mounting hardware.(43) The installation 

geometry is illustrated in Figure 5-71. The sensor contains an integrated selftest on startup and online, real-time fault monitoring. Optional solar power 

and spread spectrum wireless transmission link are available to transmit 

sensor data to a controller. The line of sight range of the wireless link 

depends on the transmitted power level and bit error rate. Minimum range is 

800 ft (244 m) and maximum range is at least 4,800 ft (1,463 m). 
 Traffic Detector Handbook—2006 

October 2006 Page 5-91 Federal Highway Administration

12 to 18 ft

Maximum diameter

of circular footprint formed

by detection cone

4 ft

2 to 3 ft

Road Surface Curb 1 ft = 0.305 m

Figure 5-70. TC-30C ultrasonic sensor overhead mounting pattern (Source: Microwave Sensors, 

TC-30C installation instructions, Ann Arbor, MI). 

20 to 35 ft

Side pole or

Mounting bracket overhead mounting 

SmartSonic Sensor 

Aimpoint

10 to 25 ft

from aperture

to center of

monitored lane

6 to 12 ft

Up to 25 ft

Top View

Side View

1 ft = 0.305 m

4 to 8 ft

Shoulder

Figure 5-71. SmartSonic acoustic sensor installation geometry (Source: IRD, Inc., Smartsonic 

Acoustic Vehicle Detection System Specifications) Saskatoon, Saskatchewan. 1996. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-92 Federal Highway Administration 

The SmartSonic detects vehicle presence through an optically isolated 

semiconductor. A serial interface on the controller card installed with the 

sensor provides volume, lane occupancy, speed, vehicle classification (cars, 

light trucks, heavy trucks and buses), and sensor status messages. 

SMARTEK SAS-1 MULTIPLE-LANE ACOUSTIC SENSOR 

The SAS-1 is designed to monitor up to five lanes when mounted in a sidelooking configuration. Detection zones are created with Windows-based 

software supplied with the sensor.(44) These zones are equivalent in size to a 

6-ft (1.8-m) inductive-loop in the direction of traffic flow and are userselectable in size in the cross-road direction. The software allows monitoring 

of acoustical signatures and output data as displayed in Figure 5-72. Output 

data include volume, lane occupancy, and average vehicle speed over a serial 

interface, which may require interface software to a controller or central 

computer to be written by the user agency. 

Figure 5-72. SAS-1 acoustic sensor data display (Source: SmarTek Systems, Inc., 

SAS-1 Specifications, Woodbridge, VA). 

SENSOR COMBINATIONS 

The ground footprint dimensions for sensors that utilize combinations of 

different technologies, such as passive infrared and CW Doppler microwave 

radar, are specified for each sensor as shown in Table 5-24 and Figure 5-73. 

In this example, the manufacturer recommends mounting the sensor above 

the center of the monitored lane facing approaching traffic with the top 

surface of the unit horizontal or parallel to the road surface if an incline is 

present.(45) The infrared presence zone is activated as soon as the front of a 

vehicle enters this portion of the footprint and remains activated until the 

vehicle leaves. The switching output of the dynamic infrared zone remains 

activated as long as there is movement detected in this area. The radar 

measures high to medium speed vehicles and is used by the infrared sensor 

for speed calibration. Data are output through a serial interface that usually 

requires driver software to be written by the user agency. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-93 Federal Highway Administration

Table 5-24. ASIM DT 281 dual-technology passive infrared and CW Doppler 

radar sensor footprint dimensions. 

Mounting height (m) 5 6 7 8 

Doppler radar footprint length (m) 9.0 10.5 12.5 14.0 

Doppler radar footprint width (m) 2.2 2.6 3.0 3.5 

Passive infrared footprint length (m) 4.0 4.5 5.2 6.0 

Passive infrared footprint width (m) 0.8 1.0 1.2 1.5 

1 m = 3.28 ft 

(From: ASIM Technologies, Ltd., DT 281 data sheet, Uznach, Switzerland.) 

CW Doppler radar 

sensor field-of-view

Presence zone

Dynamic zone

Multiple detection zone 

passive infrared sensor 

field-of-view

Figure 5-73. ASIM DT 281 dual technology passive infrared and CW Doppler microwave sensor 

ground footprints. (Source: ASIM Technologies, Ltd., DT 281 data sheet, Uznach, Switzerland.) 

REFERENCES 

1. Smith, K.L., and A.R. Romine. Materials and Procedures for Sealing 

and Filling Cracks in Asphalt-Surfaced Pavements—Manual of 

Practice, FHWA-RD-99-147, Strategic Highway Research Program. 

U.S. Department of Transportation, Federal Highway 

Administration, McLean, VA. February 2001. 

2. Improving Materials and Methods for Installing Loop Detectors, 

FHWA-OR-RD 86-03, performed by Oregon Department of 

Transportation. U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC. 1986. 

3. Woods, D.L. Texas Traffic Signal Detector Manual, Report No. 

FHWA/TX-90/1163-3F. Texas Transportation Institute, College 

Station, TX. July 1992. 

4. Bikowitz, E.W., and S.P. Ross. Installation of Vehicle Detection 

Loops, Special Report 75. New York State Department of 

Transportation, Engineering Research and Development Bureau, 

Albany, NY. July 1983. 

5. Evaluation and Improvement of Inductive Loop Detectors, FHWA, 

Research Report No. 119, Performed by New York State Department 

of Transportation. U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC. 1985. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-94 Federal Highway Administration 

6. Hsu, D. Vehicle Detector Systems Study, Part 3: Recommended 

Material, Installation, Acceptance Test, and Maintenance Procedure 

for Loop Vehicle Detection Systems. Ontario Ministry of 

Transportation and Communications, Research and Development 

Division, Toronto, Ontario. March 1980. 

7. Traffic Signal Detector Loop Location Design Installation. Illinois 

Department Transportation, District 6, Springfield, IL. November 

1988. 

8. Ingram, J.W. The Inductive Loop Vehicle Detector: Installation 

Acceptance Criteria and Maintenance Techniques, Report No. TL 

631387, prepared by California Department of Transportation, 

Sacramento, CA, for Federal Highway Administration. U.S. 

Department of Commerce, National Technical Information Service, 

PB-263 948, Washington, DC. March 1976). 

9. Evans, L.D., K.L. Smith and A.R. Romine. Materials and Procedures 

for Repair of Joint Seals in Portland Cement Concrete Pavements—

Manual of Practice, FHWA-RD-99-146, Strategic Highway Research 

Program. U.S. Department of Transportation, Federal Highway 

Administration, McLean, VA. February 2001. 

10. Smith, K.D., H.T. Yu, and D.G. Peshkin. Portland Cement Concrete 

Overlays: State of the Technology Synthesis, FHWA-IF-02-045. U.S. 

Department of Transportation, Federal Highway Administration, 

Washington, DC. April 2002. 

11. Arnold, E.D., Jr., and G.L. Munn. Installation Report—State 

Demonstration Project—Loop Detectors. Virginia Highway and 

Transportation Research Council, Charlottesville, VA October 1982. 

12. Everall, P.F. Urban Freeway Surveillance and Control—The State of 

the Art. U.S. Department of Transportation, Federal Highway 

Administration, Washington, DC. June 1973. 

13. Canoga Seminar Script. 3M Company, Traffic Control Devices 

Department, Canoga Controls, St. Paul, MN. No date. 

14. Puerto Rico Traffic Signal Slab Loop Detectors. U.S. Department of 

Transportation, Federal Highway Administration, Puerto Rico 

Division Office, San Juan, Puerto Rico. 1988. 

15. Max Cutter. Ralph M. Fergerson and Associates, Product Literature. 

1989. 

16. Klein, L.A. Sensor Technologies and Data Requirements for ITS.

Artech House, Norwood, MA. 2001. 

17. Klein, L.A., and M.R. Kelley. Detection Technology for IVHS, Vol. I:

Final Report, FHWA-RD-95-100. U.S. Department of 

Transportation, Federal Highway Administration, Washington, DC. 

December 1996. 

18. Traffic Surveillance and Detection Technology Development, Final 

Report, JPL Publication 97-10. Jet Propulsion Laboratory, California 

Institute of Technology, Pasadena, CA. March 1997. 
 Traffic Detector Handbook—2006 

October 2006 Page 5-95 Federal Highway Administration

19. Traffic Surveillance and Detection Technology Development: Phase II 

and III Test Results, Troy, MI, JPL Pub. D-1577. Jet Propulsion 

Laboratory, California Institute of Technology, Pasadena, CA. May 

1998. 

20. Kranig, J., E. Minge, and C. Jones, Field Test of Monitoring of 

Urban Vehicle Operations Using Non-Intrusive Technologies, 

FHWA-PL-97-018. U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC. May 1997. 

21. Middleton, D., and R. Parker. Initial Evaluation of Selected 

Detectors to Replace Inductive Loops on Freeways, FHWA/TX00/1439-7. Texas Transportation Institute, College Station, TX. 

2000. 

22. NIT Phase II: Evaluation of Non-Intrusive Technologies for Traffic 

Detection, Final Report, SRF No. 3683, prepared by Minnesota 

Department of Transportation and SRF Consulting Group, Inc. U.S. 

Department of Transportation, Federal Highway Administration, 

Washington, DC. September 2002. 

23. Middleton, D., and R. Parker. Evaluation of Promising Vehicle 

Detection Systems, Research Report FHWA/TX-03/2119-1, Draft. 

Texas Transportation Institute, College Station, TX. October 2002. 

24. Autoscope Consultant’s Designers Guide. Econolite Control Products, 

Anaheim, CA. 1998. 

25. Grenard, J., D. Bullock, and A. Tarko. Evaluation of Selected Video 

Detection Systems at Signalized Intersections: Final Report, 

FHWA/IN/JTRP-2001/22bbbb. Indiana Department of Transportation, 

Division of Research, and Purdue University, West Lafayette, IN. 

November. 2001. 

26. Bullock, D., and A. Tarko. Evaluation of Selected Video Detection 

Systems at Signalized Intersections: Technical Summary, 

FHWA/IN/JTRP-2001/22, SPR-2391. Indiana Department of 

Transportation, Division of Research, and Purdue University, West 

Lafayette, IN. November 2001. 

27. Sturdevant, J. Video Design Recommendations. Indiana Department 

of Transportation, Division of Research, West Lafayette, IN. 2003. 

28. Abbas, M., and J. Bonneson. “Video Detection for Intersection and 

Interchange Control,” FHWA/TX-03/4285-1. Texas Transportation 

Institute, College Station, TX. 2002. 

29. Smith, W.J. “Optical Design,” Chapter 8 in The Infrared Handbook, 

3rd printing, G.J. Zissis and W.L. Wolfe (eds.). The Infrared 

Information Analysis (IRIA) Center, Environmental Research 

Institute of Michigan (ERIM), Ann Arbor, MI. 1989. pp. 8-8–8-9. 

30. Jenkins, F.A., and H.E. White. Fundamentals of Optics. McGrawHill, New York, NY. 1957. 

31. VideoTrak®-900 Technical Specification. Peek Traffic, Tallahassee, 

FL. 1997. 
Chapter 5—Sensor-Installation Techniques 

October 2006 Page 5-96 Federal Highway Administration 

32. MacCarley, C.A., B.M. Hemme, and L.A. Klein. “Evaluation of 

Infrared and Millimeter-Wave Imaging Technologies Applied to 

Traffic Management,” paper 2000-01-1303. SAE 2000 World 

Congress Technical Paper Series, Warrendale, PA. March 2000. 

33. CCATS Initialization Manual. Traficon N.V., Heule, Belgium. 

February 1993. 

34. Klein, L.A. Final Report: Mobile Surveillance and Wireless 

Communication Systems Field Operational Test—vols. 1, 2, 3, 

California PATH Research Reports UCB-ITS-PRR-99-6, 99-7, 99-8. 

University of California Berkeley, Richmond, CA. 1999. 

35. CCATS® VIP2 Specification. VDS, Inc., San Diego, CA. 1997. 

36. TrackerTM: TDN-30 Narrow Beam Vehicle Detector, Manual 04-

0181620-00C. Whelen Engineering Company, Chester, CT. 1991. 

37. TC-20 installation instructions. Microwave Sensors, Ann Arbor, MI. 

38. RTMS User Manual, issue 3. Electronic Integrated Systems, Inc., 

Toronto, Ontario. 

39. Accuwave 150LX Instruction Manual. Naztec, Inc., Sugar Land, TX. 

40. Autosense II User’s Manual and System Specification. Schwartz 

Electro-Optics, Inc., Orlando, FL. 1998. 

41. Model 842 presence sensor data sheet. Eltec Instruments, Inc., 

Daytona Beach, FL. 

42. TC-30C installation instructions. Microwave Sensors, Inc., Ann 

Arbor, MI. 

43. Smartsonic Acoustic Vehicle Detection System Specifications. IRD, 

Inc., Saskatoon, Saskatchewan. 1996. 

44. SAS-1 Specifications. SmarTek Systems, Inc., Woodbridge, VA. 

45. DT 281 data sheet. ASIM Technologies, Ltd., Uznach, Switzerland. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-i Federal Highway Administration 

TABLE OF CONTENTS: CHAPTER 6. 

CHAPTER 6. SENSOR MAINTENANCE .................................................................6-1

NATURE OF THE PROBLEM ................................................................................6-1

FAILURE MODES IN INDUCTIVE-LOOP DETECTOR SYSTEMS ..................6-2

FAILURE MECHANISMS.................................................................................6-2

Omitted Phase..................................................................................................6-2

Stuck Signal .....................................................................................................6-2

Phase Extending to Maximum .......................................................................6-2

Intermittent Problems.....................................................................................6-3

Crosstalk ..........................................................................................................6-3

Splashover........................................................................................................6-4

CAUSES OF LOOP SYSTEM FAILURE.........................................................6-4

FACTORS AFFECTING REQUIRED MAINTENANCE..............................6-4

TROUBLESHOOTING PROCEDURES FOR INDUCTIVE-LOOP 

DETECTORS.........................................................................................................6-5

SAWCUT MAINTENANCE ..............................................................................6-5

REARRANGEMENT OF LOOP CONNECTIONS ........................................6-6

ELIMINATING CROSSTALK..........................................................................6-6

SUBSTITUTION OF ELECTRONICS UNITS................................................6-8

OPERATIONAL CHECK OF MALFUNCTIONING INDUCTIVELOOP DETECTOR ........................................................................................6-9

Adjacent Lane Detection ................................................................................6-9

Intermittent Operation ...................................................................................6-9

System Sensitivity............................................................................................6-9

ELECTRICAL TESTING OF AN INDUCTIVE-LOOP DETECTOR 

SYSTEM ........................................................................................................6-10

TEST PROCEDURES.......................................................................................6-11

Sequential Test Procedure............................................................................6-14

Step 1. Conduct Visual Inspection ............................................................6-14

Step 2. Check Operation of Inductive-Loop Electronics Unit .................6-15

Step 3. Measure Parameters Needed To Determine Quality Factor Q....6-15

Step 4. Determine Q...................................................................................6-15

Step 5. Measure Sensitivity of Inductive-Loop System.............................6-16

Method 1................................................................................................6-16

Method 2................................................................................................6-16

Step 6. Analysis..........................................................................................6-16

MAGNETOMETER SYSTEM MAINTENANCE..................................................6-16

CAUSES OF MAGNETOMETER FAILURES .............................................6-17

Probe Burial Depth .......................................................................................6-17

Probe Movement ...........................................................................................6-17

Probe Cable....................................................................................................6-17

Sawcut Maintenance .....................................................................................6-18

TROUBLESHOOTING PROCEDURES........................................................6-18

MAGNETIC DETECTOR MAINTENANCE ........................................................6-19

OVER-ROADWAY SENSOR MAINTENANCE....................................................6-21

VIDEO IMAGE PROCESSORS......................................................................6-21

MICROWAVE RADAR SENSORS ................................................................6-21

MAINTENANCE COST COMPARISONS AMONG SENSOR 

TECHNOLOGIES .............................................................................................6-21

REFERENCES........................................................................................................6-22
Chapter 6—Sensor Maintenance 

October 2006 Page 6-ii Federal Highway Administration 

LIST OF FIGURES: CHAPTER 6. 

Figure 6-1. Pavement failure near an inductive-loop installation. ...........6-5

Figure 6-2. Model ILA-550 inductive-loop system analyzer. ...................6-11

Figure 6-3. Quality factor Q data form......................................................6-12

Figure 6-4. Sensitivity S data form. ..........................................................6-13

Figure 6-5. Loop finder. ..............................................................................6-14

LIST OF TABLES: CHAPTER 6. 

Table 6-1. Inductive-loop detector malfunctions and 

potential causes. ......................................................................6-10

Table 6-2. Inductive-loop detector troubleshooting 

guide. ........................................................................................6-18

Table 6-3. Annualized per-lane sensor cost comparison 

to instrument a six-lane freeway sensor station. ..................6-22
 Traffic Detector Handbook—2006 

October 2006 Page 6-1 Federal Highway Administration

CHAPTER 6. SENSOR 

MAINTENANCE 

Use of appropriate sensor installation techniques and specification of 

suitable materials and products will minimize maintenance and other life 

cycle costs. However, even with superior design and installation, systems do 

not operate as intended for extended periods if maintenance is not provided. 

Therefore, proper and regularly scheduled sensor maintenance is critical to 

effective and prolonged operation of traffic signal control systems and 

freeway surveillance and management systems. 

Many factors can contribute to lack of maintenance. Inadequate budget and 

staffing deficiencies have a profound effect on the level and quality of 

maintenance activities. Budgetary problems that continue to plague traffic 

agencies have resulted in a cost consciousness that frequently focuses only on 

initial cost, rather than on lifetime cost. Consequently, less expensive 

products, materials, and processes are used in the original installation 

because of their lower initial cost. However, this is not always a cost effective 

life-cycle solution. 

While funding problems might not be easily resolved, they may be 

ameliorated by increased attention to cost effectiveness in all phases of 

design, installation, operation, and maintenance. Staffing concerns related to 

numbers and skill levels of maintenance personnel may be reduced 

somewhat by selecting only equipment that can be realistically maintained 

by available agency personnel or local contractors. 

This chapter is designed to assist maintenance operations managers, 

supervisors, and technicians in identifying and resolving maintenancerelated issues. Design engineers and those responsible for installation and 

operations should be fully aware of the cause-and-effect relationship between 

their activities and maintenance issues. 

A number of excellent reference sources address the maintenance process. 

These include the ITE Traffic Signal Installation and Maintenance 

Manual,(1) the TRB publication, Maintenance Management of Traffic Signal 

Equipment and Systems,(2) and the FHWA Traffic Control Devices 

Handbook.

(3) 

NATURE OF THE PROBLEM 

Sensor maintenance issues associated with inductive-loop detectors have 

changed considerably over the years. For example, the inductive-loop 

detector electronics unit, which formerly accounted for a considerable portion 

of sensor malfunctions, has matured to the point where many currently 

available digital models seldom experience failure. As stressed continually 

throughout this handbook, proper installation using appropriate materials 

and processes is the key to longer sensor system life, lower failure rates, and 

reduced required maintenance. 

Maintenance is essential for 

successful traffic signal and 

freeway surveillance and 

management systems operation 

over time. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-2 Federal Highway Administration 

There is little question that sensor malfunctions and associated signal 

failures increase motorists’ time and delay, maintenance costs, accidents, and 

liability. For example, a simple failure of an inductive-loop detector can cause 

delay in the traffic flow. If a call is locked in to the controller, it will cause the 

green to extend to its maximum limit regardless of traffic demand. 

In cases where the inductive-loop detector fails without a call, the most 

common temporary response is to place the phase on maximum recall in the 

controller. This can increase intersection delay by 50 percent or more. 

Although failures cannot be totally eliminated, adherence to proper 

installation and maintenance procedures can assuredly reduce the incidence 

of failure and the number of unnecessary maintenance calls. Some of the 

common causes of failure are discussed below. 

FAILURE MODES IN INDUCTIVE-LOOP DETECTOR 

SYSTEMS 

When an inductive-loop detector system experiences a failure, it exerts a 

negative effect on the total traffic signal control system. As discussed in 

Chapter 5, failures can be categorized by the failure mechanism and cause. 

FAILURE MECHANISMS 

A failure mechanism may be defined as the malfunction that is observed in 

the operation of an inductive-loop detector system. The following are common 

failure mechanisms. 

Omitted Phase 

An omitted phase occurs when the signal does not service one phase because 

of the lack of a call from the electronics unit. This is usually caused by a loop 

failure. Some early electronics units with solid-state outputs would fail in the 

open position, resulting in no call to the controller. Newer models have 

circuitry that provides a continuous call if the loop circuit is incomplete. (See 

“Phase Extending to Maximum.”) 

Stuck Signal 

A stuck signal is characterized by a signal indication (or phase) not changing 

as programmed. In most instances, a stuck signal is not caused by the sensor 

system, but by a controller failure. On occasion, a vehicle not being properly 

detected may cause a stuck signal. Another cause of a stuck signal is an 

inductive-loop electronics unit, operating with the delay feature, not 

retaining the call for a sufficient time to be transmitted to the controller. 

Phase Extending to Maximum 

This situation occurs when a specific phase of the signal operation extends to 

the maximum time set on the controller regardless of traffic demand. This is 

typically caused by a continuous call from the inductive-loop electronics unit 

and can generate extra delays of up to one-half the normal daily delay. The 

continuous call may be the result of a faulty electronics unit or an open loop 

circuit. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-3 Federal Highway Administration

Intermittent Problems 

Low sensitivity of the loop or the electronics unit (or a combination of both) 

can result in intermittent problems. Unstable oscillators in the electronics 

unit may also cause intermittent problems during periods of rapid 

temperature shifts. However, many current electronics unit models are 

capable of retuning. When the problem persists for no apparent reason 

(called ghost problems), the electronics unit is usually replaced. 

A broken loop wire that will ordinarily cause a malfunction may reconnect 

itself with shifts in pavement and cause intermittent operations. The only 

cure for this situation is to replace the loop when the process of elimination 

has determined that the only remaining cause could be a broken wire. 

External devices that have been installed to provide additional lightning 

protection may have been damaged by a power surge. This may result in 

intermittent operation. Since this damage may not be visible, these devices 

should be checked if intermittent operation occurs. 

Crosstalk 

Crosstalk is the result of inductive or capacitive coupling between loops or 

lead-in wires. Crosstalk may produce false detections when there are no 

vehicles in the detection zone. It may also result in lockup following a vehicle 

detection. It is usually associated with motion in either the generating or 

receiving loop channels due to transient frequency alignment caused by a 

vehicle-induced signal. 

Crosstalk occurs when the normal operating frequency of one loop 

installation is influenced by the frequency of another. This normally arises 

when the frequency of one loop system is decreasing and passes through or 

near the frequency of the other loop. Electrical engineers refer to this as a 

frequency lock between two oscillators. 

This condition occurs only when the frequencies of the two oscillators are 

close to each other and electronic coupling exists between the two oscillators. 

There are numerous sources of electronic coupling. Some of the more common 

include: 

• Poor quality connections in the loop system. 

• Field coupling due to close physical proximity of two loops. 

• Coupling between two closely spaced lead-in wires. 

• Coupling between lead-in cables sharing a common conduit. 

• Grounding of the lead-in cable shield at controller cabinet field 

terminals. 

• Coupling between closely spaced harness wires from the field 

terminals to the electronics unit. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-4 Federal Highway Administration 

Splashover 

The false detection of vehicles outside the detection zone is called splashover. 

This problem often occurs when long loops are operated at a sensitivity level 

required to detect small vehicles (i.e., motorcycles). With this high 

sensitivity, vehicles from adjacent lanes may be falsely detected. This 

problem also occurs when the loop is placed too close to the lane line. 

Splashover is a problem between lanes controlled by different phases. It is 

not a problem between lanes on the same phase unless accurate vehicle 

counts are desired. 

CAUSES OF LOOP SYSTEM FAILURE 

As discussed in Chapter 5 and Appendix M, surveys of traffic agencies 

indicate that there are eight major causes of loop failure. These are: 

• Pavement cracking and moving. 

• Breakdown of wire insulation. 

• Poor sealants or inadequate sealant application. 

• Inadequate splices or electrical connections. 

• Damage caused by construction activities. 

• Improper electronics unit tuning. 

• Electronics unit failure. 

• Lightning/electrical surges. 

FACTORS AFFECTING REQUIRED MAINTENANCE 

Many agencies report that most of their maintenance problems involving 

inductive-loop detectors can be directly attributed to installation errors. 

These errors can be a direct result of sloppy installation, poor inspection, and 

the use of low-grade components not suited to the particular environment. 

Many agencies do not use their own personnel to install inductive-loop 

detector systems. In these cases, the contract to install the systems is usually 

awarded to the lowest bidder. The lowest-bidder concept has been successful 

in encouraging competitive bidding and generally lowering costs. To be 

effective, however, a procuring agency must apply stringent prequalification 

guidelines to avoid awarding the contract to an electrical contractor with 

inadequate experience or knowledge. 

If inspection is inadequate, the potential for contractor expediency and error 

is enormous. The consequences of improper shortcuts or errors may not 

surface until the contractor’s responsibility has elapsed and, therefore, the 

loop must be repaired at the expense of the agency. For example, piercing, 

cutting, or otherwise damaging the insulation on the loop wire by the use of 

improper tools or careless handling may not show up until some time after 

the wire is sealed in the slot. 

In Chicago, IL, IDOT maintains more than 18,000 inductive-loop detectors. 

Because of their past experience with loop failure, they initiated an active 

inspection and maintenance program to monitor each loop. This program has 

reduced replacements to about 35 recuts per year. They report that no more 

than 5 percent of their loops are inoperative at any given time.(4) 
 Traffic Detector Handbook—2006 

October 2006 Page 6-5 Federal Highway Administration

TROUBLESHOOTING PROCEDURES FOR INDUCTIVELOOP DETECTORS 

The analysis of a malfunctioning inductive-loop detector can be a difficult 

task. The root cause of the failure may be associated with environmental 

conditions or other factors attributed to installation. The technician 

analyzing a faulty inductive-loop system should consider all the causes of 

failures listed above. Initially, of course, it is necessary to isolate the problem 

to one associated with the sensor system. 

An experienced technician is frequently able to pinpoint the troubled area or 

faulty part by visual examination. Figure 6-1 shows a readily apparent 

pavement failure along the saw slot, which has exposed loop wires. When 

visual inspection does not immediately disclose the problem, systematic 

troubleshooting is required. Such procedures for identifying and correcting 

malfunctioning inductive-loop detector systems are discussed below. 

Figure 6-1. Pavement failure near an inductive-loop installation. 

SAWCUT MAINTENANCE 

The hostile street environment makes the loop wire the most vulnerable 

component of an inductive-loop detector system. Therefore, a scheduled 

visual inspection of the roadway at and around the sawcut should be 

conducted every 6 months to determine if the integrity of the pavement, the 

sealant, or the slot has been violated. The inspection should include looking 

for wires that have floated to the top of the sealant. Loops with exposed or 

shallow-buried wires should be replaced. The early detection and correction 

of saw slot problems can prevent many future inductive-loop detector system 

malfunctions. 

Unstable, cracked or deteriorating sealant should be removed with a blunt 

instrument. The slot should be blown clean with compressed air and new 

sealant poured over the old. This is especially important in areas where snow 

removal equipment is used and salt applications are prevalent. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-6 Federal Highway Administration 

Potholes within the loop area should also be repaired. The two major factors 

influencing quality pothole patching are material selection and repair 

procedures. Thus, these factors control the cost effectiveness of the overall 

patching operation through associated material, labor, and equipment costs. 

The combinations of materials and procedures that produce optimum cost 

effectiveness vary from agency to agency.(5) 

Materials available for pothole repair in asphalt concrete include stockpiled 

cold mix materials, spray-injection heated emulsions, and hot mix asphalt 

concrete. FHWA reports provide detailed instructions for repairing potholes 

and spalls in asphalt surfaced pavements and portland cement concrete. (See 

references 5–10.) 

REARRANGEMENT OF LOOP CONNECTIONS 

A low value of inductance is the usual cause for inadequate loop sensitivity to 

detect a vehicle. Low inductance can be caused by an insufficient number of 

turns of wire, by turns of wire shorted together, or by steel mesh in the 

roadway that has produced a shorted-turn effect. If any of these factors are 

present in a multiple-loop configuration, the condition can be temporarily 

corrected by removing the defective loop from operation. 

Multiple loops can be connected in series or series/parallel to maintain an 

equivalent inductance within the range needed by the electronics unit. 

Quantitative measurements are made to determine if the inductance of a 

malfunctioning system has dropped below the acceptable value. Several test 

meters are commercially available to measure inductance. Some of these 

perform direct measurements, while others measure frequency, which is 

inversely related to the inductance value. In the latter case, accompanying 

charts or graphs are used to determine inductance. 

ELIMINATING CROSSTALK 

Crosstalk may occur when any of the following conditions are present: 

• Two loop lead-in cables share a common conduit. 

• Operations occur at close to the same frequencies. 

• Cable shields are not properly connected. 

• Two loops are installed within a few feet of each other. 

• Two sets of lead-in wires occupy the same saw slot. 

• Poor quality splices and coupling between closely spaced wires in the 

cabinet harness. 

Potential solutions to these problems are listed below. 

• Poor quality connections in the loop system: Solutions 

include using high-quality soldered joints, soldering of all crimp 

connections, minimizing the number of joints, using highquality connectors and replacement if worn, using safeguards 

against corrosion, tightening terminal block screws, and 

twisting and soldering of wire ends together and placing under 

one terminal block screw rather than two individual screws. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-7 Federal Highway Administration

• Field coupling due to close physical proximity of two 

loops: Not much can be done to eliminate this source. However, 

frequency separation and/or use of a multichannel multiplexed 

electronics unit are effective solutions. 

• Coupling between two closely spaced lead-in wires: 

Solutions include independent routing in slots physically 

separated by at least 1 ft (0.3 m), twisting-loop wire pairs a 

minimum of 5 turns per foot (16.5 turns per meter), use of 

multichannel multiplexed electronics units, and frequency 

separation. 

• Coupling between lead-in cables sharing a common 

conduit: Solutions include the use of individual, shielded, 

twisted-pair cables; proper grounding of cable shields; use of 

multichannel, multiplexed electronics units; and frequency 

separation. 

• Grounding the lead-in cable shield at controller cabinet 

field terminals: A potential solution is to use the threeterminal connection illustrated in Figure 5-35. The first step in 

the procedure consists of stripping the insulation on the 

individual lead-in cables back a maximum of 2 to 3 inches (5 to 

8 cm) and then grounding each lead-in cable independently. 

• Coupling between closely spaced harness wires from the 

field terminals to the electronics unit: The solution is to 

use either shielded twisted cable with proper grounding of the 

shield or twisted wire pair. 

Caltrans uses a modified loop tester and the following four-step procedure for 

identifying and correcting crosstalk:(11) 

1. Using the auxiliary input to the tester, measure the operating 

frequency of the inductive-loop detector suspected of crosstalk. 

2. If the operating frequencies of two or more inductive-loop 

detectors are less than 2 kHz apart and they share one or more 

conduit runs, they are likely to crosstalk. This can usually be 

corrected by grounding the cable shields at the controller 

cabinet. 

3. If the electronics units have a range switch or other means of 

selecting frequencies, adjust the frequencies so that at least 2 

kHz separates operating frequencies. 

4. If adjustments cannot be made, adding parallel capacitors to 

the field terminals may shift the operating frequencies. A 

quality capacitor such as one made of polycarbonate or ceramic 

should be used to assure good temperature stability. The 

capacitance value should be adequate to shift the resonant 

frequency by about 2 kHz. This procedure cannot be used with 

electronics units where the operating frequency is independent 

of loop inductance. Some manufacturers do not approve of 

adding capacitance external to the electronics unit because 

temperature-stable capacitors are difficult to procure and are 

still prone to value change with rapid temperature shifts. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-8 Federal Highway Administration 

If wire loops are causing the crosstalk, they can be reassigned to different 

channels or the lead-in wires in the pull box can be retwisted and respliced, 

thus eliminating one cause for potential crosstalk. 

To identify which electronics unit is cross talking, one general procedure is to 

disconnect all units except those that have the most frequent false calls. If no 

new false calls or lockups occur, then crosstalk from another unit is 

indicated. By selectively reconnecting the other electronics units, the unit 

causing the crosstalk between wire loops can be identified. 

Specific multichannel electronics units have a scanning feature that reduces 

potential crosstalk. In these units, the sequential scanning system activates 

one loop channel at a time, while the remaining loops, connected to other 

channels, are deenergized and thus unable to provide coupling to the active 

channel. This does not, however, eliminate crosstalk between channels on 

different electronics units. Frequency separation will reduce crosstalk 

between adjacent loops as well as other wire loop detectors. 

SUBSTITUTION OF ELECTRONICS UNITS 

Experience indicates that older electronics units cannot be freely substituted 

for each other. Tests in Los Angeles, CA, revealed that “...there are some 

brands of electronics units that appear...to have more sensitivity than 

others...Certain brands and/or types will not operate with adverse conditions 

such as very low circuit Q or a low value of shunt resistance to the conduit 

ground.”(12) It was concluded that the inductance value at the cabinet end of 

the loop system must fall within the operating range of the units if they are 

to be interchanged. Newer electronics units having the same characteristics 

and quality can generally be interchanged without problem, since different 

manufactured units operate on different frequencies; however, any unit that 

passes the NEMA tests can usually be interchanged. 

Each unit within an agency’s inventory should be checked for sensitivity to 

ensure that all electronics units achieve minimum sensitivity. After this 

check has been made, the substitution process can be used to determine if the 

electronics unit at the malfunctioning installation is the cause of the failure. 

Thus, if the electronics unit is replaced and the system functions normally, 

this may indicate a failure in the electronics unit. Conversely, if loop system 

operation does not improve with the substituted unit, then the original unit 

is reinserted and another problem source determined. 

Since many malfunctions are crosstalk-related, any change that affects the 

loop frequency will improve the situation, at least temporarily. Therefore, 

substitution of another unit, even of the same model and frequency setting, 

will often return the loop to normal operation due to small differences in the 

values of the input capacitors in the electronics unit. Substitution of a 

different model (especially from a different manufacturer) is often successful 

due to differences in front-end electronics design. The best solution is to 

eliminate, to the maximum practical extent, those elements that contribute 

to crosstalk. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-9 Federal Highway Administration

OPERATIONAL CHECK OF MALFUNCTIONING 

INDUCTIVE-LOOP DETECTOR 

Several operational checks can be conducted to expedite analysis of a 

malfunctioning inductive-loop detector system. These checks can be 

performed utilizing a maintenance vehicle and a vehicle simulator that 

analyze adjacent lane detection, motion in the loop wire, intermittent 

detection, and sensitivity of the loop system. 

Adjacent Lane Detection 

At locations where adjacent lane detection is suspected (such as where long 

loops are used on left-turn lanes), one technique is to maneuver the 

maintenance vehicle close to the lane and monitor the electronics unit. The 

sensitivity is then adjusted so that the maintenance vehicle does not cause an 

output. However, the sensitivity must not be set too low to prevent the 

electronics unit from detecting a small vehicle in the detection zone. 

It should be noted that this procedure does not test the worst case. The worst 

case occurs when several vehicles are stopped in the adjacent lane and 

occupy the entire length of the long loop. 

Several loop configurations avoid or minimize adjacent lane detection, e.g., a 

series of small loops, quadrupole loops, adjustable diamond loops, and others 

that are described in Chapter 4. When it is not possible to change loop 

configurations, delay timing can be utilized to detect a vehicle only if it is in 

the detection zone for a specified amount of time (say, 5 seconds). With this 

approach, passing vehicles in an adjacent lane are not detected, although 

stopped vehicles may present a signal large enough to hold the call in the 

empty lane. 

Intermittent Operation 

The beginning of loop failure is often associated with intermittent operation. 

As stated earlier, causes of this type of operation are generally poor 

connections, open loops, short circuits, or leakages to ground. These, in turn, 

may cause the electronics unit to lock up when a motor vehicle passes over 

the point of the fault. 

To identify this problem, the operation of the inductive loop is observed while 

the maintenance vehicle is driven over the various loops suspected of having 

intermittent operation. Operation of the vehicle in the detection area can also 

assist in identifying problems associated with wire motion. Another approach 

to identifying sources of intermittent operation is to visually inspect all 

connections and test them with a 500-volt megger and a low-ohm midscale 

ohmmeter. 

System Sensitivity 

A simulated vehicle that represents the smallest detectable vehicle can be 

used to determine if the inductive-loop system sensitivity is in balance and to 

determine the minimum sensitivity required of the system. The same 

procedure can also be used to test new installations. Caltrans uses a shortedturn vehicle model to simulate a 100 cc Honda motorcycle. The simulator is 

placed in the detection zone, and the electronics unit is observed to determine 

if the vehicle is detected. Reference 11 provides the construction details and 

procedures for using this model. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-10 Federal Highway Administration 

The simulated vehicle is utilized to measure the change in inductance, which 

is a measure of the sensitivity. For this purpose, a frequency reading is made 

with no vehicle present; then the simulated vehicle is introduced into the 

detection area and a new frequency reading is taken. 

The difference between the two frequencies provides a measure of the change 

in inductance due to the presence of the vehicle according to: 

 Δf/f = 0.5 ΔL/L. (6-1) 

This is an approximate relationship valid for changes in inductance up to 10 

percent and for loop quality factor Q greater than 5. This equation shows 

that the percent change in inductance is twice the percent change in 

frequency as was discussed in Chapter 2. 

ELECTRICAL TESTING OF AN INDUCTIVE-LOOP 

DETECTOR SYSTEM 

Table 6-1 identifies several malfunctions that occur in inductive-loop detector 

operation and their causes. Potential causal factors for each listed 

malfunction are identified by an “X” in the appropriate table cell. 

Maintaining the correct resistance values and knowledge of the inductance 

values required for proper operation of the electronics units will eliminate 

many of the problems associated with loop performance. 

Table 6-1. Inductive-loop detector malfunctions and potential causes. 

 Malfunction 

 

Potential 

cause 

No call Constant 

call 

False 

call 

Drifting Crosstalk Will not 

tune 

Unbalanced 

sensitivity 

Low quality factor X X X 

Low resistance to 

ground X X X X X 

High series 

resistance X X X X X 

Improper 

inductance X X X X 

Poor ground X X 

Improper 

installation X X X X X X 

Electronics unit 

malfunction X X X X X 

Four devices are needed to analyze loop systems: 

• High voltage resistance tester (megger) to measure insulation 

resistance. 

• Frequency tester, inductive-loop analyzer (such as the model 

illustrated in Figure 6-2) or inductance meter to measure inductance. 

• Digital VOM to measure series resistance. 

• Low-ohm (measurement range ≈ 2 to 3 ohms) midscale ohmmeter to 

measure the dynamic change in the series resistance. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-11 Federal Highway Administration

Low resistance-to-ground and high series resistance are major contributors to 

loop system malfunctions. Low resistance-to-ground can result from poor 

insulation on the loop or lead-in wire or inadequate sealing of splices. These 

problems can be avoided if, during installation, the materials are handled 

properly and appropriate splice sealing methods are followed. Low 

resistance-to-ground can also occur when wires are exposed as a result of 

pavement cracking, sealant cracking, frost action, and wear to pavement 

from studded tires. In addition, excess wire coiled in the pull box or controller 

cabinet can add inductance to the circuit. This may result in tuning 

problems. There is also an increased potential for coupling to another loop 

system. 

Poor splices, corroded or loose screw terminals, poor crimping, or inadequate 

wire size can cause high series resistance. High series resistance is a problem 

exacerbated by humidity, temperature, and vibration, which cause drifting 

and false calls. These are the same problems that occur when the system has 

low resistance-to-ground. 

Figure 6-2. Model ILA-550 inductive-loop system analyzer. Measures the leakage resistance of a 

loop to ground at 500 V dc; frequency and relative signal strength of a loop network; loop 

inductance, resistance, and Q; and change in inductance caused by vehicle driving 

over a loop (Photo courtesy of US Traffic Corp., Santa Fe Springs, CA). 

TEST PROCEDURES 

The following equipment is required to isolate the causes of erratic operation 

or malfunction of an inductive-loop detector system using the procedures 

discussed below:(12) 

• Ohmmeter, battery operated. 

• Voltmeter (ac), high-impedance input to 100 kHz. 

• Digital frequency counter, input impedance > 50 Kohms to 200 

Kohms. 

• Quality factor test meter. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-12 Federal Highway Administration 

• Loop test meter. 

• Data recording forms for quality factor Q and sensitivity S, 

samples of which are illustrated in Figures 6-3 and 6-4, 

respectively. 

• Vehicle simulator, sensitivity standard. 

QUALITY FACTOR (Q)

Location: ______________________________ Date: _______________________ 

Description: ____________________________ By: _________________________ 

Loop 

Position

Connection Elps fLT L (μH) fr fh fl BW Q calc. RP Remarks 

 

Analysis: 

Elps = voltage across loops; fLT = frequency of Loop Test Unit; fr, fh, fl = frequency for 

resonance, high, low, respectively; BW = bandwidth = fh – fl; Q calculated = fr/BW; 

RP = resistance to ground. 

Figure 6-3. Quality factor Q data form. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-13 Federal Highway Administration

SENSITIVITY (S)

Location: ______________________________ Date: _______________________ 

Description: ____________________________ By: _________________________ 

Loop 

Position

Sensitivity 

Standard 

fr

(kHz) 

f2 

(kHz)

2 2

2 r f − f

(kHz) 

Sensitivity

S (%)

Presence 

(minutes) 

Remarks 

 

Analysis: 

fr = resonant or tuning frequency; f2 = loop frequency with test standard or vehicle; 

S = [( 2 2

2 r f − f )/ 22f ] x 100 = change in inductance in percent. 

Figure 6-4. Sensitivity S data form. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-14 Federal Highway Administration 

Figure 6-5 shows a loop finder, which can be used to locate a loop when 

pavement overlays cover the loop installation. 

Figure 6-5. Loop finder. 

Sequential Test Procedure 

The six steps described below assist in isolating the causes of inductive- loop 

detector system malfunction or erratic operation. 

Step 1. Conduct Visual Inspection 

Check for indication of broken or cut loop or lead-in wires. Check for open 

leads within the controller and for the connection of the power source to the 

electronics unit.
 Traffic Detector Handbook—2006 

October 2006 Page 6-15 Federal Highway Administration

Step 2. Check Operation of Inductive-Loop Electronics 

Unit 

To eliminate the electronics unit as the source of the problem, replace the 

existing unit with one having a known sensitivity. If the operation is not 

considerably improved, remove the substituted unit, replace the original unit, 

and continue to Step 3. 

One manufacturer notes that since many problems are crosstalk-related, any 

change that will affect the loop frequency will improve the situation, at least 

temporarily. As discussed above, substitution of an electronics unit, even of 

the same model and frequency switch settings, will frequently be successful 

in eliminating the problem due to subtle differences in the values of 

electronics unit input capacitors. Therefore, substitution of electronics units 

may solve the problem temporarily without addressing the real problem. If 

the problem is crosstalk-related, the most effective solution is to identify and 

eliminate the causes of the crosstalk. 

Step 3. Measure Parameters Needed To Determine Quality 

Factor Q 

Measure and record the following data on the quality factor data form. 

• With the electronics unit disconnected and power removed from 

the loop, measure the resistance from either loop terminal to 

the bus or conduit ground. Record as RP on the quality factor 

data form. Check for series continuity between the terminals. 

• With the loop test meter attached to the controller tie points 

and the electronics unit disconnected, measure the inductance 

of the loop system. Record the frequency fLT and the inductance 

L on the form. 

Step 4. Determine Q 

To determine the Q of the loop system, follow the procedure described below 

and record the results on the quality factor data form: 

• Attach the Q test unit to the existing cable or adaptor cable as 

required. 

• Connect the ac voltmeter and digital frequency counter to the Q

test unit to read loop voltage and frequency in kHz. 

• Adjust the frequency for maximum voltage across the loops; 

record the frequency as fr and the voltage across the loops as 

Elps. 

• Successively increase and decrease the frequency to obtain the 

frequency points having 70 percent of the resonant voltage 

value. Record these as higher fh and lower fl. 

• Calculate and record the bandwidth BW and Q, where BW = 

fh – fl and Q = fr/BW. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-16 Federal Highway Administration 

Step 5. Measure Sensitivity of Inductive-Loop System 

Record the frequencies obtained from either of the following methods on the 

sensitivity data form. 

Method 1

• Set the function switch to “Q MEAS” on the Q test unit. 

• Adjust the frequency to obtain resonance, and record as fr. 

• Record the reference number of the sensitivity standard to be 

used. 

• Place the simulator or vehicle on the desired loop. 

• Retune the frequency of the electronics unit to the new 

resonant point and record as f2. 

• Calculate the sensitivity as shown on the data form. 

Method 2

• Set the function switch to “INTERNAL” and the mode switch to 

“S” on the Q test unit. 

• Allow the electronics unit to tune downward in frequency to the 

lock-on point. 

• Note the frequency, and record as fr on the form. 

• Place the sensitivity standard on the desired loop. 

• Note the new lock-on point frequency. Record as f2 on the form. 

• Calculate the sensitivity on the data form. 

Step 6. Analysis 

Perform the analysis by comparing the measured values for each of the items 

with the values shown in the “Indicated Conditions” column in Table 6-2. 

Determine the required maintenance from the suggested “Corrective Actions” 

listed in the table. 

MAGNETOMETER SYSTEM MAINTENANCE 

Conventional magnetometer systems are composed of the sensor probe, probe 

cable, lead-in cable, and the electronics unit. The same precautions should be 

observed for the probe cable and lead-in (home run) cable as with an 

inductive-loop detector lead-in wire and lead-in cable. Magnetometers that 

transmit data via RF transmission to the electronics unit do not require a 

probe cable, but do need a lead-in cable that connects the antenna (that 

receives the transmitted data) to the electronics unit in the controller 

cabinet. A magnetometer probe cable, when required, is placed in a slot in 

the road surface and should be sealed with the same care and procedures as 

used for inductive-loop detector lead-ins. The splices connecting the probe 

cable and the lead-in cable must be mechanically and electrically sound and 

environmentally protected, as described in Chapter 5. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-17 Federal Highway Administration

Failure can occur in any of the subsystems. The principal causes of 

magnetometer failures are described below. 

CAUSES OF MAGNETOMETER FAILURES 

Four key areas can affect the operation of a magnetometer system. These are 

proper burial depth of the sensor probe, stability of the probe in the 

pavement, characteristics of the probe cable with regard to moisture 

penetration, and saw slot maintenance. 

Probe Burial Depth 

Vertical placement of the probe is critical to system performance. Deep 

placement of approximately 24 inches (60 cm) will provide good single-count 

detection, but will result in lower signal levels. Shallow placement of about 6 

inches (15 cm) will provide a stronger signal, but will also increase the 

potential for double counting. Therefore, the burial depth must be 

appropriate for the intended application, as described in Chapters 4 and 5. 

Probe Movement 

Probes must be firmly supported in their holes. Any displacement of the 

vertical alignment of the probes may result in performance instability. A 

recommended installation approach is to utilize PVC conduit as a shell, with 

sand tamped around the probe to prevent lateral displacement. 

Probe Cable 

Water-blocked cable must be used to prevent moisture penetration. Any 

moisture can cause excessive capacitance or leakage between wires or to 

ground. In addition, moisture across the connections to a magnetometer may 

induce drift. Wherever possible, it is recommended that the probe cable run 

directly to the electronics unit, thus eliminating splices. If this is not possible, 

splices must be electrically sound and environmentally protected. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-18 Federal Highway Administration 

Table 6-2. Inductive-loop detector troubleshooting guide.a

Item Indicated conditions Corrective actions 

Loop wires Broken, cut, or exposed 

insulation worn away 

Install new loops shifted by 6 inches (15 cm) from 

the old loop and cut each old loop at least twice 

Loop slots Sealant missing 

Wire exposed 

Surface eroded 

Clean slot of loose material and refill or replace 

entire loop and lead-in as required 

Patch street surface if loop wire is not exposed 

Resistance to 

ground 

100 MΩ (megohms) or more 

Shorted 

Acceptable if Q > 5 

Replace as required 

Series loop 

resistance 

Open circuit 

Greater than specified (based 

on wire size and length) 

Locate and correct open circuit 

Isolate cause (poor splice, inadequate crimp, etc.) 

and repair as required 

Quality factor >5 if RP >100 MΩ

<5 for any other value of RP

Acceptable with modern electronics units

Replace electronics unit 

Sensitivity of 

loop system 

Measures lower than design 

value for the configuration 

Consider Q, shunt RP, and series RS as possible 

causes and correct as indicted 

Determine loop interconnection and rework to 

accepted design values 

Sensitivity of 

electronics unit 

Remains actuated 

Does not actuate 

Insufficient sensitivity 

Substitute known serviceable electronics unit 

Substitute known serviceable electronics unit 

Substitute and return unit removed to shop 

a The limiting values in the table are nominal values determined from the original investigation. They may 

require modification to incorporate additional experience. 

Sawcut Maintenance 

The sawcut for the magnetometer should be visually inspected every 6 

months to evaluate the condition of the sealant and the surrounding 

roadway. As with the inductive-loop sawcut, any cracking or deteriorating 

sealant should be chipped away, blown clean, and replaced with new sealant. 

TROUBLESHOOTING PROCEDURES 

The initial step for troubleshooting problems with magnetometer systems is 

to visually inspect all system components, including the connections at the 

terminal strip in the controller cabinet, any splices in pull boxes, and the 

street installation of the probe and probe cable. These components should be 

examined for loose connections, poor sealant, exposed wires, bad splices, or 

evidence of recent construction. 

If none of these elements show evidence of problems, the next step is to 

examine the probe for tilting. Without retuning the electronics unit, use a bar 

magnet oriented in a direction which adds to the vertical component of the 

Earth’s magnetic field intensity (as described in Chapter 5) to measure the 

distance from the road surface that will cause the detection to occur. 

Compare this measurement with the one taken at the time of installation. 

If the distance is smaller than the original measurement, then the probe may 

have tilted due to pavement movement. If the probe has tilted, the electronics 

unit may be retuned and this location monitored frequently for evidence of 

further tilting. If the unit cannot be retuned, the probe should be pulled and 

reinstalled with proper vertical alignment. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-19 Federal Highway Administration

One way to compensate for the tilting problem is to use a Digital Nulling 

Loop such as that developed for the City of Baltimore, MD.(13) This device has 

automatic compensating circuitry, which is incorporated into the electronics 

unit. It works similarly to an inductive-loop detector in tracking 

environmental changes. One of these devices is required for each channel in 

the electronics unit. 

If detection does not occur during the test for tilting, the problem could be in 

the cable, wire, probe, splice, or electronics unit. Series resistance and 

resistance-to-ground measurements should then be made at the controller 

cabinet with a VOM. The series resistance should be within 10 percent of the 

measurements made at installation and the other readings should be high, 

indicating no breaks in the insulation or in the integrity of the environmental 

splice. 

If the measurements do not fall within these ranges, the next step is to 

determine whether the problem lies in the probe, the probe cable, the splice, 

or the lead-in cable. 

The splice should be taken apart and the following tests conducted. A series 

resistance and resistance-to-ground measurement toward the probe should 

be made and compared with measurements taken during installation. Large 

variations indicate faulty probe or probe cable. The best solution is to replace 

the probe and probe cable. 

On the other hand, if the measured values compare favorably with the 

original values, the top pair of the lead-in cable wires should be tied together 

and the VOM read. This should be repeated with the bottom two wires. If 

these values are high, the lead-in cable should be replaced. If not, the 

problem originates in the splice and a proper reconnection of the splice is 

needed to eliminate the problem. 

If this systematic method is used, the problem will usually be identified. The 

original reference sheet should always be referred to during this process to 

compare measurements. When the problem has been rectified, new values 

should be noted and retained for future reference. 

MAGNETIC DETECTOR MAINTENANCE 

Magnetic detectors have an extremely good maintenance record. As they are 

installed under the road surface and all lead-in wires consist of cable encased 

in underground conduit, the opportunity for failure is relatively small. User 

surveys indicate that these sensors do not present any major maintenance 

problems, and that some of these devices have been installed for more than 

20 years without a failure. 

In the rare case where a magnetic detector fails, the first step is to examine 

the electronics unit. When false responses occur (the electronics unit output 

relay closes when no cars are crossing the detection area), replace the 

existing unit with another tested unit, tune, and monitor. If the system then 

works, it can be assumed that the original unit was at fault. If the system 

still does not work, a series resistance test followed by a resistance-to-ground 

test is required. This should identify any leakage in the system. 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-20 Federal Highway Administration 

The total circuit resistance-to-ground of the detector should be measured 

after disconnecting all of the jumpers to ground and the leads to the relay. In 

cases where leakage is between detector cables and signal cables, a 1-

megohm leakage may result in erratic operation. Proper cable layout and 

good splicing can avoid this condition. Occasionally, one probe circuit will 

pick up more extraneous signal than another probe circuit on the same 

phase. If this occurs, interchange probes at the terminal strip so that the 

probe circuit picking up the largest extraneous signal is at the “Magnetic 

Detector Minus” side of the system. 

If the voltmeter needle moves erratically when there are no vehicles passing 

the probes, it generally indicates surges in nearby power wires. If the probe 

can be located closer to traffic, the sensitivity of the electronics unit can be 

decreased to reduce the effect of power lines. Changing the orientation of the 

detector axis can eliminate or minimize the effect of a particular power line. 

For minimum disturbance, the detector axis should be parallel to the power 

lines. 

Relay units are designed to be immune to all line voltage changes except 

extreme ones. If line voltage changes are suspected of causing the voltmeter 

needle to behave in an erratic manner, disconnect the “Magnetic Detector 

Plus” and the “Magnetic Detector Minus” leads, and short them together. If 

the voltmeter needle is still erratic, the trouble is due to line voltage changes. 

In this case, examine the power service connections to make sure they are 

properly soldered and insulated. If there are no obvious problems, request 

that the power company check the regulation of the power being supplied and 

make any necessary modifications to their equipment. 

If the voltmeter needle tends to behave erratically only when the signals 

change (with the magnetic detector terminals short-circuited), the trouble is 

insufficient current-carrying capacity in the service wires between the 

cabinet and power source. 

If the voltmeter needle is steady when the electronics unit operates with the 

detector leads shorted and tends to be erratic when the signal lights change 

with the detectors reconnected, then the difficulty is caused by 

electromagnetic or electrostatic induction between the wires carrying signal 

currents and the wires leading to the magnetic detectors. Make sure that the 

capacitors are in place across the terminals to which the magnetic detectors 

are connected, as these capacitors are designed to absorb such surges. 

Fluctuations on the neutral power wire with respect to ground might also 

cause intermittent response. A connection to a conducting rod driven into the 

ground (required in most modern installations) can stabilize this condition. If 

the connections are stable, check for leakage in the detector circuit by 

making the same tests that were described above with a VOM. 

Other problems could involve induced voltage response, which can occur 

when the detector leads are placed in the conduit with the power supply 

leads. Induced voltage can cause false responses. Consequently, the lead-in 

cable should not be inserted into conduit with other signal wiring. Another 

cause of false response is setting the gain too high. Reducing the gain on the 

electronics unit will generally solve this problem. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-21 Federal Highway Administration

OVER-ROADWAY SENSOR MAINTENANCE 

Maintenance and life cycle costs may be determined, in part, by published 

values of the mean time between failures. Some over-roadway sensors are 

designed with mean time between failures of 64,000 to 90,000 hours. The 

effects of lightning strikes and other natural or human-induced failure modes 

are not included in this number. Thus, maintenance and replacement costs 

for these devices may be significantly less than for inductive loops over a 10-

year period, especially if commercial vehicle loads, poor subsoil, inclement 

weather, and utility improvements frequently require road resurfacing and 

loop replacement. 

Some agencies require lane closures at night and traffic diversion for 

installing or repairing in-roadway sensors on freeways. Such requirements 

create significant differences between the cost of freeway and arterial sensor 

maintenance. Other maintenance items, such as electronics unit retuning, 

are generally not needed with over-roadway sensors. However, new 

maintenance requirements, such as camera lens cleaning, can arise. 

VIDEO IMAGE PROCESSORS 

The most pervasive maintenance operation for video image processors is the 

periodic cleaning of the camera lens. The cleaning frequency varies from six 

months to one year, depending on the camera mounting height, the truck 

traffic volume, and precipitation and dust characteristics of the monitored 

area. Cleaning may be performed by the agency responsible for traffic 

management or may be contracted out. Cameras mounted on the tallest poles 

(e.g., 70 feet (21 m)) may require more specialized boom trucks for cleaning. 

MICROWAVE RADAR SENSORS 

Radar sensors do not appear to require much maintenance. EIS Electronic 

Integrated Systems Inc., the manufacturer of the RTMS, reports that of 624 

units shipped to one North American client, 10 (or 1.6%) were returned for 

repair from April 1994 through June 2002. 

MAINTENANCE COST COMPARISONS AMONG SENSOR 

TECHNOLOGIES 

A 10-year study of inductive-loop maintenance costs in Houston, TX, found as 

few as 42 failures and as many as 341 failures per year in the 600 to 1000 

intersections maintained during the 1989–1998 study period. The calculated 

loop replacement costs per intersection varied from $107 to $628 (U.S.).(14) 

Actual costs per intersection are probably higher because the calculation 

assumes all intersections had loops (some were not actuated and hence did 

not use loops), 100 percent of loop failures were discovered (some were not), 

and no maintenance besides replacement was performed. 

In a summary of maintenance costs of four Autoscope VIP systems used by 

the Road Commission of Oakland County, MI, TTI found that monthly 

camera maintenance averaged $5.05 and monthly processor maintenance 

averaged $26.71 from 1995 through 1998. A total of 692 cameras and 194 

controllers were included in the study. Costs included labor, fringe benefits, 

and truck, lift, and radio equipment. Because the VIPs were under warranty 

for at least part of the time, the manufacturer or distributor paid for some 

repair parts and new replacement units. Therefore, older units whose 

warranty period has expired may experience higher maintenance costs.(14) 
Chapter 6—Sensor Maintenance 

October 2006 Page 6-22 Federal Highway Administration 

Table 6-3 compares the annualized per-lane cost for inductive-loop detectors, 

VIPs, multiple detection zone presence-detecting microwave radar, and 

acoustic array sensors for a six-lane freeway sensor station.(14) Motorist delay 

and excess fuel consumption incurred during installation further add to the 

annualized cost of the inductive loops. 

Table 6-3. Annualized per-lane sensor cost comparison to instrument 

a six-lane freeway sensor station.(14) 

Sensor Number 

required 

for 6 lanes 

Expected 

life 

Annualized 

cost 

ILD 12 10 $746 

VIP 2 cameras, 

1 processor 

10 $580 

Multidetection zone microwave presencedetecting radar 1 7a $314 

Acoustic array 6 5 $486 

a Manufacturer’s mean time between failures suggests a 10-year expected life. 

REFERENCES 

1. Giblin, J.M., et al. Traffic Signal Installation and Maintenance 

Manual, prepared by Institute of Transportation Engineers, 

Washington, DC. Prentice Hall, Englewood Cliffs, NJ. 1989. 

2. Parsonsons, P.S. Maintenance Management of Traffic Signal 

Equipment and Systems, NCHRP Project 20-5. Transportation 

Research Board, National Research Council, Washington, DC. 1984. 

3. Fullerton, I.J., and J.H. Kell. Traffic Control Devices Handbook. U.S. 

Department of Transportation, Federal Highway Administration, 

Washington, DC. 1983. 2001 edition available from ITE, 

Washington, DC. 

4. Traffic Signal Detector Loop Location Design Installation. Illinois 

Department of Transportation, District 6, Springfield, IL. November 

1988. 

5. Wilson, T.P., and A.R. Romine. Materials and Procedures for Repair 

of Potholes in Asphalt Surfaced Pavements—Manual of Practice, 

FHWA-RD-99-168, Strategic Highway Research Program. U.S. 

Department of Transportation, Federal Highway Administration, 

McLean, VA. June 1999. 

6. Smith, K.L., and A.R. Romine. Materials and Procedures for Sealing 

and Filling Cracks in Asphalt-Surfaced Pavements—Manual of 

Practice, FHWA-RD-99-147, Strategic Highway Research Program. 

U.S. Department of Transportation, Federal Highway 

Administration, McLean, VA. June 1999. 

7. Wilson, T.P., K.L. Smith, and A.R. Romine. Materials and 

Procedures for Rapid Repair of Partial-Depth Spalls in Concrete 

Pavements—Manual of Practice, FHWA-RD-99-152, Strategic 

Highway Research Program. U.S. Department of Transportation, 

Federal Highway Administration, McLean, VA. June 1999. 
 Traffic Detector Handbook—2006 

October 2006 Page 6-23 Federal Highway Administration

8. Evans, L.D., K.L. Smith, and A.R. Romine. Materials and Procedures 

for Repair of Joint Seals in Portland Cement Concrete Pavements—

Manual of Practice, FHWA-RD-99-146, Strategic Highway Research 

Program. U.S. Department of Transportation, Federal Highway 

Administration, McLean, VA. June 1999. 

9. Bentz, D.P., M.A. Ehlen, C.F. Ferraris, and J.A. Winpigler. Service 

Life Prediction Based on Sorptivity for Highway Concrete Exposed to 

Sulfate Attack and Freeze-Thaw Conditions, FHWA-RD-01-162, 

performed by National Institute of Standards and Technology. U.S. 

Department of Transportation, Federal Highway Administration, 

McLean, VA. March 2002. 

10. Smith, K.D., H.T. Yu, and D.G. Peshkin. Portland Cement Concrete 

Overlays: State of the Technology Synthesis, FHWA-IF-02-045, 

performed by Applied Pavement Technology, Inc. U.S. Department 

of Transportation, Federal Highway Administration, Washington, 

DC. April 2002. 

11. Ingram, J.W. The Inductive Loop Vehicle Detector: Installation 

Acceptance Criteria and Maintenance Techniques, Report No. TL 

631387, prepared by California Department of Transportation, 

Sacramento, CA. U.S. Department of Transportation, Federal 

Highway Administration, PB-263 948, Washington, DC. March 

1976. 

12. Pinnell, C., et al. Inductive Loop Detectors: Theory and Practice, 

FHWA-RD-IMP 76-2. U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC. January 1976. 

13. Scarzello, J.F., et al. An Analysis of the Baltimore Freeway 

Surveillance and Control System Magnetic Sensors, prepared by 

Naval Surface Weapons Center. Maryland State Highway 

Administration, Baltimore, MD. February 1983. 

14. Middleton, D., and R. Parker. Initial Evaluation of Selected Detectors 

to Replace Inductive Loops on Freeways, FHWA/TX-00/1439-7. Texas 

Transportation Institute, College Station, TX. April 2000. 

 Traffic Detector Handbook—2006 

October 2006 Page A-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX A. 

APPENDIX A. INDUCTIVE-LOOP SYSTEM EQUIVALENT 

CIRCUIT MODEL ..........................................................................A-1

ABSTRACT...............................................................................................................A-1

INTRODUCTION.....................................................................................................A-1

LOOP CAPACITANCE THEORY...........................................................................A-2

INTERNAL-LOOP CAPACITANCE ..............................................................A-2

EXTERNAL-LOOP CAPACITANCE .............................................................A-4

LOOP RESISTANCE THEORY..............................................................................A-6

LOOP RESISTANCE.........................................................................................A-6

INTERNAL INDUCTANCE AND RESISTANCE PER UNIT LENGTH 

FOR A CYLINDRICAL CONDUCTOR ..................................................A-6

LOOP-INDUCTANCE THEORY ............................................................................A-7

SELF-INDUCTANCE OF SINGLE-TURN CIRCULAR LOOP ..................A-7

SELF-INDUCTANCE OF MULTITURN CIRCULAR LOOP .....................A-8

EXTERNAL INDUCTANCE OF SINGLE-TURN RECTANGULAR 

LOOP ............................................................................................................A-8

SELF-INDUCTANCE OF SINGLE-TURN RECTANGULAR LOOP.........A-9

SELF-INDUCTANCE OF MULTITURN RECTANGULAR LOOP ...........A-9

MUTUAL INDUCTANCE OF PARALLEL FILAMENTARY 

CIRCUITS....................................................................................................A-9

MUTUAL INDUCTANCE OF TWO COAXIAL AND PARALLEL 

RECTANGULAR LOOPS........................................................................A-10

SELF-INDUCTANCE OF MULTITURN QUADRUPOLE LOOP ............A-10

GENERAL FORMULA FOR MUTUAL INDUCTANCE OF 

PARALLEL FILAMENTS .......................................................................A-11

INDUCTIVE-LOOP CIRCUIT MODEL.......................................................A-12

LOOP TRANSMISSION LINE THEORY ............................................................A-13

LOOP TRANSMISSION LINE MODEL.......................................................A-13

FREQUENCY SHIFT ELECTRONICS UNIT SENSITIVITY...................A-14

LOOP TRANSFORMER THEORY.......................................................................A-15

INDUCTIVE-LOOP TRANSFORMER MODEL.........................................A-15

LOOP-DETECTOR ANALYSIS SYSTEM PROGRAM.......................................A-17

COMPARISON OF CALCULATED AND MEASURED LOOP 

SELF-INDUCTANCE AND QUALITY FACTOR ...........................................A-17

CONCLUSIONS ...............................................................................................A-19

APPENDIX A-1 ......................................................................................................A-20

LOOP GROUND-RESISTANCE DERIVATION.........................................A-20

APPENDIX A-2 ......................................................................................................A-20

REAL PART OF COMPLEX BESSEL FUNCTION OF FIRST KIND.....A-20

DERIVATIVE OF REAL PART.....................................................................A-21

IMAGINARY PART OF COMPLEX BESSEL FUNCTION 

OF FIRST KIND........................................................................................A-21

DERIVATIVE OF IMAGINARY PART .......................................................A-21

APPENDIX A-3 ......................................................................................................A-21

COMPLETE ELLIPTIC INTEGRAL OF FIRST KIND.............................A-21

COMPLETE ELLIPTIC INTEGRAL OF SECOND KIND........................A-21

APPENDIX A-4 ......................................................................................................A-22

SERIES TO PARALLEL CIRCUIT TRANSFORMATION.......................A-22

APPENDIX A-5 ......................................................................................................A-23

TRANSFORMER MODEL INPUT IMPEDANCE ......................................A-23

REFERENCES.......................................................................................................A-24
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-ii Federal Highway Administration 

LIST OF FIGURES: APPENDIX A. 

Figure A-1. Capacitance between adjacent isolated loop turns. ..................A-2

Figure A-2. Connection of adjacent loop turns..............................................A-3

Figure A-3. Multiturn loop equivalent circuit model....................................A-3

Figure A-4. Capacitance between loop conductors and slot. ........................A-4

Figure A-5. Capacitance between loop conductors and infinitely 

conducting planes. ....................................................................A-5

Figure A-6. Balanced inductive loop. .............................................................A-5

Figure A-7. Pair of parallel current elements. ............................................A-10

Figure A-8. Geometry for calculating mutual inductance of coaxial and 

parallel rectangular loops. .....................................................A-10

Figure A-9. Two-turn quadrupole loop.........................................................A-11

Figure A-10. Multiturn quadrupole loop. ....................................................A-11

Figure A-11. Mutual inductance of two parallel segments i and k............A-12

Figure A-12. Inductive-loop system circuit model.......................................A-12

Figure A-13. Equivalent loop circuit model.................................................A-13

Figure A-14. Loop transformer model..........................................................A-15

Figure A-15. Equivalent transformer model. ..............................................A-16

Figure A-16. Series-circuit to parallel-circuit equivalency (general).........A-22

Figure A-17. Loop-series to parallel-circuit equivalency............................A-22

Figure A-18. Transformer nodal analysis diagram.....................................A-23

LIST OF TABLES: APPENDIX A. 

Table A-1. Measured inductive-loop parameters of 3-turn 6- x 6-ft (1.8- 

by 1.8-m) loop with no vehicle present..................................A-17

Table A-2. Comparison of measured and computed inductive-loop 

parameters of 3-turn 6- x 6-ft (1.8- x 1.8-m) loop with no 

vehicle present........................................................................A-18

Table A-3. Rectangular-loop parameters.....................................................A-18

Table A-4. Quadrupole-loop parameters......................................................A-19

Table A-5. Circular-loop parameters............................................................A-19
 Traffic Detector Handbook—2006 

October 2006 Page A-1 Federal Highway Administration 

APPENDIX A. INDUCTIVE-LOOP 

SYSTEM EQUIVALENT CIRCUIT 

MODEL 

ABSTRACT 

Appendix A describes the derivation and application of an equivalent circuit 

model and computer program that calculates the apparent self-inductance 

and quality factor versus frequency of square, rectangular, quadrupole, and 

circular loops of round wire buried in a roadway. The effects of transmission 

lines and matching transformers between the loop in the roadway and 

roadside electronics units are included in the model. The capacitance 

between the loop conductors and surrounding pavement material is shown to 

have a major effect on the magnitude of the loop’s apparent self-inductance. 

Inductive-loop detectors are commonly used in actuated and computercontrolled traffic surveillance systems. The size, shape, and number of turns 

of wire in the loop should be designed to provide adequate vehicle detection 

sensitivity and prevent the lead-in wire and lead-in cable from reducing 

sensitivity. The inductance value at the input to the electronics unit must be 

within the range specified by the unit’s manufacturer. The computer program 

described in this appendix allows engineers to calculate the apparent loop 

inductance and quality factor versus operating frequency (20–60 kHz) for 

selected loop size, shape, wire diameter, number of turns, turn spacing, slot 

width, pavement loss tangent, and slot sealant dielectric constant. 

INTRODUCTION 

An inductive-loop detector system is composed of a buried loop of wire in the 

roadway pavement, which is connected with a transmission line (i.e., lead-in 

wire and lead-in cable) to a roadside electronics unit. When a vehicle is 

sensed by the loop, a small decrease in loop inductance is detected by the 

electronics unit. Furthermore, since the series inductance of the transmission 

line decreases the magnitude of the inductance change at the input to the 

electronics unit, the loop inductance should be larger than the transmission 

line series inductance. The loop inductance can be increased by winding 

additional turns into the loop and adding a transformer between the loop and 

transmission line. 

The frequency range of typical electronics units is 20–60 kHz, although 

newer units that provide vehicle classification can operate at hundreds of 

kilohertz. Loop capacitance can cause the inductance sensed by the 

electronics unit to change significantly with frequency if too many turns are 

used in the roadway loop. To better predict inductive-loop system 

performance, an equivalent circuit model of the loop system was developed 

and programmed on a computer. The computer program allows inductiveloop system designers and maintenance technicians to calculate the detection 

system inductance and quality factor as a function of frequency, wire gauge 

size, wire spacing, etc. 

The equivalent electrical circuit 

model for an inductive-loop 

system allows the loop’s 

inductance and quality factor to 

be calculated. 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-2 Federal Highway Administration 

The equivalent loop system model contains a roadway inductive-loop model, a 

transformer model, and a transmission line model. The calculation of the selfinductance of square, rectangular, and quadrupole loops is described 

elsewhere.(1,2) King describes the calculation of the self-inductance of circular 

loops.(3) This appendix expands the model to include the internal and 

external capacitance of such loops so that the high-frequency performance of 

the loop system can be calculated. A wideband transformer model is used. 

The transmission line model incorporates a complex characteristic 

impedance. All equations used in the loop computer program are included in 

this appendix. 

LOOP CAPACITANCE THEORY

INTERNAL-LOOP CAPACITANCE 

The capacitance between loop turns is calculated using a low-frequency, 

multilayer, transformer model.(4) The model assumes uniform flux coupling 

through the loop turns with minimum leakage flux. Figure A-1 illustrates the 

capacitance between two adjacent isolated loop turns. 

+

-

Figure A-1. Capacitance between adjacent isolated loop turns. 

The loop wires are modeled as parallel transmission lines that have a 

capacitance per unit length C' of(5) 

( )

( ) D a C

36 cosh / 2

/ 10 ' 1

9

0

−

− ε ε × = F/m (A-1) 

where 

D = spacing between conductor centers (m) 

a = conductor radius (m) 

ε/ε0 = relative dielectric constant of the 

 material that surrounds the turns of wire 

 (equal to 1 for air) 

ε0 = permittivity of free space = 8.854 x 10–12 F/m. 

The total capacitance between adjacent but isolated loop turns is given by 

C C P i = ' , (A-2) 

The stray capacitance 

between turns of wire in an 

inductive loop is calculated 

using a multilayer 

transformer model. 
 Traffic Detector Handbook—2006 

October 2006 Page A-3 Federal Highway Administration 

where P is the loop perimeter (m). A similar method was used by Palermo.(6)

The actual loop turns are connected as shown in figure A-2. 

+

-

Figure A-2. Connection of adjacent loop turns. 

The parallel transmission line is shorted at the end. The input capacitance of 

the shorted transmission line is given by 

∫ = = P i

T C dx C P

P

X C

0

2 ' 3

1 ( ) ' . (A-3) 

Figure A-3 shows the circuit model for a multiturn loop, where LL represents 

the low-frequency inductance of the loop and CL

i

 the lumped internal 

capacitance across the loop terminals. 

Figure A-3. Multiturn loop equivalent circuit model. 

The total internal energy stored in the capacitor is 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = = ∑

=

n

j

j

i

T j

i E T C LV C V

2

2 2

2

1

2

1

 . (A-4) 

When the voltage drop is linear, the voltage across each turn is directly 

proportional to the total voltage drop and inversely proportional to the 

number of turns. Accordingly, 

j VT

n

V 2 = (A-5) 

and 

∑

= ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎠

⎞ ⎜

⎝

⎛ = n

j

i

L

n

C P C

2

2 2

3

'

 (A-6) 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-4 Federal Highway Administration 

where n is equal to the number of loop turns. 

The lumped internal capacitance across the loop terminals is given by 

( )

C P

n

n C i

L ' 1

3

4

2

− = . (A-7) 

This equation is identical to the equation for the capacitance between 

transformer winding layers with the exception of C'.

(7) 

EXTERNAL-LOOP CAPACITANCE 

External capacitance is created when a conductor is located in the vicinity of 

a dielectric material. This does not occur when a conductor is placed near 

another conducting surface such as a metal. Figure A-4 illustrates capacitive 

coupling between turns of loop wire (i.e., the conductor) and the edge 

boundaries of a sawcut or sealant (i.e., the dielectric). The dielectric constant 

of the material surrounding the wire is denoted by ε and its conductivity by σ. 

Since loop wires are typically closer to the top of the slot (i.e., sealant in the 

bottom of the slot supports the loop wire), the capacitive coupling between 

the wire and bottom of the slot was neglected. 

Figure A-4. Capacitance between loop conductors and slot. 

Galejs determines the impedance of a buried insulated wire.(8) The 

capacitance is calculated assuming the region surrounding the slot or cavity 

is finitely conducting. If region one is composed of low-loss dielectric, we can 

write(9) 

ωε1 > σ1 j . (A-8) 

Stratton also shows that a perfectly conducting outer conductor, as found in a 

coaxial line, provides a good approximation to a finitely conducting one when 

calculating shunt admittance.(10) The slot walls are approximated by 

infinitely conducting ground planes, as illustrated in Figure A-5. These 

observations allow the combination of a coaxial conductor and TEM 

transmission line for use in modeling the capacitance of loop wires embedded 

in a slot. 
 Traffic Detector Handbook—2006 

October 2006 Page A-5 Federal Highway Administration 

h/2 h/2

d

Figure A-5. Capacitance between loop conductors and infinitely conducting planes. 

The characteristic impedance Z0 of a coaxial conductor is given by(11) 

⎟

⎠

⎞ ⎜

⎝

⎛

π = d

h

e

Z

r

4 log 138

0 10 (A-9) 

where 

h = sawcut slot width 

d = diameter of loop wire. 

The capacitance per unit length of a TEM transmission line is expressed as 

0

120 0 ' Z

C r πε ε = . (A-10) 

Thus 

⎟

⎠

⎞ ⎜

⎝

⎛

π

π ε ε =

d

h

C r

4 ln

2 ' 0

 (A-11) 

or 

⎟

⎠

⎞ ⎜

⎝

⎛

π

ε × =

−

d

h

C r

4 ln

10

18

1 '

9

F/m

 (A-12) 

where ln(x) = (2.303)[log10(x)]. 

The total external capacitance for the loop conductor is 

C = C′P . (A-13) 

Inductive loops are typically balanced, as shown in Figure A-6. 

+

-

Figure A-6. Balanced inductive loop. 

Because of the balanced configuration and zero potential point at the 

conductor perimeter center, 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-6 Federal Highway Administration 

6

'

3 2

' 1 2

C P C P C C ⎟ =

⎠

⎞ ⎜

⎝

⎛

⎟

⎠

⎞ ⎜

⎝

⎛ = = . (A-14) 

Thus

12

' 1 2

C P C C C e

L = = (A-15) 

where C1‖C2 denotes the parallel connection of C1 and C2. 

If only one side of the loop is grounded, then 

3

C'P Ce

L = . (A-16) 

LOOP RESISTANCE THEORY 

LOOP RESISTANCE 

The series loop resistance RL is composed of the direct-current wire 

resistance R, the high-frequency skin-effect resistance Rac, and the ground 

resistance Rg. The dominant resistance is the ground resistance, which is 

caused by currents induced in the conductive pavement and subgrade 

material. The ground resistance may limit loop sensitivity in locations with 

large moisture content. Ground resistance is calculated by assuming that the 

pavement and subgrade materials cause a magnetic loss similar to that of a 

ferrite or iron core in an inductor. The permeability μg of the pavement and 

subgrade material is assumed to be one. Appendix A-1, of this Appendix A, 

contains a derivation of the ground resistance. 

The series loop resistance RL is expressed as 

R L = R + R ac + R g (A-17) 

where 

R = direct-current loop resistance (Ω) 

Rac = skin-effect resistance (Ω) 

Rg = ground resistance (Ω) 

R g LL g = ω tan δ (A-18)

tanδg = loss tangent of pavement material 

LL = loop self-inductance (H) 

ω = operating radian frequency = 2πf (radians) 

f = operating frequency (Hz). 

INTERNAL INDUCTANCE AND RESISTANCE PER UNIT 

LENGTH FOR A CYLINDRICAL CONDUCTOR 

Johnson shows that the ratio of actual internal inductance to low-frequency 

internal inductance and the ratio of internal resistance to low-frequency 

internal resistance are given by(12) 
 Traffic Detector Handbook—2006 

October 2006 Page A-7 Federal Highway Administration 

( )( ) ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

′ + ′

× ′ + × ′ = 2 2

0 ( ) ( )

4 ( ) ( ) ( ) ( )

bei q ber q

bei q bei q ber q ber q

L q

L

i

i

 (A-19) 

and 

( )( ) ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

′ + ′

× − × ′ = 2 2

0 ( ) ( )

( ) '( ) ( ) ( )

2 bei q ber q

q ber q bei q bei q ber q

R

R

i

 (A-20) 

respectively, where 

( )

δ = a 2

q (A-21) 

Li = internal inductance (H/m) 

a = radius of wire (m) 

δ = skin depth (m), 

μ μ π σ

δ = f r 0

1

 (A-22) 

μ0 = permeability of free space = 4π x 10–7 H/m 

μr = relative permeability of copper wire = 1 

f = operating frequency (Hz) 

σ = conductivity of copper wire = 0.58 x 108 mhos/m 

Li0 = low-frequency internal inductance (H/m), 

π

μ μ = 8

0

0

r Li (H/m) (A-23) 

 = 0.5 x 10-7 H/m for copper wire 

Ri0 = low-frequency internal resistance (Ω), 

2 0 a R r i

π

ρ = (Ω) (A-24) 

ρr = resistivity (ohm-meter = Ωm) 

 = 1.74 x 10–8 Ωm for copper wire. 

LOOP-INDUCTANCE THEORY 

SELF-INDUCTANCE OF SINGLE-TURN CIRCULAR 

LOOP 

The self-inductance L0 of a single-turn circular loop is given by(14)

( ) () ()⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡ − ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = μ μ − − K k E k k L r r a

2

2 1

2

0 0 (A-25) 

where 

ber(q) is the real part of the 

complex Bessel function of the first 

kind. ber’(q) is the first derivative of 

the real part of the complex Bessel 

function of the first kind. bei(q) is 

the imaginary part of the complex 

Bessel function of the first kind. 

bei’(q) is first derivative of the 

imaginary part of the complex 

Bessel function of the first kind. 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-8 Federal Highway Administration 

( )2

2

2

4 ( )

r a

r r a k

−

− = (A-26) 

E(k) = complete elliptic integral of first kind 

K(k) = complete elliptic integral of second kind 

r = radius of the loop coil relative to an axis normal to the 

plane of the loop that passes through the center of the 

loop (m) 

a = radius of the loop wire (m). 

Therefore, the radius from the axis to the inside edge of the loop wire is r – a. 

The formula for E(k) is found in Appendix A-3 of this Appendix A.(15) 

SELF-INDUCTANCE OF MULTITURN CIRCULAR LOOP 

Following King(17), the inductance for a circular coil with N equally spaced 

identical turns is given by 

2( 1) 2( 2) ... LT = NL0 + N − M 12 + N − M 13 + (A-27) 

or 

= + ∑ ( ) − = +

N

i

T M i L NL N i

1

0 2 1,( 1) (A-28) 

where M1,(i+1) is the mutual inductance between turn 1 and turn i+1. 

According to Ramo,(18)

⎥

⎦

⎤ ⎢

⎣

⎡ ⎟ − ⎠

⎞ ⎜

⎝

⎛ = μ μ − ( ) 2 ( ) 2

0 1 2 E k

k

k K k

k

M r r r (A-29) 

( )2

1 2

2

2 1 2 4

d r r

r r k

+ + = (A-30) 

where 

M = mutual inductance (H) 

r1 = radius of turn one (m) 

r2 = radius of turn two (m) 

d = spacing between turns (m). 

EXTERNAL INDUCTANCE OF SINGLE-TURN 

RECTANGULAR LOOP 

The external inductance of a single-turn rectangular loop is given by the sum 

of the inductance of two pairs of conductors. Thus 

e

p epl eL0 = L + L 2 . (A-31) 

The external inductance of a single turn rectangular loop is(1)
 Traffic Detector Handbook—2006 

October 2006 Page A-9 Federal Highway Administration 

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛

+ + +

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − + +

⎪

⎩

⎪

⎨

⎧

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛

+ +

π

μ =

2

2 2

2

2

2

1

2

1

1

2

1 1

1

0

0 ln 1 ln 1 ln 1 a a a a Le l l l

l

l

l

l l l l l

2

2

2

1

2 2

2

2 2

1

2

1

2

1

2

2 ln 1 l l 2 l l

l

l

l

l l − + − + + +

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − + + a a

− ( ) l 1 +l 2 + 2a} (A-32) 

where 

l1 = width of loop (m) 

l2 = length of loop (m) 

a = radius of the conductor (m). 

This equation can be written in more compact form by combining the 

logarithmic terms. 

SELF-INDUCTANCE OF SINGLE-TURN RECTANGULAR 

LOOP 

The self-inductance of a single-turn rectangular loop is given by the sum of 

internal and external inductances as 

i e L0 = L 0 + L 0 (A-33) 

( ) i i L0 = 2 l 1 + l 2 L (A-34) 

where Li is given by Equation A-19 and e L0 is given by Equation A-32. 

SELF-INDUCTANCE OF MULTITURN RECTANGULAR 

LOOP 

The self-inductance of a coil with N equally spaced identical turns is given by 

2( 1) 2( 2) ... LT = NL0 + N − M 12 + N − M 13 + (A-35) 

MUTUAL INDUCTANCE OF PARALLEL FILAMENTARY 

CIRCUITS 

The mutual inductance between a pair of filamentary circuits located in free 

space and illustrated in Figure A-7 is given by(19) 

⎪

⎭

⎪

⎬

⎫

⎪

⎩

⎪

⎨

⎧

⎟ + ⎠

⎞ ⎜

⎝

⎛ − + ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎠

⎞ ⎜

⎝

⎛ + +

π

μ = ±

l l

l l l l d d

d d

M d

2 2

0 ln 1 1

2 ( , ) (A-36) 

where 

μ0 = permeability of free space = 4π x 10–7 H/m 

l = filamentary length (m) 

d = filamentary spacing (m) 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-10 Federal Highway Administration 

M(l,d) = mutual inductance (H). 

The mutual inductance is positive when the current flow in the filaments is 

in the same direction and negative when the current flow is in opposite 

directions. 

d

l

Figure A-7. Pair of parallel current elements. 

MUTUAL INDUCTANCE OF TWO COAXIAL AND 

PARALLEL RECTANGULAR LOOPS 

Equation A-36 can be used to calculate the total mutual inductance of the 

coaxial and parallel rectangular loops displayed in Figure A-8. 

3

1

2

4

1

2

3

4

A 

B

H

M11

Figure A-8. Geometry for calculating mutual inductance of coaxial and parallel rectangular loops. 

Accordingly, the total mutual inductance is given by the sum of the mutual 

inductances between the parallel sides as 

 (A-37) 

where M11 is the mutual inductance between side 1 of the bottom loop turn 

and side 1 of the top loop turn. All mutual inductances are symmetrical, e.g., 

M13 = M31 and M24 = M42. 

SELF-INDUCTANCE OF MULTITURN QUADRUPOLE 

LOOP 

Figure A-9 illustrates a two-turn quadrupole loop(20) and Figure A-10 a 

multiturn quadrupole loop. 

The external inductance of an N-turn quadrupole loop is 

 (A-38) 

( ) ( )⎥

⎦

⎤ ⎢

⎣

⎡ ⎟ −

⎠

⎞ ⎜

⎝

⎛ ⎟ − + + ⎠

⎞ ⎜

⎝

⎛ M = −2 M A, H + B M A, H M B, H B2 M 22 B, H 2

11 24

2 2

13

2 2 4( 1) 4( 1) 4( 2) () 4 2 ... LT = NL0 + NM12 + N − M13 + N − M14 + N − M15 + N − M16
 Traffic Detector Handbook—2006 

October 2006 Page A-11 Federal Highway Administration 

Figure A-9. Two-turn quadrupole loop. 

N – 1 N

4

2

3

1

Figure A-10. Multiturn quadrupole loop. 

GENERAL FORMULA FOR MUTUAL INDUCTANCE OF 

PARALLEL FILAMENTS 

In order to calculate the mutual inductance between the offset loops in the 

quadrupole loop model, a general formula for the mutual inductance of 

parallel, offset filaments is required. In Figure A-11, these filaments are 

labeled Segment i and Segment k. 

Following Jefimenko(21), the mutual inductance between the two parallel 

current filaments illustrated in Figure A-11 is given by 

( )( )

( )( ) ( )( )

ik

c d

a b

ik D C B A c C d D

a A b B M

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

+ + − + ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+ +

+ +

π

μ = ± ln

4

0 (A-39) 

where the positive sign is used for filaments with current flow in the same 

direction. 

This expression assumes that the filament lengths are much less than the 

wavelength divided by 2π and the conductor radius is much less than the 

filament length. Grover(22) shows that this type of general formula can also be 

derived by applying the laws of summation of mutual inductance to Equation 

A-36. 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-12 Federal Highway Administration 

c

d

a

A

C B

D

x axis

Segment k

Segment i

y axis

b

Figure A-11. Mutual inductance of two parallel segments i and k. 

INDUCTIVE-LOOP CIRCUIT MODEL 

Figure A-12 illustrates an inductive-loop system electrical circuit model that 

contains resistive, capacitive, and inductive components. Ohms Farads Farads Henrys

R

cp

G

L C

e

L C

i Ohms Ohms

ac R

Ohms

g

R

Henrys

o L

i L

Figure A-12. Inductive-loop system circuit model. 

Let 

LS = L 0 + L i (A-40) 

R S = R + R g (A-41) 

e

L

i C P = C L +C . (A-42) 

Following Johnson,(23) the slot dielectric loss conductance is 

GP CP c = ω tan δ (A-43) 

where tanδc is the loss tangent of the slot sealer material. 

Appendix A-4, of this Appendix A, shows that the inductive-loop system 

circuit model of Figure A-12 reduces to the circuit model of Figure A-13, 

where 

2

2 1

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

ω

+ ω −

=

P

P P

L P

in

L

G C

G R

 (A-44) 

and 

2

2 1

1

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

ω

+ ω −

− ω

ω =

P

P P

P

L P

in

L

G C

C

L X

. (A-45) 
 Traffic Detector Handbook—2006 

October 2006 Page A-13 Federal Highway Administration 

The loop quality factor is given by 

P

P

P

L

in

L

L in

in G

C

L

R

X Q

− ω

ω = =

1

. (A-46) 

The self-resonant frequency of the loop is given by 

LPC P f

π = 2

1

0 . (A-47) 

FaradsFarads

P P C L Henrys

PC in

R

L

in

X

L

Figure A-13. Equivalent loop circuit model. 

LOOP TRANSMISSION LINE THEORY 

LOOP TRANSMISSION LINE MODEL 

A transmission line connects the roadway loop with the roadside electronics 

unit. The complex impedance ZL of the loop is transformed to a complex 

impedance Zin by the transmission line cable according to(24) 

l

l

+ γ

+ γ = tanh

tanh

0

0

0

L

c L

in Z Z

Z Z Z Z (A-48) 

where 

L L LL Z = R + j ω (A-49) 

G j C

R j L Z

+ ω

+ ω

0 = (A-50) 

γ = (R + j ωL)(G + j ωC ) (A-51) 

and where 

R = transmission line per unit resistance (Ω/m) 

G = transmission line per unit conductance (mhos/m) 

L = transmission line per unit inductance (H/m) 

C = transmission line per unit capacitance (F/m) 

RL = loop equivalent resistance (Ω) 

LL = loop equivalent inductance (H) 

ω = radian frequency (radians) 

l = length of transmission line 

γ = complex propagation constant. 

The ratio of current to voltage 

is defined as conductance 

G = i / v. The unit of 

conductance is the mho (for 

inverse ohms) and is denoted 

by an inverted capital omega. 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-14 Federal Highway Administration 

A useful equation for computing Zin is given by(25) 

( ) x y

x j y x ky cosh 2 cos 2sinh 2 sin 2 tanh

+

±

± = . (A-52) 

FREQUENCY SHIFT ELECTRONICS UNIT SENSITIVITY 

The frequency shift ΔfD at the terminals of the electronics unit is required to 

compute the sensitivity of the inductive-loop system as a function of loop and 

cable inductance. 

Let 

fV − fNV = −Δf D (A-53) 

and 

NV D f = f (A-54) 

where fV, fNV are the frequencies measured by the electronics unit with and 

without a vehicle present. 

The sensitivity SD of the frequency shift electronics unit is defined as 

D

D

D S

f

f

2

1 = − Δ

. (A-55) 

The total inductance LD at the electronics unit is the sum of the loop and 

cable inductances given by 

LD = LL + LC . (A-56) 

Therefore, the change in inductance created by a vehicle passing over the 

loop is 

ΔLD = ΔLL (A-57) 

since it is only the loop inductance that varies in the presence of a vehicle. 

To convert the expression for sensitivity in terms of ΔfD/fD to one in terms of 

ΔLL/LL, note that 

fD = 1/[2π(LDCD)

1/2]. (A-58)

Thus, 

L C

L

D

D

L L

L

f

f

+

Δ = − Δ

2

1 . (A-59)

By inspection, 

L C

L

D L L

L

S

+

Δ = . (A-60)

Therefore, 

)

1

1 ) (

1

1 ) ( 1 (

L

C

L

L

L C

L

L

L L C

L D

L

L

S

L

L L

L

L

L L L

L

S

+

=

+

Δ = +

Δ = (A-61)
 Traffic Detector Handbook—2006 

October 2006 Page A-15 Federal Highway Administration 

and Equation A-55 becomes 

⎟

⎟

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎜

⎜

⎝

⎛

+

= − Δ

L

C

L

D

D

L

L

S

f

f

1

1

2

1

 (A-62) 

where 

L

L S L

Δ = . (A-63) 

Equation A-62 shows that the cable inductance LC strongly affects the 

sensitivity of the frequency shift electronics unit. If the cable inductance is 

one-tenth or less of the loop inductance, the cable has a negligible effect on 

inductive-loop system sensitivity, provided the quality factor QD is five or 

greater. The frequency shift electronics unit sensitivity results also apply to 

period shift electronics units. 

LOOP TRANSFORMER THEORY 

INDUCTIVE-LOOP TRANSFORMER MODEL 

A transformer with low leakage inductance (e.g., total series leakage 

inductance less than transmission line inductance) can be placed between the 

loop and transmission line to transform the loop inductance to a value larger 

than the transmission line inductance.(26) The transformer will remove the 

reduction in sensitivity caused by the transmission line. 

The transformer model is shown in Figure A-14.(27) 

PS C /n

Ohms

C R

SC n

2

L

M=kLP Z

Ohms Henrys

Henrys

Ohms HenrysFarads

VS

T

in Z

P R L P(1-k) n

2

S R

n:1

PC

L P(1-k)

Farads

Figure A-14. Loop transformer model. 

The definitions of the parameters that appear in Figure A-14 are referred to 

the primary of the transformer. Thus, 

RP = primary winding resistance 

RC = resistance corresponding to core loss 

n2

RS = referred secondary winding resistance 

n2

ZL = referred load impedance 

CP = primary capacitance 

CS = secondary capacitance 

CS'

 = modified secondary shunting capacitance, 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-16 Federal Highway Administration 

⎟

⎠

⎞ ⎜

⎝

⎛ = + −1 1 ' 2 n n

C

n

C C S PS

S (A-64) 

CPS/n = referred primary to secondary capacitance 

CP' = modified primary capacitance, 

⎟

⎠

⎞ ⎜

⎝

⎛ = + − n CP CP CPS

1 ' 1 (A-65) 

n = ratio of primary to secondary turns, 

S

P

L

L n = (A-66) 

LP = open circuit primary inductance at low frequency 

LP (1–K) = one-half total leakage inductance 

K = coupling coefficient, 

LP LS

M K = (A-67) 

M = mutual inductance. 

The equivalent transformer model of Figure A-15 was used to derive an 

expression for the transformed load impedance ZinT. 

1 Y

Farads

in Z

LZ

2 Y

3 Y

4 Y

5 Y

Farads

6Y

Ohms Ohms

Henrys Henrys

T

Figure A-15. Equivalent transformer model. 

Appendix A-5, of this Appendix A, shows that the transformed load 

impedance is given by 

[( )( ) ]

( ) ( )( ) [ ] ( )( ) 4 5 6 2 2 3 4 2 2 2 4 6 6 2 1 2 3 2 3 4 4 5 6 4

2 2 3 4 4 5 6 4

Y Y Y Y Y Y Y Y Y Y 2Y Y Y Y Y Y Y Y Y Y Y

Y Y Y Y Y Y Y ZT

in + + + + + + − − − + + − + +

+ + + + − =

. 

(A-68) 
 Traffic Detector Handbook—2006 

October 2006 Page A-17 Federal Highway Administration 

LOOP-DETECTOR ANALYSIS SYSTEM PROGRAM 

The loop-detector analysis system (LDAS) computer program calculates loop 

inductance and quality factor for rectangular, quadrupole, and circular loops. 

The loop inductance and quality factor are transformed by the transmission 

line (nonshielded, twisted-loop wire) to the roadside junction box and 

transmission line (shielded, twisted pair) between the junction box and 

electronics unit in the controller box. The program also models a transformer 

between the loop and transmission line or between the two types of 

transmission lines. 

The LDAS program is menu driven and written in Microsoft QuickBasic. All 

mathematical functions are computed using double precision calculations. 

COMPARISON OF CALCULATED AND MEASURED LOOP 

SELF-INDUCTANCE AND QUALITY FACTOR 

Table A-1 contains measured self-inductance and quality factor data for a 6- 

x 6-ft (1.8- x 1.8-m) three-turn inductive loop. The measured and LDAS 

computed data compare favorably as shown in Table A-2. Measured loop 

inductance and quality factor data versus frequency were unavailable for 

quadrupole and circular loops. 

Table A-1. Measured inductive-loop parameters of 3-turn 6- x 6-ft 

(1.8- x 1.8-m) loop with no vehicle present. 

f0

 (kHz) 

f2 3dB 

(kHz) 

f1 3dB 

(kHz) 2 1

0

f f

f Q − = C 

(μF) ( ) f C

L 2 2 0

1

π =

(μH) 

Q

f L R 2π 0 =

(Ω)

20 20.334 19.702 31.7 0.856490 73.9 0.29 

25 25.372 24.688 35.5 0.548109 73.9 0.33 

30 30.388 29.644 40.3 0.380000 74.1 0.35 

35 35.427 34.607 42.7 0.278693 74.2 0.38 

40 40.470 39.573 44.6 0.212056 74.3 0.42 

45 45.514 49.472 45.7 0.167887 74.5 0.46 

50 50.571 49.472 45.5 0.135704 74.7 0.52 

55 55.633 54.407 44.9 0.111845 74.9 0.58 

60 60.704 59.342 44.1 0.093450 75.3 0.64 

Loop size: 6 ft x 6 ft (1.83 m x 1.83 m) 

Loop number of turns: 3 

Loop wire size: #14 AWG 

Loop lead-in length: 60 in (1.5 m) 

Loop self-resonant frequency: 697.06 kHz 

50 pF residual capacitance in decade box neglected in C value 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-18 Federal Highway Administration 

Table A-2. Comparison of measured and computed inductive-loop parameters of 3-turn 

6- x 6-ft (1.8- x 1.8-m) loop with no vehicle present. 

f0 

(kHz) 

Measured L

(μH) 

Calculated L

(μH) 

Measured Q Calculated Q

20 73.9 74.4 31.7 30.4 

25 73.9 74.4 35.5 33.9 

30 74.1 74.3 40.3 36.6 

35 74.2 74.3 42.7 38.8 

40 74.3 74.3 44.6 40.6 

45 74.5 74.3 45.7 42.2 

50 74.7 74.3 45.5 43.7 

55 74.9 74.3 44.9 44.9 

60 75.3 74.3 44.1 46.1 

Calculated loop parameters are based on: 

• Loop size: 6 ft x 6 ft (1.83 m x 1.83 m) 

• Pavement loop slot width: 375 mils (0.95 cm) 

• Loop slot sealant dielectric constant: 6 

• Pavement material loss tangent: 0.01 

• Loop wire insulation dielectric constant: 2.5 

• Effective loop wire insulation loss tangent: 0.001 

• Loop conductor spacing: 200 mils (0.51 cm) 

• Wire gauge: #14 AWG 

Tables A-3 through A-5 list calculated loop inductance and quality factor as a 

function of conductor size. The quality factor decreases with increasing wire 

gauge as expected. The addition of a transmission line of 240-ft (73.2-m) 

length approximately halves the quality factor. Inductive-loop detector 

applications requiring transmission lines over 200 ft (61 m) in length should 

use number #12 AWG wire for the loop and nonshielded transmission line. 

Three to four turns of loop wire have an adequate quality factor. One to two 

turn loops should be used with a transformer. 

Table A-3. Rectangular-loop parameters. 

Wire 

gauge 

(AWG) 

1-Turn 

inductance 

(μH) 

1-Turn 

quality 

factor 

2-Turn 

inductance 

(μH) 

2-Turn 

quality 

factor 

3-Turn 

inductance 

(μH) 

3-Turn 

quality 

factor 

4-Turn 

inductance 

(μH) 

4-Turn 

quality 

factor 

5-Turn 

inductance 

(μH) 

5 Turn 

quality 

factor 

12 10.13 19.68 35.22 29.88 73.28 37.13 123.14 42.65 184.00 47.03 

14 10.50 15.61 35.96 24.06 74.39 30.40 124.62 35.41 185.85 39.51 

14* 63.45 11.59 89.16 14.11 128.18 17.51 179.61 21.20 242.96 24.86 

14** 351.70 1.77 853.20 4.90 1433.69 9.99 1985.51 17.24 2464.16 26.76 

16 10.85 11.57 36.68 18.10 75.46 23.25 126.04 27.50 187.62 31.09 

18 11.20 8.11 37.37 12.84 76.50 16.73 127.42 20.05 189.39 22.95 

* Transmission line 

** Transformer loop 

Notes: 1. 20 kHz; loop size:6 ft x 6 ft (1.83 m x 1.83 m); other parameters are the same as those given for Table A-2. 

 2. All inductance and quality factors in Table A-3 are apparent values (i.e., the effect of loop capacitance and 

resistance is included). 

 3. Transformer parameters: 

 Primary resistance (ohms) = 1 

 Primary capacitance (picofarads) = 10 

 Primary inductance (millihenrys) = 5 

 Secondary resistance (ohms) = 1 
 Traffic Detector Handbook—2006 

October 2006 Page A-19 Federal Highway Administration 

 Secondary capacitance (picofarads) = 10 

 Primary to secondary turns ratio = 5 

 Core loss resistance (ohms) = 1,000,000 

 Coupling coefficient = 0.99 

 Primary to secondary capacitance (picofarads) = 10 

 4. Transmission line parameters 

 Length = 240 ft (73.2 m) 

 Resistance (milliohms/ft) = 2.5 (6.25 milliohms/m) 

 Inductance (microhenrys/ft) = 0.22 (0,72 microhenrys/m) 

 Conductance (microohms/ft) = 0.000076 (0.000249 microohms/m) 

 Capacitance (picofarads/ft) = 26 (85.3 picofarads/m) 

Table A-4. Quadrupole-loop parameters. 

Wire 

gauge 

(AWG) 

1-Turn 

inductance 

(μH) 

1-Turn 

quality 

factor 

2-Turn 

inductance 

(μH) 

2-Turn 

quality 

factor 

3-Turn 

inductance 

(μH) 

3-Turn 

quality 

factor 

4-Turn 

inductance 

(μH) 

4-Turn 

quality 

factor 

5-Turn 

inductance 

(μH) 

5-Turn 

quality 

factor 

12 17.14 21.72 60.15 32.74 125.42 40.32 210.78 45.93 314.77 50.27 

14 17.69 17.26 61.26 26.53 127.08 33.28 212.98 38.48 317.49 42.64 

16 18.22 12.81 62.32 20.07 128.67 25.67 215.09 30.18 320.10 33.91 

18 18.74 8.99 63.36 14.32 130.22 18.61 217.15 22.21 322.65 25.29 

Notes: 1. 20 kHz; loop size:6 ft x 6 ft (1.83 m x 1.83 m); lateral conductor spacing: 200 mils (0.51 cm), other parameters 

are the same as those given for Table A-2. 

2. All inductance and quality factor values in Table A-4 are apparent values (i.e., the effect of loop capacitance and 

resistance is included). 

Table A-5. Circular-loop parameters. 

Wire 

gauge 

(AWG) 

1-Turn 

inductance 

(μH) 

1-Turn 

quality 

factor 

2-Turn 

inductance 

(μH) 

2-Turn 

quality 

factor 

3-Turn 

inductance 

(μH) 

3-Turn 

quality 

factor 

4-Turn 

inductance 

(μH) 

4-Turn 

quality 

factor 

5-Turn 

inductance 

(μH) 

5-Turn 

quality 

factor 

12 9.70 20.39 33.95 30.95 70.91 38.42 119.50 44.07 179.00 48.53 

14 10.04 16.19 34.63 24.98 71.93 31.55 120.86 36.73 180.69 40.95 

16 10.37 12.00 35.29 18.83 72.91 24.21 122.16 28.63 182.31 32.36 

18 10.68 8.42 35.92 13.38 73.86 17.47 123.43 20.96 183.89 24.00 

Notes: 1. 20 kHz; loop diameter: 7 feet (2.1 m); other parameters are the same as those given for Table A-2. 

2. All inductance and quality factor values in Table A-5 are apparent values (i.e., the effect of loop capacitance and 

resistance is included). 

CONCLUSIONS 

Loop inductance should be measured at 1 kHz to remove effects of 

capacitance when determining the number of turns in a buried loop. All loop 

measurements at frequencies of 20 kHz or greater should be made with a 

balanced instrument since the loop electronics unit is balanced. An 

unbalanced measurement produces incorrect results because of the different 

capacitance-to-ground values. Since the external capacitance is determined 

by the dielectric constant of the slot sealing material, the loop wire should be 

completely sealed to prevent water from entering the loop slot. The large 

dielectric constant of water produces a significant change in the external 

capacitance and causes the apparent loop inductance to change. Thus, 

unstable loop operation can result from incomplete sealing of the loop 

sawcut. 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-20 Federal Highway Administration 

APPENDIX A-1 

LOOP GROUND-RESISTANCE DERIVATION 

The complex impedance ZL of the loop results from a complex permeability μg, 

where μg represents a complex number. The complex impedance is given by 

L gLL Z = j ωμ (A-69) 

( ) g g LL = j ω μ' −jμ" . (A-70) 

The material loss tangent, denoted as tanδg, is given by 

g

g

g '

" tan

μ

μ δ = . (A-71) 

Letting μ'g = 1, the loss tangent becomes 

g g tanδ = μ" . (A-72) 

Then 

( ) L g LL Z = j ω1 − j tan δ (A-73) 

L g L LL Z = ωtan δ L + j ω .

 (A-74) 

When the complex impedance is written in the form 

L g L Z = R + jX , (A-75) 

the real and imaginary parts of ZL become 

R g LL g = ω tan δ (A-76a) 

and 

X L = ωLL (A-76b) 

where Rg is the material ground loss and XL is the inductive reactance. 

APPENDIX A-2 

REAL PART OF COMPLEX BESSEL FUNCTION 

OF FIRST KIND 

( ) ( ) ... 4!

2

1

2!

2

1

1

2

8

2

4

−

⎟

⎠

⎞ ⎜

⎝

⎛

+

⎟

⎠

⎞ ⎜

⎝

⎛

= −

x x

ber x (A-77) 

or 

 ( ) ( )2

4

1 2 !

2

1

1 1

n

x

ber x

n

n

n

⎟

⎠

⎞ ⎜

⎝

⎛

= + ∑ − ∞

=

 (A-78) 
 Traffic Detector Handbook—2006 

October 2006 Page A-21 Federal Highway Administration 

DERIVATIVE OF REAL PART 

... 5!6!

2

1

3!4!

2

1

1!2!

2

1 3 7 11

+

⎟

⎠

⎞ ⎜

⎝

⎛

−

⎟

⎠

⎞ ⎜

⎝

⎛

+

⎟

⎠

⎞ ⎜

⎝

⎛

′ = −

x x x

ber x (A-79) 

or

( )

( )

( ) 2 1 !2 !

2

1

1

4 1

1 n n

x

ber x

n

n

n

−

⎟

⎠

⎞ ⎜

⎝

⎛

′ = ∑ −

−

∞

=

 (A-80) 

IMAGINARY PART OF COMPLEX BESSEL FUNCTION 

OF FIRST KIND 

( ) ( ) ( ) ... 5!

2

1

3!

2

1

1!

2

1

2

10

2

6

2

2

−

⎟

⎠

⎞ ⎜

⎝

⎛

+

⎟

⎠

⎞ ⎜

⎝

⎛

−

⎟

⎠

⎞ ⎜

⎝

⎛

=

x x x

bei x (A-81) 

or

( )

( )

[ ] ( ) 2

4 2

1 2 1 !

2

1

1 −

⎟

⎠

⎞ ⎜

⎝

⎛

= ∑ − −

−

∞

= n

x

bei x

n

n

n

 (A-82) 

DERIVATIVE OF IMAGINARY PART 

... 4!5!

2

1

2!3!

2

1

2

1

5 9

−

⎟

⎠

⎞ ⎜

⎝

⎛

+

⎟

⎠

⎞ ⎜

⎝

⎛

′ = −

x x

bei x x (A-83) 

or

( )

( )

( )( ) ∑ − −

⎟

⎠

⎞ ⎜

⎝

⎛

′ = − − ∞

=

−

1

4 3

2 2 ! 2 1 !

2

1

1 n

n

n

n n

x

bei x (A-84) 

APPENDIX A-3 

COMPLETE ELLIPTIC INTEGRAL OF FIRST KIND 

( ) ∏ ( ) +

π = ∞

= + 0

1 1

2 m

K k km (A-85) 

( ) ( ) 1 ' 1 ' k m+ = 1 − k m / + k m (A-86) 

2 k m' = 1 − k m (A-87) 

k ≡ k 0 (A-88) 

COMPLETE ELLIPTIC INTEGRAL OF SECOND KIND 

( ) ( ) ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

+ × ×

× + × + + −

π = ... 2 4 6

1 3

2 4

1

2

1

2 1

6

2 2 2

2 2

4

2 2

2

2

2

k k k

k

E k (A-89) 

or
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-22 Federal Highway Administration 

( ) ( )

( )

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎥

⎦

⎤ ⎢

⎣

⎡ − + ∑ −

π = ∞

=

N

N

k

N

N

k

E k 2

2

1 (2 )!!

2 3 !! 1

2 1

 (A-90) 

k = (1− k ′)/(1+ k ′) (A-91) 

k' = 1 − m (A-92) 

2 M = k (A-93) 

where 2N!! ≡ 2NN!. 

APPENDIX A-4 

SERIES TO PARALLEL CIRCUIT TRANSFORMATION 

S L

S R

Henrys

Ohms

Ohms

Henrys

PL P R

Figure A-16. Series-circuit to parallel-circuit equivalency (general). 

From Figure A-16, 

in S LS Z = R + jω and

P P

in L j R

Y

ω = − 1 1 (A-94) 

setting 

in

in

Y

Z = 1

,

 (A-95) 

we find 

S

S S

P R

R L R

2 2 2 + ω = (A-96) 

and

S

S S

P L

R L L 2

2 2 2

ω

+ ω =

.

 (A-97) 

PL PL PC CP G LP G PG PC

Figure A-17. Loop-series to parallel-circuit equivalency. 

From Figure A-17, 

GP = GLP + GCP (A-98) 
 Traffic Detector Handbook—2006 

October 2006 Page A-23 Federal Highway Administration 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

ω = + ω −

P

P P

L

in L

Y G j C 1

 (A-99) 

in

L

in Y

Z 1 = . (A-100) 

APPENDIX A-5 

TRANSFORMER MODEL INPUT IMPEDANCE 

The transformed load impedance shown in equation A-68 is derived from 

nodal analysis of the transformer model. The transfer function is defined as 

the ratio of the output of a system to the input. It is thus used to find the 

output of a system or module for a given input. 

in I

T

Ohms

6 Y

2 Y

1 Y 3Y 5 Y

4 Y

1 V

2 V 3V

3I

1I

2I

Figure A-18. Transformer nodal analysis diagram. 

From Figure A-18, we find 

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

+ +

−

−

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

−

+ +

−

⎟

⎟

⎟

⎠

⎞

⎜

⎜

⎜

⎝

⎛

−

−

+ +

=

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

3

2

1

4 5 6

4

6

4

2 3 4

2

6

2

1 2 6

0

0

1

V

V

V

Y Y Y

Y

Y

Y

Y Y Y

Y

Y

Y

Y Y Y

 (A-101) 

1 2 3 I I I I T

in = + + (A-102) 

( ) ( ) Y1V1 Y 6 V1 V3 Y 2 V1 V2 I

T

in = + − + − (A-103) 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+ − ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+ −

= =

1

2

2

1

3

1 6

1

1 1

1

V

V Y

V

V Y Y

I

V Z

in

T

in

 (A-104) 

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

=

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎣

⎡

′ ′ ′

′ ′ ′

′ ′ ′

=

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

=

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡ −

0

0

1

0

0

1

31 32 33

21 22 23

11 12 13

1

31 32 33

21 22 23

11 12 13

3

2

1

Y Y Y

Y Y Y

Y Y Y

Y Y Y

Y Y Y

Y Y Y

V

V

V

 (A-105) 

Y

V Y Y Y V Y 22 33 23 32

1 11' − = = (A-106) 
Appendix A—Inductive-Loop System Equivalent Circuit Model 

October 2006 Page A-24 Federal Highway Administration 

Y

V Y Y Y V Y ( ) ' 21 33 23 31

2 21

− − = = (A-107) 

Y

V Y Y Y V Y 21 32 22 31

3 31' − = = (A-108) 

where 

21 2 Y = −Y (A-109) 

22 2 3 4 Y =Y +Y +Y (A-110) 

23 4 Y = −Y (A-111) 

31 6 Y = −Y (A-112) 

32 4 Y = −Y (A-113) 

33 4 5 6 Y =Y +Y +Y . (A-114) 

REFERENCES 

1. Mills, M.K. “Self Inductance Formulas for Multi-Turn Rectangular 

Loops Used with Vehicle Detectors,” 33rd [Institute of Electrical and 

Electronics Engineers] IEEE VTG Conference Record, IEEE. 1983. 

pp. 64–73. May 1983. 

2. Mills, M.K. “Self Inductance Formulas for Quadrupole Loops Used 

with Vehicle Detectors,” 35th IEEE VTG Conference Record, IEEE. 

1985. pp. 81–87. May 1985. 

3. King, R.W.P. Fundamentals of Electro-Magnetic Theory. Dover 

Publishing, Inc. 1962. pp. 450–456. 

4. Langford-Smith, F. Radiotron Designer’s Handbook. Wireless Press, 

Australia. 1953. pp. 219–221. 

5. Johnson, W.C. Transmission Lines and Networks. McGraw-Hill Book 

Company, Inc. 1950. p. 85. 

6. Palermo, A.J. “Distributed Capacity of Single-Layer Coils,” 

Proceedings of the [Institute of Radio Engineers] IRE, vol. 22, no. 7. 

July 1934. p. 897. 

7. Langford-Smith, F. op. cit., p. 221. 

8. Galejs, Janis. Antennas in Inhomogeneous Media, Pergamon Press. 

1969. pp. 60–64. 

9. Ramo, S., J.R. Whinnery, and T. Van Duzer. Fields and Waves in 

Communication Electronics. John Wiley and Sons, Inc. 1965. p. 336. 

10. Stratton, J.A. Electromagnetic Theory, McGraw-Hill Book Company, 

Inc. 1941. pp. 551–554. 

11. ITT. Reference Data for Radio Engineers, 5th Edition. Howard W. 

Sams and Company, Inc., Indianapolis, IN. 1974. pp. 22–23. 

12. Johnson, W.C. op. cit., p. 78. 
 Traffic Detector Handbook—2006 

October 2006 Page A-25 Federal Highway Administration 

13. Dwight, H.B. Tables of Integrals and Other Mathematical Data. The 

Macmillan Publishers. 1957. p. 184. 

14. Ramo, S., et al., op. cit., p. 311. 

15. Murdock, B.K. Handbook of Electronic Design and Analysis 

Procedure Using Programmable Calculators. Van Nostrand Reinhold 

Company. 1979. p. 475. 

16. Dwight, H.B. op. cit., p. 171. 

17. King, R.W.P. op. cit. 

18. Ramo, S. et al., op. cit., p. 307. 

19. Seely, S. Introduction to Electromagnetic Fields. McGraw Hill Book 

Company. 1958. pp. 185–186. 

20. Burmeister, D. “Traffic Signal Detector Loop Location Design 

Installation.” Illinois Department of Transportation, Division of 

Highways, District 6. November 1, 1988. p. 18. 

21. Jefimenko, O.D. Electricity and Magnetism. 2nd Ed. Electret 

Scientific Co., P. O. Box 4132, Star City, WV. 1989. p. 372. 

22. Grover, F.W. Inductance Calculators: Working Formulas and Tables.

Dover Publications, Inc. 1962. pp. 45–46. 

23. Johnson, W.C. op. cit., p. 88. 

24. Ibid., p. 105. 

25. Dwight, H.B. op. cit., p. 144. 

26. Mills, M.K. “Inductance Loop Detector Analysis,” 31st IEEE VTG 

Conference Record, IEEE. April 1981. p. 404. 

27. Grossner, N.R. Transformers for Electronic Circuits. McGraw-Hill 

Book Company. 1967. pp. 176–177. 

 Traffic Detector Handbook—2006 

October 2006 Page B-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX B. 

APPENDIX B. CURRENT SHEET FORMULA FOR CALCULATION 

OF LOOP INDUCTANCE..............................................................B-1

SAMPLE INDUCTANCE CALCULATION ...........................................................B-2

REFERENCES.........................................................................................................B-3

LIST OF TABLES: APPENDIX B. 

Table B-1. Values of β coefficients for short rectangular solenoids. ............B-1

 Traffic Detector Handbook—2006 

October 2006 Page B-1 Federal Highway Administration 

APPENDIX B. CURRENT SHEET 

FORMULA FOR CALCULATION OF 

LOOP INDUCTANCE 

A formula for calculating the inductance produced by a rectangular current 

sheet is given by(1) 

0.004 ' 2 1 F

b

a a L N ⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛ = π μH (B-1) 

where 

... 1 ' ' ln 5

5

3

3

2

1 1 + β2 γ + β γ − β γ + ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

γ

F = β γ + β γ (B-2) 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⋅ ⋅ ⋅

⋅ ⋅ ⋅ ⎟ =

⎠

⎞ ⎜

⎝

⎛

γ = longer side of rectangle

length of current sheet

a

b

 (B-3) 

N = number of turns and 

a1 = length of shorter side of loop. 

The term, “length of current sheet,” is defined as the axial length of a coil or 

solenoid. For an inductive loop, the length of the current sheet is equivalent 

to the height of the wires in the slot, as described in the sample calculation 

that follows. Since the length of the current sheet is very small compared to 

the longer side of the rectangle, γ is also very small. The factor F’

adjusts for 

the fact that inductive loops perform as very short solenoids (i.e., the loop 

area is much greater than the turn spacing). 

The values of β are obtained from Table B-1. The factor K that appears in 

the table is given by 

⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛ = a

a K 1 (B-4) 

Table B-1. Values of β coefficients for short rectangular solenoids.(2) 

K β1 β1

′ β2 β3 β5 β7

1.00 0.4622 0.6366 0.2122 −0.0046 0.0046 −0.0382 

0.95 0.4574 0.6534 0.2234 −0.0046 0.0053 

0.90 0.4512 0.6720 0.2358 −0.0046 0.0064 −0.0525 

0.85 0.4448 0.6928 0.2496 −0.0042 0.0080 

0.80 0.4364 0.7162 0.2653 −0.0031 0.0103 −0.0831 

0.75 0.4260 0.7427 0.2829 −0.0010 0.0141 

0.70 0.4132 0.7730 0.3032 0.0026 0.0198 −0.1564 

0.65 0.3971 0.8080 0.3265 0.0085 0.0291 

0.60 0.3767 0.8488 0.3537 0.0179 0.0432 −0.3372 

0.55 0.3500 0.8970 0.3858 0.0331 0.0711 

Inductive loops are modeled 

as very short solenoids. 
Appendix B—Current Sheet Formula for Calculation of Loop Inductance 

October 2006 Page B-2 Federal Highway Administration 

Table B-1. Values of β coefficients for short rectangular solenoids (continued). 

K β1 β1

′ β2 β3 β5 β7

0.50 0.3151 0.9549 0.4244 0.0578 0.1183 −0.7855 

0.40 0.1836 1.1141 0.5305 0.1697 0.3898 −2.403 

0.30 0.0314 1.3359 0.7074 0.5433 2.0517 −7.85 

0.20 −0.6409 1.9099 1.0610 2.3230 14.507 15.51 

0.10 −3.2309 3.5014 2.1220 22.5480 497.36 14280.00 

SAMPLE INDUCTANCE CALCULATION 

An example from reference 1 is used to demonstrate the calculation of the 

self inductance for a short, three-turn, 6- by 6-foot (1.8- by 1.8-m) loop, using 

the current sheet model described above. 

The following parameters are used: 

a1 = 6 ft = 182.88 cm 

a = 6 ft = 182.88 cm 

N = 3 turns 

P = turn spacing = 150 mils = 0.381 cm 

b = N P = (3)(0.381) = 1.143 cm 

γ = ⎟

⎠

⎞ ⎜

⎝

⎛

a

b = 1.143/182.88 = 0.00625 cm 

K = ⎟

⎠

⎞ ⎜

⎝

⎛

a

a1 = 1. 

From Table B-1, 

β1 = 0.4622 

β1

′

 = 0.6366 

β2 = 0.2122 

β 3 = -0.0046 

β5 = 0.0046 

β7 = -0.0382. 

Solving Equation B-2 for F'

 yields 

... 1 ' ' ln 5

5

3

3

2

1 1 + β2 γ + β γ −β γ + ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

γ

F = β γ + β γ (B-5)

' 0.0029 0.0200 8.16 10 0.0229 6 = + + × = − F (B-6) 

where 

5.0754 1 ln =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

γ . (B-7) 
 Traffic Detector Handbook—2006 

October 2006 Page B-3 Federal Highway Administration 

Using Equation B-1, the inductance of the loop is found as 

0.004 ' 2 1 F

b

a a L N ⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛ = π μH (B-8)

or 

 L = 0.004π(32

)(182.88*182.88/1.143)(0.0229) = 75.8 μH. (B-9)

REFERENCES 

1. Grover, F.W. Inductance Calculations. Dover Publications, Inc. 1962. 

2. Y. Niwa, “A Study of Coils Wound on Rectangular Frames with 

Special Reference to the Calculation of Inductance,” Research of the 

Electrotechnical Laboratory, No. 141, Tokyo, Japan, 1924. 

 Traffic Detector Handbook—2006 

October 2006 Page C-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX C. 

APPENDIX C. LOOP INDUCTANCE AND QUALITY FACTOR TABLES ......C-1

ABSTRACT...............................................................................................................C-1

LIST OF TABLES: APPENDIX C. 

Table C-1. Rectangular-loop inductance in μH at f = 1 kHz.........................C-2

Table C-2. Rectangular-loop quality factor at f = 1 kHz...............................C-3

Table C-3. Rectangular-loop inductance in μH at f = 20 kHz.......................C-4

Table C-4. Rectangular-loop quality factor at f = 20 kHz.............................C-5

Table C-5. Rectangular-loop inductance in μH at f = 40 kHz.......................C-6

Table C-6. Rectangular-loop quality factor at f = 40 kHz.............................C-7

Table C-7. Rectangular-loop inductance in μH at f = 60 kHz.......................C-8

Table C-8. Rectangular-loop quality factor at f = 60 kHz.............................C-9

Table C-9. Quadrupole-loop inductance in μH at f = 1 kHz........................C-10

Table C-10. Quadrupole-loop quality factor at f = 1 kHz............................C-11

Table C-11. Quadrupole-loop inductance in μH at f = 20 kHz. ..................C-12

Table C-12. Quadrupole-loop quality factor at f = 20 kHz..........................C-13

Table C-13. Quadrupole-loop inductance in μH at f = 40 kHz. ..................C-14

Table C-14. Quadrupole-loop quality factor at f = 40 kHz..........................C-15

Table C-15. Quadrupole-loop inductance in μH at f = 60 kHz. ..................C-16

Table C-16. Quadrupole-loop quality factor at f = 60 kHz..........................C-17

Table C-17. Circular-loop inductance in μH at f = 1 kHz.. .........................C-18

Table C-18. Circular-loop quality factor at f = 1 kHz..................................C-18

Table C-19. Circular-loop inductance in μH at f = 20 kHz. ........................C-18

Table C-20. Circular-loop quality factor at f = 20 kHz................................C-18

Table C-21. Circular-loop inductance in μH at f = 40 kHz. ........................C-19

Table C-22. Circular-loop quality factor at f = 40 kHz................................C-19

Table C-23. Circular-loop inductance in μH at f = 60 kHz. ........................C-19

Table C-24. Circular-loop quality factor at f = 60 kHz................................C-19

 Traffic Detector Handbook—2006 

October 2006 Page C-1 Federal Highway Administration 

APPENDIX C. LOOP INDUCTANCE 

AND QUALITY FACTOR TABLES 

ABSTRACT 

Loop inductance and quality factor values calculated for rectangular, 

quadrupole, and circular loops at excitation frequencies of 1, 20, 40, and 60 

kHz are given in these tables. The frequency f at which the values apply is 

shown as part of the table title. The appropriate length or diameter of the 

loop is given in the first column, while the number of turns is stated in the 

first row. 

Rectangular and quadrupole loop inductance and quality factor values for 6-

ft (1.8-m) wide loops are displayed for 6–20-ft (1.8–6.1-m) loop lengths in 1-ft 

(0.3-m) increments and for 20–50-ft (6.1–15.2-m) loop lengths in 5-ft (1.5-m) 

increments. Circular loop inductance and quality factor values are listed for 

loop diameters of 4, 5, 6, 7, and 8 feet (1.2, 1.5, 1.8, 2.1, and 2.4 m, 

respectively). All tables show inductance and quality factor values for 1 

through 7 turns of wire. 

The loop inductance shown in these tables is the apparent loop inductance 

value expressed in μH that includes loop resistance and capacitance effects. It 

thus represents the value which the user will actually measure when an 

appropriate measuring instrument is used. 

Detector loops are in fact very 

short solenoids. 

Inductance values shown in 

the tables are apparent or 

measured values that include 

loop resistance and 

capacitance effects. 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-2 Federal Highway Administration 

Table C-1. Rectangular-loop inductance in μH at f = 1 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 10.53 36.02 74.46 124.67 185.81 257.24 338.44 

7 (2.1) 11.52 39.49 81.72 136.94 204.23 282.91 372.40 

8 (2.4) 12.51 42.94 88.94 149.12 222.52 308.38 406.08 

9 (2.7) 13.50 46.38 96.12 161.25 240.71 333.71 439.58 

10 (3.0) 14.48 49.80 103.28 173.32 258.83 358.94 472.94 

11 (3.3) 15.46 53.22 110.42 185.37 276.90 384.09 506.20 

12 (3.7) 16.44 56.63 117.54 197.39 294.93 409.19 539.38 

13 (3.9) 17.42 60.04 124.65 209.39 312.93 434.25 572.51 

14 (4.3) 18.40 63.44 131.76 221.38 330.91 459.27 605.59 

15 (4.6) 19.37 66.84 138.85 233.35 348.87 484.27 638.63 

16 (4.9) 20.35 70.23 145.95 245.32 366.81 509.24 671.64 

17 (5.2) 21.32 73.63 153.03 257.27 384.74 534.09 704.63 

18 (5.5) 22.30 77.02 160.12 269.22 402.66 559.13 737.59 

19 (5.8) 23.27 80.41 167.20 281.16 420.57 584.06 770.54 

20 (6.1) 24.25 83.80 174.27 293.10 438.47 608.97 803.47 

25 (7.6) 29.12 100.74 209.63 352.73 527.89 733.42 967.96 

30 (9.1) 33.98 117.66 244.95 412.31 617.22 857.75 1132.29 

35 (10.7) 38.84 134.57 280.25 471.85 706.51 982.00 1296.52 

40 (12.2) 43.71 151.48 315.55 531.38 795.76 1106.21 1460.70 

45 (13.7) 48.57 168.38 350.83 590.89 885.00 1230.40 1624.85 

50 (15.2) 53.43 185.28 386.11 650.39 974.22 1354.56 1788.98 
 Traffic Detector Handbook—2006 

October 2006 Page C-3 Federal Highway Administration 

Table C-2. Rectangular-loop quality factor at f = 1 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 1.07 1.81 2.48 3.09 3.67 4.21 4.72 

7 (2.1) 1.08 1.83 2.51 3.14 3.72 4.27 4.79 

8 (2.4) 1.09 1.85 2.54 3.17 3.76 4.32 4.85 

9 (2.7) 1.09 1.87 2.56 3.20 3.80 4.36 4.90 

10 (3.0) 1.10 1.88 2.58 3.22 3.83 4.40 4.94 

11 (3.3) 1.11 1.89 2.59 3.24 3.85 4.43 4.97 

12 (3.7) 1.11 1.90 2.61 3.26 3.87 4.45 5.00 

13 (3.9) 1.11 1.91 2.62 3.28 3.89 4.47 5.03 

14 (4.3) 1.12 1.91 2.63 3.29 3.91 4.49 5.05 

15 (4.6) 1.12 1.92 2.64 3.30 3.92 4.51 5.07 

16 (4.9) 1.12 1.92 2.65 3.31 3.94 4.53 5.09 

17 (5.2) 1.13 1.93 2.65 3.32 3.95 4.54 5.11 

18 (5.5) 1.13 1.93 2.66 3.33 3.96 4.56 5.12 

19 (5.8) 1.13 1.94 2.67 3.34 3.97 4.57 5.14 

20 (6.1) 1.13 1.94 2.67 3.35 3.98 4.58 5.15 

25 (7.6) 1.14 1.96 2.70 3.38 4.02 4.62 5.20 

30 (9.1) 1.15 1.97 2.71 3.40 4.05 4.66 5.23 

35 (10.7) 1.15 1.98 2.73 3.42 4.07 4.68 5.26 

40 (12.2) 1.15 1.98 2.73 3.43 4.08 4.70 5.28 

45 (13.7) 1.16 1.99 2.74 3.44 4.09 4.71 5.30 

50 (15.2) 1.16 1.99 2.75 3.45 4.10 4.72 5.31 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-4 Federal Highway Administration 

Table C-3. Rectangular-loop inductance in μH at f = 20 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 10.50 35.96 74.39 124.62 185.85 257.48 339.07 

7 (2.1) 11.49 39.43 81.65 136.90 204.32 283.27 373.23 

8 (2.4) 12.48 42.88 88.87 149.10 222.66 308.88 407.26 

9 (2.7) 13.46 46.31 96.05 161.24 240.92 334.37 441.10 

10 (3.0) 14.44 49.73 103.21 173.34 259.12 359.80 474.86 

11 (3.3) 15.42 53.15 110.35 185.42 277.28 385.17 508.58 

12 (3.7) 16.40 56.56 117.48 197.47 295.41 410.53 542.29 

13 (3.9) 17.37 59.96 124.60 209.51 313.54 435.87 576.01 

14 (4.3) 18.35 63.36 131.71 221.54 331.65 461.22 609.76 

15 (4.6) 19.32 66.75 138.82 233.57 349.76 486.58 643.54 

16 (4.9) 20.30 70.15 145.92 245.59 367.87 511.95 677.38 

17 (5.2) 21.27 73.54 153.02 257.61 385.99 537.35 711.28 

18 (5.5) 22.24 76.93 160.12 269.62 404.11 562.78 745.24 

19 (5.8) 23.21 80.32 167.21 281.65 422.25 588.24 779.28 

20 (6.1) 24.19 83.71 174.31 293.67 440.40 613.73 813.41 

25 (7.6) 29.04 100.65 209.79 353.85 531.41 741.89 985.51 

30 (9.1) 33.90 117.58 245.31 414.22 622.99 871.46 1160.59 

35 (10.7) 38.75 134.52 280.89 474.83 715.29 1002.77 1339.33 

40 (12.2) 43.60 151.47 316.54 535.75 808.46 1136.14 1522.43 

45 (13.7) 48.45 168.43 352.28 597.01 902.63 1271.89 1710.61 

50 (15.2) 53.30 185.41 388.13 658.67 997.93 1410.37 1904.64 
 Traffic Detector Handbook—2006 

October 2006 Page C-5 Federal Highway Administration 

Table C-4. Rectangular-loop quality factor at f = 20 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 15.61 24.06 30.40 35.41 39.51 42.95 45.87 

7 (2.1) 15.75 24.28 30.67 35.73 39.85 43.30 46.24 

8 (2.4) 15.86 24.46 30.90 35.98 40.12 43.59 46.52 

9 (2.7) 15.95 24.61 31.08 36.18 40.35 43.81 46.75 

10 (3.0) 16.03 24.73 31.23 36.36 40.53 44.00 46.93 

11 (3.3) 16.10 24.84 31.36 36.50 40.68 44.16 47.08 

12 (3.7) 16.15 24.93 31.48 36.63 40.82 44.29 47.21 

13 (3.9) 16.20 25.01 31.58 36.74 40.93 44.40 47.31 

14 (4.3) 16.25 25.08 31.66 36.83 41.03 44.49 47.39 

15 (4.6) 16.29 25.14 31.74 36.92 41.11 44.57 47.46 

16 (4.9) 16.32 25.20 31.81 36.99 41.18 44.64 47.51 

17 (5.2) 16.36 25.25 31.87 37.06 41.25 44.69 47.55 

18 (5.5) 16.39 25.30 31.93 37.12 41.30 44.74 47.59 

19 (5.8) 16.41 25.34 31.98 37.17 41.35 44.78 47.61 

20 (6.1) 16.44 25.38 32.02 37.22 41.39 44.81 47.62 

25 (7.6) 16.54 25.53 32.20 37.39 41.53 44.88 47.59 

30 (9.1) 16.60 25.63 32.31 37.48 41.58 44.85 47.44 

35 (10.7) 16.65 25.71 32.38 37.53 41.57 44.74 47.18 

40 (12.2) 16.69 25.76 32.43 37.54 41.51 44.57 46.84 

45 (13.7) 16.72 25.80 32.46 37.53 41.42 44.34 46.43 

50 (15.2) 16.75 25.83 32.47 37.49 41.29 44.07 45.95 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-6 Federal Highway Administration 

Table C-5. Rectangular-loop inductance in μH at f = 40 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 10.44 35.86 74.28 124.61 186.13 258.43 341.24 

7 (2.1) 11.43 39.33 81.55 136.94 204.76 284.58 376.22 

8 (2.4) 12.41 42.77 88.78 149.19 223.28 310.63 411.12 

9 (2.7) 13.39 46.20 95.97 161.40 241.75 336.65 446.04 

10 (3.0) 14.37 49.62 103.14 173.59 260.21 362.21 481.07 

11 (3.3) 15.34 53.03 110.30 185.76 278.66 388.76 516.24 

12 (3.7) 16.32 56.43 117.45 197.92 297.14 414.92 551.61 

13 (3.9) 17.29 59.83 124.60 210.09 315.64 441.17 587.21 

14 (4.3) 18.26 63.23 131.74 222.27 334.18 467.54 623.08 

15 (4.6) 19.23 66.62 138.88 234.46 352.77 494.04 659.24 

16 (4.9) 20.20 70.02 146.03 246.66 371.42 520.68 695.72 

17 (5.2) 21.17 73.41 153.18 258.89 390.13 547.49 732.56 

18 (5.5) 22.14 76.81 160.33 271.13 408.91 574.47 769.77 

19 (5.8) 23.11 80.20 167.48 283.40 427.76 601.63 807.38 

20 (6.1) 24.07 83.59 174.64 295.69 446.70 628.99 845.42 

25 (7.6) 28.91 100.57 210.56 357.63 542.74 769.19 1043.04 

30 (9.1) 33.75 117.57 246.71 420.50 641.54 916.19 1255.80 

35 (10.7) 38.58 134.62 283.16 484.52 743.73 1071.72 1488.10 

40 (12.2) 43.42 151.72 319.96 549.90 849.98 1237.75 1745.46 

45 (13.7) 48.25 168.88 357.16 616.85 961.04 1416.63 2035.03 

50 (15.2) 53.09 186.12 394.82 685.62 1077.75 1611.21 2366.36 
 Traffic Detector Handbook—2006 

October 2006 Page C-7 Federal Highway Administration 

Table C-6. Rectangular-loop quality factor at f = 40 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 22.45 33.19 40.64 46.18 50.48 53.89 56.63 

7 (2.1) 22.63 33.46 40.95 46.51 50.80 54.19 56.89 

8 (2.4) 22.78 33.67 41.19 46.76 51.05 54.41 57.07 

9 (2.7) 22.90 33.85 41.39 46.97 51.24 54.57 57.18 

10 (3.0) 23.01 33.99 41.56 47.13 51.38 54.68 57.24 

11 (3.3) 23.09 34.12 41.70 47.27 51.50 53.76 57.26 

12 (3.7) 23.17 34.23 41.82 47.38 51.59 54.80 57.24 

13 (3.9) 23.24 34.32 41.92 47.47 51.65 54.82 57.19 

14 (4.3) 23.30 34.41 42.01 47.55 51.70 54.82 57.12 

15 (4.6) 23.35 34.48 42.08 47.61 51.73 54.80 57.02 

16 (4.9) 23.39 34.54 42.15 47.66 51.75 54.76 56.90 

17 (5.2) 23.44 34.60 42.21 47.70 51.75 54.71 56.77 

18 (5.5) 23.48 34.65 42.25 47.73 51.74 54.64 56.61 

19 (5.8) 23.51 34.70 42.30 47.75 51.72 54.56 56.44 

20 (6.1) 23.54 34.74 42.33 47.76 51.69 54.46 56.25 

25 (7.6) 23.67 34.90 42.45 47.75 51.44 53.83 55.08 

30 (9.1) 23.76 35.00 42.48 47.61 51.01 52.96 53.59 

35 (10.7) 23.82 35.05 42.45 47.39 50.46 51.89 51.80 

40 (12.2) 23.87 35.09 42.28 47.10 49.78 50.64 49.74 

45 (13.7) 23.91 35.10 42.27 46.74 49.00 49.21 47.41 

50 (15.2) 23.94 35.09 42.14 46.33 48.11 47.61 44.82 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-8 Federal Highway Administration 

Table C-7. Rectangular-loop inductance in μH at f = 60 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 10.40 35.79 74.25 124.77 186.85 260.30 345.26 

7 (2.1) 11.38 39.25 81.53 137.19 205.75 287.11 381.59 

8 (2.4) 12.36 42.69 88.78 149.56 224.60 313.94 418.12 

9 (2.7) 13.34 46.12 96.00 161.90 243.46 340.88 454.97 

10 (3.0) 14.31 49.54 103.21 174.24 262.36 367.98 492.24 

11 (3.3) 15.28 52.95 110.41 186.59 281.34 395.31 530.03 

12 (3.7) 16.25 56.36 117.61 198.96 300.41 422.89 568.41 

13 (3.9) 17.22 59.76 124.81 211.37 319.59 450.76 607.46 

14 (4.3) 18.19 63.16 132.02 223.81 338.89 478.96 647.24 

15 (4.6) 19.16 66.57 139.24 236.29 358.34 507.52 687.83 

16 (4.9) 20.12 69.97 146.47 248.83 377.94 536.47 729.31 

17 (5.2) 21.09 73.37 153.70 261.41 397.71 565.85 771.74 

18 (5.5) 22.05 76.78 160.96 274.06 417.66 595.69 815.22 

19 (5.8) 23.02 80.18 168.23 286.77 437.80 626.02 859.83 

20 (6.1) 23.98 83.59 175.51 299.55 458.15 656.87 905.65 

25 (7.6) 28.81 100.67 212.62 364.62 563.43 820.32 1156.67 

30 (9.1) 33.63 117.83 249.52 432.01 675.89 1003.04 1456.51 

35 (10.7) 38.45 135.10 287.53 502.29 797.51 1211.88 1828.33 

40 (12.2) 43.28 152.49 326.39 576.05 930.74 1456.41 2312.27 

45 (13.7) 48.11 170.04 366.24 653.98 1078.66 1750.56 2979.87 

50 (15.2) 52.95 187.75 407.21 736.88 1245.26 2115.67 3977.12 
 Traffic Detector Handbook—2006 

October 2006 Page C-9 Federal Highway Administration 

Table C-8. Rectangular-loop quality factor at f = 60 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 26.56 38.32 46.10 51.65 55.77 58.86 61.17 

7 (2.1) 26.77 38.60 46.40 51.94 56.02 59.04 61.24 

8 (2.4) 26.93 38.82 46.64 52.16 56.19 59.13 61.21 

9 (2.7) 27.07 39.01 46.83 52.33 56.30 59.15 61.10 

10 (3.0) 27.18 39.16 46.98 52.46 56.37 59.12 60.93 

11 (3.3) 27.28 39.29 47.11 52.55 56.39 59.04 60.71 

12 (3.7) 27.37 39.40 47.21 52.61 56.39 58.93 60.44 

13 (3.9) 27.44 39.49 47.30 52.66 56.36 58.78 60.12 

14 (4.3) 27.51 39.58 47.37 52.68 56.30 58.60 59.77 

15 (4.6) 27.56 39.65 47.42 52.69 56.22 58.39 59.37 

16 (4.9) 27.62 39.71 47.46 52.68 56.12 58.15 58.94 

17 (5.2) 27.66 39.76 47.50 52.66 56.01 57.89 58.48 

18 (5.5) 27.71 39.81 47.52 52.63 55.88 57.61 57.99 

19 (5.8) 27.74 39.85 47.54 52.59 55.73 57.30 57.46 

20 (6.1) 27.78 39.89 47.55 52.53 55.57 56.98 56.91 

25 (7.6) 27.92 40.01 47.51 52.14 54.56 55.05 53.70 

30 (9.1) 28.01 40.06 47.36 51.56 53.26 52.67 49.83 

35 (10.7) 28.08 40.07 47.12 50.82 51.69 49.87 45.34 

40 (12.2) 28.13 40.03 46.81 49.95 49.88 46.68 40.25 

45 (13.7) 28.16 39.97 46.43 48.94 47.84 43.10 34.59 

50 (15.2) 28.19 39.87 45.99 47.81 45.56 39.15 28.35 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-10 Federal Highway Administration 

Table C-9. Quadrupole-loop inductance in μH at f = 1 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 17.73 61.33 127.13 212.86 316.90 438.00 575.12 

7 (2.1) 20.06 69.53 144.29 241.80 360.23 498.17 654.45 

8 (2.4) 22.38 77.72 161.44 270.73 403.55 558.32 733.75 

9 (2.7) 24.70 85.91 178.59 299.65 446.85 618.46 813.03 

10 (3.0) 27.02 94.09 195.74 328.57 490.15 678.59 892.31 

11 (3.3) 29.34 102.28 212.88 357.49 533.45 738.71 971.58 

12 (3.7) 31.67 110.47 230.03 386.41 576.75 798.84 1050.85 

13 (3.9) 33.99 118.65 247.18 415.33 620.05 858.96 1130.11 

14 (4.3) 36.31 126.84 264.32 444.24 663.34 919.08 1209.38 

15 (4.6) 38.63 135.03 281.47 473.16 706.64 979.20 1288.65 

16 (4.9) 40.95 143.21 298.61 502.08 749.93 1039.33 1367.91 

17 (5.2) 43.27 151.40 315.76 530.99 793.23 1099.45 1447.18 

18 (5.5) 45.60 159.59 332.91 559.91 836.52 1159.57 1526.45 

19 (5.8) 47.92 167.77 350.05 588.83 879.82 1219.70 1605.73 

20 (6.1) 50.24 175.96 367.20 617.74 923.12 1279.82 1685.00 

25 (7.6) 61.85 216.89 452.92 762.33 1139.61 1580.47 2081.42 

30 (9.1) 73.45 257.83 538.65 906.92 1356.11 1881.15 2477.90 

35 (10.7) 85.06 298.76 624.38 1051.52 1572.63 2181.88 2874.48 

40 (12.2) 96.67 339.69 710.11 1196.12 1789.18 2482.66 3271.16 

45 (13.7) 108.28 380.63 795.85 1340.73 2005.75 2783.49 3667.95 

50 (15.2) 119.89 421.56 881.59 1485.36 2222.35 3084.39 4064.88 
 Traffic Detector Handbook—2006 

October 2006 Page C-11 Federal Highway Administration 

Table C-10. Quadrupole-loop quality factor at f = 1 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 1.20 2.06 2.82 3.52 4.16 4.77 5.33 

7 (2.1) 1.22 2.10 2.88 3.59 4.25 4.87 5.45 

8 (2.4) 1.24 2.13 2.93 3.66 4.33 4.96 5.55 

9 (2.7) 1.25 2.16 2.97 3.71 4.39 5.03 5.63 

10 (3.0) 1.26 2.18 3.00 3.75 4.44 5.09 5.70 

11 (3.3) 1.28 2.20 3.03 3.79 4.49 5.14 5.76 

12 (3.7) 1.28 2.22 3.05 3.82 4.52 5.19 5.81 

13 (3.9) 1.29 2.23 3.07 3.84 4.56 5.22 5.85 

14 (4.3) 1.30 2.25 3.09 3.87 4.59 5.26 5.89 

15 (4.6) 1.30 2.26 3.11 3.89 4.61 5.29 5.93 

16 (4.9) 1.31 2.27 3.13 3.91 4.46 5.32 5.96 

17 (5.2) 1.31 2.28 3.14 3.93 4.66 5.34 5.98 

18 (5.5) 1.32 2.29 3.15 3.94 4.68 5.36 6.01 

19 (5.8) 1.32 2.29 3.16 3.96 4.69 5.38 6.03 

20 (6.1) 1.33 2.30 3.17 3.97 4.71 5.40 6.05 

25 (7.6) 1.34 2.33 3.21 4.02 4.77 5.47 6.13 

35 (10.7) 1.35 2.35 3.24 4.06 4.81 5.52 6.19 

40 (12.2) 1.36 2.36 3.26 4.08 4.84 5.56 6.23 

40 (10.7) 1.36 2.37 3.28 4.10 4.87 5.59 6.26 

45 (13.7) 1.37 2.38 3.29 4.12 4.89 5.61 6.29 

50 (15.2) 1.37 2.39 3.30 4.13 4.90 5.63 6.31 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-12 Federal Highway Administration 

Table C-11. Quadrupole-loop inductance in μH at f = 20 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 17.69 61.26 127.08 212.98 317.49 439.56 578.46 

7 (2.1) 20.01 69.45 144.26 242.03 361.15 500.51 659.37 

8 (2.4) 22.32 77.64 161.44 271.10 404.88 561.64 740.66 

9 (2.7) 24.64 85.82 178.63 300.21 448.71 622.98 822.40 

10 (3.0) 26.96 94.01 195.83 329.35 492.64 684.58 904.66 

11 (3.3) 29.28 102.20 213.05 358.53 536.70 746.45 987.48 

12 (3.7) 31.59 110.40 230.27 387.77 580.88 808.63 1070.92 

13 (3.9) 33.91 118.59 247.51 417.05 625.20 871.14 1155.04 

14 (4.3) 36.23 126.79 264.76 446.39 669.68 934.00 1239.90 

15 (4.6) 38.55 134.99 282.03 475.79 714.33 997.25 1325.55 

16 (4.9) 40.86 143.19 299.32 505.25 759.15 1060.91 1412.05 

17 (5.2) 43.18 151.40 316.62 534.78 804.16 1125.01 1499.47 

18 (5.5) 45.50 159.61 333.95 564.38 849.37 1189.58 1587.86 

19 (5.8) 47.82 167.82 351.29 594.05 894.79 1254.64 1677.30 

20 (6.1) 50.14 176.03 368.66 623.81 940.43 1320.24 1767.86 

25 (7.6) 61.73 217.17 455.85 773.93 1172.42 1657.20 2239.86 

30 (9.1) 73.32 258.42 543.75 926.67 1411.89 2012.31 2751.56 

35 (10.7) 84.92 299.81 632.50 1082.59 1660.59 2390.42 3315.54 

40 (12.2) 96.53 341.36 722.23 1242.29 1920.49 2797.33 3948.06 

45 (13.7) 108.14 383.08 813.10 1406.39 2193.80 3240.10 4670.95 

50 (15.2) 119.75 425.01 905.26 1575.59 2483.09 3727.59 5514.46 
 Traffic Detector Handbook—2006 

October 2006 Page C-13 Federal Highway Administration 

Table C-12. Quadrupole-loop quality factor at f = 20 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 17.26 26.53 33.28 38.48 42.64 46.05 48.87 

7 (2.1) 17.51 26.91 33.74 38.98 43.16 46.56 49.37 

8 (2.4) 17.71 27.22 34.10 39.38 43.57 46.96 49.73 

9 (2.7) 17.88 27.47 34.40 39.70 43.89 47.26 50.00 

10 (3.0) 18.02 27.69 34.65 39.97 44.15 47.50 50.20 

11 (3.3) 18.14 27.87 34.87 40.18 44.36 47.68 50.33 

12 (3.7) 18.25 28.02 35.04 40.37 44.52 47.81 50.42 

13 (3.9) 18.34 28.16 35.20 40.52 44.66 47.91 50.46 

14 (4.3) 18.42 28.28 35.33 40.65 44.77 47.98 50.47 

15 (4.6) 18.49 28.38 35.45 40.76 44.85 48.02 50.44 

16 (4.9) 18.55 28.47 35.55 40.85 44.92 48.04 50.39 

17 (5.2) 18.61 28.55 35.63 40.93 44.96 48.03 50.31 

18 (5.5) 18.66 28.63 35.71 40.99 45.00 48.01 50.21 

19 (5.8) 18.71 28.69 35.78 41.04 45.01 47.97 50.08 

20 (6.1) 18.75 28.75 35.84 41.09 45.02 47.92 49.94 

25 (7.6) 18.91 28.98 36.04 41.18 44.89 47.44 48.95 

30 (9.1 ) 19.03 29.12 36.14 41.12 44.56 46.68 47.58 

35 (10.7) 19.11 29.21 36.15 40.96 44.07 45.68 45.88 

40 (12.2) 19.17 29.27 36.12 40.71 43.44 44.48 43.86 

45 (13.7) 19.22 29.30 36.04 40.38 42.69 43.08 41.56 

50 (15.2) 19.26 29.31 35.92 39.99 41.82 41.49 38.97 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-14 Federal Highway Administration 

Table C-13. Quadrupole-loop inductance in μH at f = 40 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 17.61 61.14 127.09 213.56 319.54 444.65 589.12 

7 (2.1) 19.92 69.33 144.35 242.97 364.23 508.03 675.03 

8 (2.4) 22.23 77.52 161.64 272.50 409.28 572.25 762.72 

9 (2.7) 24.54 85.72 178.97 302.18 454.75 637.45 852.47 

10 (3.0) 26.85 93.92 196.35 332.02 500.67 703.75 944.54 

11 (3.3) 29.16 102.14 213.77 362.04 547.11 771.28 1039.25 

12 (3.7) 31.47 110.36 231.25 392.26 594.11 840.17 1136.92 

13 (3.9) 33.78 118.59 248.78 422.70 641.74 910.56 1237.90 

14 (4.3) 36.09 126.83 266.38 453.37 690.03 982.62 1342.58 

15 (4.6) 38.40 135.08 284.04 484.29 739.06 1056.49 1451.40 

16 (4.9) 40.71 143.34 301.78 515.49 788.88 1132.37 1564.81 

17 (5.2) 43.02 151.61 319.59 546.97 839.56 1210.42 1683.35 

18 (5.5) 45.33 159.90 337.48 578.76 891.16 1290.86 1807.62 

19 (5.8) 47.64 168.20 355.46 610.89 943.75 1373.91 1938.27 

20 (6.1) 49.96 176.51 373.53 643.36 997.41 1459.80 2076.06 

25 (7.6) 62.53 218.33 465.40 811.78 1284.60 1942.02 2907.06 

30 (9.1) 73.11 260.61 560.30 992.54 1612.56 2548.33 4123.51 

35 (10.7) 84.72 303.46 658.95 1189.34 1998.78 3357.78 6162.33 

40 (12.2) 96.40 346.97 762.14 1406.81 2466.77 4524.99 10461.3 

45 (13.7) 107.99 391.24 870.79 1650.98 3058.41 6402.00 26153.2 

50 (15.2) 119.66 436.39 985.93 1929.88 3841.27 10005.70 68070.5 
 Traffic Detector Handbook—2006 

October 2006 Page C-15 Federal Highway Administration 

Table C-14. Quadrupole-loop quality factor at f = 40 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 24.62 36.14 43.82 49.33 53.42 56.48 58.74 

7 (2.1) 24.95 36.58 44.31 49.79 53.81 56.76 58.85 

8 (2.4) 25.21 36.94 44.68 50.14 54.08 56.89 58.80 

9 (2.7) 25.43 37.23 44.98 50.40 54.24 56.91 58.61 

10 (3.0) 25.61 37.47 45.22 50.58 54.33 56.83 58.31 

11 (3.3) 25.77 37.67 45.41 50.72 54.34 56.68 57.90 

12 (3.7) 25.90 37.84 45.57 50.80 54.31 56.45 57.41 

13 (3.9) 26.02 37.99 45.69 50.85 54.22 56.16 56.84 

14 (4.3) 26.12 38.11 45.78 50.86 54.09 55.81 56.18 

15 (4.6) 26.21 38.22 45.86 50.85 53.93 55.41 55.46 

16 (4.9) 26.29 38.31 45.91 50.81 53.72 54.96 54.67 

17 (5.2) 26.36 38.39 45.95 50.75 53.49 54.47 53.82 

18 (5.5) 26.42 38.46 45.97 50.66 53.22 53.93 52.90 

19 (5.8) 26.48 38.52 45.98 50.56 52.92 53.34 51.92 

20 (6.1) 26.54 38.57 45.98 50.44 52.60 52.72 50.88 

25 (7.6) 26.74 38.72 45.81 49.59 50.61 49.02 44.85 

30 (9.1) 26.88 38.75 45.44 48.40 48.04 44.43 37.50 

35 (10.7) 26.97 38.70 44.91 46.91 44.96 39.01 28.93 

40 (12.2) 27.04 38.59 44.25 45.14 41.38 32.80 19.22 

45 (13.7) 27.08 38.43 43.46 43.11 37.11 25.85 8.45 

50 (15.2) 27.11 38.22 42.55 40.83 32.83 18.19 -3.31 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-16 Federal Highway Administration 

Table C-15. Quadrupole-loop inductance in μH at f = 60 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 17.55 61.08 127.31 214.81 323.36 453.84 698.32 

7 (2.1) 19.85 69.28 144.73 244.86 369.89 521.58 703.48 

8 (2.4) 22.15 77.50 162.23 275.21 417.28 591.44 803.28 

9 (2.7) 24.46 85.73 179.82 305.90 465.67 663.76 908.57 

10 (3.0) 26.76 93.97 197.52 336.98 515.20 738.91 1020.35 

11 (3.3) 29.06 102.23 215.32 368.49 566.00 817.32 1139.78 

12 (3.7) 31.37 110.52 233.25 400.48 618.23 899.44 1268.22 

13 (3.9) 33.68 118.82 251.32 432.99 672.04 985.81 1407.32 

14 (4.3) 35.98 127.15 269.53 466.07 727.63 1077.63 1559.08 

15 (4.6) 38.29 135.50 287.90 499.78 785.18 1173.78 1725.94 

16 (4.9) 40.60 143.88 306.43 534.17 844.91 1276.87 1910.97 

17 (5.2) 42.90 152.28 325.15 569.29 907.05 1387.23 2118.05 

18 (5.5) 45.21 160.72 344.06 605.20 971.87 1505.96 2352.17 

19 (5.8) 47.53 169.18 363.18 641.98 1039.65 1634.37 2619.87 

20 (6.1) 49.84 177.68 382.51 679.68 1110.73 1774.02 2929.90 

25 (7.6) 61.41 220.74 482.97 884.97 1530.25 2725.24 5781.94 

30 (9.1) 73.02 264.88 591.16 1127.49 2116.11 4590.35 24158.9 

35 (10.7) 84.67 310.38 709.46 1425.35 3027.21 10214.52 −14095.7 

40 (12.2) 96.36 357.47 840.85 1808.24 4699.56 −77203.2 −5948.3 

45 (13.7) 108.11 406.48 989.27 2329.10 8926.13 −10112.4 −3900.7 

50 (15.2) 119.91 457.72 1160.02 3093.03 40376.3 −5506.1 −2956.9
 Traffic Detector Handbook—2006 

October 2006 Page C-17 Federal Highway Administration 

Table C-16. Quadrupole-loop quality factor at f = 60 kHz. 

Length ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

6 (1.8) 28.99 41.41 49.24 54.51 58.09 60.40 61.66 

7 (2.1) 29.35 41.87 49.68 54.85 58.23 60.25 61.11 

8 (2.4) 29.64 42.23 50.00 55.05 58.22 59.91 60.32 

9 (2.7) 29.88 42.52 50.24 55.15 58.08 59.41 59.32 

10 (3.0) 30.08 42.75 50.42 55.17 57.83 58.77 58.14 

11 (3.3) 30.25 42.94 50.53 55.12 57.50 58.01 56.79 

12 (3.7) 30.40 43.09 50.61 55.01 57.08 57.13 55.28 

13 (3.9) 30.52 43.22 50.64 54.84 56.58 56.14 53.62 

14 (4.3) 30.63 43.32 50.65 54.63 56.02 55.05 51.82 

15 (4.6) 30.73 43.40 50.62 54.38 55.38 55.39 49.88 

16 (4.9) 30.82 43.47 50.57 54.09 54.69 52.59 47.81 

17 (5.2) 30.89 43.52 50.49 53.76 53.94 51.22 45.61 

18 (5.5) 30.96 43.56 50.40 53.39 53.14 49.77 43.28 

19 (5.8) 31.03 43.58 50.28 53.00 52.27 48.23 40.83 

20 (6.1) 31.08 43.60 50.15 52.57 51.36 46.61 36.26 

25 (7.6) 31.29 43.54 49.24 49.97 46.02 37.30 23.69 

30 (9.1) 31.42 43.32 47.99 46.69 39.49 26.15 6.52 

35 (10.7) 31.50 42.97 46.42 42.78 31.85 13.30 -12.97 

40 (12.2) 31.54 42.51 44.57 38.28 23.18 -1.11 -34.45 

45 (13.7) 31.56 41.96 42.46 33.21 13.53 -16.91 -57.64 

50 (15.2) 31.56 41.31 40.08 27.59 2.97 -33.96 -82.23 
Appendix C—Loop Inductance and Quality Factor Tables 

October 2006 Page C-18 Federal Highway Administration 

Table C-17. Circular-loop inductance in μH at f = 1 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 5.32 18.10 37.28 62.23 92.52 127.80 167.79 

5 (1.5) 6.87 23.49 48.53 81.21 120.99 167.44 220.21 

6 (1.8) 8.45 29.02 60.12 100.80 150.42 208.47 274.51 

7 (2.1) 10.07 34.68 71.99 120.91 180.66 250.65 330.38 

8 (2.4) 11.71 40.46 84.12 141.46 211.58 293.82 387.60 

Table C-18. Circular-loop quality factor at f = 1 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 1.03 1.74 2.37 2.95 3.49 4.00 4.48 

5 (1.5) 1.06 1.81 2.47 3.08 3.65 4.19 4.69 

6 (1.8) 1.09 1.86 2.55 3.18 3.78 4.34 4.87 

7 (2.1) 1.11 1.90 2.61 3.27 3.88 4.46 5.01 

8 (2.4) 1.13 1.94 2.67 3.34 3.98 4.57 5.14 

Table C-19. Circular-loop inductance in μH at f = 20 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 5.31 18.07 37.24 62.18 92.47 127.76 167.79 

5 (1.5) 6.85 23.45 48.47 81.15 120.94 167.44 220.30 

6 (1.8) 8.43 28.98 60.06 100.75 150.40 208.54 274.78 

7 (2.1) 10.04 34.63 71.93 120.86 180.69 250.85 330.93 

8 (2.4) 11.68 40.40 84.05 141.43 211.68 294.21 388.55 

Table C-20. Circular-loop quality factor at f = 20 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 15.16 23.32 29.46 34.34 38.35 41.72 44.61 

5 (1.5) 15.57 23.99 30.31 35.32 39.42 42.86 45.79 

6 (1.8) 15.90 24.53 30.99 36.09 40.26 43.75 46.72 

7 (2.1) 16.19 24.98 31.55 36.73 40.95 44.47 47.46 

8 (2.4) 16.43 25.37 32.03 37.26 41.53 45.07 48.07 
 Traffic Detector Handbook—2006 

October 2006 Page C-19 Federal Highway Administration 

Table C-21. Circular-loop inductance in μH at f = 40 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 5.28 18.02 37.16 62.09 92.40 127.75 167.92 

5 (1.5) 6.81 23.38 48.38 81.07 120.92 167.58 220.76 

6 (1.8) 8.39 28.90 59.96 100.68 150.48 208.94 275.80 

7 (2.1) 9.99 34.54 71.83 120.84 180.92 251.66 332.81 

8 (2.4) 11.62 40.30 83.96 141.48 212.16 295.61 391.66 

Table C-22. Circular-loop quality factor at f = 40 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 21.85 32.30 39.59 45.08 49.38 52.86 55.74 

5 (1.5) 22.40 33.11 40.56 46.13 50.47 53.97 56.83 

6 (1.8) 22.84 33.77 41.33 46.95 51.31 54.80 57.63 

7 (2.1) 23.21 34.31 41.95 47.61 51.98 55.44 58.23 

8 (2.4) 23.53 34.77 42.48 48.16 52.52 55.95 58.66 

Table C-23. Circular-loop inductance in μH at f = 60 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 5.26 17.98 37.10 62.05 92.40 127.89 168.30 

5 (1.5) 6.79 23.33 48.32 81.05 121.04 167.99 221.73 

6 (1.8) 8.35 28.84 59.90 100.72 150.79 209.83 277.76 

7 (2.1) 9.95 34.48 71.79 120.98 181.53 253.27 336.29 

8 (2.4) 11.58 40.23 83.95 141.76 214.20 298.26 397.32 

Table C-24. Circular-loop quality factor at f = 60 kHz. 

Diameter ft (m) 1 Turn 2 Turn 3 Turn 4 Turn 5 Turn 6 Turn 7 Turn 

4 (1.2) 25.89 37.38 45.06 50.65 54.92 58.28 60.99 

5 (1.5) 26.50 38.25 46.05 51.69 55.95 59.26 61.88 

6 (1.8) 27.00 38.94 46.83 52.47 56.70 59.95 62.45 

7 (2.1) 27.42 39.51 47.45 53.09 57.27 60.41 62.76 

8 (2.4) 27.77 39.99 47.97 53.59 57.70 60.71 62.89 

 Traffic Detector Handbook—2006 

October 2006 Page D-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX D. 

APPENDIX D. ELECTRICAL CHARACTERISTICS OF WIRE AND CABLE.D-1

CAPACITANCE OF PARALLEL CONDUCTORS................................................D-1

INDUCTANCE OF PARALLEL CONDUCTORS .................................................D-3

LIST OF FIGURES: APPENDIX D. 

Figure D-1. Wire spacing and size definitions used in wire inductance 

and capacitance calculations....................................................D-1

Figure D-2. Dielectric field surrounding a pair of wires...............................D-2

Figure D-3. Simplified geometry that includes most of the electromagnetic 

field surrounding a twisted pair of wires................................D-2

LIST OF TABLES: APPENDIX D. 

Table D-1. Measured lead-in cable electrical characteristics of #14 

AWG twisted pair. ....................................................................D-4

Table D-2. Measured lead-in cable electrical characteristics: Shielded 

cable lead-in. .............................................................................D-5

Table D-3. Measured inductive-loop electrical characteristics: Belden 

8718 shielded lead-in................................................................D-6

Table D-4. Measured inductive-loop electrical characteristics: Belden 

8720 shielded lead-in................................................................D-7

Table D-5. Measured inductive-loop electrical characteristics: Belden 

9438 twisted pair lead-in..........................................................D-8

Table D-6. Measured inductive-loop electrical characteristics: Loop 

without lead-in cable. ...............................................................D-9

 Traffic Detector Handbook—2006 

October 2006 Page D-1 Federal Highway Administration 

APPENDIX D. ELECTRICAL 

CHARACTERISTICS OF 

WIRE AND CABLE 

This appendix describes the calculation of the electrical characteristics of 

twisted lead-in wire composed of Belden #14 AWG copper conductor wire. 

The wire size and spacing definitions are shown in Figure D-1. Here, D

represents the distance between the center of the wire cores in the twisted 

pair and d is the diameter of the wire core. 

#14 AWG copper conductor d

Wire cover material D

Figure D-1. Wire spacing and size definitions used in wire 

inductance and capacitance calculations. 

CAPACITANCE OF PARALLEL CONDUCTORS 

The capacitance C of the twisted wire pair is given by 

F/m

36 cosh

10

1

9

0

⎟

⎠

⎞ ⎜

⎝

⎛

×⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

ε

ε

= −

−

d

D

C (D-1) 

where the factor ε/ε0 is the relative dielectric constant of the wire cover 

material. 

For Belden 9438, the relative dielectric constant of the polyethylene covering 

of the wire core is 2.3. Therefore, 

⎟

⎠

⎞ ⎜

⎝

⎛

× = −

−

64.1mils

139 mils 36 cosh

2.3 10

1

9

C

. (D-2) 

The capacitance can also be expressed as 

⎟

⎠

⎞ ⎜

⎝

⎛

× =

−

32.05 mils

139 mils 36 ln

2.3 10 9

C

 (D-3) 

or 

C = 4.355 x 10-11 F/m = 43.55 pF/m = 13.3 pF/ft. (D-4)

Figure D-2 depicts the electromagnetic field lines that surround the dielectric 

material in a twisted pair of wires. 

#14 AWG Belden 9438 is 

commonly used to 

construct inductive loops. 
Appendix D—Electrical Characteristics of Wire and Cable 

October 2006 Page D-2 Federal Highway Administration 

Figure D-2. Dielectric field surrounding a pair of wires. 

This geometry is simplified as shown in Figure D-3 by including the region 

that encompasses most of the electromagnetic field. 

2L

L

Figure D-3. Simplified geometry that includes most of the electromagnetic field 

surrounding a twisted pair of wires. 

The following heuristic approach modifies the capacitance value given by 

Equation D-1 for estimating the capacitance of a twisted wire pair based on 

the geometry of Figure D-3. It assumes that all the electromagnetic field 

energy is contained in a box surrounding the two wires and their insulation. 

It gives a reasonable approximation to the capacitance in a closed form 

equation. An exact closed form solution for the capacitance is not known. 

The area of the cable is given by 

4 4 2

2 2 2 L L L Acable

π = π

+

π = , (D-5) 

where L is the diameter of the wire, including the polyethylene insulation. 

The area of the box surrounding the twisted wire pair is given as 

( ) ( ) 2

rectangle A = 2L L = 2L . (D-6) 

The ratio of the cable area to the box area is thus: 

0.79

2 4

2

Ratio of Areas 2

2

= π =

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ π

= L

L

. (D-7) 

From Equation D-3, the capacitance of the twisted wire pair becomes 

C = (0.79) (13.3 pF/ft) = 10.45 pF/ft. (D-8) 
 Traffic Detector Handbook—2006 

October 2006 Page D-3 Federal Highway Administration 

For 100 ft (30 m) of wire, 

C = 1045 pF . (D-9)

Table D-1 shows that the actual measured capacitance is in the range of 997 

to 1006 pF for 100 ft (30 m) of wire. All units of picofarads per foot may be 

converted to picofarads per meter by multiplying by 0.305 meters per foot. 

Thus, the heuristic calculation produces a reasonable value. The heuristic 

result may be fed into finite element analysis models as a first approximation 

to a final “exact” answer. 

INDUCTANCE OF PARALLEL CONDUCTORS 

The inductance L of the twisted wire pair is given by 

) 2 2 0.4 ln( Li d

D L = + μH/m

.

 (D-10) 

The internal inductance Li of copper at 1 kHz is 0.05 μH/m. Thus, 

) 0.1 2 = 0.4 ln( +

d

D L μH/m . (D-11) 

For Belden 9438 wire, 

) 0.1

64.1mils

2 139mils 0.4ln( + × L = μH/m (D-12) 

or 

L = 0.69 μH/m = 0.21 μH/ft . (D-13) 

For 100 ft (30 m) of wire, 

L = 21 μH . (D-14) 

The internal inductance Li is 

calculated using Equation 

A-23 of Appendix A with μr

=1 and 

μ0 =4π x 10-7 H/m. 

μH/m can be converted to 

to μH/ft by dividing by 

0.3048. 
Appendix D—Electrical Characteristics of Wire and Cable 

October 2006 Page D-4 Federal Highway Administration 

Table D-1. Measured lead-in cable electrical characteristics of #14 AWG twisted pair. 

Frequency 

(kHz) 

Open circuit 

measurement 

of capacitance 

(pF) 

Open circuit 

measurement 

of conductance 

(μmhos) 

Short circuit 

measurement 

of inductance 

(μH) 

Short circuit 

measurement 

of resistance 

(Ω) 

0.1 997 −0.0005 22.87 0.57 

1.0 999 −0.0082 22.95 0.57 

5.0 1000 −0.056 22.97 0.58 

10.0 1001 −0.14 22.91 0.60 

15.0 1002 −0.24 22.83 0.63 

20.0 1002 −0.36 22.72 0.67 

25.0 1002 −0.51 22.60 0.72 

30.0 1002 −0.67 22.47 0.77 

35.2941 1003 −0.85 22.33 0.82 

40.0 1003 −1.02 22.21 0.87 

45.4545 1003 −1.21 22.09 0.92 

50.0 1003 −1.40 21.99 0.97 

54.5454 1003 −1.58 21.90 1.01 

60.0 1003 −1.77 21.8 1.06 

66.6666 1003 −2.03 21.69 1.12 

71.4286 1003 −2.26 21.62 1.16 

75.0 1004 −2.43 21.57 1.19 

80.0 1004 −2.67 21.51 1.24 

85.714 1004 −2.95 21.44 1.28 

96.0 1004 −3.46 21.33 1.37 

100.0 1005 −3.79 21.29 1.4 

120.0 1005 −4.74 21.12 1.56 

125.0 1006 −5.02 21.09 1.59 

150.0 1006 −6.52 20.93 1.78 

Type: Twisted pair of Belden 9438 wire (not shielded) 

Gauge: #14 AWG 

Twists per foot: 5.5 (18.1 twists per meter) 

Pair length: 100 ft (30 m) 

Wire location: Laboratory floor, Turner-Fairbank Highway Research Center 

Measuring Instrument: HP 4284A 

Note: Balun unavailable; instrument unbalanced during measurements 
 Traffic Detector Handbook—2006 

October 2006 Page D-5 Federal Highway Administration 

Table D-2. Measured lead-in cable electrical characteristics: Shielded cable lead-in. 

Belden 8718 cable 

(#12 AWG) 

Belden 8720 cable 

(#14 AWG) 

Frequency 

(kHz) 

Inductance 

(μH) 

Resistance 

(Ω) 

Inductance 

(μH) 

Resistance 

(Ω) 

0.1 19.78 0.35 21.00 0.59 

1 19.96 0.35 21.14 0.59 

5 19.94 0.37 21.16 0.60 

10 19.78 0.43 21.07 0.64 

15 19.54 0.51 20.94 0.71 

20 19.26 0.62 20.76 0.80 

25 18.96 0.74 20.56 0.91 

30 18.65 0.87 20.33 1.04 

35.2941 18.33 1.02 20.08 1.19 

40 18.05 1.16 19.84 1.33 

45.4545 17.74 1.33 19.57 1.50 

50 17.49 1.47 19.34 1.64 

54.5454 17.24 1.62 19.11 1.79 

60 16.96 1.79 18.84 1.97 

66.6666 16.63 2.00 18.51 2.20 

71.4286 16.39 2.16 18.28 2.36 

75 16.23 2.27 18.11 2.48 

80 16.00 2.43 17.88 2.65 

85.714 15.75 2.60 17.78 2.89 

96 15.32 2.91 17.18 3.18 

100 15.16 3.03 17.01 3.30 

120 14.43 3.58 16.25 3.90 

125 14.27 3.711 16.07 4.04 

150 13.52 4.29 15.27 4.67 

Type: Shielded cable 

Gauge: #12 or #14 AWG 

Length: 100 ft (30 m) 

Wire location: Laboratory floor, Turner-Fairbank Highway Research Center 

Measuring Instrument: HP 4284A 

Note: Balun unavailable; instrument unbalanced during measurements 
Appendix D—Electrical Characteristics of Wire and Cable 

October 2006 Page D-6 Federal Highway Administration 

Table D-3. Measured inductive-loop electrical characteristics: Belden 8718 shielded lead-in. 

Frequency 

(kHz) 

Inductance 

(μH) 

Resistance 

(Ω) 

Quality factor 

(Q) 

0.1 94.80 0.53 0.1 

1 94.52 0.53 1 

5 94.08 0.56 5 

10 93.27 0.63 9 

15 93.37 0.73 12 

20 92.98 0.86 14 

25 92.59 1.01 15 

30 92.21 1.16 15 

35.2941 91.81 1.34 15 

40 91.47 1.49 15 

45.4545 91.09 1.68 16 

50 90.79 1.84 16 

54.5454 90.50 2.00 16 

60 90.17 2.18 16 

66.6666 89.79 2.41 16 

71.4286 89.53 2.57 16 

75 89.34 2.69 16 

80 89.09 2.85 16 

85.714 88.82 3.03 16 

96 88.36 3.33 16 

100 88.20 3.44 16 

120 87.47 3.93 17 

125 87.31 4.03 17 

150 86.65 4.44 18 

Loop size: 6 x 6 ft (1.8 x 1.8 m) 

Number of turns: 3 (closely wound) 

Gauge: #14 AWG 

Loop location: 3 ft (0.9 m) above electronics laboratory floor, Turner-Fairbank 

Highway Research Center 

Lead-in cable: Belden 8718 (# 12 AWG) 

Lead-In cable length: 100 ft (30 m) 

Measuring Instrument: HP 4284A 

Note: Balun unavailable; instrument unbalanced during measurements 
 Traffic Detector Handbook—2006 

October 2006 Page D-7 Federal Highway Administration 

Table D-4. Measured inductive-loop electrical characteristics: Belden 8720 shielded lead-in. 

Frequency 

(kHz) 

Inductance 

(μH) 

Resistance 

(Ω) 

Quality factor 

(Q) 

0.1 96.80 0.77 0.1 

1 95.78 0.77 0.8 

5 95.37 0.79 4 

10 95.10 0.85 7 

15 94.85 0.94 10 

20 94.59 1.05 11 

25 94.29 1.18 13 

30 94.00 1.33 13 

35.2941 93.67 1.49 14 

40 93.38 1.65 14 

45.4545 93.04 1.84 14 

50 92.76 2.00 15 

54.5454 92.48 2.17 15 

60 92.16 2.37 15 

66.6666 91.78 2.60 15 

71.4286 91.51 2.77 15 

75 91.20 2.90 15 

80 91.06 3.07 15 

85.714 90.77 3.26 15 

96 90.28 3.59 15 

100 90.10 3.70 15 

120 89.30 4.24 16 

125 89.12 4.35 16 

150 88.35 4.80 17 

Loop size: 6 x 6 ft (1.8 x 1.8 m) 

Number of turns: 3 (closely wound) 

Gauge: #14 AWG 

Loop location: 3 feet (0.9 m) above electronics laboratory floor, Turner-Fairbank 

Highway Research Center 

Lead-in cable: Belden 8720 (# 14 AWG) 

Lead-in cable length: 100 ft (30 m) 

Measuring instrument: HP 4284A 

Note: Balun unavailable; instrument unbalanced during measurements 
Appendix D—Electrical Characteristics of Wire and Cable 

October 2006 Page D-8 Federal Highway Administration 

Table D-5. Measured inductive-loop electrical characteristics: Belden 9438 twisted pair lead-in. 

Frequency 

(kHz) 

Inductance 

(μh) 

Resistance 

(Ω) 

Quality factor 

(Q) 

0.1 98.19 0.75 0.1 

1 97.83 0.75 0.8 

5 97.43 0.77 4 

10 97.20 0.81 8 

15 97.04 0.86 11 

20 96.88 0.92 13 

25 96.74 0.99 15 

30 96.61 1.06 17 

35.2941 96.50 1.14 19 

40 96.43 1.21 20 

45.4545 96.37 1.30 21 

50 96.34 1.36 22 

54.5454 96.34 1.43 23 

60 96.36 1.51 24 

66.6666 96.43 1.60 25 

71.4286 96.50 1.67 26 

75 96.56 1.72 27 

80 96.68 1.78 27 

85.714 96.83 1.86 28 

96 97.18 2.00 29 

100 97.34 2.05 30 

120 98.34 2.31 32 

125 98.63 2.38 33 

150 100.42 2.7 35 

Loop size: 6 x 6 ft (1.8 x 1.8 m) 

Number of turns: 3 (closely wound) 

Gauge: #14 AWG 

Loop location: 3 ft (0.9 m) above electronics laboratory floor, Turner-Fairbank 

Highway Research Center 

Lead-in cable: Belden 9438 twisted-pair (5.5 twists/ft) (15/m) 

Lead-in cable length: 100 ft (30 m) 

Measuring instrument: HP 4284A 

Note: Balun unavailable; instrument unbalanced during measurements 
 Traffic Detector Handbook—2006 

October 2006 Page D-9 Federal Highway Administration 

Table D-6. Measured inductive-loop electrical characteristics: Loop without lead-in cable. 

Frequency 

(kHz) 

Inductance 

(μH) 

Resistance 

(Ω) 

Quality factor 

(Q) 

0.1 75.02 0.18 0.3 

1 74.33 0.19 3 

5 74.35 0.20 12 

10 74.15 0.21 22 

15 74.04 0.23 30 

20 73.95 0.25 37 

25 73.86 0.28 42 

30 73.79 0.30 46 

35.2941 73.71 0.33 50 

40 73.66 0.36 52 

45.4545 73.61 0.39 55 

50 73.57 0.41 57 

54.5454 73.53 0.43 59 

60 73.48 0.46 61 

66.6666 73.44 0.49 63 

71.4286 73.41 0.51 64 

75 73.39 0.53 65 

80 73.36 0.55 67 

85.714 73.34 0.58 68 

96 73.30 0.62 71 

100 73.29 0.64 72 

120 73.25 0.73 76 

125 73.25 0.75 77 

150 73.25 0.84 82 

Loop size: 6 x 6 ft (1.8 x 1.8 m) 

Number of turns: 3 (closely wound) 

Gauge: #14 AWG 

Loop location: 3 ft (0.9 m) above electronics laboratory floor, Turner-Fairbank 

Highway Research Center 

Lead-in cable: None 

Lead-in Length: 0 ft (0 m) 

Measuring instrument: HP 4284A 

Note: Balun unavailable; instrument unbalanced during measurements

 Traffic Detector Handbook—2006 

October 2006 Page E-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX E. 

APPENDIX E. VEHICLE DETECTION SENSITIVITY FORMULAS FOR 

RECTANGULAR LOOPS............................................................E-1

ABSTRACT...............................................................................................................E-1

INTRODUCTION.....................................................................................................E-1

VEHICLE-DETECTION SENSITIVITY THEORY...............................................E-2

VEHICLE MODEL ............................................................................................E-2

REINFORCING STEEL MODEL....................................................................E-2

CIRCUIT MODEL FOR ONE-TURN LOOP .................................................E-3

Circuit Equations for One-Turn Loop .......................................................................E-3

Inductive-Loop Driving-Point Impedance .................................................................E-4

Inductive-Loop Sensitivity ..........................................................................................E-4

CIRCUIT MODEL FOR TWO-TURN LOOP ................................................E-5

Circuit Equations for Two-Turn Loop.......................................................................E-6

CIRCUIT EQUATIONS FOR MULTITURN LOOP.....................................E-6

VEHICLE-DETECTION SENSITIVITY FOR A MULTITURN LOOP.....E-7

INDUCTIVE-LOOP DETECTOR SENSITIVITY .........................................E-8

LOOP-DETECTOR SENSITIVITY MEASURED DATA AND CALCULATED 

RESULTS.............................................................................................................E-9

LOOP-DETECTOR SENSITIVITY COMPUTER PROGRAM...................E-9

MEASURED LOOP-DETECTOR SENSITIVITY DATA.............................E-9

COMPARISON BETWEEN MEASURED AND CALCULATED 

SENSITIVITY DATA....................................................................................E-9

EFFECT OF LOOP TURNS ON SENSITIVITY..........................................E-10

EFFECT OF LOOP VOLUME ON SENSITIVITY .....................................E-10

EFFECT OF LEAD-IN CABLE INDUCTANCE ON SENSITIVITY ........E-11

EFFECT OF MESH ON SENSITIVITY........................................................E-12

EFFECT OF LEAD-IN CABLE AND MESH ON SENSITIVITY..............E-12

RESULTS AND CONCLUSIONS .........................................................................E-13

REFERENCES.......................................................................................................E-14
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops 

October 2006 Page E-ii Federal Highway Administration 

LIST OF FIGURES: APPENDIX E. 

Figure E-1. Two-turn loop in roadway...........................................................E-3

Figure E-2. Single-turn inductive-loop circuit model. ..................................E-3

Figure E-3. Two-turn inductive-loop circuit model.......................................E-6

LIST OF TABLES: APPENDIX E. 

Table E-1. Comparison of measured and calculated loop detector 

sensitivity data. ......................................................................E-10

Table E-2. Vehicle undercarriage detection height versus number of 

loop turns and spacing (calculated results). .........................E-11

Table E-3. Vehicle undercarriage detection height versus loop-turn 

spacing for 3-turn loop in English and metric units 

(calculated results). ................................................................E-11

Table E-4. Vehicle undercarriage detection height versus number of 

loop turns and spacing with lead-in cable and reinforcing 

steel mesh for 5- x 5-ft loop (calculated results)...................E-12

Table E-5. Vehicle undercarriage detection height versus number of loop 

turns and spacing with lead-in cable and reinforcing steel 

mesh for 1.52- x 1.52-m loop (calculated results) 

(see Table E-4 for English units version)..............................E-13

 
 Traffic Detector Handbook—2006 

October 2006 Page E-1 Federal Highway Administration 

APPENDIX E. VEHICLE DETECTION 

SENSITIVITY FORMULAS FOR 

RECTANGULAR LOOPS

ABSTRACT 

This appendix describes the derivation and application of formulas to 

calculate the vehicle detection sensitivity of single-turn and multiturn 

rectangular loops of coaxial wire. The effect on detection sensitivity of 

reinforcing steel mesh in the pavement and lead-in cable inductance is 

accounted for in the models. The loop is installed in the roadway pavement 

and connected to a roadside electronics unit by lead-in cable, which is 

modeled as a transmission line. When a vehicle passes over the loop, eddy 

currents induced in the vehicle undercarriage cause a decrease in loop 

inductance, which is sensed by the electronics unit. 

The multiturn rectangular loop is modeled as a number of series connected, 

single-turn, rectangular loops that exhibit mutual coupling between each 

turn of wire. The multiturn loop is represented by the primary of an air core 

transformer, which contains two secondary windings composed of shorted 

turns that simulate the vehicle undercarriage and reinforcing steel mesh. 

The derived equations are used to calculate tables of vehicle detection 

sensitivity, expressed as vehicle undercarriage detection height, as a function 

of number of loop turns, loop size, and loop spacing. 

INTRODUCTION 

The inductive-loop detector (ILD) system is used nationwide to detect 

stopped or moving vehicles for traffic surveillance and control systems. A 

typical ILD system consists of a 3-turn, 6- x 6-ft (1.83- x 1.83-m) loop of #14 

AWG wire embedded in the pavement, a lead-in cable to roadside, and 

roadside electronics unit. The vehicle-detection sensitivity is the normalized 

inductance change at the terminals of the electronics unit when a vehicle is 

sensed by the loop. High-ground-clearance vehicles such as trucks cause a 

small change in loop inductance, which sometimes results in nondetection of 

part of the vehicle. Increasing the sensitivity of the electronics unit to sense 

such small inductance changes usually increases the response time of the 

electronics unit, which causes errors in ILD applications for measuring 

vehicle speed. 

The vehicle-detection sensitivity of an inductive-loop detection system is 

maximized when the spacing between loop turns is correctly selected. The 

increased sensitivity should improve detection of high-ground-clearance 

trucks. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-2 Federal Highway Administration 

This appendix is based on a previous paper that developed a detectionsensitivity formula for a single-turn rectangular loop.(1) It was shown that, 

with no reinforcing steel, the sensitivity is proportional to the mutual 

inductance squared, divided by the product of the self-inductance of the 

single-turn loop and the self-inductance of the shorted-turn loop simulating 

the vehicle. If the loop turns of a multiturn, rectangular loop are tightly 

coupled, the sensitivity is independent of the number of loop turns, provided 

the loop inductance is approximately ten times larger than the inductance of 

the lead-in cable. A formula for the self-inductance of a multiturn 

rectangular loop, which accounts for leakage flux, was developed in a second 

paper.(2) As the loop turns become more widely spaced, the increased leakage 

flux causes the loop self-inductance to decrease, which results in increased 

detection sensitivity until the decrease in mutual coupling to the shorted 

turn becomes significant. 

VEHICLE-DETECTION SENSITIVITY THEORY 

VEHICLE MODEL 

The vehicle is modeled as a flat, perfectly conducting plate at a height above 

the loop approximately equal to the average undercarriage height of the 

vehicle. The width and length of the plate are equal to the width and length 

of the vehicle. The continuous, perfectly conducting plate can be simulated by 

a wire grid, provided the mesh elements are sufficiently small. 

In order to simplify the calculations of loop system sensitivity, the wire grid 

simulating the vehicle is set equal to the size of the inductive loop in the 

roadway. When the wire grid is coaxially located over the loop, the interior 

currents induced in the grid by the loop cancel, leaving a current flowing 

around the perimeter of the grid, provided the grid width and length are 

equal to those of the loop. Thus, the grid can be replaced with a wire loop or 

shorted turn equal to the grid perimeter. This case represents a vehicle 

centered over the loop (i.e., maximum percentage inductance change of the 

loop). Magnetic effects on the loop due to the permeable material of the 

vehicle are assumed to be negligible. 

REINFORCING STEEL MODEL 

The mesh elements of the reinforcing steel in the pavement are considered 

sufficiently small and the size of the mesh sufficiently large compared to the 

loop so that the steel mesh can be replaced with a perfectly conducting plane 

of infinite extent. Based on image theory, the infinitely conducting plane is 

replaced with an image loop at twice the distance from the loop to the 

reinforcing steel. Figure E-1 illustrates the geometry of the vehicle and 

reinforcing steel shorted turn model for a two-turn rectangular inductive 

loop. The direction of induced currents determines the sign of the mutual 

inductance terms used in the circuit model. A plus sign indicates current flow 

in the same direction and a minus sign current flow in opposite directions. 
 Traffic Detector Handbook—2006 

October 2006 Page E-3 Federal Highway Administration 

l

1

l

2

HV

Shorted turn loop 

simulating vehicle 

undercarriage

Two turn loop

in roadway 

Shorted turn loop 

simulating 

reinforcing steel

2HS

Figure E-1. Two-turn loop in roadway. 

CIRCUIT MODEL FOR ONE-TURN LOOP 

Figure E-2 shows the air core transformer model of a one-turn inductive loop, 

vehicle undercarriage, and reinforcing steel mesh. The inductive loop is 

modeled as the primary winding of the transformer, while the vehicle and 

reinforcing steel are modeled as shorted turn secondary windings. The 

secondary winding simulating the vehicle is movable to simulate changes in 

vehicle undercarriage height HV. The secondary winding modeling the 

shorted turn reinforcing steel mesh is located at a distance of twice the steel 

mesh to inductive loop spacing HS. 

V1 Eg

R11

Single Turn

Inductive Loop

22 L R22

M13 R33

M12

L33

11 L M23

i2

i

3

i1

Rg Shorted Turn

Secondary

Simulates

Vehicle

Shorted Turn

Secondary

Simulates

Reinforcing 

Steel Mesh

Figure E-2. Single-turn inductive-loop circuit model. 

Circuit Equations for One-Turn Loop 

The circuit equations corresponding to Figure E-2 are: 

1 11 1 12 2 12 3 V = Z I + Z I + Z I (E-1) 

0 21 1 22 2 23 3 = Z I + Z I + Z I (E-2) 

0 31 1 32 2 33 3 = Z I + Z I + Z I (E-3) 

where 

11 11 L11 Z = R + jω (E-4) 

22 22 L22 Z = R + jω (E-5) 

33 33 L33 Z = R + jω (E-6) 

Z is impedance in ohms. 

I is current in amperes. 

Equations E-2 and E-3 are 

set equal to 0 since there 

is no external driving 

voltage in the two 

secondary windings in 

Figure E-2 that contain L22

and L33. 

R is resistance in ohms. 

j represents the square root 

of -1 in the imaginary term. 

ω is frequency in radians. 

L is inductance in henries. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-4 Federal Highway Administration 

12 M 12 Z = jω (E-7) 

13 M 13 Z = jω . (E-8) 

All mutual impedances are symmetrical (i.e., Z12 = Z21, Z13 = Z31, Z23 = Z32). 

Experimental measurements show that the quality factor ωL11 /R11 of the loop 

driving point impedance Z11 is ≥ 10. 

Assuming small circuit loss to simplify calculations gives 

11 L11 Z = jω (E-9) 

22 L 22 Z = jω (E-10) 

33 L33 Z = jω . (E-11) 

Inductive-Loop Driving-Point Impedance 

The loop driving-point impedance Z1 from Equations E-1, E-2, and E-3 is 

( ) ( ) ( )

( )2

22 33 32

33 21 31 32

2

11 21

2

22 32

2

11 22 33 31

1

1

1

2

Z Z Z

Z Z Z Z Z Z Z Z Z Z Z Z

I

V Z −− − − + × = = . (E-12) 

If the reinforcing mesh is spaced a great distance from the inductive loop, 

1. .

Then 

( )

22

2

11 22 21

1 Z

Z Z Z Z − = . (E-13)

In this case, the loop driving-point impedance Z1 depends only on the effect of 

the vehicle. 

Inductive-Loop Sensitivity 

The sensitivity SL of an inductive loop is defined as 

L NV Z

Z S

1

Δ 1 = . (E-14)

The change in loop driving-point impedance ΔZ1 is given by 

NV V ΔZ 1 = Z 1 − Z 1 (E-15) 

where ΔZ1

NV is the loop driving-point impedance with no vehicle present and 

ΔZ1

V is the loop driving-point impedance when sensing a vehicle. 

The loop driving-point impedance Z1 can also be expressed as 

( ) ( )

( )2

22 33 23

33 12 13 23

2

22 12

2

13

1 11

Z

Z Z 2

−

+ − = − Z ZZ Z Z Z Z Z Z . (E-16) 

Then, if no vehicle is present, 

NV represents the value of 

the variable when no vehicle 

is present. 

V represents the value of the 

variable when a vehicle is 

present. 

NV represents the value of 

the variable when no vehicle 

is present. 

V represents the value of the 

variable when a vehicle is 

present. 

Z13 = Z23 = 0
 Traffic Detector Handbook—2006 

October 2006 Page E-5 Federal Highway Administration 

( ) ( )

33

2

11 33 13

33

2

13

1 11

12 23

Z Z

0

Z

Z Z

Z

Z Z

Z Z

NV − = − =

= =

 (E-17) 

() ()

( )2

22 33 23

33 12 13 33 23

2

22 12

2

13

1 11

Z

Z Z 2

−

+ − = − Z ZZ Z Z Z Z Z Z Z V

(E-18) 

( )( ) ( ) ( )

( ( ) Z )

Z Z Z Z 2

2

33 22 33 23

12 13 33 23

2

33

2

12

2

23

2

13

1

−

+ − Δ = Z Z ZZ Z Z Z Z . (E-19) 

The loop sensitivity SL is given by 

( )( ) ( )( )

( () () Z )( Z )

Z Z Z Z 2

2

22 33 23

2

11 33 13

12 13 33 23

2

33

2

12

2

23

2

13

1

1

− −

+ − = Δ = Z Z Z ZZ Z Z Z

Z

Z S L NV (E-20)

if 

( )

11 22

2

12

13 23

Z

0

Z Z

S

Z Z

L =

= =

. (E-21) 

Applying Equations E-9, E-10, and E-11 to Equation E-20 yields the loop 

sensitivity SL in percent as 

( )( )

2

100 2

22 33 23

2

13

2

11 33 13

12 13 23 33

2

23

2

13

2

33

2

12

L L M M L L M

M L M M M M M L S L − − −+ − = (E-22) 

where 

SL = inductive loop sensitivity (percent) 

M12 = mutual inductance between inductive loop and loopsimulating vehicle (H) 

M13 = mutual inductance between inductive loop and loopsimulating reinforcing mesh (H) 

M23 = mutual inductance between loop-simulating vehicle and 

loop-simulating reinforcing mesh (H) 

L11 = self-inductance of inductive loop (H) 

L22 = self-inductance of loop-simulating vehicle (H) 

L33 = self-inductance of loop-simulating reinforcing steel mesh (H). 

When reinforcing steel mesh is not present, 

( )

11 22

2

12 100

L L

M S L = . (E-23) 

CIRCUIT MODEL FOR TWO-TURN LOOP 

Figure E-3 shows the air core transformer model of a two-turn inductive loop, 

vehicle undercarriage, and reinforcing steel mesh. Mutual inductance terms 

that model the effects of the multiple-turn loop have been added to the model. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-6 Federal Highway Administration 

V1 Eg

Rg

Two Turn

Inductive Loop

22 L R22

M13

22 R33

Shorted Turn

Secondary

Simulates

Vehicle

Shorted Turn

Secondary

Simulates

Reinforcing 

Steel Mesh

M12

11

33 L

11 L11

M23

R11

11

11 L

22

13

22

M13

11

i2

i3

i

1 M

Figure E-3. Two-turn inductive-loop circuit model. 

Circuit Equations for Two-Turn Loop 

From Figure E-3, the circuit equations for the two-turn loop are given by 

3

22

13

11

2 13

22

12

11

1 12

21

11

12

11

22

11

11

1 11 V = (Z + Z + Z + Z )I + (Z + Z )I + (Z + Z )I (E-24) 

3

22

2 23

22

1 22

22

21

11

21 0 = (Z + Z )I + Z I + Z I (E-25) 

3

33

2 33

22

1 32

22

31

11

31 0 = (Z + Z )I + Z I + Z I (E-26) 

where 

11 Z 12 = mutual impedance between primary turn 1 of the transformer 

and secondary shorted turn 2, which models the vehicle, 

22 Z 12 = mutual impedance between primary turn 2 of the transformer 

and secondary shorted turn 2, which models the vehicle, 

12 Z 11 = mutual impedance between primary turn 1 and primary turn 2. 

Then 

21

11

12

11

22

11

11 Z11 = Z11 + Z + Z + Z (E-27) 

22

12

11 Z12 = Z12 + Z (E-28) 

22

13

11 Z13 = Z13 + Z (E-29) 

22

21

11 Z21 = Z21 + Z (E-30) 

22 Z22 = Z22 (E-31) 

22 Z23 = Z23 (E-32) 

22

31

11 Z31 = Z31 + Z (E-33) 

22 Z32 = Z32 (E-34) 

33 Z33 = Z33 . (E-35) 

As compared to the one-turn loop model, the self-impedance Z11 is replaced 

by the total two-turn loop impedance. Each mutual impedance is replaced by 

the sum of the mutual impedances from each turn. 

CIRCUIT EQUATIONS FOR MULTITURN LOOP 

Circuit equations for a multiturn loop are given by 
 Traffic Detector Handbook—2006 

October 2006 Page E-7 Federal Highway Administration 

= + δ ∑ ∑ + δ ∑ ∑ = = = =

N

i

N

j

ij

ij

N

i

N

j

ij

ij V ZI Z I Z I

1 1

13 3

1 1

1 1 12 2 (E-36) 

= δ ∑ ∑ + + = =

N

j

ij N

i ij Z I Z I Z I

1

21 1 22 2 23 3

1

0 (E-37) 

1 32 2 33 3

1 1

31 0 Z I Z I Z I

N

i

N

j

ij = δij ∑ ∑ + + = =

(E-38) 

where δij is the Kronecker Delta function (i.e., δij = 1 for i = j and δij = 0 for i

not equal to j). Then 

Z 11 = Z N = multiturn loop self impedance (E-39) 

= δ ∑ ∑ = =

N

j

ij N

i

Z ij Z

1

12

1

12 (E-40) 

= δ ∑ ∑ = =

N

j

ij N

i

Z ij Z

1

13

1

13 (E-41) 

= δ ∑ ∑ = =

N

j

ij N

i

Z ij Z

1

21

1

21 (E-42) 

= δ ∑ ∑ = =

N

j

ij N

i

Z ij Z

1

31

1

31 . (E-43) 

Equations E-39–E-43 are generalized for the multiturn case by letting 

* L11 = L N (E-44) 

= δ ∑ ∑ = =

N

j

ij N

i

M ij M

1

12

1

12 (E-45) 

= δ ∑ ∑ = =

N

j

ij N

i

M ij M

1

13

1

13 (E-46) 

where L*

N is the total self-inductance of the loop. 

VEHICLE-DETECTION SENSITIVITY FOR A 

MULTITURN LOOP 

Detection sensitivity is expressed as 

[ ] ( ) ⎢

⎢

⎢

⎢

⎢

⎢

⎣

⎡

− ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − δ ∑ ∑

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟ + δ ∑ ∑ ⎠

⎞

⎜

⎜

⎝

⎛

δ ∑ ∑

=

= =

= = = =

2

22 33 23

2

1 1 33 13

*

23

2

1 1 33 13

2

1 1 12

(%) 100

L L M L L M

M L M M

S

N

i

N

j

ij

N ij

N

i

N

j

ij

ij

N

i

N

j

ij

ij

L

[ ] ( ) ⎥

⎥

⎥

⎥

⎥

⎥

⎦

⎤

− ⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − δ ∑ ∑

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

⎟

⎟ δ ∑ ∑ ⎠

⎞

⎜

⎜

⎝

⎛ − δ ∑ ∑

= =

= = = =

2

22 33 23

2

1 1 33 13

*

23 33 1 1 13 1 1 12 2

L L M L L M

M M M L

N

i

N

j

ij N ij

N

i

N

j

ij

ij

N

i

N

j

ij

ij . (E-47) 

For a two-turn (N = 2) loop, 

( )( ) ( )( )

( ) [ ] 2 22 33 23

2 22

13 11 33 13 *

23 33 2213 1113 2212 11 23 12

2 22

13 11 33 13

2 22

12 1112

( )

2 (%) 100

L L M M L L M

M M L M M M M M M M M L S

N

L

− ⎥⎦

⎤ ⎢⎣⎡ − +

+ + + − + + = . (E-48) 

The formula for multiturn loop-sensitivity can be applied to any loop 

geometry, provided the vehicle and reinforcing steel mesh can be modeled as 

shorted turns. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-8 Federal Highway Administration 

When no reinforcing steel mesh is present, the sensitivity of the electronics 

unit to a change in loop-system inductance caused by vehicle passage or 

presence is given by 

22

*

2

1

12

1 (%) 100

L L

M

S

N

N

j

ij N

i ij

L

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

δ ∑ ∑

= = = . (E-49) 

Equation E-47 can be applied to a multiturn rectangular loop with N coaxial, 

equal-spaced, identical turns. The low-frequency inductance LN for such a 

loop is given by(3) 

2( 1) 2( 2) ... LN = NL11 + N − M12 + N − M13 + (E-50) 

or 

= + ∑ ( ) − = +

N

i

N M i L NL N i

1

11 2 1, 1 (E-51)

where L11 is the self-inductance of a single turn and M1,i+1 is the mutual 

inductance between turn one and the i + 1 turn. This formula assumes that 

the smallest dimension of a loop turn length or width is much greater than 

the largest spacing between loop turns. 

For a two-turn loop, 

LN = 2L11 + 2M12 . (E-52) 

The total self-inductance L*

N of the loop is equal to the sum of the lowfrequency inductance LN and the high-frequency, skin-effect inductance LH

and is expressed as 

LN = LN + LH * (E-53) 

where 

( ) i LH = 2 l 1 + l 2 L . (E-54) 

l1 and l2 are turn width (m) and turn length (m), respectively, and the 

internal inductance Li

 is 0.036 μH/m (0.111 μH/ft) at 47 kHz. 

INDUCTIVE-LOOP DETECTOR SENSITIVITY 

The series inductance LC of the transmission line connecting the loop in the 

roadway to the electronics unit reduces the inductive-loop system sensitivity 

SL at the electronics unit to 

⎥

⎥

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎢

⎢

⎣

⎡

+ =

* 1

1

N

P L C

L

S S L

. (E-55) 
 Traffic Detector Handbook—2006 

October 2006 Page E-9 Federal Highway Administration 

LOOP-DETECTOR SENSITIVITY MEASURED DATA AND 

CALCULATED RESULTS 

LOOP-DETECTOR SENSITIVITY COMPUTER 

PROGRAM 

Equations E-47, E-51, E-53, and E-55 were programmed in BASIC and run 

on an IBM computer. The main program calls a subroutine, which calculates 

the sum of the mutual inductance from each loop turn to the image loop 

simulating the vehicle and image loop simulating the reinforcing steel. The 

main program also calls a subroutine, which calculates the external, or lowfrequency inductance of a coaxial, multiturn, rectangular loop of round wire. 

This low-frequency inductance subroutine calls a subroutine, which 

calculates the mutual inductance between two coaxial, single-turn, 

rectangular loops. The single-turn, mutual inductance subroutine calls a 

subroutine, which calculates the mutual inductance between two parallel 

current elements. 

MEASURED LOOP-DETECTOR SENSITIVITY DATA 

The only known measured loop-detector sensitivity data(4) versus number of 

loop conductor turns are presented in Table E-1. These data were obtained by 

driving a compact automobile over a 6- x 6-ft (1.83- x 1.83-m) loop with 10 ft 

(3.05 m) of #14 underground feeder (UF) cable connected between the loop 

and the laboratory equipment. Loop sensitivity as a function of the number of 

loop turns was calculated from the change in loop inductance recorded with 

and without the vehicle. The sensitivity magnitude increased approximately 

5 percent when the operating frequency changed from 15 to 100 kHz for four 

or more loop turns. 

COMPARISON BETWEEN MEASURED AND 

CALCULATED SENSITIVITY DATA 

Since the average undercarriage height of the compact automobile used was 

unknown, the automobile was modeled with a 6- x 6-ft (1.83- x 1.83-m) 

shorted turn coaxially located 0.71 ft (216 mm) over the loop of #14 AWG 

wire. A loop-wire spacing of 150 mils (38 mm) and lead-in cable inductance of 

0.22 μH/ft (0.72 μH/m) were assumed. 

Table E-1 shows that the agreement between measured and calculated loopdetector sensitivity is good. Therefore, values calculated from the computer 

program model can be used to infer the effects of inductive-loop design 

parameters on loop sensitivity as described below. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-10 Federal Highway Administration 

Table E-1. Comparison of measured and calculated loop detector sensitivity data. 

Loop turns Measured loop detector 

sensitivity** 

(%) 

Calculated loop 

detector sensitivity 

(%) 

Difference 

(%) 

1 3.25 3.72 14.47 

2 4.75 4.75 0 

3* 5.20 5.20 0 

4 5.50 5.47 −0.55 

5 5.60 5.68 1.43 

6 5.65 5.83 3.19 

8 5.75 6.07 5.57 

10 6.05 6.25 3.31 

* The height of 0.71 ft (0.22 m) was selected so that the difference equaled zero. 

** Data measured at f = 50 kHz. 

EFFECT OF LOOP TURNS ON SENSITIVITY 

A survey by Dorsey(5) found that 15 of 21 States cut loop slots greater than 2 

inches (50.8 mm) in the pavement. The spacing between the center of the 

conductor of the top loop turn and the center of the conductor of the bottom 

loop turn was assumed to be 2 inches (50.8 mm) for analysis. In practice, a 

much closer top-to-bottom turn spacing is used. A loop slot depth of 1-3/8 

inches (35 mm) is recommended in the Traffic Detector Handbook for a twoturn loop. A nominal sensitivity of 0.098 percent was assumed for the 

detection threshold. 

A lead-in cable length of zero was used so that the effect of loop turns on 

sensitivity would be independent of lead-in cable length. 

The effect of the reinforcing steel mesh was removed by spacing the mesh 

1000 ft (304.8 m) from the loop. Results from the loop-detector system 

sensitivity computer program are presented in Table E-2 for 1 to 8 loop turns. 

The vehicle undercarriage detection height is approximately proportional to 

the volume enclosed by the loop conductors and is approximately 

independent of the number of loop turns for a given volume. For example, the 

volume of the 6- x 6-ft (1.83- x 1.83-m) by 2000-mils (50.8-mm) loop is 6 ft3

(0.171 m3

), as illustrated by the results in Table E-2. 

EFFECT OF LOOP VOLUME ON SENSITIVITY 

Since Table E-2 demonstrates that the undercarriage detection height is 

approximately independent of the number of loop turns for three or more 

turns, the effect on loop sensitivity and hence detection height of increasing 

the spacing between turns in a three-turn loop was calculated as shown in 

Table E-3. The vehicle undercarriage detection height slowly increases when 

the turn spacing exceeds 1 inch (25 mm). The volume of the 6- x 6-ft (1.83- x 

1.83-m) by 300-mils (7.6-mm) loop is 0.90 ft3

 (0.25 mm3) with a detection 

height of 4.7 ft (1.43 m). Increasing the loop volume by a factor of 10 results 

in a 0.3-ft (9.1-cm) increase in vehicle detection height. 

Vehicle-detection height is 

approximately 

proportional to volume 

enclosed by the loop 

windings and is 

approximately 

independent of the 

number of loop turns for a 

given volume when 

effects of lead-in cable 

length and steel mesh are 

noncontributory. 

Vehicle-detection height 

slowly increases when the 

turn spacing exceeds 1 

inch (25 mm). 
 Traffic Detector Handbook—2006 

October 2006 Page E-11 Federal Highway Administration 

Table E-2. Vehicle undercarriage detection height versus number 

of loop turns and spacing (calculated results). 

Loop 

turns 

Loop 

turn 

spacing 

(mils) 

Loop 

turn 

spacing 

(mm) 

Vehicle 

detection height 

for 

5- x 5-ft 

(1.52- x 1.52-m) 

loop 

(ft) 

Vehicle 

detection 

height for 

5- x 5-ft 

(1.52- x 1.52-m) 

loop 

(m) 

Loop 

inductance for 

6- x 6-ft 

(1.83- x 1.83-m)

loop 

(μh) 

Vehicle 

detection 

height for 6- 

x 6-ft (1.83- x 

1.83-m) loop 

(ft) 

Vehicle 

detection 

height for 6- 

x 6-ft (1.83- x 

1.83-m) loop

(m) 

1 NA NA 3.8 1.16 10.42 4.5 1.37 

2 2,000 50.8 4.1 1.25 29.15 4.8 1.46 

3 1,000 25.4 4.2 1.28 60.15 5.0 1.52 

4 660 16.8 4.3 1.31 103.47 5.0 1.52 

5 500 12.7 4.3 1.31 158.72 5.0 1.52 

6 400 10.2 4.3 1.31 226.48 5.0 1.52 

7 330 8.4 4.3 1.31 307.23 5.0 1.52 

8 286 7.3 4.3 1.31 399.10 5.0 1.52 

Conductor type: #14 AWG Cable 

Lead-in cable length: 0 ft 

Mesh spacing: 1,000 ft (305 m) 

Detection sensitivity threshold: 0.098 percent 

Table E-3. Vehicle undercarriage detection height versus loop-turn spacing for 3-turn loop in inchpound and metric units (calculated results). 

Loop turn spacing 

(mils) 

Vehicle detection height 

for 6- x 6-ft loop (ft) Loop turn spacing (mm) 

Vehicle detection height for 

1.83- x 1.83-m loop (m) 

150 4.7 3.8 1.43 

300 4.8 7.6 1.46 

450 4.8 11.4 1.46 

600 4.9 15.2 1.49 

750 4.9 19.1 1.49 

900 4.9 22.9 1.49 

1,050 5.0 26.7 1.52 

1,200 5.0 30.5 1.52 

1,350 5.0 34.3 1.52 

1,500 5.0 38.1 1.52 

Conductor type: #14 AWG cable 

Lead-in cable length: 0 

Mesh spacing: 1,000 ft (305m) 

Detection sensitivity threshold: 0.098 percent 

EFFECT OF LEAD-IN CABLE INDUCTANCE ON 

SENSITIVITY 

Lead-in cable inductance reduces loop sensitivity to vehicles and hence 

vehicle undercarriage detection height. Additional loop turns increase the 

loop inductance, which reduces the detrimental effect of the lead-in cable 

inductance. Tables E-4 and E-5 illustrate this effect. 

Lead-in cable inductance 

reduces loop sensitivity 

and hence vehicle 

detection height. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-12 Federal Highway Administration 

EFFECT OF MESH ON SENSITIVITY 

Reinforcing steel mesh also reduces loop sensitivity to vehicles and hence 

vehicle undercarriage detection height. Adding loop turns has little effect on 

negating the reduced sensitivity caused by the mesh, as shown in Tables E-4 

and E-5. 

EFFECT OF LEAD-IN CABLE AND MESH ON 

SENSITIVITY 

The combined effects of lead-in cable inductance and steel mesh accentuate 

the reduction of loop sensitivity and hence vehicle undercarriage detection 

height. Tables E-4 and E-5 show that the number of loop turns should be five 

or more for detection of 4-ft-high (1.22-m-high) ground-clearance trucks when 

lead-in cable and steel mesh are present. 

Table E-4. Vehicle undercarriage detection height versus number of loop turns and spacing 

with lead-in cable and reinforcing steel mesh for 5- x 5-ft loop (calculated results) 

(see Table E-5 for metric units version). 

Loop 

turns 

Loop turn 

spacing 

(mils) 

Vehicle detection 

height with lead-in 

cable (ft)

Vehicle detection 

height with mesh 

(ft) 

Vehicle detection 

height with lead-in 

cable and mesh (ft)

1 NA 2.3 3.9 1.9 

2 2,000 3.5 4.3 3.0 

3 1,000 4.1 4.4 3.6 

4 660 4.4 4.4 3.9 

5 500 4.6 4.5 4.1 

6 400 4.7 4.5 4.2 

7 330 4.8 4.5 4.3 

8 286 4.9 4.5 4.3 

Conductor type: #14 AWG Cable 

Lead-in cable length: 250 ft 

Mesh spacing: 3,000 mils 

Detection sensitivity threshold: 0.098 percent 

Reinforcing steel mesh 

reduces loop sensitivity and 

hence vehicle detection 

height. 

Combined effects of lead-in 

cable inductance and steel 

mesh accentuate the 

reduction of loop sensitivity 

and hence vehicle detection 

height. 
 Traffic Detector Handbook—2006 

October 2006 Page E-13 Federal Highway Administration 

Table E-5. Vehicle undercarriage detection height versus number of loop turns and spacing 

with lead-in cable and reinforcing steel mesh for 1.52- x 1.52-m loop (calculated results) 

(see Table E-4 for inch-pound units version) 

Loop 

turns 

Loop turn 

spacing 

(mm) 

Vehicle detection height 

with lead-in cable 

(m) 

Vehicle detection

height with mesh 

(m) 

Vehicle detection height with 

lead-in cable and mesh 

(m) 

1 NA 0.70 1.19 0.58 

2 50.8 0.94 1.31 0.91 

3 25.4 1.25 1.34 1.10 

4 16.8 1.34 1.34 1.19 

5 12.7 1.40 1.37 1.25 

6 10.2 1.43 1.37 1.28 

7 8.4 1.46 1.37 1.31 

8 7.3 1.49 1.37 1.31 

Conductor type: #14 AWG Cable 

Lead-in cable length: 76.2 m 

Mesh spacing: 72.6 mm 

Detection sensitivity threshold: 0.098 percent 

RESULTS AND CONCLUSIONS 

A formula for calculating the vehicle detection sensitivity for a multiturn loop 

was presented. Results from this formula indicate that the vehicle detection 

sensitivity for a 6- x 6-ft (1.83- x 1.83-m), 3-turn loop of #14 AWG wire 

increases as the spacing between turns increases. Although a maximum 

detection height of 5.05 ft (1.54 m) occurs with a turn spacing of 2 inches 

(50.8 mm), a detection height of 4.99 ft (1.52 m) occurs at the more practical 

turn spacing of 1 inch (2.54 cm). A very small increase in detection height to 

5.06 ft (1.54 cm) was noted when #12 AWG wire was used instead of #14 

AWG wire. 

When the loop turns are spaced to use the maximum practical installation 

space in the pavement, the undercarriage detection height is approximately 

independent of the number of turns, provided that the loop inductance is 

approximately ten times larger than the inductance of the lead-in cable. For 

example, the self-inductance of a 6- x 6-ft (1.83- x 1.83-m), 3-turn loop of #14 

AWG wire with a turn spacing of 1 inch (25.4 mm) is 60.15 μH. If the 

maximum inductance of the lead-in cable is 6 μH and the maximum lead-in 

cable length is 27 ft (8.23 m), then the lead-in cable inductance is 0.22 μH/ft 

(0.72 μH/m). The self-inductance of a 6- x 6-ft (1.83- x 1.83-m), 6-turn loop of 

#14 AWG wire with a turn spacing of 0.4 inches (10.2 mm) is 226 μH, which 

allows a lead-in cable length of 104 ft (31.7 m) when constrained by the 

requirement to limit the lead-in cable inductance to no more than 23 μH (i.e., 

1/10 the value of the loop inductance). 

If a vehicle undercarriage detection height of 4.1 ft (1.25 m) is acceptable, 

then the 3-turn loop with a loop-turn spacing of 1 inch (25.4 mm) can be used 

with 250 ft (76.2 m) of lead-in cable, as shown in Table E-4. The presence of 

reinforcing steel 3 inches (76.2 mm) from the top loop turn reduces the 

vehicle detection height to 3.6 ft (1.10 m) for the 3-turn loop. 
Appendix E—Vehicle Detection Sensitivity Formulas for Rectangular Loops

October 2006 Page E-14 Federal Highway Administration 

A 5-turn loop spaced 0.5 inches (12.7 mm) between turns with a detector 

sensitivity of 0.098 percent has adequate vehicle detection height sensitivity 

for most applications. One possible method to obtain the 0.5-inch (12.7-mm) 

spacing is to use loop wire centered in a flexible, water-blocked, 0.5-inch 

(12.7-mm) height density polyethylene tube. Lead-in cables longer than 250 

ft (76.2 m) require increasing the sensitivity of the electronics unit to a value 

greater than 0.098 percent. 

REFERENCES 

1. Mills, M.K. “Inductive Loop Detector Analysis,” 31st IEEE VTG 

Conference Record. April 1981. 

2. Mills, M.K. “Self Inductance Formulas for Multi-Turn Rectangular 

Loops used with Vehicle Detectors,” 33rd IEEE VTG Conference 

Record. May 1983. pp 65–73. 

3. Mills, M.K. op. cit., p. 70. 

4. Link, Advanced Product Division. Link Inductance Loop Analysis 

Project, Final Report. August 1968. p. 21. 

5. Dorsey, W.F. Status Report on Vehicle Detectors, Report No. FHWARD-77-137. U.S. Department of Transportation, Federal Highway 

Administration, Washington, DC. November 1976. 

6. DeLaski, A.B., and P.S. Parsonson. Traffic Detector Handbook, 

Report No. FHWA-IP-81-1. U.S. Department of Transportation, 

Federal Highway Administration, Washington, DC. 1985. p. 113. 
 Traffic Detector Handbook—2006 

October 2006 Page F-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX F. 

APPENDIX F. DIGITAL FREQUENCY-SHIFT ELECTRONICS UNIT 

ANALYSIS .......................................................................................F-1

ABSTRACT...............................................................................................................F-1

ANALYSIS ................................................................................................................F-1

LIST OF FIGURES: APPENDIX F. 

Figure F-1. Digital frequency-shift electronics unit block diagram.............F-1

Figure F-2. Loop-inductive and capacitive elements at electronics unit 

terminals. ..................................................................................F-4

 Traffic Detector Handbook—2006 

October 2006 Page F-1 Federal Highway Administration 

APPENDIX F. DIGITAL 

FREQUENCY-SHIFT 

ELECTRONICS UNIT ANALYSIS 

ABSTRACT 

Appendix F describes the operation of the digital frequency-shift electronics 

unit, which measures the difference in oscillator frequency fD that occurs 

with and without a vehicle present. A frequency counter computes the 

frequency difference by determining a count difference that is proportional to 

the frequency difference. The frequency-shift electronics unit sensitivity is 

shown to be proportional to the threshold count Nft divided by the total 

number of counts Nfc in the frequency counter. 

ANALYSIS 

A simplified block diagram of a single-channel, digital frequency-shift 

electronics unit is illustrated in Figure F-1. The change in inductance at the 

electronics unit terminals causes a change in the oscillator frequency. The 

frequency is multiplied by the frequency multiplier. The resultant frequency 

is transformed into a frequency count by the frequency counter. 

Inductance at

Electronics Unit

Terminals

LD

Electronics Unit

Oscillator 

Frequency

f

D

Frequency

Multiplier

m

Frequency

Counter

Comparator

(Subtractor)

Output Call

(when ΔNfc

= Nft)

Transfer

Reset

Crystal

Clock 

Frequency

fnc

Comparator

(Subtractor)

Threshold

Sensitivity Divider

Reference

Frequency

Count

Nfc

nv

Nfc ΔNfc Nft v

•

mfD

One

Shot

One

Shot

Figure F-1. Digital frequency-shift electronics unit block diagram. 

The frequency counter is first initialized to zero. Frequency count data are 

then collected for a preset time frame. The first one-shot module then causes 

the frequency count to be transferred to the frequency counter and first 

comparator at the end of the time frame. The second one-shot module resets 

the frequency counter to zero so that it can collect data for the next time 

frame. The cycle then repeats. 

The first comparator subtracts the reference frequency count stored in 

memory, which corresponds to the count when no vehicle is present, from the 

frequency count just transferred, which may or may not correspond to the 

presence of a vehicle. The count difference is next sent to the second 

comparator. If the second comparator finds that the count difference is 

greater than the threshold sensitivity count, then a vehicle is presumed 

present and a call is output. 

Using a frequency counter, the 

frequency-shift electronics unit 

uses the change in inductance 

caused by a vehicle passing 

over a loop to measure the 

altered electronics unit 

oscillator frequency. 
Appendix F—Digital Frequency-Shift Electronics Unit Analysis

October 2006 Page F-2 Federal Highway Administration 

The reference frequency count in memory updates at a specified time interval 

whenever no change has occurred in the call status over that interval. 

The number of oscillator cycles counted by the frequency counter is 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = =

nc fc ft f

m f N m f T D

D (F-1)

where 

Tft = frame time of the frequency counter in s 

m = frequency multiplier 

fD = frequency of electronics unit oscillator in Hz 

fnc = crystal clock frequency in Hz. 

The frame time is assumed to be constant at each sensitivity setting. The 

response time of the digital frequency-shift electronics unit is equal to the 

frame time Tft, which in turn is equal to the inverse of the divided crystal 

clock frequency. 

For example, with a frame time of one second, the number of counts Nfc made 

by the frequency counter is the electronics unit oscillator frequency 

multiplied by m. 

Since the frequency of the electronics unit oscillator changes when a vehicle 

is present, the count given by the frequency counter will also change. Thus, 

the presence of a vehicle is ascertained by noting when the change in the 

output of the frequency counter is above some threshold. 

The reference count is measured with no vehicle present and is equal to 

nc

nv nv D

fc f

mf N = . (F-2)

With a vehicle present, the count becomes 

nc

v v D

fc f

mf N = . (F-3)

The electronics unit computes the difference in count ΔNfc with and without a 

vehicle present at the output of the first comparator as 

nv

fc

v ΔN fc = N fc − N . (F-4) 

Substituting Equations F-2 and F-3 into the above equation yields 

nc

nv

D

nc

v

D

f c f

m f

f

m f ΔN = − . (F-5) 

Factoring out m and fnc yields 

( ) v nv

nc fc fD fD f

m N − ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ = . (F-6) 
 Traffic Detector Handbook—2006 

October 2006 Page F-3 Federal Highway Administration 

Denoting ( ) v nv fD − fD by ΔfD, we get 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ Δ =

nc fc f

m f N D (F-7)

as the difference in count that occurs when a vehicle is present and not 

present. 

A vehicle call is output when the difference count ΔNfc equals the threshold 

count Nft. 

Accordingly when a call is made, one can write 

ΔN fc = N ft (F-8) 

or 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ =

nc

D

ft f

m f N . (F-9)

From Equation 2-51 in Chapter 2, the sensitivity of the digital frequency 

shift electronics unit is given by 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ = −

D

f D

D f

f S 2 . (F-10) 

Upon substituting ΔfD from Equation F-9 into Equation F-10, we obtain 

 

D

f ft nc

D mf

N f S −2 = (F-11) 

Since 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

π =

D D

D L C

f

2

1

, (F-12) 

the sensitivity becomes 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − π = m

N f L C

S f f t nc D D

D

4

. (F-13)

Thus, the sensitivity of the digital frequency-shift electronics unit is directly 

proportional to the threshold frequency count Nft, the crystal clock frequency 

fnc, and the square root of the product of the total inductance LD across the 

electronics unit terminals and total capacitance CD across the electronics unit 

terminals (CD includes internal tuning capacitance). Sensitivity is inversely 

proportional to the oscillator multiplier factor m. 

An equivalent electrical circuit showing the inductive and capacitive 

elements that contribute to LD and CD is shown in Figure F-2. 
Appendix F—Digital Frequency-Shift Electronics Unit Analysis

October 2006 Page F-4 Federal Highway Administration 

Active

devices:

Amplifiers,

filters, 

sensitivity 

control, etc.

Loop

inductance

LL

Lead-in cable

inductance LC

Oscillator

tank circuit

CD

CD = CSC + internal 

capacitance in 

electronics unit

LD = 

LL + LC

Electronics unit

output frequency CSC

CSC = wire-to-wire and 

wire-to-sawcut 

wall capacitance

Figure F-2. Loop-inductive and capacitive elements at electronics unit terminals. 

If we define 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − π = m

N f K ft nc

f

4

 , (F-14)

then 

f D D

f S D = K L C . (F-15)

Another form for the sensitivity equation is obtained by substituting 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =

nc

D

fc f

m f N (F-16)

from Equations F-2 or F-3 into Equation F-11 as 

mN f m

N f

m f

N f S

fc nc

ft nc

D

f ft nc

D /

2 2 = − = −

or 

fc

ft f

D N

N

S

2 = − . (F-17) 

Thus, the frequency shift electronics unit sensitivity is proportional to the 

frequency threshold count Nft divided by the total number of counts Nfc in the 

frequency counter. 
 Traffic Detector Handbook—2006 

October 2006 Federal Hi Page G-i ghway Administration 

TABLE OF CONTENTS: APPENDIX G. 

APPENDIX G. DIGITAL RATIOED FREQUENCY-SHIFT 

ELECTRONICS UNIT ANALYSIS ............................................. G-1

ABSTRACT...............................................................................................................G-1

ANALYSIS ................................................................................................................G-1

LIST OF FIGURES: APPENDIX G. 

Figure G-1. Digital ratioed frequency-shift electronics unit block 

diagram. ....................................................................................G-1 

 Traffic Detector Handbook—2006 

October 2006 Page G-1 Federal Highway Administration 

APPENDIX G. DIGITAL RATIOED 

FREQUENCY-SHIFT ELECTRONICS 

UNIT ANALYSIS 

ABSTRACT 

Appendix G describes the derivation of the digital ratioed frequency-shift 

electronics unit sensitivity, which is shown to be independent of the 

electronics unit operating frequency. The response time is proportional to the 

electronics unit oscillator frequency. 

ANALYSIS 

A simplified block diagram of a single-channel, digital ratioed frequency-shift 

electronics unit is illustrated in Figure G-1. 

Inductance at

Electronics Unit

Terminals

LD

Electronics Unit

Oscillator 

Frequency

fD

Frequency

Multiplier

m

Frequency

Counter

Comparator

(Subtractor)

Output

Call

Transfer

Reset

Crystal

Clock

Frequency

fnc

Variable

Frame Time

Logic

Comparator

(Subtractor)

Threshold

Sensitivity

Divider Frame 

Time

Reference

Frequency

Count

Increase Decrease Nfc

nv

Nfc ΔNfc Nft v

•

mfD

Figure G-1. Digital ratioed frequency-shift electronics unit block diagram. 

The sensitivity f

DS of a digital ratioed frequency-shift electronics unit is given 

by 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ − =

D

f fc nc

D m f

N f S2

 (G-1)

where the parameters in equation G-1 are defined in Figure G-1. 

If 

D fc

nc

m f N

f 1 = constant proportional to , (G-2) 

the sensitivity is independent of the electronics unit oscillator frequency fD. 

The frame time is automatically adjusted so that the no-call frequency count 

is constant. The sensitivity is changed by adjusting the frame time for a 

different number of frequency counts. 

The response time of the electronics unit is primarily dependent on the time 

required to fill the frequency counter and is equal to 

The ratioed frequency-shift 

electronics unit utilizes the 

change in inductance caused 

by a vehicle passing over a 

loop to measure the altered 

electronics unit oscillator 

frequency by using a 

frequency counter with a 

selected crystal clock 

frequency and frequency 

multiplier that make the 

sensitivity independent of the 

oscillator frequency. 
Appendix G—Digital Ratioed Frequency-Shift Electronics Unit Analysis

October 2006 Page G-2 Federal Highway Administration 

f

D D

ft

D

f nc

m f S

N

m f

f t

2 = = . (G-3) 

Since N ft and f

DS are constant, the response time changes in proportion to 

the electronics unit oscillator frequency. 
 Traffic Detector Handbook—2006 

October 2006 Page H-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX H. 

APPENDIX H. DIGITAL PERIOD-SHIFT ELECTRONICS UNIT 

ANALYSIS ...................................................................................... H-1

ABSTRACT.............................................................................................................. H-1

ANALYSIS ............................................................................................................... H-1

LIST OF FIGURES: APPENDIX H. 

Figure H-1. Digital period-shift electronics unit block diagram. ................ H-1

 Traffic Detector Handbook—2006 

October 2006 Page H-1 Federal Highway Administration

APPENDIX H. DIGITAL PERIODSHIFT ELECTRONICS UNIT 

ANALYSIS 

ABSTRACT 

Appendix H describes the derivation of the digital period-shift electronics 

unit sensitivity, which is shown to be proportional to the threshold count Npt

divided by the total number of counts Npc in the frequency counter. The 

response time is proportional to the threshold count, divided by the product 

of the sensitivity and the frequency of the crystal clock. 

ANALYSIS 

The oscillator in a digital period-shift electronics unit operates at a frequency

fD, which is determined by the loop, lead-in cable, and the electronics unit 

tuning network. The number m of oscillator cycles from the period scaler 

determines the length of the gating pulse applied to the AND gate. When the 

gating pulse is m times as long as the loop is occupied, more clock pulses are 

sent to the period counter, making the electronics unit more sensitive. 

A simplified block diagram of a single channel, digital period-shift vehicle 

electronics unit is illustrated in Figure H-1. The change in inductance at the 

electronics unit terminals caused by a vehicle produces a change in the 

oscillator frequency. The sine wave oscillator frequency is converted by a 

period scaler into a square wave. The square wave is input to an AND gate, 

which compares the signal level (i.e., high or low) to that of the crystal clock 

frequency pulse. 

Inductance at

Electronics Unit

Terminals

LD

Electronics Unit

Oscillator 

Frequency

fD

Period

Counter

Comparator

(Subtractor)

Output

Call

Sensitivity

Control

Period

Scaler

Comparator

(Subtractor)

Threshold

Control

Crystal Clock

Frequency

Reference

Period in

Memory

Npcnv

Npc ΔNpc Npt v

AND

Gate

Memory 

Update 

Logic

Figure H-1. Digital period-shift electronics unit block diagram. 

If both the square wave and the crystal clock pulse are high, a count is sent 

to the period counter. The count is equal to the number of clock pulses that 

occur in a period. 

The period-shift electronics 

unit utilizes the change in 

inductance caused by a 

vehicle passing over a loop to 

measure the altered 

electronics unit oscillator 

frequency using a counter to 

count the number of pulses 

that occur during the 

electronics unit oscillator 

period. 
Appendix H—Digital Period-Shift Electronics Unit Analysis 

October 2006 Page H-2 Federal Highway Administration 

At the end of the period, the count is sent to the first comparator. The 

comparator subtracts the reference period count stored in memory, which 

represents the count when no vehicle is present, from the current period 

count. The difference enters the second comparator. If the second comparator 

finds that the difference is in excess of the threshold sensitivity, then a 

vehicle is presumed present and a call is output. 

The period TDC of the electronics unit oscillator is 

TDC = mTD (H-1)

where 

TD = 1/fD = period of electronics unit oscillator in s 

fD = electronics unit oscillator frequency in Hz 

m = number of oscillator cycles from the period scaler. 

The length of the gating pulse is 

⎟

⎟

⎠

⎞ ⎜

⎜

⎝

⎛ ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = 2 = 2

DC D

g

T mT

T

 . (H-2)

The number Npc of clock pulses counted by the period counter is 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

C

D

C

g

pc T

mT

T

T

N

2 , (H-3) 

where TC is the period of the crystal clock equal to the inverse of the clock 

frequency. 

The number of clock pulses stored in the reference memory when no vehicle 

is present is 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

C

nv nv D

pc T

mT

N

2 . (H-4)

The number of clock pulses counted when a vehicle is present is 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

C

v v D

pc T

mT

N

2 . (H-5)

Since the loop inductance decreases when a vehicle is sensed, the electronics 

unit oscillator frequency fD increases and the oscillator period TD decreases. 

Then 

v

c nvc nvD v f D > f implies N > N . (H-6)

The output of the first comparator is 

( ) v

D nvD

C

v

c nv pc pc T T

T

m N N N − ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ = − = 2 . (H-7)

The count of the period 

counter is proportional to 

the electronics unit 

oscillator period TD. 
 Traffic Detector Handbook—2006 

October 2006 Page H-3 Federal Highway Administration

Since 

( ) v

D nv ΔTD = TD − T , (H-8) 

the output of the first comparator can be written as 

D

C

pc T

T

m N Δ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ = 2 . (H-9)

When the output count ∆Npc of the first comparator equals the threshold 

count Npt in the second comparator, a vehicle call results. 

Then, 

ΔN pc = N pt . (H-10)

Thus, 

D

C

pt T

T

m N Δ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = 2 . (H-11)

For a high quality factor tuned circuit, the normalized period shift is 

approximated by 

⎟

⎠

⎞ ⎜

⎝

⎛ = Δ

⎟

⎠

⎞ ⎜

⎝

⎛ =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ p

D

D

D

D

D S

L

L

T

T

2

1

2

1

 . (H-12)

Solving for sensitivity, we get 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ Δ =

D

p D

D T

T

S

2

 (H-13)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

D

pt C

mT

4N T

 . (H-14)

Since 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =

C

C f

T 1

 (H-15)

and 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =

D

D f

T 1

 (H-16)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

C

p pt D

D m f

N f

S

4

 (H-17)

when 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

π =

D D

D

L C

f

2

1

 (H-18)

The output count of the first 

comparator is proportional to 

the change ∆TD in the 

oscillator period. 
Appendix H—Digital Period-Shift Electronics Unit Analysis 

October 2006 Page H-4 Federal Highway Administration 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

π =

C D D

p pt

D m f L C

N

S

2

 . (H-19)

Letting 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

π =

C

pt

p m f

N

K

2

, (H-20) 

the sensitivity becomes 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

D D

p p

D L C

K

S (H-21)

since 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ =

D

C

C

D

pc f

m f

T

mT

N

2 2 (H-22) 

from Equation H-3, 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

pc

p pt

D N

N

S

2

 . (H-23)

The response time of the digital period-shift electronics unit is primarily due 

to the time required to fill the period counter with clock pulses. Thus, 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = =

C

pc

pc C

p

f

N

t N T . (H-24) 

Rewriting equation H-23 to solve for Npc, we get 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

= p

D

pt

pc S

N

N

2

 . (H-25) 

Upon substituting this result into equation H-24, the response time becomes 

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

=

C

p

D

p pt

S f

N

t

2

 . (H-26)

The sensitivity of the digital 

period-shift electronics unit 

is twice the threshold count 

divided by the maximum 

number of counts on the 

period counter. 
 Traffic Detector Handbook—2006 

October 2006 Page I-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX I. 

APPENDIX I. DIGITAL RATIOED PERIOD-SHIFT ELECTRONICS UNIT 

ANALYSIS ........................................................................................I-1

ABSTRACT................................................................................................................I-1

ANALYSIS .................................................................................................................I-1

LIST OF FIGURES: APPENDIX I. 

Figure I-1. Digital ratioed period-shift electronics unit block diagram........ I-1
 
 Traffic Detector Handbook—2006 

October 2006 Page I-1 Federal Highway Administration 

APPENDIX I. DIGITAL RATIOED 

PERIOD-SHIFT ELECTRONICS 

UNIT ANALYSIS

ABSTRACT 

Appendix I describes the derivation of the digital ratioed period-shift 

electronics unit sensitivity, which is shown to be equal to the threshold count 

Npt divided by 2. The response time is identical to the response time of the 

digital period-shift electronics unit. Thus, the response time is proportional 

to the threshold count divided by the product of the sensitivity and the 

frequency of the crystal clock oscillator. 

ANALYSIS 

A simplified block diagram of a single channel, digital ratioed period-shift 

electronics unit is illustrated in Figure I-1. This electronics unit is similar to 

the digital period shift electronics unit except for the variable threshold 

count, which allows the sensitivity to be independent of the electronics unit 

oscillator frequency. 

Inductance at

Electronics Unit

Terminals

LD

Electronics Unit

Oscillator 

Frequency

f

D

Period

Counter

Comparator

(Subtractor)

Output

Call

Memory 

Update 

Logic

Period

Scaler

Comparator

(Subtractor)

Threshold

Sensitivity

ST

Crystal Clock

Frequency

Reference

Period in

Memory

Npcnv

STNpt

ΔNpc

Npcv

AND

Gate

• •

Figure I-1. Digital ratioed period-shift electronics unit block diagram. 

The ratioed period-shift electronics unit sensitivity p

DS is given by 

pc

p pt

D N

N

S −2 = (I-1) 

where the threshold count Npt is a constant and the count established by the 

period counter Npc is inversely proportional to the electronics unit oscillator 

frequency. The oscillator frequency is a function of loop inductance. 

A variable threshold Npt is selected by multiplying the count established by 

the period counter by a preset sensitivity factor. Thus, 

pt t N pc N = S (I-2) 

The ratioed period- shift 

electronics unit utilizes the 

change in inductance caused 

by a vehicle passing over a 

loop to measure the altered 

electronics unit oscillator 

frequency using a counter to 

count the number of pulses 

that occur during the 

electronics unit oscillator 

period and a variable 

threshold count to make the 

sensitivity independent of the 

oscillator frequency. 
Appendix I—Digital Ratioed Period-Shift Electronics Unit Analysis 

October 2006 Page I-2 Federal Highway Administration 

or 

t

pc

t pc

pt S

N

S N

N 2

2 = − − = . (I-3) 
 Traffic Detector Handbook—2006 

October 2006 Page J-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX J. 

APPENDIX J. NEMA DETECTOR STANDARDS EXCERPTS ........................... J-1

INTRODUCTION ................................................................................................... J-1 

NEMA TS 1 AND TS 2 TRAFFIC CONTROL SYSTEMS ................................ J-2 

DETECTOR TERMS AND DEFINITIONS......................................................... J-2 

1.2.4 Detection................................................................................................ J-2 

1.2.4.1 Advisory Detection ....................................................................... J-2 

1.2.4.2 Passage Detection ......................................................................... J-2 

1.2.4.3 Presence Detection........................................................................ J-3 

1.2.5 Detector ................................................................................................. J-3 

1.2.5.1 Bidirectional Detector .................................................................. J-3 

1.2.5.2 Calling Detector............................................................................ J-3 

1.2.5.3 Classification Detector ................................................................. J-3 

1.2.5.4 Directional Detector ..................................................................... J-3 

1.2.5.5 Extension Detector ....................................................................... J-3 

1.2.5.6 Infrared Detector.......................................................................... J-3 

1.2.5.7 Light-Sensitive Detector............................................................... J-3 

1.2.5.8 Loop Detector ............................................................................... J-3 

1.2.5.9 Magnetic Detector ........................................................................ J-3 

1.2.5.10 Magnetometer Detector ............................................................. J-3 

1.2.5.11 Nondirectional Detector............................................................. J-4 

1.2.5.12 Pedestrian Detector .................................................................... J-4 

1.2.5.13 Pneumatic Detector .................................................................... J-4 

1.2.5.14 Pressure-Sensitive Detector ....................................................... J-4 

1.2.5.15 Radar Detector ........................................................................... J-4 

1.2.5.16 System Detector .......................................................................... J-4 

1.2.5.17 Side-Fire Detector....................................................................... J-4 

1.2.5.18 Sound-Sensitive Vehicle Detector.............................................. J-4 

1.2.5.19 Ultrasonic Detector..................................................................... J-4 

1.2.6 Detector Mode ...................................................................................... J-4 

1.2.7 Inductive Loop Detector System......................................................... J-4 

1.2.8 Inductive Loop Detector Unit.............................................................. J-4 

1.2.9 Lead-in Cable ....................................................................................... J-5 

1.2.10 Output ................................................................................................. J-5 

1.2.10.1 Extension Output........................................................................ J-5 

1.2.10.2 Delayed Output........................................................................... J-5 

1.2.11 Probe ................................................................................................... J-5 

1.2.12 Sensor .................................................................................................. J-5 

1.2.13 Vehicle Detector System .................................................................... J-5 

1.2.14 Zone of Detection (Sensing Zone) ..................................................... J-5 

ENVIRONMENTAL TESTING OF DETECTORS ............................................ J-5 

2.8 LOOP DETECTOR UNIT TESTS............................................................. J-5 

2.8.1 Environmental Requirements ............................................................. J-5 

2.8.1.1 Voltage, DC Supply ...................................................................... J-5 

2.8.1.2 Temperature and Humidity......................................................... J-6 

2.8.1.3 Transients, DC Powered Units .................................................... J-6 

2.8.1.4 Transients, Loop Detector Input Terminals .............................. J-6 

2.8.1.5 Vibration ....................................................................................... J-6 

2.8.1.6 Shock ............................................................................................. J-8 

MINIMUM REQUIREMENTS FOR DETECTORS .......................................... J-8 

6.5 INDUCTIVE LOOP DETECTOR UNITS................................................. J-9 

6.5.1 Loop Detector Unit Definitions ............................................................ J-9 

6.5.1.1 Channel........................................................................................... J-9 

6.5.1.2 Crosstalk......................................................................................... J-9 

Numbered headings 

correspond to the numbering 

of sections in the NEMA 

standard. Unnumbered 

sections are narrative 

explaining the context of the 

document. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-ii Federal Highway Administration 

6.5.1.3 Detector Mode................................................................................ J-9 

6.5.1.4 Lead-In Cable ................................................................................ J-9 

6.5.1.5 Loop Detector System ................................................................... J-9 

6.5.1.6 Loop Detector Unit ........................................................................ J-9 

6.5.1.7 Reset Channel .............................................................................. J-10 

6.5.1.8 Reset Unit ..................................................................................... J-10 

6.5.1.9 Sensor Loop.................................................................................. J-10 

6.5.1.9 Vehicle Detector System.............................................................. J-10 

6.5.1.9 Zone of Detection......................................................................... J-10 

6.5.2 Functional Standards.......................................................................... J-10 

6.5.2.1 Operation...................................................................................... J-10 

6.5.2.2 Configurations and Dimensions ................................................. J-10 

6.5.2.2.1 Configurations ....................................................................... J-10

6.5.2.2.2 Dimensions ............................................................................ J-11

6.5.2.3 Accessibility.................................................................................. J-12 

6.5.2.4 Material and Construction of Rigid Printed Circuit 

Assemblies ................................................................................................ J-12 

6.5.2.4.1 Materials ................................................................................ J-12

6.5.2.4.2 Component Identification ...................................................... J-12

6.5.2.5 Power Inputs ................................................................................ J-12 

6.5.2.5.2 Low Supply Voltage Automatic Reset................................... J-13

6.5.2.6 Logic Ground ............................................................................... J-13 

6.5.2.7 Earth Ground............................................................................... J-13 

6.5.2.8.1 Low or Active State ............................................................... J-13

6.5.2.8.2 High or Inactive State ............................................................ J-13

6.5.2.8.3 Transition Voltage Zone Of Input Circuitry .......................... J-13

6.5.2.8.4 External Transition Time ....................................................... J-13

6.5.2.8.5 Maximum Current.................................................................. J-13

6.5.2.8.6 Signal Recognition................................................................. J-13

6.5.2.8.7 Activation of Delay/Extension Feature.................................. J-14

6.5.2.8.8 Activation of Detector Unit Address Feature ........................ J-14

6.5.2.9 Data Receive (RX) Input .......................................................... J-14

6.5.2.9.1 Mark State (Binary 1) ............................................................ J-14

6.5.2.9.2 Space State (Binary 0) ........................................................... J-14

6.5.2.9.3 Other States............................................................................ J-14

6.5.2.9.4 Transient Withstand............................................................... J-14

6.5.2.10 Loop Inputs ................................................................................ J-14 

6.5.2.11 Loop/Lead in Electrical Properties .......................................... J-14 

6.5.2.12Test Loop Configurations .......................................................... J-15 

6.5.2.13 Test Vehicle Definition .............................................................. J-15 

6.5.2.14 Sensitivity ................................................................................... J-16 

6.5.2.15 Sensitivity Control ..................................................................... J-16 

6.5.2.16 Approach Speed......................................................................... J-16 

6.5.2.17 Modes of Operation................................................................... J-17 

6.5.2.17.1 Presence ............................................................................... J-17

6.5.2.17.2 Pulse..................................................................................... J-17

6.5.2.18 Recovery from Sustained Occupancy ...................................... J-17 

6.5.2.19 Response Time ........................................................................... J-17 

6.5.2.19.1 Variation in Response Time................................................. J-17

6.5.2.20 Tuning......................................................................................... J-18 

6.5.2.21 Self-Tracking.............................................................................. J-18 

6.5.2.22 Recovery From Reset ................................................................ J-18 

6.5.2.23 Crosstalk Avoidance.................................................................. J-18 

6.5.2.24 Delay/Extension ......................................................................... J-18 

6.5.2.24.1 Delay.................................................................................... J-18
 Traffic Detector Handbook—2006 

October 2006 Page J-iii Federal Highway Administration 

6.5.2.24.2 Extension ............................................................................. J-19

6.5.2.25 Controls and Indicators ............................................................ J-19 

6.5.2.26 Outputs....................................................................................... J-20 

6.5.2.26.1 Solid State Channel Detection Outputs ............................... J-20

6.5.2.26.2 Channel Status Outputs........................................................ J-20

6.5.2.26.3 Channel Status Reporting .................................................... J-21

6.5.2.26.4 Data Transmit Output (TX) ................................................. J-22

6.5.2.26.4.1 Mark State (Binary 1) .................................................. J-22

6.5.2.26.4.2 Space State (Binary 0) ................................................. J-22

6.5.2.26.4.3 High Impedance State .................................................. J-22

6.5.2.26.4.4 Output Impedance During Power Off.......................... J-22

6.5.2.26.4.5 TX Output Shorts......................................................... J-23

6.5.2.26.4.6 Rise/Fall Time.............................................................. J-23

6.5.2.26.4.7 Transient Withstand..................................................... J-23

6.5.2.27 Communication Port Functional Requirements..................... J-23 

6.5.2.27.1 Communication Port Electrical Requirements..................... J-23

6.5.2.27.2 Baud Rate ............................................................................ J-23

6.5.2.27.3 Communication Parameters................................................. J-23

6.5.2.27.4 Slot Addresses ..................................................................... J-23

6.5.2.28 Electrical Connections............................................................... J-24 

6.5.2.28.1 Connector Description......................................................... J-24

6.5.2.28.2 Connector Terminations ...................................................... J-24

6.5.2.28.3 Type A Two Channel Without Delay / Extension Timing .. J-24

6.5.2.28.4 Type B Four Channel Without Delay / Extension Timing .. J-24

6.5.2.28.4 Type C Two Channel With Delay / Extension Timing........ J-25

LIST OF FIGURES: APPENDIX J. 

Figure 2-4 Test Configurations.................................................................. J-6

Figure 2-5 Loop Input Terminal Transient Tests..................................... J-7

Figure 6-4 Two Channel Card Rack Unit................................................ J-11

Figure 6-5 Four Channel Card Rack Unit............................................... J-12

Figure 6-6 Test Loop Configurations....................................................... J-16

Figure 6-7 Delay Operation...................................................................... J-19

Figure 6-8 Extension Operation............................................................... J-19

LIST OF TABLES: APPENDIX J. 

Transient Test Configurations ....................................................................... J-8

Table 6-1 DETECTOR UNIT TYPES........................................................... J-11

Table 6-2 CONNECTOR TERMINATIONS ................................................ J-24

Numbers for figures and 

tables correspond to the 

numbering in the NEMA 

standard. Therefore, figure 

and table numbering are 

NOT sequential in this 

appendix. 

NEMA captions are in all 

capital letters. 

 Traffic Detector Handbook—2006 

October 2006 Page J-1 Federal Highway Administration 

APPENDIX J. NEMA DETECTOR 

STANDARDS EXCERPTS 

INTRODUCTION 

The following excerpts are provided from NEMA Standards Publication TS 2-

2003 v02.06, Traffic Controller Assemblies with NTCIP [National 

Transportation Communications for Intelligent Transportation Systems 

(ITS) Protocol] Requirements. TS 2-2003 v02.06 is a revised and reballoted 

version of TS 2-1998. Version v02.06 includes the revisions from TS 2 

amendment 1 v01 and also includes minor revisions to sections 3, 5, and 6. 

TS 2-2003 v02.06 was balloted in January 2003 and approved by NEMA in 

May 2003. 

NEMA TS 2-2003 is published by the National Electrical Manufacturers 

Association, 1300 North 17th Street, Suite 1847, Rosslyn, VA 22209. For 

further information and to order a complete copy of the standard, visit 

www.nema.org. NEMA TS 2 was developed and is maintained by the 

member companies in the NEMA Transportation Management Systems and 

Associated Control Devices Product Group. 

This appendix is reprinted by permission from the National Electrical 

Manufacturers Association. Excerpt is © 2003 by NEMA. All rights reserved. 

For clarity, text in [square brackets] has been revised from the original. 

NOTES: 

1. NEMA standards have headers and text set in Arial, normal, 10-

point type. Heading, figure and table numbers and font type in 

this appendix correspond to those in the actual NEMA standard 

rather than those found in rest of the Traffic Detector Handbook. 

2. The term detector as used by NEMA is replaced by the terms 

sensor or electronics unit, depending on context, in the other 

chapters and sections of this Handbook. 

3. In case of conflict with Detector Terms and Definitions beginning 

on page J-2, those listed in Chapter 1 and Appendix P are 

recommended for the applications described in this Handbook. 

This NEMA standard excerpt 

is reprinted with permission of 

the National Electrical 

Manufacturers Association, 

1300 North 17th Street,Suite 

1847, Rosslyn, VA 22209. For 

further information, and to 

order a complete copy of the 

standard, visit www.nema.org
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-2 Federal Highway Administration 

NEMA TS 1 AND TS 2 TRAFFIC CONTROL SYSTEMS 

The NEMA TS 2 standard expands on the older NEMA TS 1 Traffic Control 

Systems standard. The TS1 standard was based on the philosophy that 

controllers would provide a basic set of features and standard connectors. 

Manufacturers would compete based on the hardware and software they 

provided inside the controllers. The NEMA TS1 standard was successful for 

isolated actuated intersection control, but it lacked sufficient detail for 

implementing more advanced features, such as coordinated-actuated 

operation and preemption. Type 1 systems include the controller unit, 

conflict monitor, and the included features of each. Individual vendors 

supplemented the TS 1 standard by providing the complement of features 

necessary for deploying coordinated-actuated traffic signal systems. This 

introduced incompatibility and procurement issues, particularly when 

government agencies needed to upgrade existing signal systems at a later 

date and had to solicit competitive bids. Nevertheless, the competitive 

market forces continued to rapidly advance the state of the practice and 

created a following that led many States to adopt the NEMA standard. In the 

late 1980s and early 1990s, the NEMA TS1 specification was updated with 

NEMA TS2 to provide coordinated-actuated operation, preemption, and an 

optional serial bus to simplify wiring.(1) 

The detectors, referred to as electronics units in other chapters of this 

Handbook, are described in the TS 2 standard by the following attributes: 

• Rack mounted, 16 detector channels per rack, up to four racks. 

• Port 1 connector: pluggable, interchangeable Bus Interface unit to convert 

Port 1 high speed serial data to format required by individual detectors. 

• Communications with controller unit, as described above. 

• Per channel diagnostic data: open loop; shorted loop; excessive 

inductance change; watchdog failure. 

• Detector reset capability. 

• Operation from either 12 volt or 24 volt DC power supply. 

DETECTOR TERMS AND DEFINITIONS 

These definitions reflect the consensus of the traffic control equipment industry 

as represented by NEMA and are intended to be in harmony with terminology in 

current usage, such as is published in the Manual on Uniform Traffic Control 

Devices and various technical reports of the Institute of Transportation 

Engineers. 

1.2.4 Detection 

1.2.4.1 Advisory Detection 

The detection of vehicles on one or more intersection approaches solely for the 

purpose of modifying the phase sequence and/or length for other approaches to 

the intersection. 

1.2.4.2 Passage Detection 

The ability of a vehicle detector to detect the passage of a vehicle moving 

through the zone of detection and to ignore the presence of a vehicle stopped 

within the zone of detection. 
 Traffic Detector Handbook—2006 

October 2006 Page J-3 Federal Highway Administration 

1.2.4.3 Presence Detection 

The ability of a vehicle detector to sense that a vehicle, whether moving or 

stopped, has appeared in its zone of detection. 

1.2.5 Detector 

A device for indicating the presence or passage of vehicles or pedestrians. 

1.2.5.1 Bidirectional Detector 

A detector that is capable of being actuated by vehicles proceeding in either of 

two directions and of indicating in which of the directions the vehicles were 

moving. 

1.2.5.2 Calling Detector 

A detector that is installed in a selected location to detect vehicles which may not 

otherwise be detected and whose output may be modified by the controller unit. 

1.2.5.3 Classification Detector 

A detector that has the capability of differentiating among types of vehicles. 

1.2.5.4 Directional Detector 

A detector that is capable of being actuated only by vehicles proceeding in one 

specified direction. 

1.2.5.5 Extension Detector 

A detector that is arranged to register actuations at the controller unit only during 

the green interval for that approach so as to extend the green time of the 

actuating vehicles. 

1.2.5.6 Infrared Detector 

A detector that senses radiation in the infrared spectrum. 

1.2.5.7 Light-Sensitive Detector 

A detector that utilizes a light-sensitive device for sensing the passage of an 

object interrupting a beam of light directed at the sensor. 

1.2.5.8 Loop Detector 

A detector that senses a change in inductance of its inductive loop sensor by the 

passage or presence of a vehicle near the sensor. 

1.2.5.9 Magnetic Detector 

A detector that senses changes in the earth's magnetic field caused by the 

movement of a vehicle near its sensor. 

1.2.5.10 Magnetometer Detector 

A detector that measures the difference in the level of the earth's magnetic forces 

caused by the passage or presence of a vehicle near its sensor. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-4 Federal Highway Administration 

1.2.5.11 Nondirectional Detector 

A detector that is capable of being actuated by vehicles proceeding in any 

direction. 

1.2.5.12 Pedestrian Detector 

A detector that is responsive to operation by or the presence of a pedestrian. 

1.2.5.13 Pneumatic Detector 

A pressure-sensitive detector that uses a pneumatic tube as a sensor. 

1.2.5.14 Pressure-Sensitive Detector 

A detector that is capable of sensing the pressure of a vehicle passing over the 

surface of its sensor. 

1.2.5.15 Radar Detector 

A detector that is capable of sensing the passage of a vehicle through its field of 

emitted microwave energy. 

1.2.5.16 System Detector 

Any type of vehicle detector used to obtain representative traffic flow information. 

1.2.5.17 Side-Fire Detector 

A vehicle detector with its sensor located to one side of the roadway. 

1.2.5.18 Sound-Sensitive Vehicle Detector 

A detector that responds to sound waves generated by the passage of a vehicle 

near the surface of the sensor. 

1.2.5.19 Ultrasonic Detector 

A detector that is capable of sensing the passage or presence of a vehicle 

through its field of emitted ultrasonic energy. 

1.2.6 Detector Mode 

A term used to describe the operation of a detector channel output when a 

presence detection occurs. 

1. Pulse Mode: Detector produces a short output pulse when detection occurs. 

2. Controlled Output: The ability of a detector to produce a pulse that has a 

predetermined duration regardless of the length of time a vehicle is in the 

zone of detection. 

3. Continuous-Presence Mode: Detector output continues if any vehicle (first or 

last remaining) remains in the zone of detection. 

4. Limited-Presence Mode: Detector output continues for a limited period of time 

if vehicles remain in zone of detection. 

1.2.7 Inductive Loop Detector System 

See 1.6.56.5.1.5. 

1.2.8 Inductive Loop Detector Unit 
 Traffic Detector Handbook—2006 

October 2006 Page J-5 Federal Highway Administration 

See 6.5.1.6. 

1.2.9 Lead-in Cable 

See 1.6.56.5.1.4 

1.2.10 Output 

1.2.10.1 Extension Output 

The ability of a detector to continue its output for a predetermined length of time 

after the vehicle has left its zone of detection. 

1.2.10.2 Delayed Output 

The ability of a detector to delay its output for a predetermined length of time 

after a vehicle has entered its zone of detection. 

1.2.11 Probe 

The sensor form that is commonly used with a magnetometer type detector. 

1.2.12 Sensor 

The sensing element of a detector. 

1.2.13 Vehicle Detector System 

See 6.5.1.10. 

1.2.14 Zone of Detection (Sensing Zone) 

That area of the roadway within which a vehicle will be detected by a vehicle 

detector system. 

ENVIRONMENTAL TESTING OF DETECTORS 

 [NEMA TS 2-2003] Section 2 relates to environmental standards and operating 

conditions for intersection traffic control equipment. This section establishes the 

limits of the environmental and operation conditions in which the Controller 

Assembly will perform. This section defines the minimum test procedures which 

may be used to demonstrate conformance of a device type with the provisions of 

the standard. These test procedures do not verify equipment performance under 

every possible combination of environmental requirements covered by this 

standard. 

2.8 LOOP DETECTOR UNIT TESTS 

The Loop Detector Unit shall perform its specified functions under the conditions 

set forth in this Section. This Clause defines the test procedures required to 

demonstrate the conformance of a Loop Detector Unit with the provisions of the 

standards. 

2.8.1 Environmental Requirements 

Loop detector units shall operate in accordance with requirements listed herein 

under the following environmental conditions. 

2.8.1.1 Voltage, DC Supply 

1. Voltage Range—The voltage range shall be 10.8 VDC minimum to 26.5 VDC 

maximum. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-6 Federal Highway Administration 

2. Ripple—The maximum supply ripple shall be 500 millivolts peak to peak. 

2.8.1.2 Temperature and Humidity 

Temperature and humidity shall be in accordance with 2.1.5. 

2.8.1.3 Transients, DC Powered Units 

Loop detector units shall operate normally when the test impulse described in 

2.1.7 is applied as follows: 

1. Between Logic Ground and the DC Supply power input. The test setup 

shown in Figure 2-4 shall be used for this test. 

2. Across the output terminals of each channel while in both the detect and nondetect condition. 

3. Between Logic Ground and the control inputs. 

Detector loop inputs are specifically excluded from this test. 

Figure 2-4 

TEST CONFIGURATIONS 

TEST CONDITIONS: 

The transient generator is described in 2.1.7. 

The input voltage shall be 10.8 to 26.5 volts DC measured at the input terminal to the loop detector 

unit under test. 

The DC power source must be capable of supplying at least 50 milliamperes per channel. 

When testing for the reverse polarity transient, the diode shown shall be reversed. 

2.8.1.4 Transients, Loop Detector Input Terminals 

The loop detector unit shall be capable of withstanding the eight nondestructive 

transient tests described in Figure 2-5. Each test shall be repeated 10 times with 

the loop detector unit operating from its normal power source. The time between 

repetitions shall not exceed 10 seconds. 

The energy source shall be a capacitor, oil filled, 10 microfarads + 5 percent, 

internal surge impedance less than 1 ohm connected in accordance with Figure 

2-5. The voltage on the capacitor shall be adjusted as described herein. The test 

push button shall be activated for at least one second for each of the 80 test 

repetitions. 

After the foregoing tests, the loop detector unit shall operate normally. 

In the NEMA standard, 

tables and figures are 

delineated by a double 

ruled bar at the top and 

bottom of the table or 

figure. When the figure or 

table has notes at the 

bottom, the double rule is 

below the notes. This 

delineation is kept in this 

appendix for consistency 

with the standard. 
 Traffic Detector Handbook—2006 

October 2006 Page J-7 Federal Highway Administration 

1. Test Numbers 1 and 5. The Loop detector unit shall withstand the discharge 

of a 10 microfarad capacitor charged to + 1000 volts applied directly across 

the loop detector unit sensor loop input pins with no sensor loop load present. 

2. Test Numbers 2, 3, 4, 6, 7, and 8. The loop detector unit shall withstand the 

discharge of a 10 microfarad capacitor charged to + 2000 volts applied 

directly across either the loop detector unit sensor loop input pins or from 

either side of the sensor loop input pins to equipment ground. The loop 

detector unit sensor loop input pins shall have a dummy resistive load 

attached equal to 5.0 ohms + 10 percent. 

Figure 2-5 

LOOP INPUT TERMINAL TRANSIENT TESTS 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-8 Federal Highway Administration 

Transient Test Configurations

Test 

Number 

Test Selector

Position 

Power Supply

Setting +5%

Polarity 

Selector 

Load Across 

Detector Input 

Tested 

Inputs 

1 1 1000 Positive None D to E 

2 2 2000 Positive 5 ohms D to E 

3 3 2000 Positive 5 ohms D to L 

4 4 2000 Positive 5 ohms E to L 

5 1 1000 Negative None D to E 

6 2 2000 Negative 5 ohms D to E 

7 3 2000 Negative 5 ohms D to L 

8 4 2000 Negative 5 ohms E to L 

*The pin designations shown are for Channel 1. Similar tests shall be performed on all channels (i.e., 

pin pairs J and K, P and R, and U and V, respectively and as applicable) of the loop detector unit. 

2.8.1.5 Vibration 

The loop detector unit shall maintain both its physical integrity and operating 

characteristics after being subjected to the vibration test in 2.2.8. This test shall 

be run at nominal voltage and room environmental conditions. 

2.8.1.6 Shock 

The loop detector unit shall suffer neither permanent mechanical deformation nor 

any damage which renders it permanently inoperable after being subjected to the 

shock test described in 2.2.9. 

This test shall be run at room environmental conditions without power applied to 

the unit. 

MINIMUM REQUIREMENTS FOR DETECTORS 

[NEMA TS 2-2003 Section 6] defines the minimum requirements for auxiliary 

devices within the cabinet, consisting of solid state load switches, solid state 

flashers, flash transfer relays, and inductive loop detector units. 

[In the] TS 2-1998 update of TS 2-1992, the SECTION 6 [was revised to 

include these changes in detector specifications:] 

Detector Configurations has been modified to add four new types (AC, BC, 

CC, & DC) with communications port TX & RX capability. 

Detection Outputs & Status Outputs condition has been added for the Disable 

and Reset states. 

Detector Connector Terminations has been modified to add Detector Address 

Bit #3. 

The Transient Test 

Configurations data are 

not listed as a table in the 

NEMA specifications. 

Therefore, for consistency 

they are not listed as a 

table in this appendix. 
 Traffic Detector Handbook—2006 

October 2006 Page J-9 Federal Highway Administration 

6.5 INDUCTIVE LOOP DETECTOR UNITS 

Clause 0 responds to the need for a series of Inductive Loop Detector Units 

which provide inputs for traffic-actuated or traffic-responsive control, surveillance, 

or data collection systems. The Inductive Loop Detector Unit responds to the 

presence of vehicles on the roadway by relying upon the effect of the conductive 

mass of the vehicle on the alternating magnetic field of a loop. When a vehicle 

passes over the loop of wire embedded in the surface of the roadway, it reacts 

with the alternating magnetic field which is associated with that loop. On 

standard loops this reaction is a reduction in loop inductance. 

These standards cover the performance and design requirements of 

interchangeable Inductive Loop Detector Units. A Detector Unit used with a 

sensor loop embedded in the surface of a roadway detects vehicles moving or 

standing in the detection zone of the sensor loop. The output of the Detector Unit 

may be used directly to provide an input to a vehicle-actuated traffic CU or 

provide inputs to traffic-responsive control and surveillance systems. Detector 

Units generate outputs indicative of vehicles passing through the sensor loop 

zone of detection. This output may be used for counting (volume) or for detecting 

presence time representative of the time that vehicles are in the sensor loop 

zone of detection (occupancy), or both. When two loops are placed in a lane, 

one downstream from the other, the time between the sequential responses of 

detectors attached to these loops may be used to measure speed. 

6.5.1 Loop Detector Unit Definitions 

6.5.1.1 Channel 

Electronic circuitry which functions as a Loop Detector System. 

6.5.1.2 Crosstalk 

The adverse interaction of any channel of a Detector Unit with any other channel. 

6.5.1.3 Detector Mode 

A term used to describe the duration and conditions of the occurrence of a 

detector output. 

6.5.1.4 Lead-In Cable 

The electrical cable which serves to connect the sensor loop(s) to the input of the 

Detector Unit. 

6.5.1.5 Loop Detector System 

A vehicle detector system that senses a decrease in inductance of its sensor 

loop(s) during the passage or presence of a vehicle in the zone of detection of 

the sensor loop(s). 

6.5.1.6 Loop Detector Unit 

An electronic device which is capable of energizing the sensor loop(s), of 

monitoring the sensor loop(s) inductance, and of responding to a predetermined 

decrease in inductance with output(s) which indicates the passage or presence 

of vehicles in the zone(s) of detection. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-10 Federal Highway Administration 

6.5.1.7 Reset Channel 

A command for the Detector Unit to calculate a new reference frequency (the 

frequency that the loop oscillates at when no vehicle is influencing the loop) for 

the channel being reset and to appropriately adjust other channel related 

parameters. 

6.5.1.8 Reset Unit 

A command for the Detector Unit to set all parameters to the states they would 

be set at if power had been applied at the moment the RESET command is 

received or released. 

6.5.1.9 Sensor Loop 

An electrical conductor arranged to encompass a portion of roadway to provide a 

zone of detection and designed such that the passage or presence of a vehicle in 

the zone causes a decrease in the inductance of the loop that can be sensed for 

detection purposes. 

6.5.1.9 Vehicle Detector System 

A system for indicating the presence or passage of vehicles. 

6.5.1.9 Zone of Detection 

That area of the roadway within which a vehicle is detected by a vehicle detector 

system. 

6.5.2 Functional Standards 

6.5.2.1 Operation 

The Detector Unit defined and described in this standard shall respond to 

changes in the inductance of the sensor loop/lead-in combination(s) connected 

to its loop input terminals. It shall develop a detection output when there is a 

sufficiently large decrease in the magnitude of the connected inductance. 

The sensor loop(s) connected to the Detector Unit input terminals shall be 

located at the intended zone(s) of detection. The sensor loop(s) shall be 

connected to the Detector Unit by means of lead-in cable. 

The sensor loop(s) shall be so configured that the presence of a vehicle in each 

zone of detection causes a sufficient decrease in inductance to cause an output 

response from the Detector Unit. 

6.5.2.2 Configurations and Dimensions 

6.5.2.2.1 Configurations 

This standard covers Detector Unit configurations shown in Table 6-1. 
 Traffic Detector Handbook—2006 

October 2006 Page J-11 Federal Highway Administration 

Table 6-1 DETECTOR UNIT TYPES 

Rack Mount Detector 

Unit Type 

NEMA Detector Unit 

Designations 

Delay/Extension 

Timing 

Communication 

Port RX & TX 

2 Channel Type A None No 

4 Channel Type B None No 

2 Channel Type C Included No 

4 Channel Type D Included No 

2 Channel Type AC None Yes 

4 Channel Type BC None Yes 

2 Channel Type CC Included Yes 

4 Channel Type DC Included Yes 

Each channel shall be provided with independent loop input terminals and shall 

deliver detection information on independent output terminals. 

6.5.2.2.2 Dimensions 

1. Two-channel card rack units shall be 28.96 mm (1.14 in.) max. W x 114.3 mm 

(4.5 in.) H x 177.8 mm (7.00 in.) D, excluding the handle as shown in Figure 6-4. 

Figure 6-4 

TWO CHANNEL CARD RACK UNIT 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-12 Federal Highway Administration 

2. Four-channel card rack units shall be 59.44 mm (2.34 in.) max. W x 114.3 mm 

(4.5 in.) H x 177.8 mm (7.00 in.) D, excluding the handle as shown in Figure 6-5. 

Figure 6-5 

FOUR CHANNEL CARD RACK UNIT 

6.5.2.3 Accessibility 

The Detector Unit shall be easily disassembled to gain access for maintenance. 

When thus disassembled, the Detector Unit shall be operational for trouble 

shooting. 

6.5.2.4 Material and Construction of Rigid Printed Circuit Assemblies 

6.5.2.4.1 Materials 

All printed circuit boards shall be per 3.2.3.1. 

6.5.2.4.2 Component Identification 

All components shall be identified per 3.2.3.3. 

6.5.2.5 Power Inputs 

Detector Unit DC Supply Voltage 

1. Voltage Range–The voltage range shall be 10.8 VDC minimum to 26.5 VDC 

maximum. 

2. Ripple–The maximum supply ripple shall be 500 millivolts peak to peak. 

This input supplies power. The current consumption shall not exceed 50 

milliamperes per channel. The return for this input is Logic Ground as described 

in 6.5.2.6. This input shall not be connected within the unit to any loop input. The 

input shall draw a surge current not to exceed 5 amperes at the time of power 

application to the input. The Detector Unit shall not be damaged by insertion to or 

removal from a powered Detector Rack. 
 Traffic Detector Handbook—2006 

October 2006 Page J-13 Federal Highway Administration 

6.5.2.5.2 Low Supply Voltage Automatic Reset 

A Reset Unit condition shall be activated anytime the input DC supply voltage 

falls below that which is required for operation, as defined in this standard. 

A Reset Unit condition shall not be activated when the DC Supply Input falls 

below 10 volts for less than 0.25 milliseconds. 

6.5.2.6 Logic Ground 

This input is the return for the Detector Unit DC Supply input. This point shall not 

be connected within the unit to AC Neutral, Earth Ground, or to any loop input 

terminal. 

6.5.2.7 Earth Ground 

The loop Detector Unit shall have a terminal for connection to the chassis of the 

unit. This input shall not be connected to Logic Ground, AC Neutral, or to any 

other point within the unit, except that it shall be permissible to use this input as a 

return for transient protection devices. 

If the unit has a metallic case or front panel, the case and/or front panel shall be 

connected to Earth Ground. 

6.5.2.8 DC Control Inputs 

Control inputs shall have the following characteristics as measured from Logic 

Ground. 

6.5.2.8.1 Low or Active State 

A voltage between 0 and 8 volts shall be considered the Low or active state. 

6.5.2.8.2 High or Inactive State 

A voltage greater than 16 volts shall be considered the High or inactive state. 

6.5.2.8.3 Transition Voltage Zone Of Input Circuitry 

Transition zone of input circuitry from Low state to High state and vice versa 

shall occur between 8 and 16 volts. 

6.5.2.8.4 External Transition Time 

External transition from Low state to High state and vice versa shall be 

accomplished within 0.1 milliseconds. 

6.5.2.8.5 Maximum Current 

Over the voltage range 0 to 26 volts DC, the maximum current In or Out of any 

input control terminal shall be less than 10 milliamperes. The input circuitry shall 

be returned to the Detector Unit DC supply in such a manner that the removal of 

all connections to the input shall allow the voltage at the input terminal to rise to 

the High or inactive state. Rising to Detector Unit DC supply voltage is permitted. 

6.5.2.8.6 Signal Recognition 

Any input signal (including External Reset, Pin C) shall respond as defined in 

3.3.5.1.3. 

All channels shall remain in a RESET UNIT or RESET CHANNEL condition, see 

6.5.1.7 and 6.5.1.8, while Pin C is held in an Active State (Low) voltage. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-14 Federal Highway Administration 

6.5.2.8.7 Activation of Delay/Extension Feature 

The application of a Low state voltage to a Delay/Extension input shall function 

to inhibit the delay timing function and/or enable the extend timing function. 

This input is provided for downward compatibility. (Authorized Engineering 

Information.) 

6.5.2.8.8 Activation of Detector Unit Address Feature 

The application of a Low state voltage to a Detector Unit Address input shall 

activate the input, e.g. it is a logic 1. Four address input pins shall provide for a 

maximum of 15 hard wired addresses. Address FF(hex) shall be a broadcast 

address. 

6.5.2.9 Data Receive (RX) Input 

The Data Receive input, RX, must have an input impedance 45 kohm to 105 kohm 

and an input capacitance less than 250 picofarads. This input impedance shall 

terminate to DC common. The input open circuit voltage shall be less than 0.9 

VDC. The input shall be functional when voltages over the range of ±25 VDC are 

applied to it. The Transition Region is any voltage between a Mark and a Space. 

6.5.2.9.1 Mark State (Binary 1) 

During Mark, the RX input shall recognize as a Mark voltages less than 0.9 VDC. 

6.5.2.9.2 Space State (Binary 0) 

During Space, the RX input shall recognize as a Space voltages greater than 3.0 

VDC. 

6.5.2.9.3 Other States 

The RX input shall not be damaged when connected to any voltage up to 26 VDC 

and: 

1. Shall rise to >8.5 VDC when connected to +12 VDC through 30 kohm. 

2. Shall rise to >16 VDC when connected to +22 VDC through 11 kohm. 

6.5.2.9.4 Transient Withstand 

The RX input shall not be damaged by application of the transients described in 

2.1.7.1 through 2.1.7.5. 

6.5.2.10 Loop Inputs 

Two loop input terminals shall be provided for each sensor channel. These inputs 

shall be isolated (resistance > 106

 and breakdown voltage >1000 V rms) from 

Logic Ground, AC Neutral, and the control input and output circuits. 

6.5.2.11 Loop/Lead in Electrical Properties 

Each channel of the Detector Unit shall function in accordance with the specific 

requirements of this standard and in addition shall operate without significant 

degradation with any sensor loop/lead-in combination which exhibits the 

following electrical properties as measured at the Detector Unit terminals of the 

lead-in: 
 Traffic Detector Handbook—2006 

October 2006 Page J-15 Federal Highway Administration 

1. Inductance at 50 KHz – 50 to 1000 microhenrys. 

2. Q at 50 KHz–greater than 5. 

3. Resistance to earth ground–greater than 1 megohm. 

4. Field installation practices or Detector Unit design may require grounding the 

shield of the loop lead-in cable. Such grounding should be in accordance with 

the Detector Unit manufacturer's recommendation. 

6.5.2.12 Test Loop Configurations 

Sensor loop and lead-in combinations used to verify the performance 

requirements of this standard shall consist of the following combinations of 1.828 

m x 1.828 m (6 ft. x 6 ft) three-turn loops and shielded lead-in cable as illustrated 

in Figure 6-6. 

1. Single-loop 1.828 m x 1.828 m (6 ft by 6 ft), three turns, with 30.48 m (100 ft) 

of lead-in (80–105 microhenrys). 

2. Single-loop 1.828 m x 1.828 m (6 ft by 6 ft), three turns, with 304.8 m (1,000 

ft) of lead-in (260–320 microhenrys). 

3. Four loops 1.828 m x 1.828 m (6 ft by 6 ft), three turns, in a row in the 

direction of travel and separated by 2.743 m (9 ft), series/parallel connected 

with 76.0 m (250 ft) of lead-in (100–140 microhenrys). 

6.5.2.13 Test Vehicle Definition 

Detector Units shall detect all vehicles which ordinarily traverse the public streets 

and highways and which are comprised of sufficient conductive material suitably 

located to permit recognition and response by the Detector System. 

Vehicles are classified by this standard in accordance with the reduction in 

inductance resulting when they are centered in the single 1.828 m x 1.828 m (6 ft 

x 6 ft), three-turn-test loop with 30.48 m (100 ft) of lead-in. 

These minimum reductions are as follows: 

1. Class 1: 0.13 percent (ΔL/L) or 0.12 microhenrys (ΔL) inductance change with 

a single 1.83 m x 1.83 m (6 ft x 6 ft), three turn loop, with 30.48 m (100 ft) 

lead-in (small motorcycle). 

2. Class 2: 0.32 percent (ΔL/L) or 0.3 microhenrys (ΔL) inductance change with 

a single 1.83 m x 1.83 m (6 ft x 6 ft), three turn loop, with 30.48 m (100 ft) 

lead-in (large motorcycle). 

3. Class 3: 3.2 percent (ΔL/L) or 3 microhenrys (ΔL) inductance change with a 

single 1.83 m x 1.83 m (6 ft x 6 ft), three turn loop, with 30.48 m (100 ft) leadin (automobile). 

The maximum reduction caused by a class three vehicle shall be 5.4 percent 

(ΔL/L) or 5 microhenrys (ΔL) inductance change with a single 1.83 m x 1.83 m 

(6 ft x 6 ft), three-turn loop with 30.48 m (100 ft) lead-in. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-16 Federal Highway Administration 

Figure 6-6 

TEST LOOP CONFIGURATIONS 

Construction–Loop dimension tolerances shall be +50.8 mm (2 in.). Connections shall be soldered 

and waterproofed. Loops shall be installed in a non-reinforced pavement and located at least 

0.914 m (3 feet) from any conductive material. Lead-in cable shall be spooled. Loop leads shall 

exit at one corner of the loop structures. All loop corners shall be chamfered 0.305 m (12 in.). 

Loop Wire–Each loop shall be three turns of AWG #14 cross-linked polyethylene insulated, stranded 

copper wire, such as IMSA (International Municipal Signal Association) Specification 51-3, 1984, 

or equivalent. Loop inductance shall be between 60–80 microhenrys. 

Lead-in Wire–The lead-in wire shall be AWG #14 twisted pair, aluminum polyester shield, 

polyethylene insulation, polyethylene jacket, inductance between 20 uH and 24 uH per 30.48 m 

(100 ft), such as IMSA Specification 50-2, 1984, or equivalent. For standardized test purposes, 

the shield shall be insulated from ground. 

Field installation practices or Detector Unit design may require grounding the shield of the loop lead-in 

cable. Such grounding should be in accordance with the Detector Unit manufacturer’s 

recommendation. (Authorized Engineering Information). 

Sawslot–The conductors shall be placed at the bottom of a 38.1 mm + 6.35 mm (1-1/2" +1/4 in.) deep 

by 6.35 mm (1/4 in.) wide sawslot. Pavement sawslot shall be filled with a suitable polyurethane 

or equivalent sealant. 

6.5.2.14 Sensitivity 

The Detector Unit shall be capable of detecting any of the vehicles defined in 

6.5.2.14 on any of the test loops defined in 6.5.2.13. 

6.5.2.15 Sensitivity Control 

When detecting test vehicles as described in 6.5.2.13 and operating on any of 

the test loop configurations described in 6.5.2.12, each channel of the Detector 

Unit shall include means to adjust the sensitivity such that it shall not produce an 

output when the nearest point of any test vehicle of 6.5.2.13 is 0.914 m (36 in.) or 

more outside the loop(s) perimeter. A minimum of three sensitivity selections 

shall be provided for each detection channel. 

6.5.2.16 Approach Speed 

The Detector Unit shall detect any vehicle described in 6.5.2.13 over any of the 

single loops described in 6.5.2.12 traveling within the speed range of 8.045 km to 

128.72 km (5 to 80 miles) per hour. 
 Traffic Detector Handbook—2006 

October 2006 Page J-17 Federal Highway Administration 

The Detector Unit shall detect any vehicle described in 6.5.2.13 over all of the 

loops of the four loop configurations described in 6.5.2.12 traveling within the 

speed range of 8.045 km to 64.36 km (5 to 40 miles) per hour. 

All channels of a multichannel Detector Unit shall be operating at the same 

sensitivity and connected to equivalent inductances for the purpose of these tests. 

6.5.2.17 Modes of Operation 

Each Detector Unit channel shall be capable of functioning in the following two 

front panel selectable modes: 

6.5.2.17.1 Presence 

When a Class 2 vehicle defined in 6.5.2.13, or larger vehicle occupies the center 

of any test loops described in 6.5.2.12, the Detector Unit shall be capable of 

maintaining a detection output for a minimum of 3 minutes. 

6.5.2.17.2 Pulse 

A detection output between 100 and 150 milliseconds shall be initiated when a 

vehicle enters the sensor loop zone of detection. 

If this vehicle remains in the zone of detection, the Detector Unit shall become 

responsive within a maximum of 3 seconds to additional test vehicles entering 

the zone of detection. The Detector Unit shall produce one and only one output 

pulse for a test vehicle traveling at 16.09 km (10 miles) per hour across the zone 

of detection of the single sensor loops defined in 6.5.2.12. 

6.5.2.18 Recovery from Sustained Occupancy 

When operating in the presence mode, and following an occupancy of any duration, 

the Detector Unit shall recover to normal operation with at least 90 percent of its 

selected sensitivity within five seconds after the zone of detection is vacated. 

6.5.2.19 Response Time 

When operating in the presence mode, the Detector Unit shall be capable of 

being set to produce an output in response to a step decrease in inductance 

equivalent to the minimum decrease from a class one vehicle as defined in 

6.5.2.13 within not more than 100 milliseconds when tested on either of the 

single loop test configurations described in 6.5.2.12. In response to step return to 

the original inductance, the Detector Unit shall terminate its output within no 

more than 100 milliseconds. 

6.5.2.19.1 Variation in Response Time 

The difference between the minimum measured response time and the maximum 

measured response time for input changes in either direction, over any number of 

tests, to an input change equivalent to a Class 1 vehicle shall not exceed 10 

milliseconds per channel multiplied by the number of active channels. The Detector 

Unit must be set to the proper sensitivity to detect a Class 1 vehicle. 

The difference between the minimum measured response time and the maximum 

measured response time for input changes in either direction, over any number of 

tests, to an input change equivalent to a Class 3 vehicle shall not exceed 5 

milliseconds per channel multiplied by the number of active channels. The Detector 

Unit must be set to the proper sensitivity to detect a Class 3 vehicle. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-18 Federal Highway Administration 

All channels of a multichannel Detector Unit which are ON shall be operating at 

the same sensitivity and connected to equivalent inductances for the purpose of 

these tests. 

For certain specific surveillance applications involving vehicle speeds in excess 

of 72.405 km (45 miles) per hour, a more precise response time will be required. 

(Authorized Engineering Information.) 

6.5.2.20 Tuning 

Each Detector Unit channel shall include means for accommodating the range of 

sensor loop/lead-in inductance. 

The unit shall tune automatically upon the application of power. It shall operate with 

at least its minimum sensitivity within 2 seconds after application of power, and at 90 

percent of its selected sensitivity within 5 seconds after application of power. 

6.5.2.21 Self-Tracking 

The Detector Unit shall automatically accommodate those after-tuning changes 

in the loop/lead-in electrical characteristics as might reasonably be expected to 

occur in undamaged loops, properly installed in sound pavement and exhibiting 

the electrical properties outlined in 6.5.2.11, without producing a false output or 

change in sensitivity. 

6.5.2.22 Recovery From Reset 

After any reset, Reset Unit or Reset Channel, the Detector Unit shall operate with 

at least its minimum sensitivity within 2 seconds after the removal of the reset 

condition, and at 90 percent of its selected sensitivity within 5 seconds after the 

removal of the reset condition. 

6.5.2.23 Crosstalk Avoidance 

Each Detector Unit channel shall include means to prevent that channel from 

adversely interacting with any other channel. The means to prevent such 

interaction shall be either inherent, automatic, or manual switch. 

6.5.2.24 Delay/Extension 

Each channel of a unit with delay/extension shall have at least three modes of 

operation–delay, extension, and normal (neither delay nor extension). Channels 

1 and 2 of four-channel Detector Units shall be the channels to include the 

delay/extension feature. Delay and extension timing shall be setable on a per 

channel basis with the timing programmed independently. 

6.5.2.24.1 Delay 

When selected, the output is delayed for the time set. If the vehicle departs 

before the time set, an output does not occur and the timer is reset. If a vehicle is 

present and the delay timer is active, when the delay inhibit is applied, the output 

shall become active. See Figure 6-7. This delay timing is controlled by the 

Delay/Extension input defined in 6.5.2.8.7. 

The delay time shall be adjustable over the range from 0 to 30 seconds. The 

setability shall be within one second in the 0 to 15 second range and within two 

seconds in the 16 to 30 second range. The accuracy shall be + 1/2 second or + 5 

percent of the setting, whichever is greater. When the Delay/Extension input is 

active, the delay shall be zero (0 to 0.1 second). 
 Traffic Detector Handbook—2006 

October 2006 Page J-19 Federal Highway Administration 

6.5.2.24.2 Extension 

When selected, the output is extended after the vehicle departs the zone of 

detection for the time set. If a new vehicle arrives before the extension timer 

times out, the timer is reset, the output is maintained, and the timer resumes 

timing when the vehicle departs. See Figure 6-8. This extension timing is 

controlled by the Delay/Extension input defined in 6.5.2.8.7. 

The extension time shall be adjustable in the range from 0 to 7-1/2 seconds. The 

setability shall be within 1/2 second. The accuracy shall be + 1/2 second. When the 

Delay/Extension input is inactive, the extension shall be zero (0 to 0.1 second). 

Figure 6-7 

DELAY OPERATION 

Figure 6-8 

EXTENSION OPERATION 

6.5.2.25 Controls and Indicators 

All controls and indicators necessary for the operation of the Detector Unit shall 

be located on the front panel of the unit except as noted below. Multiple functions 

combined in a single control shall be permitted. The controls and indicators shall 

include, but are not limited to: 

1. Output Indicator—Means to visually indicate the output state of each channel. 

Each channel shall have a separate indicator. 

2. Sensitivity Control—Means to permit selection of the sensitivity of each 

channel as described in 6.5.2.15. 

3. Reset—(Reset Unit or Reset Channel or both) A control which unconditionally 

causes the Detector Unit or detection channel to retune to a non-vehicle 

present condition. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-20 Federal Highway Administration 

4. Mode Selector—Shall provide for selection of pulse or presence mode 

operation of each channel. Card mounting of this control shall be permitted 

where actuation does not require disassembly. 

5. Crosstalk Control—As required, shall provide means to prevent interaction of 

channels as described in 6.5.2.23. 

6. Delay/Extension Selection Control—The type C, D, CC, and DC Detector Units 

shall have either a control or combination of controls to allow for the selection of 

one of at least three operating modes–delay, extension, and normal (neither 

delay nor extension). It shall be permissible to have the normal mode selected by 

either setting a three position selector switch to the normal position or setting the 

delay/extension timing control to zero. Card mounting of the control(s) shall be 

permitted where actuation does not require disassembly. 

7. Delay/Extension Timing Control—The type C, D, CC, and DC Detector Units 

shall provide a means to permit setting of the time duration of the delay/extension 

period for each channel as described in 6.5.2.24. Card mounting of this control 

shall be permitted where actuation does not require disassembly. 

8. Enable/disable—Means to turn ON (enable) or OFF (disable) a channel or 

number of channels on a Detector Unit. 

6.5.2.26 Outputs 

6.5.2.26.1 Solid State Channel Detection Outputs 

The output interface of each channel shall be an independent, isolated, solidstate output. Each output device shall have the following characteristics: 

1. Output Solid-State Device—The output shall be conducting when a vehicle is 

detected or when the loop circuit is in a failed state (i.e., open loop or shorted 

loop). The channel Disabled (OFF) condition shall have the output in the nonconducting state. During a Reset (Reset Unit or Reset Channel) condition the 

output shall be in the conducting state. 

2. DC Supply Voltage Failure Condition—The output shall be conducting when it 

is terminated to a NEMA defined input, indicating a detection output condition. 

3. Output Circuit Isolation—The isolation between each output device terminal 

pair and all other terminals shall exceed: 

a. Resistance >106

 ohms 

b. Breakdown >1000 V rms 

4. Output Rating—The output shall conduct a minimum of 20 milliamperes with a 

maximum 3.5 volt drop across the output terminals in the conducting state. The 

output shall conduct a maximum of 500 microamperes with any voltage between 

0 and 26 VDC applied across the output terminals in the non-conductive state. 

5. Transition Time—When switching to or from a steady state current in the 

range of 2.4 to 20 milliamperes, the transition time from 8 to 16 volts and vice 

versa shall be 0.1 milliseconds or less. The circuit(s) to which the output is 

connected is defined in 3.3.5.1.3. 

6. Maximum Voltage—When in the non-conducting state, the output shall 

tolerate a voltage as high as 30 VDC without damage. 

6.5.2.26.2 Channel Status Outputs 

Each Detector Unit channel shall provide a channel status output. The channel 

status output shall be a solid state device with the following characteristics: 
 Traffic Detector Handbook—2006 

October 2006 Page J-21 Federal Highway Administration 

1. Output Solid State Device—The output shall be conducting when the channel 

is operating properly. The channel Disabled (OFF) condition shall have the 

output in the non-conductive state. During a Reset (Reset Unit or Reset 

Channel) condition the output shall be in the non-conducting state. 

2. DC Supply Voltage Failure Condition—Output device is nonconductive, 

indicating a fault condition. 

3. Status Outputs shall be referenced to logic ground. 

4. Output Rating—The output shall conduct a minimum of 20 milliamperes with a 

maximum 3.5 volt drop across the output terminals in the conducting state. The 

output shall conduct a maximum of 100 microamperes with any voltage between 

0 and 26 VDC applied across the output terminals in the non-conductive state. 

5. Transition Time—When switching to or from a steady state current in the 

range of 2.4 to 20 milliamperes, the transition time from 8 to 16 volts and vice 

versa shall be 0.1 milliseconds or less. The circuit(s) to which the output is 

connected is defined in 3.3.5.1.3. 

6. Maximum Voltage—When in the non-conducting state, the output shall 

tolerate a voltage as high as 30 VDC without damage. 

6.5.2.26.3 Channel Status Reporting 

The Channel Status Output shall provide for the communication of eight distinct 

status states as defined below: 

1. Normal operation (Detector Unit and loop OK). 

2. Detector Unit failure (watch dog time-out, channel currently in RESET, or 

channel disabled). 

3. Open loop (An open loop may be reported when the terminal inductance is 

>1000 microhenrys and <2500 microhenrys. The open loop shall be reported 

when the terminal inductance is >2500 microhenrys). 

4. Shorted loop (A shorted loop may be reported when the terminal inductance is 

<50 microhenrys and >20 microhenrys. A shorted loop shall be reported when 

the terminal inductance is <20 microhenrys). 

5. Excessive inductance change (+25%). 

6. Reserved. 

7. Reserved. 

8. Reserved. 

Pulse width modulation shall be utilized to encode the eight possible states as 

described below: 

1. Continuous Low or On state. 

2. Continuous High or Off state. 

3. 50 milliseconds Off time. 

4. 100 milliseconds Off time. 

5. 150 milliseconds Off time. 

6. 200 milliseconds Off time. 

7. 250 milliseconds Off time. 

8. 300 milliseconds Off time. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-22 Federal Highway Administration 

The tolerance for all times listed above shall be + 10 milliseconds; the On time 

between pulses shall be 50 milliseconds + 10 milliseconds. 

The Channel Status Output shall reflect the current fault status of the channel (i.e., 

the fault status shall be self clearing in the event that a fault condition clears). 

The Channel Status Output shall maintain an output state, other than a condition 

generated during a Reset, for a minimum of 5 seconds. The Channel Detection 

Output shall maintain the conducting state during a fault condition only while the 

fault condition exists. 

The Channel Status Output shall be in State 2 during RESET. Once the RESET 

is complete and the Detector Unit resumes normal operation, the channel status 

output shall return to State 1. 

The Channel Status Output shall be in State 2 for channels that are disabled 

(OFF). The Channel Detection Output shall maintain the non-conductive state 

(State 2) for channels that are disabled. 

The last channel failure status shall be stored in memory for future reference. 

Enunciation of the last failure status shall be accomplished through the use of the 

front panel indicators. If power is removed or the Reset is activated, the last 

channel status information shall be cleared. 

The particular method employed to enunciate past failure status is not specified 

and will be manufacturer specific. (Authorized Engineering Information.) 

6.5.2.26.4 Data Transmit Output (TX) 

This output shall have 3 states: Mark, Space, and High Impedance. The output 

shall be in the High Impedance state except when transmitting to the BIU. The 

transmitter output shall go from the High Impedance state to the Mark state until 

the first Start Bit is sent. The Transition Region shall be defined as any voltage 

between a Mark and a Space. 

6.5.2.26.4.1 Mark State (Binary 1) 

During Mark, the TX output shall be less than 0.5 VDC when sinking 4 

milliamperes current. 

6.5.2.26.4.2 Space State (Binary 0) 

During Space, the TX output shall be greater than 3.5 VDC when sourcing 4 

milliamperes current. 

6.5.2.26.4.3 High Impedance State 

The TX output shall have an impedance greater that 1 Megohm in the High 

Impedance state. When connected to the following loads, the TX output in the High 

Impedance state: 

1. Shall be >8.5 VDC when connected to +12 VDC through 30 kohm. 

2. Shall be >16 VDC when connected to +22 VDC through 11 kohm. 

6.5.2.26.4.4 Output Impedance During Power Off 

When power is OFF, the output impedance shall be greater that 300 ohms when 

measured with an applied voltage not greater than 2 volts in magnitude to circuit 

common. 
 Traffic Detector Handbook—2006 

October 2006 Page J-23 Federal Highway Administration 

6.5.2.26.4.5 TX Output Shorts 

The TX output shall not be damaged under these situations: 

1. The TX output shall be capable of withstanding a continuous short to DC 

common. 

2. The TX output shall be capable of withstanding a continuous short to +26 

VDC through 600 ohms. 

6.5.2.26.4.6 Rise/Fall Time 

The transition time from the Space to the Mark voltage or from the Mark to the 

Space voltage shall be less than 2.5 microseconds or 4% of a bit time, whichever is 

less. This shall apply with 3750 picofarads attached to the TX output. There shall 

be no reversal of the direction of voltage change while the signal is in the Transition 

Region. The Transition Region shall not be reentered until the next change of 

signal condition. 

6.5.2.26.4.7 Transient Withstand 

The TX output shall not be damaged by application of the transients described in 

2.1.7.1 through 2.1.7.5. 

6.5.2.27 Communication Port Functional Requirements 

The protocol for this port is under definition. 

Inductive Loop Detector Unit types AC, BC, CC, and DC have this 

communication port. 

6.5.2.27.1 Communication Port Electrical Requirements 

This port, RX and TX and DC Common, is an unbalanced, +5 volt nominal, 

standard NRZ (Mark/Space) format, asynchronous serial communication port. For 

compatibility with systems currently in use, when TX is shorted to the Ch 2(+) 

Output and when RX is shorted to the Ch 4(+) Output, TX and RX must not 

interfere with Ch 2(+) or Ch 4(+) operation. See 6.5.2.9 for RX specification details 

and 6.5.2.26.4 for TX specification details. 

6.5.2.27.2 Baud Rate 

The port shall be capable of operation at a baud rate of 9600 bps ±1%. The width 

of 1 bit shall be the reciprocal of the baud rate. 

6.5.2.27.3 Communication Parameters 

The standard NRZ (Mark/Space) format shall be used. Further specifications are 

under definition. 

6.5.2.27.4 Slot Addresses 

The address of a Detector Unit slot is "hard-wired" at each Detector Rack 

connector. A logic 1 shall be created by wiring an address pin to DC Common. 

Open or +24VDC shall be a logic 0. Slot 1 shall be Address 0 and Slot 15 shall 

be Address 14. 
Appendix J—NEMA Detector Standards Excerpts

October 2006 Page J-24 Federal Highway Administration 

6.5.2.28 Electrical Connections 

6.5.2.28.1 Connector Description 

Two and four channel card rack units shall mate with a 44 terminal, double row, 

3.2 mm (0.156 in.) contact spacing, Cinch Jones card edge connection 50-44A30M, or equivalent. 

6.5.2.28.2 Connector Terminations 

Input / Output connector pin terminations shall be as shown in Table 6-2. 

Table 6-2 

CONNECTOR TERMINATIONS 

Pin Function Pin Function 

A Logic Ground 1 Channel 1 Delay/Extension Input 

B Detector Unit DC Supply 2 Channel 2 Delay/Extension Input 

C External Reset 3 Detector Unit Address Bit #3 

D Channel 1 Loop Input 4 Channel 1 Redundant Loop Input (Optional) 

E Channel 1 Loop Input 5 Channel 1 Redundant Loop Input (Optional) 

F Channel 1 Output (+) 6 Detector Unit Address Bit #0 

H Channel 1 Output (-) 7 Channel 1 Status Output 

J Channel 2 Loop Input 8 Channel 2 Redundant Loop Input (Optional) 

K Channel 2 Loop Input 9 Channel 2 Redundant Loop Input (Optional) 

L Chassis Ground 10 Detector Unit Address Bit #1 

M Reserved (AC Neutral) 11 Reserved (AC Neutral) 

N Reserved (AC Line) 12 Reserved (AC Line) 

P Channel 3 Loop Input 13 Channel 3 Redundant Loop Input (Optional) 

R Channel 3 Loop Input 14 Channel 3 Redundant Loop Input (Optional) 

S Channel 3 Output (+) 15 Detector Unit Address Bit #2 

T Channel 3 Output (-) 16 Channel 3 Status Output 

U Channel 4 Loop Input 17 Channel 4 Redundant Loop Input (Optional) 

V Channel 4 Loop Input 18 Channel 4 Redundant Loop Input (Optional) 

W Channel 2 Output (+) 19 Data Transmit Output (TX) 

X Channel 2 Output (-) 20 Channel 2 Status Output 

Y Channel 4 Output (+) 21 Data Receive Input (RX) 

Z Channel 4 Output (-) 22 Channel 4 Status Output 

6.5.2.28.3 Type A Two Channel Without Delay / Extension Timing 

The following pins shall be inactive: P, R, S, T, U, V, Y, Z, 1, 2, 13, 14, 16, 17, 

18, and 22. 

6.5.2.28.4 Type B Four Channel Without Delay / Extension Timing 

The following pins shall be inactive: 1 and 2. 
 Traffic Detector Handbook—2006 

October 2006 Page J-25 Federal Highway Administration 

6.5.2.28.4 Type C Two Channel With Delay / Extension Timing 

The following pins shall be inactive: P, R, S, T, U, V, Y, Z, 13, 14, 16, 17, 18, and 

22. 

Pin 1 through 22 is on the top (component) side and pin A through Z is on the 

back (solder side). Polarization keys shall be located at three positions: 

Between B/2 and C/3 

Between M/11 and N/12 

Between E/5 and F/6 

Pins 3, 6, 10, and 15 are address pins for Type AC, BC, CC and DC Inductive 

Loop Detector Units. When one of these Detector Unit types are installed, it will 

be assigned an address associated with the Detector Unit position in the 

Detector Rack. 

Pins 19 and 21 are the TX output and RX input for communication with the 

Detector Unit. The communication protocol is under definition. (Authorized 

Engineering Information.) 

REFERENCES 

1. Bullock, D., and T. Urbanik. “Traffic Signal Systems: Addressing Diverse 

Technologies and Complex User Needs,” TRB A3A18 Committee on 

Traffic Signal Systems. http://gulliver.trb.org/ 

publications/millennium/00116.pdf/, accessed Feb. 23, 2005. 

 Traffic Detector Handbook—2006 

October 2006 Page K-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX K. 

APPENDIX K. CALTRANS TRANSPORTATION ELECTRICAL EQUIPMENT 

SPECIFICATIONS (TEES) FOR ELECTRONICS UNITS 

AND MAGNETIC DETECTORS..................................................K-1

SECTION 1. GENERAL REQUIREMENTS..........................................................K-1

5.1.1 – Channel Independence and Power Requirements.........................................K-1

5.1.2 – Sensor Unit Features........................................................................................K-1

5.1.3 – Output Device Type .........................................................................................K-1

5.1.4 – Output Signal Characteristics.........................................................................K-2

5.1.5 – Interface Requirements ...................................................................................K-2

5.1.6 – Output Device Switching Time .......................................................................K-2

5.1.7 – Serial Output Definition ..................................................................................K-2

5.1.8 – Electrical Surge Protection..............................................................................K-2

SECTION 2. MODEL 222E AND 224E LOOP DETECTOR SENSOR UNIT 

REQUIREMENTS..............................................................................................K-2

5.2.1 – Output Signal Definition..................................................................................K-2

5.2.2 – Malfunctioning Loop Signal Definition..........................................................K-3

5.2.3 – Supported Loop Configurations .....................................................................K-3

5.2.4 –Inductance and Q Range of Supported Loop and Lead-in Cable 

Assemblies.....................................................................................................K-3

5.2.5 – Transformer Isolation of Loop Inputs............................................................K-3

5.2.6 – Number of Supported Operating Frequencies ..............................................K-3

5.2.7 – Drift Compensation of Tuning Circuits .........................................................K-3

5.2.8 – Pulse and Presence Mode Support..................................................................K-3

5.2.8.1: Pulse Mode Description.............................................................................. K-3

5.2.8.2: Presence Mode Description........................................................................ K-4

5.2.9 – Sensitivity..........................................................................................................K-4

5.2.10 – Response Time................................................................................................K-5

5.2.11 – Normal Operation Time ................................................................................K-5

5.2.12 – Tracking Rate.................................................................................................K-5

5.2.13 – Tracking Range..............................................................................................K-5

5.2.14 – Temperature Change.....................................................................................K-5

SECTION 3. MAGNETIC DETECTOR REQUIREMENTS.................................K-6

5.3.1.1 – Design Requirements................................................................................ K-6

5.3.1.2 – DC Resistance ........................................................................................... K-6

5.3.2 MODEL 232E TWO CHANNEL MAGNETIC DETECTOR 

SENSING UNIT.........................................................................................K-6

5.3.2.1 – Output Signal Definition .......................................................................... K-6

REFERENCES.........................................................................................................K-6

LIST OF TABLES: APPENDIX K. 

Table K-1. Presence detection time for detector sensitivity settings 

of 2 and 6. ..................................................................................K-4

Table K-2. Sensitivity settings for presence and pulse modes. ....................K-4

Numbered headings 

correspond to the numbering 

of sections in the Chapter 5 

of the Caltrans Transportation 

Electrical Equipment 

Specifications, August 16, 

2002 Edition. Unnumbered 

sections are narrative 

explaining the context of the 

document. 

 Traffic Detector Handbook—2006 

October 2006 Page K-1 Federal Highway Administration

APPENDIX K. CALTRANS 

TRANSPORTATION ELECTRICAL 

EQUIPMENT SPECIFICATIONS 

(TEES) FOR ELECTRONICS UNITS 

AND MAGNETIC DETECTORS 

This appendix contains specifications for the following loop detectors (i.e., 

electronics units) and magnetic detectors (i.e., electronics units), as described 

in Chapter 5 of the Caltrans Transportation Electrical Equipment 

Specifications (TEES), August 16, 2002 Edition.(1) 

• MODEL 222E TWO-CHANNEL LOOP DETECTOR SENSOR UNIT 

• MODEL 224E FOUR-CHANNEL LOOP DETECTOR SENSOR UNIT 

• MODEL 231E MAGNETIC DETECTOR SENSING ELEMENT 

• MODEL 232E MAGNETIC DETECTOR SENSOR UNIT 

As in Appendix J, the numbering system used in this appendix corresponds 

to that used in the cited specification, in this case—TEES, rather than this 

Handbook. The exception is that Tables K-1 and K-2 have been numbered 

and titled so that they can be added to the table of contents. TEES places 

Metric units first with the English units in parenthesis while the style of the 

Handbook is to place English units first with Metric units in parenthesis. 

Where definitions for advanced transportation controller (ATC)- and 2070- 

related terms are not provided, they may be found in the glossary. 

SECTION 1. GENERAL REQUIREMENTS 

5.1.1 – Channel Independence and Power Requirements 

The sensor and isolator channels shall be operationally independent from 

each other. Each sensor or isolator channel shall draw no more than 50 mA 

from the +24 VDC [voltage direct current] cabinet power supply and shall be 

insensitive to 700 mV RMS [root-mean-square] ripple on the incoming +24 

VDC line. 

5.1.2 – Sensor Unit Features 

The sensor unit or isolator front panel shall be provided with the following: 

• Hand pull to facilitate insertion and removal from the input. 

• Control switches. 

• Channel visual indication of detection or incoming signal. 

5.1.3 – Output Device Type 

Each sensor or isolator channel output shall be an opto-isolated NPN open 

collector capable of sinking 50 mA at 30 VDC. The output shall be compatible 

with the controller unit inputs. 
Appendix K—Caltrans TEES for Electronics Units and Magnetic Detectors 

October 2006 Page K-2 Federal Highway Administration 

5.1.4 – Output Signal Characteristics 

A valid channel input shall cause a channel Ground True Output to the 

controller unit of a minimum 100 ms in duration. An onboard two-post shunt 

jumper shall be provided to disallow this requirement when the jumper is in 

an OPEN position. 

5.1.5 – Interface Requirements 

The sensor unit or sensing element shall operate and interface successfully 

with an associated Caltrans Standard Sensing Unit or Element. 

5.1.6 – Output Device Switching Time 

The output transistor shall switch from OFF to ON state or ON to OFF state 

in 20 μs or less. 

5.1.7 – Serial Output Definition 

Each Enhanced (E) Sensor Unit shall provide a Tri-State EIA [Electronic 

Industries Association] 423 Serial Port for interface with through pins 19 and 

21 of the unit connector to the assembly SIU [serial interface unit] Connector 

C03. Each sensor unit shall read in the Slot Address pins located at unit 

connector for unit slot Identification. Four onboard switches shall match the 

upper 5th to 8th bits associated Assembly name. Slot Pin 22 shall turn on the 

SIU Drivers allowing RXD to the ATC [advanced transportation controller]. 

Status and report messages shall meet SDLC [synchronous data link control] 

Protocol transmitting at 19,200 bps. Said Messages shall be as called out by 

the user. 

5.1.8 – Electrical Surge Protection 

Onboard protection shall be provided to enable the sensor unit or isolator to 

withstand the discharge of a 10 μF capacitor charged to ±1000 Volts directly 

across the input pins with no load present. With a dummy load of 5 Ohms, 

protection shall enable the sensor unit or isolator to withstand the discharge 

of a 10 μF capacitor charged to ±2000 Volts directly across either the input 

pins or from either side to equipment ground. 

SECTION 2. MODEL 222E AND 224E LOOP DETECTOR 

SENSOR UNIT REQUIREMENTS 

5.2.1 – Output Signal Definition 

The sensor unit channel shall produce an output signal when a vehicle passes 

or remains over loop wires embedded in the roadway. The method of 

detection shall be based upon a design that renders the output signal when a 

metallic mass (vehicle) enters the detection zone causing a change of 0.02% 

minimum decrease in inductance of the circuit measured at the input 

terminals of the sensor unit. 
 Traffic Detector Handbook—2006 

October 2006 Page K-3 Federal Highway Administration

5.2.2 – Malfunctioning Loop Signal Definition 

An open, shorted, or otherwise malfunctioning loop shall cause the sensor 

unit channel to a constant signal state. A two-post shunt jumper or approved 

switch shall be provided to dictate output state upon malfunction. The 

jumper in closed position (Ground True) shall cause a Ground True Output to 

the controller unit. An open position (Ground False) shall cause a Ground 

False Output to the controller unit. An indicator labeled “EOS” (Event 

Output Status) shall be provided on the front panel. When in closed position, 

the indicator shall be ON. A second indicator per channel labeled “EVENT” 

shall be provided indicating a malfunctioning loop or tracking condition. The 

indicators shall be reset by the Unit RESET Switch. 

5.2.3 – Supported Loop Configurations 

Each sensor unit channel shall be capable of detecting all types of AGENCY 

licensed motor vehicles when connected to the loop configuration/lead-in 

requirements of Section 5.2.9.1. 

5.2.4 – Inductance and Q Range of Supported Loop and Lead-in Cable 

Assemblies 

The sensor unit shall comply with all performance requirements when 

connected to an inductance (loop plus lead-in) from 50 to 700 μH with a Qparameter as low as 5 at the sensor unit operating frequency. 

5.2.5 – Transformer Isolation of Loop Inputs 

Loop inputs to each channel shall be transformer isolated. 

5.2.6 – Number of Supported Operating Frequencies 

Each individual channel shall have a minimum of 4 switch selectable 

operating frequencies. 

5.2.7 – Drift Compensation of Tuning Circuits 

The sensor unit channel tuning circuits shall be automatic and shall be so 

designed that drift caused by environmental changes or changes in applied 

power shall not cause an actuation. 

5.2.8 – Pulse and Presence Mode Support 

Each sensor unit channel shall have PULSE and PRESENCE selectable 

modes. 

5.2.8.1: Pulse Mode Description 

In the PULSE MODE, each new vehicle presence within the detection 

zone shall initiate a sensor unit channel output pulse of 125 (±25) ms 

in duration. Should a vehicle remain in a portion of the detection 

zone for a period in excess of 2 seconds, the sensor unit channel shall 

automatically “tune out” the presence of said vehicle. The sensor unit 

channel shall then be capable of detecting another vehicle entering 

the same detection zone. The recovery time to full sensitivity between 

the first vehicle pulse and channel capability to detect another 

vehicle shall be 3 seconds maximum. 
Appendix K—Caltrans TEES for Electronics Units and Magnetic Detectors 

October 2006 Page K-4 Federal Highway Administration 

5.2.8.2: Presence Mode Description 

In the PRESENCE MODE, the sensor unit channel shall recover to 

normal sensitivity within 1 second after termination of vehicle 

presence in the detection zone regardless of the duration of the 

presence. The channel sensitivity settings 2 and 6 shall provide 

presence detection of a vehicle in the detection zone for a specified 

time period and inductance change as shown in Table K-1. 

Table K-1. Presence detection time for detector sensitivity settings of 2 and 6. 

Setting Minimum time duration 

(minutes) 

Detector Input Inductance change 

(%) 

6 3 

10 

0.02 or more 

0.06 or more 

2 4 1.00 or more 

5.2.9 – Sensitivity 

5.2.9.1: California Standard Plan ES-5A and B Loop Configurations 

(California Department of Transportation Standard Plans) are listed 

below: 

5.2.9.1.1 – Single Type A, B, Q, or Round Loop with a 250 foot 

(76 m) lead-in cable. 

5.2.9.1.2 – Single Type A, B, Q, or Round Loop with a 1000 foot 

(305 m) lead-in cable. 

5.2.9.1.3 – Four Type A, B, or Q Loops connected in 

series/parallel with a 250 foot (76 m) lead-in cable. 

5.2.9.1.4 – Four Type A, B, Q, or Round Loops connected in 

series with a 1000 foot (305 m) lead-in cable. 

5.2.9.1.5 – One 50 foot (15 m) Type C Loop with a 250 foot 

lead-in cable. 

5.2.9.2: Sensitivity Settings – Each sensor unit channel shall be 

equipped with 7 selectable (digitally read) sensitivity settings (shown 

in Table K-2) in presence and pulse modes to accomplish the 

following under operational and environmental requirements of this 

specification. 

Table K-2. Sensitivity settings for presence and pulse modes. 

Setting Sensitivity Setting Sensitivity 

1 0.64% delta L 5 0.04% delta L 

2 0.32% delta L 6 0.02% delta L 

3 0.16% delta L 7 0.01% delta L 

4 0.08% delta L 0 Channel OFF 

5.2.9.2.1 – All sensitivity settings shall not differ by ±40% from the 

nominal value chosen. 
 Traffic Detector Handbook—2006 

October 2006 Page K-5 Federal Highway Administration

5.2.9.3: Each sensor unit channel shall not detect vehicles, moving or 

stopped, at distances of 3 feet (0.91 m) or more from any loop 

perimeter, in all configurations listed in paragraph 5.2.9.1. 

5.2.10 – Response Time 

Response time of the sensor unit channel for Sensitivity Settings 1, 2 and 3 

shall be less than 5 ± 1 ms at an approximate loop frequency of 40 kHz. That 

is, for any decreased inductive change that exceeds its sensitivity threshold, 

the channel shall output a ground true logic level within 5 ± 1 ms. When such 

change is removed, the output shall become an open circuit within 5 ± 1 ms. 

5.2.11 – Normal Operation Time 

The sensor unit channels shall begin normal operation within 2 seconds after 

the application of power or after a reset signal of 30 μs. 

5.2.12 – Tracking Rate 

The sensor unit shall be capable of compensating or tracking for an 

environmental change up to 0.001% change in inductance per second. 

5.2.13 – Tracking Range 

The sensor unit shall be capable of normal operation as the input inductance 

is changed from ±5.0% from the quiescent tuning point regardless of internal 

circuit drift. The sensor unit shall be capable of normal operation as the 

input resistance is changed from ±0.5% from the quiescent tuning point 

regardless of internal circuit drift. 

5.2.14 – Temperature Change 

The operation of the sensor unit shall not be affected by changes in the 

inductance and/or capacitance of the loop caused by environmental changes 

with the rate of temperature change not exceeding 1 degree C (1.8 degrees F) 

per 3 minutes. The opening or closing of the controller cabinet door with a 

temperature differential of up to 18 degrees C (32.4 degrees F) between the 

inside and outside air shall not affect the proper operation of the sensor unit. 
Appendix K—Caltrans TEES for Electronics Units and Magnetic Detectors 

October 2006 Page K-6 Federal Highway Administration 

SECTION 3. MAGNETIC DETECTOR REQUIREMENTS 

5.3.1 MODEL 231E MAGNETIC DETECTOR SENSING 

ELEMENT 

5.3.1.1 – Design Requirements 

Each sensing element shall be designed for ease of installation, repositioning, 

and removal. The sensing element shall be 57 mm (2.2 in) maximum in 

diameter, have no sharp edges, and its length not exceed 450 mm (18 in). The 

sensing element shall be constructed of nonferrous material and shall be 

moisture proof. The element shall contain no moving parts or active 

components. The element shall have a 15.4 m (50 ft) lead-in cable. Leakage 

resistance shall be a minimum of 10 megohms when tested with 400 VDC 

between lead wire, including lead wire entrance, and the fluid of a salt water 

bath after the device has been entirely immersed in the bath for a period of 

24 hours at 20 ±3 degrees C (36 ± 5.4 degrees F) . The salt water bath 

concentrate shall contain one fourth ounce of salt per gallon (2 gm per liter) 

of water. 

5.3.1.2 – DC Resistance 

Each sensing element including lead-in shall have a DC resistance of less 

than 3500 ohms and an inductance of 20 Henry ± 15%. 

5.3.2 MODEL 232E TWO CHANNEL MAGNETIC 

DETECTOR SENSING UNIT 

5.3.2.1 – Output Signal Definition 

When resident in an active cabinet input assembly and attached to one or 

more Model 231E Sensing Elements resident in conduit under the travelway, 

the sensing channel shall output a Ground True Output to the Controller 

Unit when sensing an induced voltage caused by a California Licensed 

Vehicle passing within 1.83 m (6 ft) from an element with a 305 m (1000 ft) of 

lead-in cable at all speeds between 5 and 130 km/h (3 and 80 mi/h). The 

sensing channel output shall be continuous as long as the vehicle is detected. 

A digital reading switch with 8 selected step positions for gain (0 to Full) and 

a momentary test switch providing a voltage test input shall be furnished for 

each channel on the front panel. 

REFERENCES 

1. Transportation Electrical Equipment Specifications (TEES). California 

Department of Transportation, Sacramento, CA, August 16, 2002. 

Available at 

http://www.dot.ca.gov/hq/traffops/elecsys/2070/reports/TEES.pdf 

(Accessed March 28, 2004.) 
 Traffic Detector Handbook—2006 

October 2006 Page L-i Federal Highway Administration

TABLE OF CONTENTS: APPENDIX L. 

APPENDIX L. CLASSIFICATION OF SENSOR SYSTEMS BY SENSOR 

DENSITY LEVEL ...........................................................................L-1

ABSTRACT............................................................................................................... L-1

TERMINOLOGY...................................................................................................... L-1

UNCOORDINATED SIGNALS or UNCOORDINATED CONTROL.........L-1

TIME-BASED COORDINATION or TIME-BASED COORDINATED 

CONTROL..................................................................................................L-1

INTERCONNECTED CONTROL ...................................................................L-2

TRAFFIC ADJUSTED CONTROL..................................................................L-2

TRAFFIC RESPONSIVE CONTROL .............................................................L-2

TRAFFIC ADAPTIVE CONTROL..................................................................L-2

SENSOR WEB DENSITY LEVELS........................................................................ L-3

SENSOR WEB DENSITY LEVEL 0.0 .............................................................L-3

SENSOR WEB DENSITY LEVEL 0.5 .............................................................L-4

SENSOR WEB DENSITY LEVEL 1.0 .............................................................L-4

SENSOR WEB DENSITY LEVEL 1.5 .............................................................L-5

SENSOR WEB DENSITY LEVEL 2.0 .............................................................L-6

SENSOR WEB DENSITY LEVEL 2.5 .............................................................L-7

SENSOR WEB DENSITY LEVEL 3.0 .............................................................L-8

SENSOR WEB DENSITY LEVEL 3.5 ...........................................................L-10

SENSOR WEB DENSITY LEVEL 4.0 ...........................................................L-10

GENERAL OBSERVATIONS ............................................................................... L-10

REFERENCES....................................................................................................... L-11

LIST OF FIGURES: APPENDIX L. 

Figure L-1. Network sensor web density level 0.0—no sensors. ..................L-3

Figure L-2. Network sensor web density level 0.5—occasional minor 

movement stop bar detection...................................................L-4

Figure L-3. Network sensor web density level 1.0 (UTCS 1.5, closed-loop 

systems—several minor movement without detectors). ........L-5

Figure L-4. Network sensor web density level 1.5 (UTCS 1.5, closed-loop 

systems—with heavily actuated controller & several minor 

movement without detectors). .................................................L-6

Figure L-5. Network sensor web density level 2.0 (traffic adjusted, 

UTCS 2 GC). .............................................................................L-6

Figure L-6. Network sensor web intensity level 2.5 (SCATS) (SCATS optimal 

sensor layout, downstream sensors, 1 per lane link). ............L-8

Figure L-7. Network sensor web density level 2.5 (SCOOT) (SCOOT optimal 

sensor layout, upstream sensors, 1 per lane link)..................L-8

Figure L-8. Network sensor web density level 3.0 (sensor layout for ACS 

systems such as RHODES, optimal sensor layout of upstream 

and downstream sensors shown, 2 per lane link)...................L-9

Figure L-9. Network sensor web density level 3.0a (advance dilemma zone 

sensors and minor movement protection).............................L-10

 Traffic Detector Handbook—2006 

October 2006 Page L-1 Federal Highway Administration

APPENDIX L. CLASSIFICATION OF 

SENSOR SYSTEMS BY SENSOR 

DENSITY LEVEL

ABSTRACT 

Appendix L describes a series of successively higher sensor density levels and 

how they relate to traffic control system requirements. Sensor density is 

described in terms of a web of sensors required for different kinds of arterial 

and network traffic control and surveillance systems by link and by lane. 

TERMINOLOGY 

The sensor web density level terminology used in this edition of the Traffic 

Detector Handbook and the 2005 edition of the Traffic Control Systems 

Handbook (TCSH) differs slightly from that used in previous editions of the 

TCSH.(1) The refinement of the terminology is due to several factors. The first 

is the need to distinguish between levels of sensor density and sensor 

characteristics. The second is the need to distinguish how different kinds of 

interconnected traffic control systems utilize sensor data and thus require 

different densities and distributions of sensors. A third factor is that traffic 

adjusted and traffic responsive systems have evolved since control systems 

were originally introduced to traffic management. The evolution was spurred 

on by increases in computing power and speed of communications. Similarly, 

traffic adaptive systems require a sensor density and data transmission rate 

that were not physically nor fiscally feasible 30 years ago. The control systems 

terminology is described in more detail in the TCSH and in Table 3-1. 

For the purposes of this Handbook, control systems are broken down into the 

following categories: 

UNCOORDINATED SIGNALS or UNCOORDINATED 

CONTROL 

This is a form of signal control in which an individual signalized intersection 

has neither control relationships between intersections nor any data flow 

between intersections. There are no system sensors. The individual isolated 

intersection may have local actuated control or Time of Day/Day of Week 

(TOD/DOW) changes based on historical traffic flows. This category has also 

been called isolated signal control and totally isolated systems. These systems 

have neither time-based nor communications-based connections between 

intersections. They present no system response to variations in traffic demand. 

TIME-BASED COORDINATION or TIME-BASED 

COORDINATED CONTROL 

These are control systems in which basic coordination is provided by a highly 

accurate real-time clock within the traffic signal controller. This information 

is used to determine the local signal timing of offset and cycle/split or 

actuated timing based on the TOD/DOW. There are no system sensors. 

System response to variations in traffic demand is measured in weeks or 

months. In a fixed timing plan, cycle, split, offset and actuated settings are 

Sensor density levels relate to 

traffic control levels and their 

requirements. Categories of control 

systems are described in more 

detail in the 2005 Traffic Control 

Systems Handbook. Their 

characteristics are described in 

Table 1 of Chapter 3 of this 

Handbook. 

The first traffic control system to 

use volume and occupancy to 

select timing plans was developed 

by the company Automatic Signal 

in the 1950s. They named their 

system a traffic adjusted system—

hence the use of the term here to 

distinguish it from systems using 

and manipulating flexible timing 

plans (see definitions on next 

page,) 

The key difference between 

categories of traffic control is how 

rapidly a system makes changes in 

response to variations in traffic 

demand. 
Appendix L—Classification of Sensor Systems by Sensor Density Level 

October 2006 Page L-2 Federal Highway Administration 

not adjusted between switching of timing plans. These are used in 

uncoordinated signal control, time-based coordinated control, interconnected 

control, and traffic adjusted control systems. 

INTERCONNECTED CONTROL 

These are control systems in which basic coordination is provided by wireline 

or wireless interconnect. There are no system sensors. This information is 

used to determine the local signal timing of offset and cycle/split or actuated 

timing based on TOD/DOW. The operator can select timing, download timing 

plans and changes, and monitor system status. System response to variations 

in traffic demand is measured in weeks or months. 

Time of Day/Day of Week (TOD/DOW) are control features which isolated 

control systems, time-based control systems, and interconnected systems may 

possess. In systems using these features, timing plan selections are based on 

the clock and calendar rather than on system traffic information. TOD/DOW 

systems operate with no system sensors (or at a sensor web density level of 

0). These systems do not measure any characteristics of traffic flow. They 

may use clocks or wireline or wireless communications to provide the timing 

plan selection. Totally isolated and TOD/DOW may be given some response 

to traffic through the use of traffic actuated controllers. 

TRAFFIC ADJUSTED CONTROL 

These systems operate with few sensors (or at sensor web density levels of 1.0 

and 1.5) and use fixed timing plans, selection of which is adjusted by a 

minimum number of system sensors at periodic intervals. The sensors in these 

systems measure systemwide variations in traffic demand using either volume 

plus weighted occupancy or using predominant direction of flow. They then use 

pattern matching to choose from among a set of preselected fixed plans. This 

definition builds on the original definition by Kell and Fullerton in the Manual 

of Traffic Signal Design.(2) This in turn is built on that of Automatic Signal. 

System response to variations in traffic demand is about 15 minutes. 

TRAFFIC RESPONSIVE CONTROL 

These systems operate at a moderate sensor density (sensor web density 

levels of 2.0 and 2.5) and use flexible timing plans in which the offsets, splits, 

and phase durations can be adjusted in reaction to changes in traffic on a 

nearly cycle-by-cycle basis and have to have at least one sensor per link up to 

one sensor per lane per link. Cycle, split, and offset are each optimized in the 

selection of the flexible timing plan to be implemented. These systems 

typically use macroscopic measures of traffic flow on individual links such as 

platoon dispersion and other characteristics. System response to variations in 

traffic demand ranges from one cycle length to 15 minutes, depending on the 

extent of the changes. 

TRAFFIC ADAPTIVE CONTROL 

These systems operate with two sensors per lane per link for optimum 

efficiency (sensor web density level of 3.0). Adaptive systems do not have 

cycle, split, or offsets in the classic sense. They forecast traffic into the near 

future and proactively change signal timing, including selection and 

sequence of phasing. These systems need to measure the flow of individual 

vehicles so that they can predict the future flow of the individual vehicles. 

Traffic adjusted systems are 

particularly beneficial where 

traffic is not disrupted by 

unpredictable incidents and 

special events and traffic 

growth is very predictable. 

In a flexible timing plan, 

cycle, split, offset, and 

actuated settings are 

adjusted once per cycle. 

This occurs between 

switching of timing plans. 

These are used in traffic 

responsive systems. 

Traffic responsive and traffic 

adaptive systems are 

particularly beneficial where 

traffic is regularly disrupted 

by unpredictable incidents 

and special events and/or 

traffic growth is strong but 

erratic.(1)
 Traffic Detector Handbook—2006 

October 2006 Page L-3 Federal Highway Administration

System response to variations in traffic demand ranges from a few seconds to 

a minute. 

The key difference between interconnected, traffic adjusted, traffic 

responsive and traffic adaptive control strategies is the rapidity of response 

to variations in traffic flow. As the response time is decreased, the frequency 

of changes to the control parameters must increase and the selection of 

timing features must change. More rapid response requires more sensor data 

and more frequent updates of the sensor data. This in turn leads to changes 

in requirements for sensor density and data communications rates. These 

differences are highlighted in Table 3-1. If a jurisdiction cannot afford to 

operate and maintain the dense sensor and communications networks 

required for traffic responsive or traffic adaptive systems, then those systems 

are not good choices for that jurisdiction. However, the additional costs 

required by denser sensor networks introduce increased capabilities to the 

control systems, and thus justify their introduction in that manner. 

Sensor web density levels are described in the next section along with figures 

illustrating them in a sample networks. 

SENSOR WEB DENSITY LEVELS 

SENSOR WEB DENSITY LEVEL 0.0 

This density level has no vehicle detection or sensors. Therefore, no 

maintenance of sensors is required. See Figure L-1 for the network layout. 

Figure L-1. Network sensor web density level 0.0—no sensors. 

Systems may operate on fixed timing plans with no actuated phases. The 

systems may operate either as totally isolated systems or as TOD/DOW, in 

which precalculated, fixed timing plans are implemented accordingly. 
Appendix L—Classification of Sensor Systems by Sensor Density Level 

October 2006 Page L-4 Federal Highway Administration 

SENSOR WEB DENSITY LEVEL 0.5 

This has a very low level of sensor density. Thus, a very low level of 

maintenance is needed. See Figure L-2 for a typical network layout. 

Figure L-2. Network sensor web density level 0.5—occasional minor movement stop bar detection. 

These systems are TOD/DOW systems made more responsive to traffic by the 

incorporation of semiactuated and fully actuated controllers at key locations. 

The purpose of the semiactuated controllers is generally to minimize the 

breakup of progressive flow when side-street traffic is not present. The traffic 

actuated controller’s actuated signal settings may be varied by time of day 

and day of week. Generally only left turns and minor side streets are 

actuated. 

SENSOR WEB DENSITY LEVEL 1.0 

This density level provides a low level of sensor density with up to one sensor 

per link. A low level of sensor maintenance is needed. See Figure L-3 for a 

typical network layout. 

This density level is used for traffic adjusted systems. For this Handbook, a 

traffic adjusted system is defined as one in which a timing plan is selected 

based on a combination of TOD/DOW and the real demands of traffic. These 

systems require system sensors for volume plus weighted occupancy 

(VPLUSKO) or peak direction of flow data to choose timing plans. However, 

the timing plans are selected from a fixed library of timing plans previously 

calculated (either on line or off line). Examples of traffic adjusted systems are 

UTCS (Urban Traffic Control System) 1st Generation traffic responsive, 

UTCS 1.5 Generation and various closed loop systems. Periodic manual 

traffic counts are required for calibration of system sensors. 

How to locate system 

detectors for traffic adjusted 

systems is described in 

“Locating Detectors for 

Advanced Traffic Control 

Strategies.” (4.5) These reports 

state that this kind of system 

“requires instrumentation on 

the entrance links and 

approximately every fourth 

block thereafter on arterials 

and on every third block 

within grid networks. Links 

that carry low volumes (peak 

hour volumes of less than 

100 vph [vehicles per 

hour]/lane should not be 

instrumented ...” 
 Traffic Detector Handbook—2006 

October 2006 Page L-5 Federal Highway Administration

Figure L-3. Network sensor web density level 1.0 (UTCS 1.5, closed-loop systems— 

several minor movement without detectors). 

SENSOR WEB DENSITY LEVEL 1.5 

This density level provides a moderate sensor density with up to one sensor 

per link and additional sensors for actuated controllers. A moderate level of 

sensor maintenance required due to the greater density of system and traffic 

actuated sensors. 

This level might be used for a traffic adjusted system that has been made 

more responsive to traffic by the incorporation of traffic actuated controllers. 

The actuated signal settings may be varied by time of day and day of week. 

Sensors are placed at key locations along with more intensive sensor 

placement at actuated locations. Periodic manual traffic counts are required. 

A moderate degree of sensor maintenance is also required due to the greater 

density of system and traffic actuated sensors. See Figure L-4 for a typical 

network layout. 
Appendix L—Classification of Sensor Systems by Sensor Density Level 

October 2006 Page L-6 Federal Highway Administration 

Figure L-4. Network sensor web density level 1.5 (UTCS 1.5, closed-loop systems—with heavily 

actuated controller & several minor movement without detectors). 

SENSOR WEB DENSITY LEVEL 2.0 

This density level provides a medium density of at least one system sensor 

per approach link. A medium level of sensor maintenance is required due to 

the greater density of system sensors. See Figure L-5 for a typical network 

layout. 

Figure L-5. Network sensor web density level 2.0 (traffic adjusted, UTCS 2 GC). 

UTCS 2 GC “requires 

instrumentation on all links 

between ‘major intersections 

which are defined as those 

intersections operating within 

the strategic optimization 

routine” 

(p. 84, ref. 5). 
 Traffic Detector Handbook—2006 

October 2006 Page L-7 Federal Highway Administration

Sensor Web Density Level 2.0 is applicable to simpler traffic responsive 

control systems found in arterial and closed loop systems. These systems are 

characterized by timing plans that are selected or computed based on realtime demands of traffic as detected by vehicle sensors. The plans are 

implemented at regular intervals of not less than 15 minutes (and rechecked 

at shorter intervals thereafter). Since traffic responsive systems have fixed 

offsets, changes of these plans more than once every 15 minutes may result 

in system degradation. An example of a system using a Level 2 density is 

UTCS 2nd Generation Traffic Responsive. Systems using Level 2 sensor web 

density and above are usually able to include some form of traffic prediction. 

Periodic manual calibration counts are required to obtain an accurate 

distribution of traffic across lanes for fully utilizing system sensors. 

Note that some major phases may need dilemma zone protection. TxDOT has 

offered several publications describing different kinds of techniques and 

dilemma zone systems that might be useful.(6,7,8) The sensor network needed 

for dilemma zone protection is discussed under sensor web density level 3.0. 

SENSOR WEB DENSITY LEVEL 2.5 

This level provides a high sensor density of at least one sensor per lane per 

link (as for example, the approach link for SCATS (Sydney Coordinated 

Adaptive Control System) or the departure leg of SCOOT (The split, cycle 

and offset optimization technique). Sensor web density level 2.5 requires a 

large amount of sensor and communications maintenance due to the heavy 

concentration of system sensors. 

This density level might be used for traffic responsive systems. A traffic 

responsive system is defined as one in which key aspects of the timing plan 

may be altered between signal timing plan updates and timing plan updates 

are possible at least every 15 minutes. These would include the distributed 

intelligence traffic control system (DITCS) that uses timing plans but 

dynamically adjusts splits. Also, SCATS(9,10) updates intersection cycle length 

using the sensors at the stop line. SCATS allows for phase skipping. Sensor 

web density level 2.5 systems might typically include some form of traffic 

prediction. SCOOT(11,12) may make modest adjustments (<4 s) to the offset, 

split, and cycle in each cycle. See Figure L-6 for a typical network layout for 

SCATS and Figure L-7 for a typical network layout for SCOOT. 
Appendix L—Classification of Sensor Systems by Sensor Density Level 

October 2006 Page L-8 Federal Highway Administration 

Figure L-6. Network sensor web intensity level 2.5 (SCATS) 

 (SCATS optimal sensor layout, downstream sensors, 1 per lane link). 

 

Figure L-7. Network sensor web density level 2.5 (SCOOT) 

 (SCOOT optimal sensor layout, upstream sensors, 1 per lane link). 

SENSOR WEB DENSITY LEVEL 3.0 

This density level has two or more sensors per lane per link. This requires a 

very high level of sensor and communications maintenance. See Figure L-8 

for a typical network layout. 
 Traffic Detector Handbook—2006 

October 2006 Page L-9 Federal Highway Administration

Figure L-8. Network sensor web density level 3.0 (sensor layout for ACS systems such as 

RHODES, optimal sensor layout of upstream and downstream sensors shown, 2 per lane link). 

Level 3 is applicable to traffic adaptive systems. A traffic adaptive system for 

this purpose is defined as one in which all signal timings are evaluated and 

changed based on forecasts of future traffic flow continuously in real time. A 

rolling prediction horizon of from 1 to 4 seconds to 20 seconds is a key 

element of traffic adaptive systems. A traffic adaptive system usually does 

not have offsets in the classical pretimed or traffic responsive sense. This is 

because the signal settings are in constant adaptation. Traffic adaptive 

systems are frequently distributed systems with the algorithms running in 

individual controllers and the controllers negotiating with each other over 

the settings. For example, RHODES requires an upstream, free-flow sensor 

and a downstream stop bar sensor and left turn bay sensors to operate at full 

efficiency. Thus, RHODES(13,15) and other ACS systems operate at a sensor 

web density level 3.0. 

Sensor density level 3.0a is really a higher density version of sensor web 

density level 2. This level presents advance systems for dilemma zone 

protection on the major approaches and stop bar sensors for minor movement 

protection on minor approaches and phases. Density level 3.0a is illustrated 

in Figure L-9. Network Sensor Web Density Level 3.0a 

Density level 3.0a is used by State highway departments on rural, high-speed 

State highways to help prevent red light running crashes. The upstream 

detectors on coordinated movements may also be used for volume-density 

operation. Volume-density, where it is configured correctly, is quite effective. 

For ACS systems, the 

upstream sensors should be 

placed to provide a rolling 

10-second traffic prediction 

window. This means at 10 

seconds travel time distance 

upstream from the stop bar. 

This will be approximately 

250 feet (76 m) at 25 ft/s (7.6 

m/s).(14) 
Appendix L—Classification of Sensor Systems by Sensor Density Level 

October 2006 Page L-10 Federal Highway Administration 

SENSOR WEB DENSITY LEVEL 3.5 

This is an enhancement of level 3.0. This level uses vehicle identification, 

classification and tracking sensors. It could be used for traffic responsive or 

traffic adaptive systems. This level uniquely identifies each vehicle entering 

a leg, know its classification, and know when that vehicle left the leg. The 

sensors to support this level are in research and development, and the first 

research networks are being built. 

Figure L-9. Network sensor web density level 3.0a (advance dilemma zone 

sensors and minor movement protection). 

SENSOR WEB DENSITY LEVEL 4.0 

This is a hypothetical level that will become possible with the widespread 

adoption of DSRC (Dedicated Short-Range Communications), high-accuracy 

GPS, (Global Positioning System) and in-vehicle computers and 

communications. Each vehicle will regularly report its location, and possibly 

its destination, to the traffic management system. This would allow the 

creation of a new category of traffic control using long time horizon traffic 

prediction, which might be called, for example, traffic predictive control. 

GENERAL OBSERVATIONS 

The higher the sensor density level number, the higher is the need for 

detectors. Level 0.0 density systems do not support real-time systems and do 

not need sensors. Level 0.5 systems only have sensors for locally actuated 

controllers. 

The more variable and unpredictable the traffic, the greater is the need for 

sensors. 

Closed-loop systems that use less communications to the central master and 

microcomputer-based masters can fit into many of the above categories. ACS 

Lite is a closed-loop subset of ACS and may utilize a sensor web density of 

level of 1.0 or 1.5. 
 Traffic Detector Handbook—2006 

October 2006 Page L-11 Federal Highway Administration

REFERENCES 

1. Traffic Control Systems Handbook, FHWA-SA-95-032. Federal 

Highway Administration. 1996. 

2. Kell, J.H., and I.J. Fullerton. Manual of Traffic Signal Design.

Institute of Transportation Engineers/Prentice-Hall, 1991. p. 164. 

3. Carter, M., B. Hicks. “What Have We Learned About ITS?” 

Chapter 3, “Arterial Management.” SAIC. Available from: 

http://www.itsdocs.fhwa.dot.gov//JPODOCS/REPTS_TE//13319.pdf 

4. Kay, J.L., R.D. Henry, and S.A. Smith. Locating Detectors for 

Advanced Traffic Control Strategies, Handbook, FHWA-RD-75-

91. Federal Highway Administration, Washington, DC. 1975. 

5. Henry, R.D., J.M. Bruggeman, and S.A. Smith. Locating Detectors for 

Advanced Traffic Control Strategies, Technical Report, FHWA-RD75-92. Federal Highway Administration, Washington, DC. 1975. 

6. Messer, C.J., S.R. Sunkari, H.A. Charara, and R.T. Parker. 

Development of Advance Warning Systems for End-of-Green Phase at 

High Speed Traffic Signals, FHWA/TX-04/0-4260-4. Texas 

Department of Transportation and Federal Highway Administration. 

2004. Available at: http://tti.tamu.edu/documents/0-4260-4.pdf 

7. Chaudhary, N.A., M.M. Abbas, H. Charara, and R. Parker. Platoon 

Identification and Accommodation System for Isolated Traffic Signals 

on Arterials, FHWA/TX-05/0-4304-2. Texas Department of 

Transportation and Federal Highway Administration. 2003. 

Available at: http://tti.tamu.edu/documents/0-4304-2.pdf 

8. Abbas, M., J. Bonneson, H. Charara, D. Middleton, K. Zimmerman. 

Intelligent Detection-Control System for Rural Signalized 

Intersections, FHWA/TX-03/4022-2. Texas Department of 

Transportation, Federal Highway Administration. 2002. Available at: 

http://tcd.tamu.edu/Documents/Products.htm 

9. Gross, N.R., “SCATS Adaptive Traffic System,” TRB Committee A3A18, 

TRB Adaptive Traffic Control Workshop, July 1998. Available at: 

http://www.signalsystems.org.vt.edu/documents/TRBWorkshop2000/-

SCATS_TRB2000Pres2.pdf 

10. “SCATS,” version 6 booklet, TEPG Australia, Unit 1, 2-8, Rydalmere, 

NSW 2116 Australia, available at www.traffictech.com/pdf/scatsbooklet.pdf 

11. El-Assar, H. “Orange County SCOOT System,” TRB 2004 Annual 

Meeting, Adaptive Signal Control Workshop, Session 2 Field 

Experience. January 2004. Available at: 

http://www.signalsystems.org.vt.edu/documents/Jan2004AnnualMeet

ing/SundayWorkshop/TRB%202004%20Hazem.ppt#292,25,Lessons 

Learned 

12. Day, I. “SCOOT Version 3.1,” TRB Midyear Meeting and Adaptive 

Traffic Signal Control Workshop, July 1998. Available at: 

http://www.signalsystems.org.vt.edu/documents/July1998AnnualMee

ting/scoot.pdf 

13. Ghaman, R. Sensor requirements for ACS. Personal communication, 

January 14, 2005. 
Appendix L—Classification of Sensor Systems by Sensor Density Level 

October 2006 Page L-12 Federal Highway Administration 

14. Curtis, D. Sensor requirements for ACS. Personal communication, 

February 4, 2005. 

15. Curtis, D. Sensor requirements for ACS. Personal communication, 

February 4, 2005. 
 Traffic Detector Handbook—2006 

October 2006 Page M-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX M. 

APPENDIX M. EXTENT AND CAUSES OF INDUCTIVE-LOOP FAILURES.M-1

STUDIES SUMMARIZED IN THIS APPENDIX.................................................M-1

OREGON STUDY....................................................................................................M-1

OREGON SURVEY RESULTS .......................................................................M-2

OREGON CONCLUSIONS AND RECOMMENDATIONS ........................M-3

WASHINGTON STUDY .........................................................................................M-4

WASHINGTON SURVEY RESULTS.............................................................M-4

WASHINGTON CONCLUSIONS AND RECOMMENDATIONS..............M-6

MINNESOTA STUDY.............................................................................................M-7

MINNESOTA SURVEY RESULTS ................................................................M-7

MINNESOTA CONCLUSIONS AND RECOMMENDATIONS .................M-9

NEW YORK STUDY.............................................................................................. M-11

NEW YORK SURVEY RESULTS ................................................................M-11

NEW YORK CONCLUSIONS AND RECOMMENDATIONS .................M-13

LIST OF FIGURES: APPENDIX M. 

Figure M-1. Oregon presence loop configuration. ........................................M-3

LIST OF TABLES: APPENDIX M. 

Table M-1. Oregon inductive-loop failure survey results. ...........................M-2

Table M-2. Washington State inductive-loop failure rate survey results. .M-5

Table M-3. New York State distribution of inductive loops by pavement 

type in failure survey. ............................................................M-11

Table M-4. Inductive-loop failure percentage by cause of failure and 

pavement type........................................................................M-12

 Traffic Detector Handbook—2006 

October 2006 Page M-1 Federal Highway Administration

APPENDIX M. EXTENT AND CAUSES 

OF INDUCTIVE-LOOP FAILURES 

A number of studies were conducted during the 1980s to determine the 

extent and causes of inductive-loop detector failures. The underlying purpose 

of those investigations was to identify potential solutions to increase 

reliability and life expectancy of inductive-loop installations. Several of the 

studies involved testing various loop configurations and selected materials in 

agency facilities. None of these particular studies, however, tested all 

possible configurations or all available materials or products. 

STUDIES SUMMARIZED IN THIS APPENDIX 

Four studies were particularly significant in that they contained the results 

of questionnaire and interview surveys, which documented the experience of 

numerous agencies. These studies are listed below and their results are 

summarized in this appendix: 

• Improving Materials and Methods for Installing Loop 

Detectors, FHWA-OR-RD 86-03. Oregon Department of 

Transportation, 1986. (Oregon study) 

• Evaluation of Inductor Loop Installation Procedures in 

Flexible Pavements, WA-RD 76.1. University of 

Washington, 1985. (Washington study) 

• Study of Traffic Loop Failures, FHWA-MN-RC-84-04. 

Minnesota Department of Transportation, 1984. 

(Minnesota study) 

• Evaluation and Improvement of Inductive Loop Detectors, 

FHWA Research Report No. 119. New York State 

Department of Transportation, 1985. (New York study) 

OREGON STUDY 

The Oregon study was designed to: 

1. Identify the causes of loop system failures and the methods used to 

remedy the identified problems. 

2. Assess the accumulated data. 

3. Test and evaluate those materials and procedures that appeared most 

likely to be effective in the Oregon state system. 

The required information was obtained by distributing a lengthy 

questionnaire to the following State departments of transportation: 

• Alaska. 

• California. 

• Idaho. 

• Montana. 

• Nevada. 

• Oregon. 

• Utah. 

• Washington. 
Appendix M—Extent and Causes of Inductive-Loop Failures 

October 2006 Page M-2 Federal Highway Administration 

Some local agencies also participated in the survey. 

OREGON SURVEY RESULTS 

Although different materials and installation methods were used by the 

agencies surveyed, two major causes of loop failure were consistently 

reported: loop sealant failure (loss of adhesion) and poor installation 

techniques (e.g., inadequately cleaned or dried saw slots). Another key 

problem common to many agencies was wire breakage due to asphalt 

pavement deterioration. The City of Albany, OR, cited pavement flexing and 

shoving, which caused the epoxy sealant to pop out of the saw slot, requiring 

resealing. The Portland, OR, metropolitan region had many loop failures 

because of road wear down to the loop wire, primarily in wheel rut locations. 

Many agencies also experienced loop loss resulting from trenching for the 

installation or maintenance of underground utility lines. 

Of particular interest was Alaska's exclusive use of the preformed loop for all 

of their roadway-installed vehicle detection. Their preformed loop is 

composed of #12 AWG cross-linked polyethylene wire encased in 1-inch (2.54-

cm), schedule 80 PVC conduit. The loop wire is placed in the conduit, which 

is then assembled to the specified loop dimensions. Alaska reported no loop 

failure using the preformed loops. Other States also reported selected use of 

preformed loops. Oregon uses preformed loops mostly in signalized gravel or 

dirt detour areas around bridge construction sites. California and Utah have 

experimented with preformed loops and have achieved satisfactory 

performance with no loop failures reported. 

Table M-1 summarizes the Oregon survey results. With the exception of the 

State of Utah, most loop installations were performed by contractors. 

Table M-1. Oregon inductive-loop failure survey results. 

Percent installed by 

State State Contractor Major failures Remarks 

Alaska 10 90 No loop failures reported Exclusive use of 

preformed loops 

California 5 95 Improper sealing and 

foreign material in saw 

slot 

Uses preformed loops 

in poor pavement and 

dirt detours 

Idaho 10 90 Improper sealing No failure for loops 

made of #20002 cable 

Montana 10 90 Improper sealing ―

Nevada 5 95 Improper sealing and 

pavement deterioration 

―

Oregon 10 90 Improper sealing ―

Utah 70 30 Improper sealing and 

pavement deterioration 

Used some preformed 

loops with no failures 

Washington 10 90 Improper sealing and 

foreign material in saw 

slot 

Need better inspection 

to improve loop 

performance 

Alaska reported no 

failures when using 

preformed loops. 

The Oregon study 

reported two major 

causes of loop failure: 

loss of adhesion in the 

sealant and poor 

installation techniques. 
 Traffic Detector Handbook—2006 

October 2006 Page M-3 Federal Highway Administration

OREGON CONCLUSIONS AND RECOMMENDATIONS 

Previous to this study, the Oregon vehicle detection system consisted of a 3- 

by 3-ft (0.9- by 0.9-m) diamond loop, constructed with four turns of THWN 

wire. The loop was installed in a 1/4-inch (6.5-mm) wide sawcut and 

encapsulated in a preapproved sealant. The splice between the loop wires 

and the lead-in cable was made in proximity to the loop and was soldered, 

covered with heat-shrinkable tubing, and encapsulated with sealant. The 

lead-in cable (Belden No. 8720) was installed in the sawcut from the loop to 

the curb or edge of pavement and then in a conduit underground to the 

controller. 

Based on the research study results, Oregon has amended many of their 

specifications and practices. They continue to specify a 3- by 3-ft (0.9- by 

0.9-m) diamond configuration when the loops are in series in the field. 

However, any single loops are now required to be 4 ft by 4 ft (1.2 m by 1.2 m). 

Backer rod to hold down the loop wires is now specified on all State projects. 

A significant change is that the splice between the loop wires and the lead-in 

cable is now made exclusively at pull box locations. 

Presence loop configuration evolved to its present specification in order to 

accommodate the detection of smaller vehicles in the traffic stream. Two 3- 

by 3-ft (0.9- by 0.9-m) diamond loops are connected in series. A single 4- by

4-ft (1.2- by 1.2-m) diamond is placed upstream to provide a long loop effect. 

Information from this detector is input into a separate electronics unit 

channel so that the Model 170 controller carry-over feature may be used. 

Presence type operation is used on left-turn lanes and most side streets. 

Loop spacing is now 4 ft, 12 ft, and 60 ft (1.2, 3.6, and 18 m) from the stop 

line as shown in figure M-1. 

Direction of travel

Loop wires to pull box

(for splice)

4 foot

diamond

loop

60 ft

(18 m)

3 foot 

diamond loops

12 ft

(3.6 m)

4 ft

(1.2 m)

Stop

line

Figure M-1. Oregon presence loop configuration. 

Because of sealant failure incidents, 16 sealant products were tested on both 

asphalt and cement pavements. Sealant installation was made in July 1983 

and was continually observed until the spring of 1984. 

The sealant was subjected to a cross section of weather. July, August, and 

September were very warm, and in December, snow and freezing were 

present. Cold and wet conditions continued though the winter months. Ten of 

the sealants tested were approved and added to the approved product list. It 

was recommended that testing of various epoxy sealants be continued. 
Appendix M—Extent and Causes of Inductive-Loop Failures 

October 2006 Page M-4 Federal Highway Administration 

Oregon State maintenance personnel expressed interest in enclosing the loop 

wire in a 0.25-inch (6-mm) vinyl tube prior to inserting the wire in the saw 

slot. While this technique was worthy of consideration, the State expressed 

concern about the need for a wider saw slot, extra sealant, and the 

requirement to twist the loop wire 4 to 6 turns per foot (13 to 20 turns per 

meter) from the loop to the pull box. 

It was apparent from the Oregon experience and from the gathered data that 

complete cleanout and drying of the loop saw slot are vital to long lasting, 

effective loop performance. Oregon became interested in the sawcut cleanout 

nozzle developed by the State of New York. This nozzle uses the Venturi 

principle to supply pressurized water for cleaning out the slot after sawing. A 

nozzle was fabricated by Oregon based on the New York design. Valves were 

added to control both air and water feed lines so that options existed to use 

air only, water only, or an air/water mixture. This nozzle is being used in loop 

installations by three of Oregon's five regions. They report a better bonding 

of sealant to the saw slot because of the more efficient cleaning of the saw 

slot by the nozzle. 

WASHINGTON STUDY 

The information base for the study consisted of a literature search and a 

State and national telephone survey of traffic engineers. A questionnaire was 

developed for the survey that could accommodate either verbal or written 

responses. The survey participants represented 23 cities and 7 counties 

within Washington State, 6 districts of the Washington State Department of 

Transportation (WSDOT), and 9 other State agencies, representing a 

geographic cross section of the Nation. Many of the State and local agencies 

did not maintain precise documented records related to inductive-loop 

detectors; accordingly, respondents were asked to provide estimates as 

necessary. As a result, much of the information obtained was qualitative. 

WASHINGTON SURVEY RESULTS 

This study was conducted by the University of Washington Civil Engineering 

Department and sponsored by WSDOT in cooperation with FHWA. The 

purpose of this research was to identify the types and frequency of loop 

detector failure and to identify possible solutions. The specific study focus 

was on the reasons for loops becoming dislodged from the saw slot, the 

frequency of failure, the optimum products, and installation techniques. 

The number of loops within the responding jurisdictions and the 

corresponding failure rates are given in table M-2. The cities and counties 

within the State of Washington had the same median number of loops, while 

the WSDOT districts had a much greater mean, median, and range for the 

number of loops. County officials reported the lowest incident of loop failures. 

The WSDOT districts and cities seemed to have relatively comparable failure 

rates. Out-of-State respondents reported much higher failure rates than 

those reported in the State of Washington. 

The majority of the responding agencies felt that their loop failures were not 

excessive. Those indicating excessive failures complained that repair 

procedures were very costly and that loop performance needs to be improved. 
 Traffic Detector Handbook—2006 

October 2006 Page M-5 Federal Highway Administration

When asked how failures were detected, the survey showed that notification 

of loop failure by Washington cities was primarily from individuals outside 

the traffic department (i.e., the general public, police, other agencies, etc.). 

For the WSDOT districts and county agencies, most failures were discovered 

by their own personnel. 

In WSDOT, failures were reported as a consequence of their 12 inspections 

per year of each installation. Out-of-State agencies reported that failures 

were most frequently identified by sources outside their responsible 

department. 

Pavement failure was frequently identified as the cause of loop failure. 

Cracking was listed as the primary mode of pavement distress, while 

permanent pavement deformation (rutting and shoving), or a combination of 

this with cracking was also reported as a primary reason for loop failure. 

Table M-2. Washington State inductive-loop failure rate survey results. 

Jurisdiction Number of loops Failure rate (% per year) 

Cities (23): 

 Mean 477 4.1 

 Median 300 2.4 

 Range 1724 11.7 

Counties (7): 

 Mean 646 3.4 

 Median 300 1.7 

 Range 2090 9.5 

WSDOT Districts (6): 

 Mean 1255 3.8 

 Median 525 3.0 

 Range 4070 9.0 

Out-of-State (9)*

: 

 Mean N/A 11.3 

 Median N/A 9.0 

 Range N/A 34.5 

* AZ, FL, GA, IL, MS, NE, NC, PA, and TX. 

The most frequent causes for electronics unit failures, as defined by the 

respondents, included lightning damage and the need for constantly retuning 

older models to accommodate temperature changes. Other respondents 

stated that, although retuning was occasionally necessary, it was not 

considered to be a failure. 

The questionnaire asked what methods were used to monitor detector loops. 

The answers were categorized as follows: periodic inspection, waiting for 

complaint about a traffic signal, continuous monitoring, or a combination of 

the above. Periodic inspections were the predominant method, with periods 

ranging from 1 to 25 times per year per installation. Continuous monitoring 

was used by some agencies, particularly in areas of high congestion where 

optimizing traffic flow was a concern. 
Appendix M—Extent and Causes of Inductive-Loop Failures 

October 2006 Page M-6 Federal Highway Administration 

When asked who installed the loops, the response was contractors 

approximately 90 percent of the time. Only one State DOT indicated that 

in-house personnel installed 98 percent of its loops. 

Most of the cities and counties within Washington State cited WSDOT 

specifications for loop detectors. The preferred loop shape was the square 

loop or quadrupole. Very few agencies specified rectangular or diamond 

loops. 

Out-of-State agencies specified a variety of loop shapes, including the 

chevron. The quadrupole was given the highest ratings for sensitivity and 

reliability. Square and rectangular loops scored high on reliability but 

somewhat less on sensitivity. Most sensitivity problems were associated with 

vehicles having a high ground clearance such as logging trucks. Most 

agencies expressed some concern for the detection of light vehicles such as 

bicycles and small motorcycles. 

The preponderance of agencies used only pressurized air to clean out saw 

cuts. High-pressure water followed by pressurized air was seldom used. The 

reasoning cited for this was the need for the cuts to be dry prior to the 

placement and sealing of the loop wire. 

The choice of loop wire was fairly consistent among the agencies surveyed. 

Although Washington State specifies #14 gauge wire with RHH-RHW 

insulation, many of the city agencies preferred #12 THHN or TWN wire. 

Out-of-State agencies were about equally divided between #12 and #14 gauge 

wire. 

The methods and products used for sealing the sawcuts and splices varied 

considerably. Many of the agencies within the State used the WSDOT 

specifications for sealants. Some city agencies used either polypropylene rope 

or cotton rope on the top and bottom of the wire to prevent the wire from 

floating to the surface. Cotton rope was substituted for polypropylene rope 

because the polypropylene rope tends to deform when covered with hot 

asphalt, forcing the rope out of the slot. Most WSDOT districts used roofing 

asphalt as a sealant, while out-of-State agencies tended to use commercial 

sealants that were tested and approved prior to their use. For splicing, more 

out-of-State agencies used sealant kits and/or heat-shrink tubing with 

silicone sealer for their splices than agencies within Washington State. 

WASHINGTON CONCLUSIONS AND 

RECOMMENDATIONS 

The failure rates reported by Washington State agencies were much lower 

than reported by out-of-State DOTs and in the literature. The largest single 

factor contributing to loop failure appeared to be pavement cracking. The 

next most important factor was breaking of loop wires due to utility repair or 

construction. Other significant factors included poor inspection procedures, 

pavement rutting, and sealant failure. 

Problems associated with the electronics unit could be attributed to the 

inability of these units to adjust to temperature changes and to moisture or 

other changes within the loop wire or lead-in cable. This could be resolved by 

the purchase of self-tuning amplifiers and by complete encapsulation of the 

loop wires. The study recommended the elimination of using rope to hold 

down wire. 

Primary cause of loop failure 

uncovered by the WSDOT 

study was pavement 

cracking. Secondary causes 

included wire breakage, poor 

inspection, pavement rutting, 

and sealant failure. 
 Traffic Detector Handbook—2006 

October 2006 Page M-7 Federal Highway Administration

WSDOT districts expressed much concern about the poor quality of 

inspection of loop installations. It was pointed out that the best remedy for 

poor inspection is training and experience. Training may be accomplished on 

the job by assigning new inspectors to more experienced personnel or through 

formal classes. Inspectors must remain in their jobs long enough to be 

effective in the enforcement of specifications. 

It was also suggested that forms and procedures be developed to track loop 

performance. Such records would provide valuable information on the 

expected life of loops, failure rates, failure modes, quality and reliability of 

products, etc. 

Finally, it was recommended that self-sealing heat-shrink tubing be 

considered for splicing wires. It is used quite successfully in the marine 

industry in deep underwater applications and adds extra strength and 

abrasion resistance to the area of the splice. Heat guns with fixed 

temperature heads should be used with the temperature selected ahead of 

time to avoid damage to the wire coating. 

MINNESOTA STUDY 

The Minnesota Department of Transportation (MnDOT) Office of Traffic 

Engineering determined that 14 percent of their signal maintenance time 

was spent repairing loop detectors because of poor installation procedures, 

pavement deterioration, or environmental effects. Consequently, the State 

Office of Research and Development was asked to conduct a study to identify 

the reasons for loop detector failure and where within the installation the 

failure usually occurred. 

A literature search revealed that Minnesota's problems were not unique. 

Loops installed by other States in the Snowbelt area were also affected by 

moisture, loop sealant failure, and pavement cracks and joints. A survey 

questionnaire was developed and distributed to all the Snowbelt States (all 

States above 36 degrees latitude were considered to be in the Snowbelt 

region). The District of Columbia and Provinces of Canada were also 

included, as were the major city and county agencies within Minnesota. The 

questionnaire covered two critical areas of interest: (1) reasons why the loop 

detector failed and (2) the materials and procedures used by each agency 

when installing loop detectors. 

Completed surveys were received from 26 States, two counties, two cities, 

and three Canadian provinces. In addition to the literature search and the 

survey, MnDOT personnel were observed while making loop repairs. 

MINNESOTA SURVEY RESULTS 

An initial review of the completed questionnaire revealed that most States, 

including Minnesota, did not keep good records concerning reasons for loop 

failures. Characteristically, if it were determined that a short or open circuit 

had occurred in the loop wire, no investigation was made as to where or why 

the short occurred. The loop wire was simply replaced. 

Many failures occurred because of broken wires or deteriorated insulation at 

concrete pavement joints, pavement cracks running through the loop, or at 

the pavement interface with a curb or shoulder. In addition, failures 

frequently occurred in pull boxes where the loop wires are spliced with the 

Inductive-loop detector 

failures reported in the 

MnDOT study were due to 

broken wires or deteriorated 

insulation at pavement joints, 

pavement cracks running 

through the loops, poor 

splice technique, treatment 

of loop corners, and 

improper sealant choice or 

application technique. 
Appendix M—Extent and Causes of Inductive-Loop Failures 

October 2006 Page M-8 Federal Highway Administration 

lead-in cables. Only one agency indicated that they did not make their splices 

in a pull box. 

Splice failures occurred because of corrosion, moisture, or poor connecting 

methods. A number of problems occurred with the "V" formed between wires 

held side by side as their ends were twisted together. Soldering was favored 

over crimping, although some agencies crimp and then solder. Many of the 

agencies sealed their connections with a waterproofing agent and then 

wrapped with tape. Other agencies encapsulated the connections in an epoxy 

or other slot sealant using drug store pill boxes or paper bags to serve as 

inexpensive molds. Commercial sealing kits were also used. 

Another frequently mentioned problem was electronics unit failure caused 

when older models of electronics units become detuned. Deteriorated wire 

insulation, poor splices, temperature changes, moisture, vibration, electrical 

storms, and wire movement all contributed to this type of failure. Many 

agencies have alleviated this problem by using digital self-tuning electronics 

units. 

As for wire type, 25 agencies used #14 AWG wire for their loops. Stranded 

wire was always used, and type THHN and THWN were favored for 

insulation. Eleven agencies enclosed the insulated wire in thin vinyl tubing, 

which creates a conduit effect and provides added protection. Three agencies 

were reported experimenting with this concept. This tube-encased wire is 

commercially available or can be made up in-house. 

For sealing the saw slots, 12 commercial brands of sealant were mentioned, 

as were many types of epoxy and polyester resins and unspecified asphalt 

compounds. Experience and price seemed to be the criteria used in making a 

selection. 

The question of most concern in this study involved the way the various 

agencies installed their traffic loops. The response indicated three particular 

procedures were most important in installing loops: (1) laying and sealing the 

loop wires, (2) addressing the loop corners, and (3) crossing expansion joints 

or pavement-shoulder joints. 

Initial MnDOT procedures used to lay and seal the loop wires merely 

specified inserting the turns of insulated wire into the slot and sealing with 

any type of inexpensive sealant. It was soon learned that these inexpensive, 

untested sealants were unsuitable for preventing water and foreign 

materials from entering the slot. The sealant would either loose its bond with 

the sides of the slot or would deteriorate to the point where it would just 

disintegrate. Foreign materials entering the slot could pierce the wire 

insulation and allow moisture to come into contact with the loop wire thus 

causing detuning or possibly a short. 

This led to the testing of various sealants to find one that would maintain 

adhesion to the side of the slot during expansion, withstand compression 

when joints were contracting, and yet be of the right viscosity to be poured 

into the slot but not run out if the slot were on an incline. It also became 

apparent that sharp edges along the slot edges and at the loop corners could 

create breaks in the wire insulation. Therefore, procedures were undertaken 

to smooth sharp edges, flush debris from slots with pressurized water, and 

blow-dry the slot with compressed air. 
 Traffic Detector Handbook—2006 

October 2006 Page M-9 Federal Highway Administration

Cutting 45-degree diagonal slots reduced sharp bends at the loop corners. 

However, this method created triangles at each corner that could eventually 

break down from the wheels of vehicles passing over the area. Several 

agencies resolved this problem by drilling 1- or 2-inch (2.5- or 5.1-cm) holes at 

each corner instead of cutting the diagonals. The corners of the hole were 

chiseled out to remove the sharp edges. Other agencies did not drill holes at 

the corners; rather, they chiseled out the inside of the corner to create small 

open triangular areas. Slack in the wire should be left at each corner, 

whether holes are drilled or chiseled to accommodate any movement of the 

pavement. 

Many ideas were presented on how to fill the slots and corners with sealant. 

Some agencies simply laid the wire in the bottom of the slot and filled it with 

sealant, hoping that most of the wire would be encased. To better 

encapsulate the wire, others partially filled the bottom of the slot with 

sealant, laid the loop wire, and then added more sealant until it was almost 

flush with the top of the slot. Still other agencies went to the trouble to put 

sealant in the bottom of the cut and between each turn of the loop wire before 

final sealing to achieve total encapsulation. 

To create a conduit effect without the need for wide sawcuts, some agencies 

chose to cut a standard slot, lay the wires, and then wedge a form of rope in 

the slot over the wire before adding sealant. This procedure also reduced the 

amount of sealant needed. One agency suggested that this method created 

problems in that the rope was not as resilient as the sealant and that voids 

could occur between the wires and rope and/or the slot. A conduit effect could 

be achieved during asphalt pavement construction by cutting the slots and 

inserting the wire before adding the final layer of bituminous asphalt. 

PVC rigid plastic tubing was used as conduit for preformed loops by a 

number of agencies. This 0.5- to 1-inch (13- to 25-mm) tubing was often 

installed during construction before the final courses were laid. However, if it 

were used in an existing installation, much wider cuts would be made to 

accommodate the tubing, thus weakening the pavement structure. 

Experience demonstrated that preformed loops were particularly effective 

where the pavement was broken or in poor condition. 

MINNESOTA CONCLUSIONS AND 

RECOMMENDATIONS 

MnDOT contracts out the installation of new traffic signals, ramp meters, 

and automatic traffic counters, all of which use loop detectors for activation. 

MnDOT inspection personnel preform inspection of new installations. 

MnDOT maintenance staff primarily carry out repairs and replacement of 

failed loops. 

Previous to this study, MnDOT did not require its in-house crews to follow 

the same procedures and State specifications as those required of contractors. 

For example, a contractor is required to place a layer of sealant in the bottom 

of the slot before installing the loop wire, while MnDOT crews simply lay in 

the wire and cover with sealant. In addition, in-house crews use #14 AWG 

wire instead of the specified #12 AWG. However, MnDOT personnel use 

insulated wire encased in a thin vinyl tubing, which is not required of the 

contractor. 
Appendix M—Extent and Causes of Inductive-Loop Failures 

October 2006 Page M-10 Federal Highway Administration 

It was found that, on occasions, MnDOT crews created problems by choosing 

poor locations for the new loops or routing the lead-in wires through areas 

already showing distress. Moreover, they did not follow specifications 

regarding expansion joints, cracks, or pavement edges. 

The primary recommendation of the MnDOT study was that MnDOT repair 

crews use the same procedures as those required of contractors installing 

new loops. The study also recommended that consistency be achieved among 

all crews making repairs. Using MnDOT standard plates as guidelines, it 

was suggested that the repairmen meet and decide how future repairs would 

be made. The major points to be addressed included: 

• Depth of loop saw slots and steps to be taken to ensure that a 

consistent depth is maintained. 

• How loop corners are to be treated. 

• Size and type of conductor used for loop wire. 

• Treatment of pavement joints or crack passing though the loop. 

• Procedures for inserting and sealing loop wire. 

• Procedures for splicing and waterproofing splices. 

Poor installation procedures by contractors were the cause of a significant 

portion of loop failures experienced in the State. Faulty inspection 

procedures contributed to these failures. It was recommended that the 

MnDOT inspectors become more knowledgeable in the proper installation of 

traffic loops and conduct more aggressive inspections during installation. 

A major conclusion from this study was consistent with other survey 

findings: that records and documentation of loop failures throughout the 

United States are wholly inadequate. It is common practice to simply replace 

a failed loop installation without investigating the cause of failure or the 

precise mechanism of failure. The study concluded that detailed information 

on loop failures remains critical to determining the procedures and products 

most appropriate to enhanced reliability and loop life expectancy. 

To resolve this inadequacy, MnDOT has recommended that all new loop 

installations be documented and stored in electronic form. Initial input would 

include type and size of wire, model of inductive-loop detector electronics 

unit, sealant type, identity of installer (contractor or in-house crew), etc. 

This installation would then be tracked throughout its lifetime with all 

repairs being fully documented. 

A new repair sheet would be developed that would include a checkoff list for 

use during troubleshooting and details on reasons for failures and on repairs 

made. All replaced equipment would be identified and input into the 

computer for future reference. 
 Traffic Detector Handbook—2006 

October 2006 Page M-11 Federal Highway Administration

NEW YORK STUDY 

This study was based on an in-depth survey of 9 New York State DOT 

regions responsible for more than 15,000 loop installations. Although the 

study did not include a survey of out-of-State agencies, several other State 

DOTs were contacted to ascertain the types of sealants they used. The survey 

was initiated after a 1980 study determined that as many as 25 percent of 

New York State's 15,000 loop detectors were not operational at any given 

time. The 1980 study further determined that loop detectors were 

maintenance-free for an average of only 2 years. Consequently, this 

investigation was designed to discover the major causes of loop failures and 

to identify steps to reduce the frequency of failure. 

NEW YORK SURVEY RESULTS 

All current loop failures in New York State were listed and a number of 

installations were inspected to determine if the actual failure causes were 

readily discernible. In most instances, failures were attributed to one or 

more of the following factors: 

• Design and installation oversights. 

• Sealant failure. 

• Wire failure. 

• Various types of pavement failure. 

• Utility construction activities. 

An evaluation sheet was designed to record failure types during the full-scale 

investigation. A total of 340 failed loops were fully documented and sorted 

into various categories of major failure type. In all cases, final failure was 

due to a broken or grounded wire resulting from installation error, material 

problems, or other difficulties. Table M-3 lists the number of loops installed 

in each of three pavement types, AC, composite, or PCC. Table M-4 

summarizes the survey results by the types of failures that occurred in each 

type of pavement. 

Table M-3. New York State distribution of inductive loops by pavement type in failure survey. 

 Total 

loops 

AC 

pavement 

Composite 

pavement 

PCC 

pavement 

Number 340 275 20 45 

Percent 100 81 6 13 
Appendix M—Extent and Causes of Inductive-Loop Failures 

October 2006 Page M-12 Federal Highway Administration 

Table M-4. Inductive-loop failure percentage by cause of failure and pavement type. 

Failure type Total loops % Failure 

AC* 

% Failure 

composite* 

% Failure PCC* 

Surface Cracking 

 Major 

 Minor 

 None Visible 

18 

29 

53 

20 

30 

50 

20 

40 

40 

2 

20 

78 

Pavement Heaving 

 Visible 

 None Visible 

10 

90 

12 

88 

0 

100 

0 

100 

 Pavement Patching 

 Patched 

 No Patching 

14 

86 

14 

86 

20 

80 

9 

91 

Sealant Loss 

 Partial 

 Total 

 None 

35 

23 

42 

40 

20 

40 

10 

30 

60 

11 

38 

51 

Loop Wire Visible 

 Visible 

 Not Visible 

39 

61 

39 

61 

35 

65 

38 

62 

Broken Loop Wire 

 Broken 

 None evident 

22 

78 

21 

71 

25 

75 

29 

71 

* AC = asphalt pavement 

 Composite = asphalt overlay on concrete pavement 

 PCC = concrete pavement 

Of the surveyed loops, approximately 40 percent had exposed wires and 

approximately 20 percent had broken wires. Broken or exposed wires were 

attributed to three major causes: sealant failure, pavement failure (cracking, 

potholes, and shoving), and wire float. Almost 60 percent of the failed loops 

that were examined had partial or complete sealant failure. 

When the sealant did not achieve a good bond to the sidewalls of the slot, 

debris and water could infiltrate the slot. Sealant could then be forced out of 

the slot, exposing the wire to traffic and inducing loop failure. Moisture could 

also enter the slot and create an electrical ground. Once moisture entered, 

expansion and contraction of the water during freeze-thaw cycles would 

further disrupt the loop and abrade the insulation, exposing the wire to an 

electrical ground. 

The most frequent pavement failure affecting the loop wire was cracking. In 

general, pavement failure caused the loop wire to be strained, resulting in 

wire breakage, wire insulation wear, or the infiltration of foreign materials. 

Of the failed loops, 50 percent of the AC pavement and asphalt-overlaid 

concrete pavement surfaces displayed some cracking. 

Loops in PCC without overlays represented approximately 20 percent of the 

cracking failures. However, this type of pavement presented other problems, 

such as gross slab movements that strained the wire at the pavementshoulder interface or where loop wires cross joints. 
 Traffic Detector Handbook—2006 

October 2006 Page M-13 Federal Highway Administration

NEW YORK CONCLUSIONS AND RECOMMENDATIONS 

This extensive examination of failed loops revealed two major areas for 

further study: sealant reliability and reduction of wire breakage due to 

pavement failure. The survey also concluded that problems existed in the 

installation process. 

Some failed wires were found very close to the surface even when the sealant 

had adequate adhesion to the sidewalls of the slot. It was concluded that the 

wire had floated to the surface either before the sealant could cure, or 

because the sealant remained plastic. 

Inspection of splicing methods and materials used in the pull box found that 

different techniques were applied depending on the individual doing the 

splice. To guarantee waterproofing integrity and longevity of the splice, 

better techniques should be evaluated and the best methods incorporated 

into a Statewide standard. 

After a thorough investigation of various installation elements and 

procedures, the following changes were recommended: 

• Use encased loop wire (which requires a 3/8-inch (9.5-mm) slot instead 

of a 1/4-inch (6.4-mm) slot). 

• Chip or core corners of loop sawcut rather than use diagonal cuts. 

• Use cold-applied sealant. Hot-asphalt sealants have proved less 

effective and can cause severe damage to wire insulation. 

• Use a higher horsepower saw for the sawcut (18 hp or higher) and use 

water with sawing operation. 

• Assure saw slot is properly cleaned and dried before application of 

sealant. 

• Assure that appropriate hold-down materials are used and properly 

applied. 

• Use proper splicing techniques. 

• Standardize installation techniques, and make sure that the work 

force are fully aware of these standards and that specifications are 

rigidly enforced on both the contractor and the agency. 

• Continue evaluation to determine best currently available methods 

and materials. 

Major causes of inductive-loop 

failure uncovered by the New York 

study were sealant reliability and 

wire breakage due to pavement 

failure. 

 Traffic Detector Handbook—2006 

October 2006 Page N-i Federal Highway Administration 

APPENDIX N. GROUNDING 

(DESIGN GUIDELINES) 

The material in this appendix is a reproduction of the information that 

appears in: 

Electrical Design Manual, volume 1 

Chapter 9, “Grounding” 

Part 2, “Component Design” 

Pages 9-1 to 9-39 

as published by the Ontario Ministry of Transportation, Ontario, Canada 

December 1989 and October 1992 updates to Section III, Subsections 3 

and 4, and Section IV. 

(Note: The format of the material has been modified to conform in 

appearance to the other parts of the Traffic Detector Handbook.) 

 Traffic Detector Handbook—2006 

October 2006 Page N-iii Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX N. 

APPENDIX N. GROUNDING (DESIGN GUIDELINES) ......................................N-1

SECTION I—REASONS FOR GROUNDING .........................................................N-1

1. SAFETY GROUNDING.......................................................................................N-1

2. SYSTEM GROUNDING ......................................................................................N-1

3. LIGHTNING DISCHARGE ................................................................................N-1

SECTION II—CALCULATION OF RESISTANCE TO GROUND......................N-2

1. GENERAL ............................................................................................................N-2

2. SOIL RESISTIVITY.............................................................................................N-2

3. GROUND ELECTRODE RESISTANCE TO GROUND ...................................N-3

3.1 GROUND RODS...........................................................................................N-3

3.2 PEDESTALS .................................................................................................N-4

3.3 PLATE ELECTRODES ...............................................................................N-4

3.4 WIRE GRIDS................................................................................................N-5

3.5 MULTIPLE RODS .......................................................................................N-5

3.6 COMBINATION ROD AND WIRE GRIDS .............................................N-6

3.7 SINGLE WIRE .............................................................................................N-7

3.8 SUMMARY OF CALCULATIONS............................................................N-8

3.9 APPLICATION.............................................................................................N-9

3.10 PROBLEM AREAS..................................................................................N-10

3.11 APPLICATION GUIDELINES...............................................................N-14

SECTION III—EFFECTS OF LIGHTNING..........................................................N-14

1. GENERAL ..........................................................................................................N-14

2. DESIGN CRITERIA ..........................................................................................N-15

3. POWER SURGES...............................................................................................N-16

4. OTHER SOURCES OF POSSIBLE DAMAGE................................................N-18

SECTION IV—SUMMARY OF DESIGN GUIDELINES.....................................N-21

1. TRAFFIC SIGNAL SYSTEMS..........................................................................N-21

SECTION V—REFERENCES .................................................................................N-22
Appendix N—Grounding (Design Guidelines)

____________________________________________________________________________________________________ _____ 

October 2006 Page N-iv Federal Highway Administration 

 LIST OF FIGURES: APPENDIX N 

Figure N-1. Voltage and current waveforms.............................................N-15

Figure N-2. Voltage in earth due to discharging lightning current 

at service pole. ........................................................................N-17

Figure N-3. Recommended improvement to system ground connections.N-18

Figure N-4. Controller cabinet location for best lightning protection.....N-20

Figure N-5. Signal grounding system (with or without lighting). No 

ground electrodes at cabinet..................................................N-21

LIST OF TABLES: APPENDIX N 

Table N-1. Representative values of soil resistivity.....................................N-2

Table N-2. Variation of resistivity with ground temperature. ....................N-2

Table N-3. Resistance to ground for various ground system 

configurations and soils.............................................................N-12
 Traffic Detector Handbook—2006 

October 2006 Page N-1 Federal Highway Administration 

APPENDIX N. GROUNDING 

(DESIGN GUIDELINES) 

SECTION I—REASONS FOR GROUNDING 

1. SAFETY GROUNDING 

(1) Grounding of all metallic electrical enclosures is required for safety. If a 

live conductor touches the metal, a large ‘short circuit current’ flows to 

ground, thus tripping the circuit breaker. If the metal were not 

grounded, it would assume the same voltage as the touching conductor 

and remain so until discharged to ground. When touched, the discharge 

could occur through the person’s body to ground depending on the 

resistance of gloves, boots and the material the person is standing on. 

2. SYSTEM GROUNDING 

(1) A low voltage system is grounded throughout to ensure that any line-toground fault is cleared by the circuit breakers prior to doing any 

permanent power system damage such as melting of cables, etc. The 

system ground is usually tied to the safety ground. If the two grounds 

are separated, the following disadvantages occur: 

(a) ‘Resistance to ground’ of both system and safety grounds is greater 

than would be the case if the two were connected together. 

(b) High currents could still flow in the safety ground in the event of 

cable insulation failures in the enclosure. 

(c) A high degree of coupling, through the earth, is difficult to avoid if 

the ground rods are in the same local area. 

(d) Where decoupling is possible, voltages (often dangerous) can be 

possible between the nearby ‘grounded points’. 

3. LIGHTNING DISCHARGE 

(1) Lightning induced currents on cables must be given a fast and easy path 

to ground through protective devices such as lightning arresters, 

varistors and gas-tube arresters. If the path to ground is not provided 

properly, the voltage surge ‘spikes’ and resultant current and energy 

will damage components. Electronic components are particularly 

susceptible to damage since they operate at very low voltages and high 

speeds and are not designed to physically absorb any significant energy. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-2 Federal Highway Administration 

SECTION II—CALCULATION OF RESISTANCE TO 

GROUND 

1. GENERAL 

(1) A minimal resistance to ground is desirable. Older versions of the Code 

called for 10 ohm (Ω) maximum to ground. This requirement is now 

replaced with a description of the physical grounding materials, or, in 

the case of a substation, limiting the voltage rise due to a fault to 5000 

V. The requirement for 10 Ω was difficult to design and perhaps even 

more difficult to obtain during installation. 

(2) The resistance to ground depends on several non-exclusive factors: 

(a) The number and length of ground rods 

(b) The number and length of connecting ground wires in the ground grid 

(c) The quality of wiring connections 

(d) The resistivity of the earth 

(e) The temperature of the earth 

(f) The water content of the earth 

(3) The last three factors are somewhat weather dependent and are 

therefore beyond precise design. 

2. SOIL RESISTIVITY 

(1) The resistivity of the soil, at the site under consideration, is a measure 

of the resistance to conducting electrical current and is measured in 

Ohm-meters (Ωm). Representative values are given Table N-1. 

Table N-1. Representative values of soil resistivity. 

Soil Type Resistivity ρ (Ωm) 

Clay, saturated silt 100 

Sandy or silty clay 250 

Clayey sand or saturated sand 500 

Sand 1500 

Gravel 5000 

Dry sand, rock >5000 

(2) The soil classification and ρ values in Table N-1 are left purposely vague 

since environmental effects can drastically change the resistivity of the 

soil. Table N-2 shows the typical variation of a nominal resistivity with 

different soil temperatures. 

Table N-2. Variation of resistivity with ground temperature. 

Ground Temperature (oC) Resistivity ρ (% of nominal) 

 20 73 

 10 100 

 0+ 139 

 0– (freeze) 303 

 – 5 798 

–10 3333 
 Traffic Detector Handbook—2006 

October 2006 Page N-3 Federal Highway Administration 

(3) Resistivity varies widely with moisture content as well as temperature, 

with values a factor of 350% higher for soil in the ‘dry’ state than in the 

‘wet’ state. 

(4) In order to custom design a grounding system, the designer would need 

to know not only the type of soil and its resistivity, but also the condition 

of future measurements. For this reason, a resistivity of ρ = 100 Ωm is 

selected as the basis of design for grounding systems. (The system is 

field measured upon installation and any deficiencies can be made up by 

installing supplementary facilities.) It should also be noted that Ontario 

has little or no lightning activity during months when the ground 

temperature is below the freezing point.

3. GROUND ELECTRODE RESISTANCE TO GROUND 

3.1 Ground Rods 

(1) Resistance to ground for a single ground rod may be calculated from 

) 1] 4 [ln(

2 − π

ρ = =

R

R

R

G R

a

L

L

R R

where 

RG = Resistance to ground in Ohms (Ω) 

ρ = Soil resistivity in Ohm-meters (Ωm) 

LR = Rod length in meters (m) 

aR = Rod radius in meters (m) 

RR = Resistance to ground of one rod in Ohms (Ω). 

Example 1: for 20 mm dia. x 3 m Rod 

Given ρ = 100 Ωm, LR = 3 m, aR = 0.01 m 

− = Ω ×

π × = ) 1] 32.20.01

4 3 [ln(

2 3

100 RG . 

If the soil is wet and ρ decreases to ρ = 50 Ωm 

− = Ω ×

π × = ) 1] 16.10.01

4 3 [ln(

2 3

50 RG . 

If the soil is dry and ρ increases to ρ = 300 Ωm 

− = Ω ×

π × = ) 1] 96.60.01

4 3 [ln(

2 3

300 RG . 

It may be seen that the nominal resistance to ground of 50 Ω usually 

quoted for a single ground may vary substantially depending on soil 

type or conditions. 

Example 2: for 20 mm dia. x 6 m Rod 

Given ρ = 100 Ωm, LR = 6 m, aR = 0.01 m 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-4 Federal Highway Administration 

− = Ω ×

π × = ) 1] 18.00.01

4 6 [ln(

2 6

100 RG

or for a 100% rod depth increase (over example 1), the resistance to 

ground is decreased by 44%. 

Example 3: for 25 mm dia. x 3 m Rod 

Given ρ = 100 Ωm, LR = 3 m, aR = 0.0125 m 

− = Ω ×

π × = ) 1] 31.10.0125

4 3 [ln(

2 3

100 RG

or for a 25% increase in rod diameter (over that of Example 1), the 

resistance to ground is decreased by 3%. 

3.2 Pedestals 

(1) Using the same formula as for a single ground rod, 

) 1] 4 [ln(

2 − π

ρ = =

R

R

R

G R

a

L

L

R R

we have the following examples. 

Example 4: for Steel Footing (220 mm dia. x 2300 mm) 

Given ρ = 100 Ωm, LR = 2.30 m, aR = 0.110 m 

− = Ω ×

π × = ) 1] 23.70.110

4 2.30 [ln(

2 2.30

100 RG

or 26 % "better" than a single rod. 

Example 5: for Steel Footing (85 mm dia. x 1830 mm) 

Given ρ = 100 Ωm, LR = 1.830 m, aR = 0.043 m 

− = Ω ×

π × = ) 1] 36.00.043

4 1.830 [ln(

2 1.830

100 RG

or 12 % "worse" than a single ground rod. 

3.3 Plate Electrodes 

General 

(1) For a single plate, 

) 1]

0.5

8 [ln(

2 − π +

ρ = =

P P

P

P

G P W T

W

L

R R

where 

RP = Resistance of plate to ground in Ohms 

LP = Length in meters 

WP = Width in meters 

TP = Thickness in meters. 
 Traffic Detector Handbook—2006 

October 2006 Page N-5 Federal Highway Administration 

Example 6: for 610 x 610 x 6 mm Plate 

Given ρ = 100 Ohm, LP = 0.61 m, WP = O.61 m, TP = O.OO6 m 

 − = Ω

+

×

π × = ) 1] 45.80.305 0.006

8 0.61 [ln(

2 0.61

100 RG

3.4 Wire Grids 

General 

(1) For the case of a grounding system consisting of a wire grid only, the 

wire shape forms a ground plane (similar to antenna design), which if 

buried deep enough, can constitute the most effective part of the 

grounding system. (Ground rods are normally driven, in any event, in 

order to penetrate below the frost line.) 

The resistance to ground for a grid system is approximated by 

5.6] 1.4 ) 2 [ln ( + − π

ρ = =

W

W

W W

W

W

G W A

L

d Z

L

L

R R

where 

RW = Resistance of wire grid in Ohms 

LW = Total Length of grid wires in meters 

dW = Diameter of wire in meters 

ZW = Burial depth of grid in meters 

AW = Plan area covered by grid in square meters. 

Example 7: Using 3 x 3 m grid with cross-tie 

Given ρ = 100 Ωm, LW = 5 x 3 = 15 m, ZW = 0.3 m 

AW = 3 x 3 = 9 sq m, dW = 0.0105 m (#2/0) 

− = Ω × + ×

×

π × = = 5.6] 16.4

9

1.4 15 )

0.0105 0.3

2 15 [ln(

15

100 R G RW

Example 8: Using 3 x 3 m triangular grid 

Given ρ = 100 Ωm, LW = 3 + 3 + 3 = 9 m, ZW = 0.3 m 

AW = 0.5 x 3 x 3 sin(60o

) = 3.90 sq m, dW = 0.0105 m (#2/0) 

− = Ω × + ×

×

π × = = 5.6] 23.3

3.9

1.4 9 )

0.0105 0.3

2 9 [ln (

9

100 R G RW

 

3.5 Multiple Rods 

General 

(1) The combined effect of several rods is similar to the rod resistance acting 

in parallel and is given by 

3 

3 

3 3

3
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-6 Federal Highway Administration 

] 2.8 ( 1) ) 1

4 [ln (

2

2

R

R

R

R

R

G MR A

L n

a

L

nL

R R − − +

π

ρ = =

where 

RMR = Combined resistance of multiple rods to ground in Ohms 

LR = Rod length in meters 

n = Number of rods 

AR = Area covered by the n rods in square meters. 

For 20 mm x 3 m rods, 

] 1.4( 1) [1

2

R

MR A

n

n

R − +

π

ρ =

Example 9: Using four rods on 3 m square 

Given ρ = 100 Ωm, aR = 0.01 m, LR = 3 m, n = 4, 

AR = 3 x 3 = 9 sq m 

= Ω − +

π × = = ] 11.7

9

1.4( 4 1) [1

4

100 2

RG R MR

(Note: rods not connected by wire) 

3.6 Combination Rod and Wire Grids 

General 

(1) It is may be necessary to include both rod and wire grids for service 

grounds, substations, etc. 

The resistance to ground of the combined system is given by 

W MR WR

W MR WR

G R R R

R R R

R 2

2

+ −

− = , 

where 

RG = Total system resistance to ground in Ohms 

RW = Resistance of wire grid in Ohms (Subsection 3.4) 

RMR = Resistance of multiple rods in Ohms (Subsection 3.5) 

RWR = Mutual resistance factor of the wires to the rods 

= 4.6] 1.4 ) 2 [ln( + − π

ρ =

W

W

R

W

W

WR A

L

L

L

L R

For 20 mm x 3 m rods, 

4.6] 1.4 [ln(0.67) + − π

ρ =

W

W

W

WR A

L

L R
 Traffic Detector Handbook—2006 

October 2006 Page N-7 Federal Highway Administration 

Example 10: Using 3 x 3 m grid with cross-tie and rods 

Given ρ = 100 Ωm, LR = 3 m, aR = 0.01 m, 

AW = AR = 3 x 3 = 9 sq m, n = 4, ZW = 0.3 m, 

dW = 0.0105 m (#2/0), LW = 5 x 3 = 15 m, 

4.6] 1.4 ) 2 [ln( + − π

ρ =

W

W

R

W

W

WR A

L

L

L

L R

 

Substituting the given data in the formula, we have 

− = Ω × × +

π × = 4.6] 10.0

9

1.4 15 [ln(0.67 15)

15

100

RWR

 

= Ω − +

π × = ] 11.7

9

1.4( 4 1) [1

4

100 2

R MR

 

(from Example 9). 

− = Ω × + ×

×

π × = 5.6] 16.4

9

1.4 15 )

0.0105 0.3

2 15 [ln(

15

100 RW

(from Example 7). 

W MR WR

W MR WR

G R R R

R R R

R 2

2

+ −

− =

Substituting the above results in the formula for RG, we 

have 

= Ω

+ −

× − = 11.3

16.4 11.7 20

16.4 11.7 100

RG

 

If the site soil was clayey sand instead of clay, ρ would be 

500 Ωm instead of 100 Ωm (Table 1) and the resistance to 

ground would be 

= × = 56.5Ω

100

500

RG 11.3

 

3.7 Single Wire 

General 

(1) A single wire or counterpoise directly buried in earth has a resistance to 

ground of 

], 2 [ln( ) ln( ) 2

2 2

2

W

W

W

W

W

W

W

W

W

G C

L

Z

L

Z

Z

L

a

L

L

R R + − + − π

ρ = =

where RC = Resistance to ground of buried conductor in Ohms. 

Example 11: Using #6 AWG wire 

Given ρ = 100 Ωm, ZW = 0.6 m, LW = 50 m, aW = 0.00252 m, 

3

 3 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-8 Federal Highway Administration 

) 1] 4 [ln(

2 − π

ρ = =

R

R

R

G R

a

L

L

R R

− = Ω × + − +

π × = ] 3.93

50

0.6

50

2 0.6 ) 2

0.6

50 ) ln(

0.00252

50 [ln(

2 50

100

2

2

RC

 

3.8 Summary of Calculations 

General Formulae 

Single Rod Only: 

Single Plate Only: 

Wire Grid Only: 

Multiple Rods Only: 

] 2.8 ( 1) ) 1

4 [ln (

2

2

R

R

R

R

R

G MR A

L n

a

L

nL

R R − − +

π

ρ = =

Multiple Rods and Wire Grid: 

where 

Single Wire Only: 

] 2 [ln( ) ln( ) 2

2 2

2

W

W

W

W

W

W

W

W

W

G C

L

Z

L

Z

Z

L

a

L

L

R R + − + − π

ρ = =

The symbols that appear in the above formulae are defined as: 

RG = Total resistance to ground of the system in Ohms 

RR = Resistance to ground of a single ground rod in Ohms 

RP = Resistance to ground of a single ground plate in Ohms 

RW = Resistance to ground of a single ground wire in Ohms 

RMR = Resistance to ground of multiple ground rods in Ohms 

RWR = Mutual resistance factor of wires to rods in Ohms 

RC = Resistance to ground of a single buried wire in Ohms 

LR = Length of ground rod in meters 

LW = Length of wire in meters 

LP = Width of plate in meters 

TP = Thickness of plate in meters 

AW = Area of wire grid in square meters 

AR = Area covered by several ground rods in square meters 

aR = Radius of ground rod in meters 

aW = Radius of wire in meters 

) 1]

0.5

8 [ln(

2 − π +

ρ = =

P P

P

P

G P W T

W

L

R R

5.6] 1.4 ) 2 [ln( + − π

ρ = =

W

W

W W

W

W

G W A

L

d Z

L

L

R R

W MR WR

W MR WR

G R R R

R R R

R 2

2

+ −

− =

4.6] 1.4 ) 2 [ln ( + − π

ρ =

W

W

R

W

W

WR A

L

L

L

L R
 Traffic Detector Handbook—2006 

October 2006 Page N-9 Federal Highway Administration 

dW = Diameter of wire in meters 

ZW = Burial depth of wire in meters 

n = Number of ground rods 

ρ = Soil resistivity in Ohm-meters. 

Useful Formulae 

(1) Since the Ministry’s grounding system uses common components of 

• 20 mm dia. x 3.0 m long ground rod 

• #2/0 and #6 AWG ground wire, 

then the general formulae can be reduced to reflect the physical 

parameters of the common items as follows: 

• Single Rod Only: RR = O.32ρ 

• Single 220 x 2300 mm Steel Footing: RR = 0.24ρ

• Single 85 x 1830 mm Steel Footing: RR = O.36ρ

• Single 610 x 610 x 6 mm Plate: RP = 0.46ρ

• Single #6 wire, 3 m length (ZW = 0.30): RW = 0.40ρ

• Single #2/0 wire, 3 m length (ZW = 0.30): RW = 0.36ρ

• 3 x 3 m grid c/w cross-tie (#2/0 wire and 4 rods): RG = O.11ρ 

• 2 Rods @ 3 m space with #6 tie: RG = O.19ρ

• 2 Rods @ 3 m space with #2/0 tie: RG = 0.19ρ. 

The foregoing formulae are approximate and may be used where special 

conditions apply. 

3.9 Application 

(1) The Ministry’s designs for grounding systems are based on the following 

premises: 

(a) A resistance to ground of 10 Ω should be obtained in accordance with 

good practice. 

(b) In cases where 10 Ω to ground is impossible to obtain using practical 

methods, 25 Ω to ground is the minimum requirement provided that 

adequate steps are taken to ensure that step and touch voltages do 

not present a safety problem to workers or the public. 

(c) Every effort is to be made to meet the 10 Ω to ground requirement, 

where practical, by addition of ground electrodes and wire in the 

field. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-10 Federal Highway Administration 

(d) Since soils and their resistivity vary widely with location and 

environment respectively, the Ministry’s standard criteria for design 

is ρ = 100 Ωm. The resistance to ground of the designed system is 

field checked and any required alterations are made at that time. 

Where it is obvious to the designer that increased grounding 

facilities will be required (sand, gravel, rock, etc.), the required 

facilities can be estimated from Table N-3 and included in the 

design. 

(e) Table N-3 is derived from the general formulae of Subsection 3.8. 

(f) The following notes apply to Table N-3: 

(i) Configuration No.9 may be used with continuous #6 

ground wire commonly used for lighting. Grounding at 

every 5th pole applies. 

(ii) Configurations No. 11 or 12 may be used for grounding in 

clay or areas that remain damp. Configuration No. 13 

should be used as the ‘standard’ design. 

(iii) Configurations No. 15 to 18 indicate the results of adding 

more wire and rods to the grid. If dry sand, gravel, or rocky 

areas are unavoidable, the principles illustrated may be 

extended by manual calculation using the formulae given. 

(iv) Values shown in brackets are for information as a simpler 

grid would normally be required. 

(v) Values shown are ‘stand alone’ values (isolated ground). 

An approximation of resistance to ground for any number 

of the systems, which are tied together with ground wire, 

may be made by considering the values to be in parallel. 

3.10 Problem Areas 

Problem areas are identified as: 

(1) Bedrock or shallow overburden of less than 1 m depth over bedrock: 

(a) It will be necessary to drill 150 mm (min) holes in the bedrock and 

backfill these with a cementous iron slag slurry mixture (trade 

name: ‘Embico’). Note that previously used methods used rock salt 

as the chief conductor and that this method is no longer 

recommended due to corrosion. Difficulty in obtaining (and 

measuring) proper resistance to ground will be encountered as the 

ground depends, to some extent, on the number of seams between 

rock layers that are encountered. In this situation, the first design 

choice would be to locate the object to be grounded away from the 

rock area. If this is unavoidable, configuration No. 18 in Table N-3 

should be used for design and added to during construction if 

necessary. 

(2) Soil overburden of 1 m to 2 m depth over bedrock: 

(a) Plates may be used as ground electrodes with the same 

configurations shown in Table N-3 for rods (depending on type of soil 

overburden). A minimum of 300 mm of soil should be left between 

the rock and plate and the #2/0 wire grid. 
 Traffic Detector Handbook—2006 

October 2006 Page N-11 Federal Highway Administration 

(3) Rock Fill 

(a) Areas of rock fill can be assumed to have a resistivity in excess of 

10,000 Ωm. A previously used method was to run two parallel runs 

of #2/0 wire through the voids in the rock fill to a location suitable 

for use of normal grounding methods. The method causes large 

voltages to appear at the cabinet due to the high inductance of the 

leads to ground and should be avoided by placing at least 2.0 m of 

earth fill over the rock fill. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-12 Federal Highway Administration 

 Table N-3. Resistance to ground for various ground system configurations and soils. 

Ground 

System 

Configuration 

Description Normal 

Use 

Resistance 

to Ground 

(Ω) in Clay 

(ρ=100Ωm) 

Resistance 

to Ground 

(Ω) in 

Sandy Clay 

(ρ=200Ωm) 

Resistance 

to Ground 

(Ω) in 

Clayey Sand

(ρ=500Ωm) 

Resistance 

to Ground 

(Ω) in Sand 

(ρ=1500Ωm) 

Resistance 

to Ground 

(Ω) in Sand, 

Gravel 

(ρ=5000Ωm) 

• 1. Single 20 

mm x 3 m rod 

Addition to 

system 

32 80 160 480 1610 

z 2. Single 220 

mm dia. x 2300 

mm steel 

footing 

Poles 

(requires 

additional 

system) 

28 70 140 420 1400 

z 3. Single 85 

mm dia. x 1830 

mm steel 

footing 

Poles, 

cabinets 

(requires 

additional 

system) 

40 100 200 600 2000 

 4. 610 x 610 x 

6 mm plate 

Rock 

overburden 

0.6 to 2.0 m 

46 115 230 690 2300 

 3 5. Single #6 

wire, bare, 3 m 

long 

Addition to 

system 

41 103 205 615 2050 

 3 6. Single #2/0 

wire, bare, 3 m 

long 

Addition to 

system 

38 95 190 570 1900 

 3 7. Single #6 

wire, 2 rods 

Service 19 38 95 290 950 

 



8. Single #2/0 

wire, 2 plates 

Service in 

overburden 

27 54 140 410 1400 

 

z 

9. 220 mm dia. 

x 2300 mm 

steel footing, 

#6 wire, 1 rod 

Poles 19 38 95 285 950 

 

z

10. 85 mm dia. 

X 1830 mm 

steel footing, 

#6 wire, 1 rod 

Poles 16 34 80 240 800 

3

•

•

1

1
 Traffic Detector Handbook—2006 

October 2006 Page N-13 Federal Highway Administration 

 Table N-3. Resistance to ground for various ground system configurations and soils (continued). 

Ground 

System 

Configuration 

Description Normal Use Resistance 

to Ground 

(Ω) in Clay 

(ρ=100Ωm) 

Resistance 

to Ground 

(Ω) in 

Sandy Clay 

(ρ=200Ωm) 

Resistance 

to Ground 

(Ω) in 

Clayey Sand

(ρ=500Ωm) 

Resistance 

to Ground 

(Ω) in Sand 

(ρ=1500Ωm) 

Resistance 

to Ground 

(Ω) in Sand, 

Gravel 

(ρ=5000Ωm) 

 

3 3 

• •

11. 85 mm dia. 

x 1830 mm 

steel footing, 

#2/0 wire, 

2 rods 

Cabinets 14 28 70 210 700 

 •

3 3 

 

• 

12. 85 mm dia. 

x 1830 mm 

steel footing, 

#2/0 wire, 

3 rods 

Cabinets 13 26 65 195 650 

• •

 z

• •

13. 85 mm dia. 

x 1830 mm 

steel footing, 

#2/0 wire, 

4 rods 

Cabinets 10 20 50 150 250 

• •

 

 

• •

14. #2/0 wire, 4 

rods 

Service 

Any for 

ρ < 125 Ωm 

11 22 55 165 550 

• •

• •

 3 

15. #2/0 wire, 4 

rods, 2 ties 

Any for 

ρ < 125 Ωm 

11 22 55 165 550 

 16. #2/0 wire, 4 

rods, 2 ties, 4 

tails 

Any for 

125 < ρ < 

150 Ωm 

(9) 18 45 135 450 

 17. #2/0 wire, 4 

rods, 2 ties, 8 

tails 

Any for 

150 < ρ < 

200 Ωm 

(6) 12 30 90 300 

 18. #2/0 wire, 8 

rods, 6 ties 

Any for 

200 < ρ < 

350 Ωm 

(5) 10 25 75 250 

3

z

z

3 3 3

• •

• •

•

3 3 3

• •

• •

3

3

• •

•

3

3

3

3 3 3

• •

• •

3

3
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-14 Federal Highway Administration 

3.11 Application Guidelines 

(1) From the examples in the foregoing sections, it is immediately obvious 

that obtaining a 10 Ω resistance to ground is difficult in soils with high 

resistivity. 

(2) The effect of ground rod diameter is small. About 8% less resistance to 

ground is obtained by using a 25 mm diameter rod instead of a 20 mm 

rod. Much better results are obtained by making ground rods longer 

rather than thicker. 

(3) The effect of electrode material (copper or steel) has negligible effect on 

results since the resistivity of all metals is much less than that of all 

soils. 

(4) Ground rod spacing should be kept within one rod length spacing of each 

other. 

(5) The effect of the size and type of wire interconnecting the ground rods 

has little effect on results. The #2/0 AWG cable usually used is sized to 

withstand a 50,000 ampere lightning discharge without complete 

melting. 

(6) The upper 1.0 m of ground rod does not have much effect, even in wet 

soil. A minimum depth of 2.0 m gives about 25% more resistance to 

ground than the 3.0 m standard depth rod. 

(7) In order to design proper grounding, a soils classification at the intended 

location should be obtained from the Regional Geotechnical Office (if not 

on the ‘Soils Profile’ or indicated on borehole logs included with contract 

drawings) and District personnel should be consulted. 

(8) If the equipment to be grounded will be in a new fill location, the fill 

should not be composed of sand, gravel, rock and the like (if practical). A 

note on the grading drawings should be added where necessary: ‘Fill in 

the area of (equipment) to be cohesive material only or similar.’ 

(9) Table N-3 gives the number of ground rods (20 mm x 3.0 m) and grid 

configurations required for various classes of soil. Where there is not an 

apparent site problem, ground designs corresponding to ρ = 100 Ωm 

should be used by the designer. Where necessary after testing, the 

design may be adjusted during construction. Where it is not practically 

possible to obtain 10 Ω to ground, an absolute minimum of 25 Ω may be 

used. 

SECTION III—EFFECTS OF LIGHTNING 

1. GENERAL 

The effects of lightning on outdoor electrical and electronic equipment can be 

costly. Damage from lightning may result from: 

• Direct strokes 

• Power surges 

• Inducted transient voltage spikes 

• Capacitive voltages. 
 Traffic Detector Handbook—2006 

October 2006 Page N-15 Federal Highway Administration 

Since it is not practical to protect outdoor equipment against direct strokes, 

protective systems apply to the prevention or handling of surges and 

transients. The protective systems consist of the application of proper 

ground, suppression, and shunting devices. 

Since weather is somewhat unpredictable, protection design is based on the 

following probabilities: 

• Probability of a storm 

• Probability of a strike 

• Probable potential energy and RF energy 

• Probable rise time of the voltage (open circuit) wave or current (short 

circuit) wave 

• Probable duration or repetition of a strike. 

2. DESIGN CRITERIA 

The design criteria adopted for protection of the Ministry’s electronic 

equipment are: 

• Peak voltage = 15,000 V 

• Peak current = 5,000 A 

• Maximum current flow duration = 500 μs 

• Current waveform = 8 x 20 μs 

• Voltage Waveform = 1.2 x 50 μs. 

Figure N-1 indicates waveforms and timing of lightning protection devices. 

Figure N-1. Voltage and current waveforms. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-16 Federal Highway Administration 

Note that the times needed for protection are much too fast to allow power 

circuit protection devices such as breakers, fuse, lightning arresters etc. to 

operate effectively. However, devices such as gas tubes and metal oxide 

varistors (MOVs) will initiate protection at about 0.15 μs and 0.007 μs, 

respectively. 

3. POWER SURGES 

Surges in any equipment including cables, poles, etc. can be induced by 

lightning strikes as far as 6 km away. Surges on overhead high voltage lines 

are grounded through lightning arresters at transformer locations. 

Figure N-2 shows the voltage and current distribution through the earth 

near the bottom of the utility pole. For the design value of resistivity ρ = 100 

Ωm, a voltage of 15,000 volts would be transferred through the earth for a 

distance of 5.3 m. It is therefore necessary to keep the service ground a 

minimum distance from the Hydro ground as indicated in Figure N-3. Since 

the designer seldom knows where the Hydro ground line is located, the 

convention of 5.5 m to the center of the pole is used as a design practice. Note 

that a large voltage will appear at the service ‘SN’ due to the Ldi/dt voltage on 

the grounding cable. 
 Traffic Detector Handbook—2006 

October 2006 Page N-17 Federal Highway Administration 

Figure N-2. Voltage in earth due to discharging lightning current at service pole. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-18 Federal Highway Administration 

Figure N-3. Recommended improvement to system ground connections. 

4. OTHER SOURCES OF POSSIBLE DAMAGE 

Traffic signal systems contain many other sources of transient voltages and 

currents within the controller cabinet. These sources are not considered as 

severe as the energy surge through the service neutral and all have 

protection devices installed within the cabinet. Some sources are: 

(a) Detector Loops—inductive loop detector electronics units are 

protected internally with their own lightning arrester and are also 

provided with external MOVs at the input file. Failure rate due to 

lightning damage is very low as the voltage impressed on a loop is 

caused by capacitive effects. 

(b) Detector Cable—the possibility of induced currents caused by 

transient voltages in the earth is minimized by shielding the 

detector cable and leaving both ends of the shield cut off. 

(c) Signal Cable—signal cable is shielded by metal poles (above 

ground), but is subject to induced currents caused by transient 

voltages in the earth. The load switches and the AC-terminals of the 

cabinet are protected by MOVs and the failure rate is low. 

(d) Direct Hits on Cabinet—although nothing can be done to ensure a 

complete lack of damage, the controller cabinet may be considered to 

be protected by an umbrella cone of 30o

 from an overhead line and 

somewhat protected by a 45o

 cone. It is not desirable, however, to 

install the cabinet directly under the lines due to possible 

electromagnetic interference. The cabinet location (Figure N-4) 

should be: 

• 11 m minimum from the supply pole 

• 3 m minimum (horizontally) clear of overhead lines 
 Traffic Detector Handbook—2006 

October 2006 Page N-19 Federal Highway Administration 

• Within the 30o to 45o cone of protection (within 15 m for 

normal height lines) of the overhead lines. 

If the controller is to be situated across the road from the hydro 

lines, then the #6 AWG (green) ground wire and the" feeder wires 

should be run in rigid steel duct, to the nearest electrical chamber. 

These conductors should then be run to the next chamber across the 

road via the under-pavement crossing, and from this chamber to the 

controller in any approved electrical duct, not necessarily of metal. 

(e) Direct Hits on Poles or Equipment—this condition would cause 

severe damage. The method of mitigating possible damage effects 

consists of installing a #6 AWG RWU 90 (green) system ground wire 

connecting all poles and intersection equipment and installing a 

ground rod on each corner. Connection of the system ground around 

the intersection should be made at one point only (the service 

ground bus) as indicated in Figure N-5. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-20 Federal Highway Administration 

Figure N-4. Controller cabinet location for best lightning protection. 
 Traffic Detector Handbook—2006 

October 2006 Page N-21 Federal Highway Administration 

Figure N-5. Signal grounding system (with or without lighting). No ground electrodes at cabinet. 

SECTION IV—SUMMARY OF DESIGN GUIDELINES 

1. TRAFFIC SIGNAL SYSTEMS 

(1) Design standard grounding system under normal circumstances 

(Figures N-3 and N-5): 

• Service ground—4 rods and #2/0 bare ground wire. 

• Controller cabinet ground connected to system ground at service 

ground bus. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-22 Federal Highway Administration 

• Equipment ground—1 rod or steel footing per intersection corner 

interconnected with #6 insulated wire. 

• System ground—interconnect the controller cabinet ground and the 

equipment ground to the service ground at service ground bus. 

(2) Use improved design as per Table N-3 for grounds in sand, gravel, or 

rock. Consult Geotechnical Information and District Maintenance. 

(3) Both ends of the detector cable shield should be cut off and left 

unconnected. 

(4) Locate controller at least 11 m from a hydro pole and at least 3 m 

horizontally from overhead lines. Locate controller 1.5 m clear minimum 

from metal objects such as poles, fences, and guide rails. 

SECTION V – REFERENCES 

(1) Biddle Instruments, Getting Down to Earth: A Manual on Earth 

Resistance Testing for Practical Man, 4th ed., 1981. 

(2) Bodle, D., Electrical Protection Guide for Land-Based Radio Facilities, 

Joslyn Electronic Systems, 1971. 

(3) Burns, G.A, “Lightning Damage to Tank-Gauging Equipment Solved by 

Modification Instead of Replacement,” Oil and Gas Journal, pg. 93, 

September 14, 1981. 

(4) Canadian Standards Association, Canadian Electrical Code Part I, 

Section 10, Grounding and Bonding, 1987. 

(5) Carpenter, R.B., “Total Isolation from Lightning Influences,” IEEE 

Transactions on Industry Applications, Vol. 1A-17, No. 3, pg. 334, 

May/June 1981. 

(6) Cunagin, W.D. and Avoub, N.A., Lightning Protection Hardware and 

Techniques for Electronic Traffic Control Equipment, Federal Highway 

Administration, February, 1986. 

(7) Dasen, M., Insulation Tester – Megger, Algonquin College. 

(8) Dasen, M., Meg-Earth Tester, A1gonquin College. 

(9) Denny, H.W. and Rohrbaugh, J.P., “Transient Protection, Grounding, 

and Shielding of Electronic Traffic Control Equipment,” NCH RP 

Report 317, Transportation Research Board, June, 1988. 

(10) Edco, Inc., Lightning Protection for Traffic Control, Edco Technical 

Bulletin No. 200-01, May, 1978. 

(11) Edco Inc. of Florida, Installation Technical Bulletin # 100484, 1984. 

(12) Epstein. B.M., “For Best Results, Treat Power and Computer 

Requirements as One System,” EC&M, pg. 130, August 1986. 

(13) Fink, D.G and Beaty, H.W., Standard Handbook for Electrical 

Engineers, 11th Edition, 1978. 

(14) Freund, A, “Protecting Computers from Transients,” EC&M, pg. 65, 

April 1987. 

(15) General Electric, Transient Voltage Suppression, 3rd Edition. 
 Traffic Detector Handbook—2006 

October 2006 Page N-23 Federal Highway Administration 

(16) Gunn, R., “Facility Noise Control from the Ground Up,” EC&M, pg. 56, 

April 1987. 

(17) Harder, J.E., Hughes, A.E., and Vosicky, J., “Analytical Method for 

Coordination of Surge Arresters with Current-Limiting Fuses,” IEEE 

Transactions on Industry Applications, Vol. lA-17, No. 5, pg. 445, 

September/October 1981. 

(18) Institute of Electrical and Electronic Engineers, Inc., IEEE Guide for 

Radio Methods of Measuring Earth Conductivity, IEEE Standard 356-

1974. 

(19) Institute of Electrical and Electronic Engineers, Inc., IEEE Guide for 

Safety in Substation Grounding. IEEE Standard 80-1976. 

(20) Institute of Electrical and Electronic Engineers, Inc., IEEE Guide for 

the Installation of Electrical Equipment to Minimize Electrical Noise 

Inputs to Controllers from External Sources, IEEE Standard 518-1982. 

(21) Institute of Electrical and Electronic Engineers, Inc., ”IEEE Standard 

Procedures for the Measurement of Radio Noise from Overhead Power 

Lines,” IEEE Transactions on Power Apparatus and Systems, Vol. Pas -

100, No. 8, August 1981. 

(22) Institute of Electrical and Electronic Engineers, Inc., “Modeling 

Current-Limiting Surge Arresters,” IEEE Transactions on Power 

Apparatus and Systems, Vol. Pas -100, No.8, August 1981. 

(23) Lee, W.R., “The Dangers of Lightning,” ETI Canada, pg. 27, November 

1978. 

(24) Michaels, E.C., “Principles and Techniques for Grounding and Bonding 

in Hazardous (Classified) Locations,” Plant Engineering, pg. 133, 

September 17, 1981. 

(25) Mims, F.M., “Introducing the Varistor,” Computers and Electronics, pg. 

88, May 1983. 

(26) Ontario Hydro Inspection Department, “Electrical Inspection, 

Provincial Government,” Electrical Inspection Guide 26-4, December 

1975. 

(27) Ontario Hydro Inspection Department, “Pub1ic Roads - Electrical 

Devices,” Electrical Inspection Guide 11-3, August 1984. 

(28) Ontario Hydro Inspection Department, “Rule 10-208: Grounding 

Connections for Two or More Buildings or Structures,” Bulletin 10-6-0, 

April 1987. 

(29) Plumber, J.A. and Crouch, K.E., Lightning Protection for Traffic 

Control Systems, National Aeronautics and Space Administration, 

1978. 

(30) Schwarz, S.J., “Analytical Expressions for the Resistance of Grounding 

Systems,” presented at the AIEE Summer and Pacific General Meeting, 

Los Angeles, June 1954. 

(31) Stifter, F.J., “Power-Line Disturbances,” Computers and Electronics, 

pg. 35, October, 1983. 
Appendix N—Grounding (Design Guidelines) 

October 2006 Page N-24 Federal Highway Administration 

(32) Thomas, P., Investigative Report - Lightning Problems on 170/332 

Traffic Control Systems, Ministry of Transportation of Ontario, June 

1985. 

(33) Waterson, A. and Maher, P., “Computer Power - Problems and 

Solutions,” EC&M, pg. 67, December 1982. 

(34) Westinghouse Electric Corporation, Electrical Transmission and 

Distribution Reference Book, 1950. 

(35) “Equipment, Manuals and Procedures Evaluation for the Design and 

Maintenance of Traffic Signal Systems,” Report No. 2, Grounding, 

Ministry of Transportation of Ontario, May 1988. 
 Traffic Detector Handbook—2006 

October 2006 Page O-i Federal Highway Administration 

APPENDIX O. GROUNDING 

(MAINTENANCE GUIDELINES) 

The material in this appendix is a reproduction of the information that 

appears in: 

Electrical Maintenance Manual, volume 2 

Chapter 9, “Grounding” 

Part 2, “Component Maintenance,” 

Pages 9-1 to 9-11 

as published by the Ontario Ministry of Transportation, Ontario, Canada 

December 1989. 

(Note: The format of the material has been modified to conform to the 

styles used in other parts of the Traffic Detector Handbook.) 

 Traffic Detector Handbook—2006 

October 2006 Page O-iii Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX O. 

APPENDIX O. GROUNDING (MAINTENANCE GUIDELINES)....................... O-1

SECTION I—TRAFFIC SIGNAL GROUNDING .................................................O-1

1. HYDRO GROUNDS...................................................................................... O-1

2. SERVICE GROUNDS................................................................................... O-1

3. CABINET GROUNDS .................................................................................. O-2

4. SPECIAL SOIL CONDITIONS ................................................................... O-2

5. SYSTEM GROUND ...................................................................................... O-2

6. ROUTINE INSPECTION ............................................................................. O-2

7. EMERGENCY INSPECTION ..................................................................... O-2

8. MEASUREMENTS ....................................................................................... O-3

9. STEEL FOOTINGS....................................................................................... O-4

10. GROUND RODS.......................................................................................... O-4

11. GOOD PRACTICE ..................................................................................... O-7

SECTION II—REFERENCES................................................................................O-7

LIST OF FIGURES: APPENDIX O. 

Figure O-1. Recommended improvement to system ground connection. ..O-1

Figure O-2. Signal grounding system (with or without lighting). 

No ground electrodes at cabinet. .............................................O-3

LIST OF TABLES: APPENDIX O. 

Table O-1. Resistance to ground for various ground system 

configurations and soils............................................................O-5 

 Traffic Detector Handbook—2006 

October 2006 Page O-1 Federal Highway Administration 

APPENDIX O. GROUNDING 

(MAINTENANCE GUIDELINES) 

SECTION I—TRAFFIC SIGNAL GROUNDING 

1. HYDRO GROUNDS 

(1) Inadequate hydro grounds at a service pole will allow energy from 

lightning discharge into the controller. Inspect Hydro grounds visually 

and arrange for the Supply Authority to repair faulty grounds. 

(2) At controller sites where lightning problems have occurred, the Hydro 

ground should be reinforced to two 20 x 3000 mm copperclad rods at 3 m 

spacing at the service pole and at one hydro pole each way. Arrange 

purchase order for this work if necessary. 

(3) Do not use Hydro ground as a Service ground. 

2. SERVICE GROUNDS 

(1) To reduce voltages transferred from the Hydro ground (under lightning 

discharge) to the Service ground, the Service ground should be located 

as far from the Hydro ground as possible. See Figure O-1. 

(2) The service ground should consist of a minimum of four 20 x 3000 mm 

copperclad ground rods on #2/0 bare copper wire grid as per Figure O-1. 

Figure O-1. Recommended improvement to system ground connection. 
Appendix O—Grounding (Maintenance Guidelines)

October 2006 Page O-2 Federal Highway Administration 

3. CABINET GROUNDS 

(1) Proper ground bus bar should be installed in the controller cabinet and 

all ground leads from equipment should connect to this one point. 

(2) The controller cabinet ground bus should be connected to the service 

solid neutral by a #6AWG RWU90 insulated system ground wire. 

4. SPECIAL SOIL CONDITIONS 

(1) Grounding system in areas of sand, gravel, or rock may require 

reinforcement due to poor soil conductivity. Refer to Electrical Design 

Manual—Volume 1. Request design aid from Electrical Engineering 

Section if necessary. 

5. SYSTEM GROUND 

(1) System ground wire for signals to be #16 AWG RWU90–40oC green. 

(2) Insta11 connections of the three systems (Service ground, cabinet 

ground, and equipment ground) as indicated in Figure O-2. 

(3) Ensure that all metal enclosures are bonded to the proper section of the 

system ground. 

(4) All connections to be thermit weld, impact or compression type (no split 

bolts, etc.). 

6. ROUTINE INSPECTION 

(1) Yearly inspection frequency for: 

(a) Tight connections of accessible ground wires (controller cabinet and 

service); corrosion. 

(b) Hydro and service grounds; visual for obvious damage from vehicles, 

etc. 

(c) Testing of resistance to ground and soil conductivity (service ground) 

as per Subsection 8. 

7. EMERGENCY INSPECTION 

(1) For controllers damaged by lightning: 

(a) Inspection Hydro ground as per Subsection 1. 

(b) Have Supply Authority check distribution arrestor and upgrade 

ground and arrestor if required. 

(c) Inspect for proper installation of grounds as per Subsections 1 to 5. 
 Traffic Detector Handbook—2006 

October 2006 Page O-3 Federal Highway Administration 

Figure O-2. Signal grounding system (with or without lighting). No ground electrodes at cabinet. 

8. MEASUREMENTS 

(1) Resistance to ground and soil conductivity should be measured at an 

‘average’ time of year. Measurements taken when soil is wet or frozen 

are not meaningful. 

(2) Use the Megger ground resistance meter and the two small ground rods 

supplied with the unit. Follow instructions exactly for distances or 

results will be meaningless. For information regarding the instrument, 

contact the Electrical Operations Unit. 

(3) Measurements to ground to be taken at the SN of the service and the 

controller cabinet ground bus. 

(4) Resistance to ground and soil resistivity are related mathematically. See 

Electrical Design Manual—Volume 1, Section II. 
Appendix O—Grounding (Maintenance Guidelines)

October 2006 Page O-4 Federal Highway Administration 

(5) Resistance to ground should be 10 Ω or less but can vary up to 25 Ω in 

high resistivity soils. Readings over 25 Ω indicate that further 

inspection and repairs or replacement should be done or that additional 

rods and wire should be added (new installations). Add additional 

elements as per Table O-1. 

9. STEEL FOOTINGS 

(1) Steel footings may be considered the equivalent of a ground rod for 

resistance to ground purposes. 

10. GROUND RODS 

(1) One rod length spacing to be obtained between rods. 

(2) Connecting wire is more effective if buried to 1/2 the normal frost depth 

(assuming snow cover). 

(3) Do not install rods at more than 45o

 angle. Driving jaws for use with a 

hydraulic drive head are available. 

(4) Use 20 x 3000 mm copperclad rods only, with thermit weld or impact 

connectors. 
 Traffic Detector Handbook—2006 

October 2006 Page O-5 Federal Highway Administration 

 Table O-1. Resistance to ground for various ground system configurations and soils. 

Ground 

System 

Configuration 

Description Normal 

Use 

Resistance 

to Ground 

(Ω) in Clay 

(ρ=100Ωm) 

Resistance 

to Ground 

(Ω) in 

Sandy Clay

(ρ=200Ωm) 

Resistance 

to Ground 

(Ω) in 

Clayey Sand

(ρ=500Ωm) 

Resistance 

to Ground 

(Ω) in Sand 

(ρ=1500Ωm) 

Resistance 

to Ground 

(Ω) in Sand, 

Gravel 

(ρ=5000Ωm) 

• 1. Single 20 

mm x 3 m rod 

Addition to 

system 

32 80 160 480 1610 

z 2. Single 220 

mm dia. x 2300 

mm steel 

footing 

Poles 

(requires 

additional 

system) 

28 70 140 420 1400 

z 3. Single 85 

mm dia. x 1830 

mm steel 

footing 

Poles, 

cabinets 

(requires 

additional 

system) 

40 100 200 600 2000 

 4. 610 x 610 x 

6 mm plate 

Rock 

overburden 

0.6 to 2.0 m 

46 115 230 690 2300 

 3 5. Single #6 

wire, bare, 3 m 

long 

Addition to 

system 

41 103 205 615 2050 

 3 6. Single #2/0 

wire, bare, 3 m 

long 

Addition to 

system 

38 95 190 570 1900 

 3 7. Single #6 

wire, 2 rods 

Service 19 38 95 290 950 

 



8. Single #2/0 

wire, 2 plates 

Service in 

overburden 

27 54 140 410 1400 

 

z 

9. 220 mm dia. 

x 2300 mm 

steel footing, 

#6 wire, 1 rod 

Poles 19 38 95 285 950 

 

z

10. 85 mm dia. 

X 1830 mm 

steel footing, 

#6 wire, 1 rod 

Poles 16 34 80 240 800 

3

•

•

1

1
Appendix O—Grounding (Maintenance Guidelines)

October 2006 Page O-6 Federal Highway Administration 

Table O-1. Resistance to ground for various ground system configurations and soils (continued). 

Ground 

System 

Configuration 

Description Normal Use Resistance 

to Ground 

(Ω) in Clay 

(ρ=100Ωm) 

Resistance 

to Ground 

(Ω) in 

Sandy Clay

(ρ=200Ωm) 

Resistance 

to Ground 

(Ω) in 

Clayey Sand 

(ρ=500Ωm) 

Resistance 

to Ground 

(Ω) in Sand 

(ρ=1500Ωm) 

Resistance 

to Ground 

(Ω) in Sand, 

Gravel 

(ρ=5000Ωm) 

 

3 3 

• •

11. 85 mm dia. 

x 1830 mm 

steel footing, 

#2/0 wire, 

2 rods 

Cabinets 14 28 70 210 700 

 •

3 3 

 

• 

12. 85 mm dia. 

x 1830 mm 

steel footing, 

#2/0 wire, 

3 rods 

Cabinets 13 26 65 195 650 

• •

 z

• •

13. 85 mm dia. 

x 1830 mm 

steel footing, 

#2/0 wire, 

4 rods 

Cabinets 10 20 50 150 250 

• •

 

 

• •

14. #2/0 wire, 4 

rods 

Service 

Any for 

ρ < 125 Ωm 

11 22 55 165 550 

• •

• •

 3 

15. #2/0 wire, 4 

rods, 2 ties 

Any for 

ρ < 125 Ωm 

11 22 55 165 550 

 16. #2/0 wire, 4 

rods, 2 ties, 4 

tails 

Any for 

125 < ρ < 

150 Ωm 

(9) 18 45 135 450 

 17. #2/0 wire, 4 

rods, 2 ties, 8 

tails 

Any for 

150 < ρ < 

200 Ωm 

(6) 12 30 90 300 

 18. #2/0 wire, 8 

rods, 6 ties 

Any for 

200 < ρ < 

350 Ωm 

(5) 10 25 75 250 

 

3

z

z

3 3 3

• •

• •

•

3 3 3

• •

• •

3

3

• •

•

3

3

3

3 3 3

• •

• •

3

3
 Traffic Detector Handbook—2006 

October 2006 Page O-7 Federal Highway Administration 

11. GOOD PRACTICE 

(1) Grounding is a safety device and many members of the electrical 

industry are somewhat lax about proper grounding practices as the only 

times they are required are under abnormal conditions, such as short 

circuits and lightning surges. Poor workmanship or practices are not 

apparent until such an abnormal problem occurs. 

(2) The Ministry’s grounding system practices should meet or exceed the 

requirements of the Code. (The Code is a minimum requirement). If 

doubt exists as to practice, the regulations contained in the Code should 

overrule other opinions. 

(3) Workers should endeavor to follow the Ministry’s practices faithfully so 

as not to endanger themselves or the Ministry to the possibility of legal 

prosecution due to mishap through poor practice or application. 

SECTION II—REFERENCES 

(1) “Equipment, Manuals and Procedures Evaluation for the Design and 

Maintenance of Traffic Signal Systems,” Report No. 2, Grounding.

Ministry of Transportation of Ontario, May 1988. 

 Traffic Detector Handbook—2006 

October 2006 Page P-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX P. 

APPENDIX P. GLOSSARY......................................................................P-1 

 Traffic Detector Handbook—2006 

October 2006 Page P-1 Federal Highway Administration

APPENDIX P. GLOSSARY 

AC (also written ac): Alternating Current. See definition of alternating 

current. 

ACTIVE SENSOR: A sensor that transmits energy, a portion of which is 

reflected or scattered from a vehicle or other objects and surfaces in its 

detection zone back toward the receiving aperture of the sensor. 

ACTUATED CONTROLLER: A traffic signal controller that receives 

information from vehicle and/or pedestrian sensors and provides 

appropriate signal timing. 

ACTUATION: The operation of any type of sensor (NEMA). The word 

operation means an output from the sensor system to the controller. 

ADVISORY SENSOR: A device that detects vehicles on one or more 

intersection approaches solely for the purpose of modifying the phase 

sequence and/or length for other approaches to the intersection (NEMA). 

ALTERNATING CURRENT (ac): A current that reverses its magnitude 

and direction of flow at regular intervals. The rate of reversal is 

expressed in hertz (cycles per second). 

AMPERE: The unit expressing the rate of flow of electrons through a 

conductor. One ampere is the current flowing through a 1-ohm resistance 

with 1-volt pressure. 

AMPLIFIER: See electronics unit. 

ANALOG: An electronic design that uses continuously varying voltages, 

rather than discrete digital values. 

ANSI: American National Standards Institute. 

ANTENNA: The radiating or receiving elements utilized in transmitting or 

receiving electromagnetic waves (NEMA). 

AREA DETECTION: The continuous detection of vehicles over a length of 

roadway wherein the call of a vehicle in the detection area is intended to 

be held for as long as the vehicle remains in the area of detection. (Most 

sensors cannot hold the call indefinitely.) Sensors used for this are 

frequently referred to as large-area detectors, long-loop detectors, or 

presence detectors. 

AREA OF DETECTION: See Detection Zone. 

AREA DETECTOR: See Large Area Detector. 

ARTERIAL: A major urban roadway usually having coordinated signals 

along its length. 

ASTM: American Society for Testing and Materials. 

ATC: Advanced Transportation Controller. 

AUXILIARY EQUIPMENT: Separate devices used to add supplementary 

features to a controller assembly (NEMA). 

AWG: American Wire Gauge. The standard measurement of wire size based 

on the circular mil system (1 mil equals 0.001 inch). 

CABLE: A group of separately insulated conductors wrapped together and 

covered with an outer jacket. 

CALL: A registration of a demand for right-of-way by traffic at a controller 

unit (NEMA). The call comes to the controller from a traffic flow sensor 

that is outputting an actuation. 
Appendix P—Glossary 

October 2006 Page P-2 Federal Highway Administration 

CALLING DETECTOR (CALLING SENSOR): A detector (sensor) that is 

installed in a selected location to detect vehicles which may not otherwise 

be detected and whose output may be modified by the controller unit 

(NEMA). This traditionally has meant a small area detector near the 

Stop Line to detect vehicles entering the roadway from a driveway during 

the red or yellow signal. The detector is disconnected when the green 

signal is displayed so that extensions of the green can only come from the 

appropriate Extension Detector. 

CAPACITANCE: That property of dielectric materials that permits the 

storage of electrically separated charges when potential differences exist 

between the surfaces of the dielectric. Capacitance is expressed as the 

ratio of an electric charge to a potential difference. 

CARD-RACK MOUNTED ELECTRONICS UNIT: See Rack Mounted 

Electronics Unit. 

CARRYOVER OUTPUT: The ability of a sensor to continue its output for a 

predetermined length of time following an actuation (NEMA). See 

Extended Call Detector. 

CHAMFER: Diagonal saw slots at the corners of square or rectangular 

inductive loops to reduce the angle of bend of the loop wires at the 

intersecting saw slots. 

CHANGE INTERVAL: The yellow interval following the green signal 

indicating the change to a conflicting phase. 

CHANNEL: Electronic circuitry that processes information from an 

inductive-loop detector or other sensor. 

CIRCUIT: A closed path followed by an electric current. 

CLEARANCE INTERVAL: See Red Clearance Interval. 

COIL: A coiled conductor, wound on a form or core, which uses 

electromagnetic induction to cause changes in a current. 

CONDUCTANCE: The measure of ability to conduct electricity expressed in 

units of inverse ohms-meter. 

CONDUCTOR: A medium for transmitting electrical current. A conductor 

usually consists of copper or other electrically conductive materials. 

CONDUIT: A tube for protecting electrical wires or cables. 

CONFLICTING CALL: See Serviceable Conflicting Call.

CONFLICTING PHASES: Two or more traffic phases which will cause 

interfering (i.e., conflicting) traffic movements if operated concurrently. 

CONTINUOUS PRESENCE MODE: Detector (sensor) output continues if 

any vehicle (first or last remaining) remains in the field of influence 

(NEMA). This definition does not imply that the use of this mode 

guarantees that the output will continue indefinitely, as most detectors 

are incapable of holding a call beyond a finite interval. See Detector 

Mode. 

CONTROLLED OUTPUT: The ability of a detector (sensor) to produce a 

pulse that has a predetermined duration regardless of the length of time 

a vehicle is in the field of influence (NEMA). See Detector Mode. 

CONTROLLER ASSEMBLY: A complete electrical mechanism mounted in 

a cabinet for controlling the operation of a traffic control signal (NEMA). 
 Traffic Detector Handbook—2006 

October 2006 Page P-3 Federal Highway Administration

A. Traffic-Actuated Controller Assembly: A controller assembly for 

supervising the operation of traffic control signals in accordance with 

the varying demands of traffic as registered with the controller by 

detectors (NEMA). 

B. Semi-Traffic-Actuated Controller Assembly: A type of traffic 

actuated controller assembly in which means are provided for traffic 

actuation on one or more, but not all approaches to the intersection 

(NEMA). 

C. Full-Traffic-Actuated Controller Assembly: A type of traffic 

actuated controller assembly in which means are provided for traffic 

actuation on all approaches to the intersection (NEMA). 

D. Pedestrian-Actuated Controller Assembly: A controller assembly 

in which intervals, such as pedestrian WALK and clearance 

intervals, can be added to or included in the controller cycle by the 

actuation of a pedestrian detector (NEMA). Pedestrian actuation may 

be part of an intersection controller or be used to control a midblock 

crosswalk. 

CONTROLLER UNIT: A controller unit is that potion of a controller 

assembly that is devoted to the selection and timing of signal displays 

(NEMA). Also referred to as the Dispatcher or Timer. Another controller 

class (i.e., Type 170, 179, and 2070) standardizes hardware modules and 

uses specialized software to implement traffic management functions. 

CRITICAL INTERSECTION CONTROL (CIC): An algorithm employed in 

traffic systems to dynamically control the split at signalized locations 

where the traffic patterns are such that special control, responsive to 

changing conditions, is needed.

CRITICAL LANE DETECTION: A system of hardware and software 

designed to provide data on traffic flow for a selected lane, usually the 

heaviest volume lane on an approach to a signalized intersection.

CROSSTALK: The adverse interaction of any channel of a sensor or sensor 

electronics unit with any other channel in that or another device. 

Crosstalk can occur via mutual coupling of magnetic fields in nearby 

inductive loops. The effect is an interaction between two or more 

electronics units in the same cabinet when the units operate at the same 

or nearby frequencies. Crosstalk results in a sensor output actuation in 

the absence of a vehicle. 

CYCLE: A complete sequence of all signal indications at an intersection. In 

an actuated controller, a complete cycle is dependent on the presence of 

sensor calls on all phases.

CYCLE LENGTH: The time period in seconds required for a complete cycle. 

Cycle length is normally variable for actuated intersections unless they 

are part of a coordinated system.

DELAYED CALL DETECTOR (ELECTRONICS UNIT): A detector 

(electronics unit) that does not issue an output until the detection zone 

has been occupied for a period of time that exceeds the time preset on an 

adjustable timer incorporated into the electronics unit. 

DELAYED OUTPUT: The ability of an electronics unit to delay its output 

for a predetermined length of time during an extended actuation 

(NEMA). 

DELTA L (∆L): Change in inductance. 
Appendix P—Glossary 

October 2006 Page P-4 Federal Highway Administration 

DEMAND: The request for service, e.g., one or more vehicles desiring to use 

a given segment of roadway during a specified unit of time. 

DEMAND CONTROL: See Loop Occupancy Control. 

DEMAND OPERATION: A mode of operation whereby the service provided 

at an intersection reflects the presence of demand for that service 

without regard to background cycles. 

DENSITY: A measure of the concentration of vehicles, stated as the number 

of vehicles per mile per lane. 

DENSITY CONTROLLER: Actuated controller that has timing 

adjustments for the selection of the allowable gap independent of the 

passage time. A volume-density controller and a modified density 

controller are each a type of density controller. 

DESIGN SPEED: The speed used as typical by the designer of the sensor/- 

controller system in the kinematic analysis of the scheme under free 

traffic flow conditions. 

DETECTION ZONE: That area of the roadway within which a vehicle will 

be detected by a vehicle sensor (NEMA). Also called zone of detection, 

sensing zone, area of detection, detection area, effective loop area, field of 

influence, field of view, or footprint. 

DETECTOR or SENSOR: A device for indicating the presence or passage of 

automotive vehicles, light and heavy rail, buses, or pedestrians. This 

general term is usually supplemented with a modifier indicating type 

(e.g., inductive-loop detector, magnetic detector, radar sensor, and video 

image processor); operation (e.g., point detector or sensor, presence 

detector or sensor, and large area sensor); or function (e.g., calling 

detector and extension detector). See also Traffic Detector or Sensor. 

DETECTOR AMPLIFIER: See Electronics Unit. 

DETECTOR (ELECRONICS UNIT) FAILURES: The occurrence of 

detector malfunctions including nonoperation, chattering, or other 

intermittently erroneous detections. 

DETECTOR (ELECTRONICS UNIT) MEMORY: The retention of an 

actuation for future utilization by the controller assembly. The phrase 

might better be detection memory to make it clearer that the memory is 

within the controller, not the sensor. 

DETECTOR MODE: See Electronics Unit Mode. 

DETECTOR SETBACK: See Sensor Setback. 

DETECTOR SYSTEM: See Sensor System. 

DIELECTRIC: Any insulating material that is a nonconductor of electricity. 

DILEMMA ZONE: A distance or time interval related to the onset of the 

yellow interval. Originally the term was used to describe that portion of 

the roadway in advance of the intersection within which a driver can 

neither stop prior to the stop line nor clear the intersection before 

conflicting traffic is released. That usage pertained to insufficient length 

of timing of the yellow and/or all-red intervals. More recently the term 

has been used to describe that portion of the roadway in advance of the 

intersection within which a driver is indecisive regarding stopping prior 

to the stop line or proceeding into or through the intersection. May also 

be expressed as the increment of time corresponding to the dilemma zone 

distance. 
 Traffic Detector Handbook—2006 

October 2006 Page P-5 Federal Highway Administration

DILEMMA-ZONE PROTECTION: Any method or procedure that attempts 

to control the end of the green interval so that no vehicle will be caught 

in the dilemma zone when the signal turns yellow. 

DIRECT CURRENT (dc): An electrical current that travels in one 

direction. 

DISPATCHER: Obsolete Name. See Controller Unit 

DRIFT: Change in the electrical properties of the sensor system or a portion 

of it due to environmental changes, particularly temperature variations 

and rain water. 

EDDY CURRENT: An electric current induced within the body of a 

conductor when that conductor moves through a time-varying magnetic 

field. 

EFFECTIVE LOOP AREA: See Detection Zone. 

EIA: Electronic Industries Association. 

ELECTRONICS UNIT: An electronic device that energizes the wire loop(s), 

monitors the loop(s) inductance by filtering and amplifying the signals it 

receives, and responds to a predetermined decrease in inductance with an 

output that indicates the passage or presence of vehicles in the detection 

zone. It is the electronics element in a loop detector system, i.e., it does 

not include the loop(s), lead-in wire, or lead-in cable. Sometimes called an 

amplifier or detector, although it performs other functions as well, e.g., 

sensitivity adjustment, failure indication, and delayed actuation. 

Electronics units are also used with magnetic detectors and 

magnetometers. 

ELECTRONICS UNIT MODE: A term used to describe the operation of an 

electronics unit channel output when a presence (or passage) detection 

occurs (NEMA). Also referred to as Detector Mode. See pulse mode, 

controlled output, continuous presence mode, and limited presence mode. 

ENCAPSULATION: The process of filling the saw slot with sealant to 

surround the wires in the slot and protect them from traffic, weather, and 

debris. 

ENCASEMENT: Material used to enclose the loop wire for protection, such 

as a polyvinyl or polyethylene tube. Often referred to as Detect-a-duct or 

other similar commercial names. 

EPOXY: A resin used in bonding. 

EXTENDED CALL DETECTOR (ELECTRONICS UNIT): When selected, 

this detector extends the output (See Carryover Output) after the vehicle 

departs the detection zone for a preset time. 

EXTENDED CALL TIMING: An electronics unit with carryover output. It 

holds or stretches the call of a vehicle for a period of seconds that has 

been set on an adjustable timer incorporated into the electronics unit. It 

can be designed to begin the timing of that period when the vehicle 

enters the detection area, or when it leaves. The latter is specified by 

NEMA. Also referred to as a Stretch Detector. 

EXTENSION DETECTOR (ELECTRONICS UNIT): An electronics unit 

that is configured to register actuations at the controller only during the 

green interval for a given approach, so as to extend the green time for 

that approach to accommodate the actuating vehicles. It is not active 

during the red or yellow intervals for that approach. 
Appendix P—Glossary 

October 2006 Page P-6 Federal Highway Administration 

EXTENSION LIMIT: The maximum length of time that the actuations on 

any traffic phase may retain the right-of-way after an actuation on an 

opposing traffic phase. Also known as maximum green. 

FAILSAFE (as in output relay design): A type of output-relay design 

that produces a constant call, thereby keeping traffic moving in the event 

that the electronics unit fails. 

FARAD: A unit of capacitance, usually expressed in microfarads (μF) (one 

millionth of a farad). 

FEEDER CABLE: See lead-in cable. 

FIELD OF INFLUENCE: See Detection Zone. 

FIXED TIMING PLAN: Timing plan in which the cycle length, split, offset, 

and actuated settings are not adjusted between switching of timing 

plans. These are used in TOD/DOW and traffic adjusted systems. 

FLEXIBLE TIMING PLAN: A dynamic timing plan in which the cycle 

length, split, offset, and actuated settings may be adjusted once per cycle 

between switching of timing plans. These are used in Traffic Responsive 

Systems such as SCOOT. 

FMCW: Frequency modulated, continuous wave. A waveform Capable of 

detecting presence and measuring the relative speed of vehicles within 

the range measurement interval. 

FREEWAY SURVEILLANCE: Process or method of monitoring freeway 

traffic performance and control system operation. 

FREQUENCY: The number of times an alternating current repeats its cycle 

in 1 second. 

FULL TRAFFIC-ACTUATED CONTROLLER ASSEMBLY: A controller 

assembly for supervising the operation of traffic control signals in which 

means are provided for traffic actuation on all approaches to the 

intersection. 

GAP: The time interval between the end of one vehicle sensor actuation and 

the beginning of the next actuation. 

GAP OUT: Terminating of a green phase due to an excessive time interval 

between the actuations of vehicles arriving on the green, allowing the 

green to be served to a competing phase. 

GAP REDUCTION: Å feature whereby the unit extension or allowed time 

spacing between successive vehicle actuations on the phase displaying 

the green in the extensible portion of the interval is reduced (NEMA). 

GREEN EXTENSION SYSTEM: Hardware assembly of extended call 

electronics units and auxiliary logic. The logic can monitor the signal 

display, enable or disable the selected extended call sensors, and hold the 

controller in artery green. 

GROUND: The earth and all parts conductively connected to the earth. 

HAND HOLE: See Pull Box. 

HENRY (H): The measure of inductance, defined as the inductance of a 

circuit in which a counter electromotive force of one volt is generated 

when the current is changing at the rate of one ampere per second. 

HERTZ (Hz): A term replacing cycles per second as the measurement unit 

for frequency. 
 Traffic Detector Handbook—2006 

October 2006 Page P-7 Federal Highway Administration

HOLD: A command that retains the existing right-of-way. A command to the 

controller, which causes it to retain the existing right-of-way. A 

momentary release of the hold command allows the controller to yield to 

other conflicting phases requesting service (often referred to as the yield 

command or yield point). 

HOLD ON LINE: A connection that modifies a controller from fully 

actuated, isolated operation to semiactuated, system-controlled 

operation. It is used as the basic tie from the local intersection to the 

system master. 

HOME-RUN CABLE: See Lead-In Cable. 

HONDA 100 cc MOTORCYCLE: A small motorcycle commonly considered 

to be the smallest registered motor vehicle. It is used as the baseline for 

motorized vehicle detection. 

μH (MICROHENRY): A measure of inductance. See Henry and Inductance. 

IMSA: International Municipal Signal Association. 

INDUCTANCE: That property of an electric circuit or of two neighboring 

circuits, whereby an electromotive force is generated in one circuit by a 

change of current in itself or in the other circuit. The ratio of the 

electromotive force to the rate of change of the current. Measured in 

microhenrys (μH). 

INDUCTIVE-LOOP DETECTOR: See Loop Detector. 

INDUCTIVE REACTANCE: The reactance (ohms) of an ideal (lossless) 

inductor is the product of the voltage across the inductor and the sine of 

the phase angle (0 to 90 degrees) between inductor voltage and current 

divided by the inductor current assuming sinusoidal excitation. 

INFRARED SENSOR: See passive infrared sensor. 

INITIAL PORTION: The first timed portion actuated controller unit: 

A. Fixed initial portion: A preset initial portion that does not change. 

B. Computed initial portion: An initial portion that is traffic adjusted. 

C. Maximum initial portion: The limit of the computed initial portion. 

D. Minimum initial portion: See fixed initial portion. 

E. Added initial portion: An increment of time added to the minimum 

initial portion in response to vehicle actuations. 

IN-ROADWAY SENSOR: A sensor that is embedded in the pavement of the 

roadway, embedded in the subgrade of the roadway, or taped or 

otherwise attached to the surface of the roadway. 

INTERCONNECTED CONTROL: Control systems in which basic 

coordination is provided by wireline or wireless interconnect. This 

information is used to determine the local signal timing of offset and 

cycle/split or actuated timing based on the Time-of-Day/Day-of-Week. The 

operator can select timing and download timing plans and changes and 

monitor system status. There are no system detectors in Interconnected 

Control systems. 

INTERVAL: The part of parts of the signal cycle during which signal 

indications do not change (NEMA). 
Appendix P—Glossary 

October 2006 Page P-8 Federal Highway Administration 

ISOLATED INTERSECTION CONTROL: The form of signal control for a 

single signalized intersection that does not have control relationships 

between intersections nor any data flow between intersections. The 

system of intersections is not responsive to traffic in real time. The 

individual isolated intersection may have local actuated control or Time 

of Day/Day of Week changes based on historical traffic flows. Also called 

uncoordinated signal control. See also totally isolated systems. 

ITE: Institute of Transportation Engineers. 

JUNCTION WELL: See Pull Box. 

JUNCTION BOX: See Pull Box. 

KILOHERTZ (kHz): Thousands of hertz. A measure of frequency. 

L: Symbol for inductance. 

∆L: Symbol for change in inductance. 

LARGE AREA SENSOR: One or more sensors or combination of sensors 

connected together in series, parallel, or series/parallel covering an area 

in the approach to an intersection or extended freeway segment. Large 

area sensors that monitor intersection approaches have detection areas 

that vary from 6 x 40 ft (1.8 x 12 m) to 6 x 100 ft (1.8 x 30 m) or larger. A 

common configuration is four 6- x 6-ft (1.8- x 1.8-m) loops spaced 10 ft (3 

m) apart for a length of 54 ft (16.5 m). Also known as large area detector, 

area detector, or area sensor. 

LASER RADAR: An active sensor that transmits energy usually in the near 

infrared spectrum for traffic management applications. Single and twolane models are manufactured. 

LAST CAR PASSAGE: A selected feature of a density controller that, upon 

gap-out, will cause the green to complete the timing of the passage time. 

The last vehicle to have been detected, known as the Last Car, will 

therefore retain the green until it reaches the stop line. Thus, it is 

assured of avoiding the dilemma zone problem and of clearing the 

intersection. This feature is not defined by NEMA; however, a number of 

manufacturers provide it. 

LEAD-IN CABLE: The electrical cable that is spliced to the lead-in wire in 

the pull box and connects to the input of the electronics unit in the 

controller cabinet (NEMA). Sometimes called home-run cable, 

transmission line, or feeder cable. 

LEAD-IN WIRE: That portion of the inductive-loop wire between the 

physical edge of the loop and the pull box, which should be twisted a 

specified number of turns per foot. The lead-in wire is located in the saw 

slot connecting the loop to the edge of the roadway. From there, it is 

placed in conduit to the controller or to a pull box, where it is spliced to 

the lead-in cable. For a magnetic detector and magnetometer, it is the 

wire that runs from the sensor (probe) to the pull box. 

LIGHT-SENSITIVE VEHICLE SENSOR: A sensor consisting of a light 

source and photoelectric cell or cells, capable of being actuated by the 

passage of a vehicle interrupting the beam or beams of light. When 

properly equipped, directional characteristics are present. 

LIMITED PRESENCE MODE: Electronics unit output continues for a 

limited period of time if vehicles remain in the field of influence. See 

electronics unit mode. 

LINK: The length of roadway between two signalized locations. 
 Traffic Detector Handbook—2006 

October 2006 Page P-9 Federal Highway Administration

LIP: See local intersection program. 

LOCAL CONTROLLER: A controller supervising the operating of traffic 

signals at a single intersection. Also see controller assembly and 

controller unit. 

LOCAL INTERSECTION PROGRAM (LIP): A Type 170 software 

program developed by Caltrans that incorporates electronics unit timing 

features within the program. 

LOCKING DETECTION MEMORY: A selectable feature of the circuit 

design for a controller phase, whereby the call of a vehicle arriving on the 

red (or yellow) is remembered or held by the controller after the vehicle 

leaves the detection area until it has been satisfied by the display of a 

green interval to that phase. 

LOOP DETECTOR: An active sensor composed, in part, of one or more 

loops of wire whose inductance decreases in response to the passage or 

presence of a vehicle in the detection zone of the loops. See also Loop 

Detector System. 

A. Motion loop detector, nondirectional: An inductive-loop detector 

that produces a signal when a vehicle passes over any portion of the 

loop. 

B. Motion loop detector, directional: An inductive-loop detector 

consisting of two separate loops either closely spaced or partially 

overlapping that is not affected by the passage of a vehicle except in 

the desired direction. 

C. Presence loop detector: An inductive-loop detector that is capable 

of detecting the presence of a standing or moving vehicle in any 

portion of the effective loop area. 

LOOP DETECTOR SYSTEM: A vehicle detection system that senses a 

decrease in inductance of the wire loop(s) during the passage or presence 

of a vehicle in the detection zone of the loop(s) (NEMA). Has the same 

meaning as loop detector, but is clearer in its inclusion of the wire loop(s), 

lead-in wire, lead-in cable, and the electronics unit. 

LOOP ELECTRONICS UNIT: See Electronics Unit. 

LOOP OCCUPANCY CONTROL: A detector/controller design using long 

detection loop(s) [normally 30 ft (9 m) or longer], and a controller 

operated in the nonlocking mode. A loop occupancy controller may, but 

need not necessarily, be designed to rest in all red in the absence of any 

traffic demand. Loop occupancy control can utilize magnetometer and 

other sensors, as well as inductive-loop detectors. 

LOOP SYSTEM: The wire loops and lead-in wire that are connected through 

lead-in cable to the input terminals of the electronics unit. 

LOOP LEAD-IN WIRE: See Lead-In Wire. 

LSI CHIP: An electrical component with more than 1,000 logic elements. 

This large-scale integration of miniature elements is used in some digital 

loop electronics units. 

MAGNETIC DETECTOR: A passive device that senses changes in the 

Earth’s magnetic field caused by the movement of a ferrous-metal vehicle 

in or near its detection zone. It is placed under or in the roadway to 

detect the passage of a vehicle over the sensor. These sensors generally 

detect only moving vehicles. Also known as induction and search coil 

magnetometers. Their output is connected to an electronics unit. 
Appendix P—Glossary 

October 2006 Page P-10 Federal Highway Administration 

A. Compensated magnetic vehicle detector: Any magnetic detector 

that is designed and structured to not be affected by changing 

electromagnetic influence other than that resulting from the passage 

of a vehicle. When properly equipped, a compensated magnetic 

detector has strong directional characteristics. 

B. Noncompensated magnetic detector: Any magnetic detector other 

than a compensated magnetic detector. 

MAGNETIC SHADOW: The distortion of the flux lines of the Earth’s 

magnetic field as they pass through a ferrous vehicle, due to the greater 

permeability of the vehicle to these flux lines as compared to air. 

MAGNETOMETER SENSOR: A passive device that detects the passage or 

presence of a ferrous metal object through the perturbation (know as a 

magnetic anomaly or magnetic shadow) it causes in the Earth’s magnetic 

field. Its output is connected to an electronics unit. Not to be confused 

with a magnetic detector. Also known as a fluxgate magnetometer. Twoaxis fluxgate magnetometers respond to changes in the vertical and 

horizontal components of the Earth’s magnetic field. 

MAXIMUM GREEN LIMIT: The maximum green time after an opposing 

actuation, which may start in the initial portion. 

MEGGER: A device used by power companies to measure very high 

resistance to earth ground. 

MEGOHM: One million ohms, which is the unit of electrical resistance. 

MEMORY OFF: A selectable feature of an actuated controller, synonymous 

with nonlocking detection memory. 

MICROHENRY: One millionth of a henry (μH), from the unit of 

measurement of inductance. 

MICROWAVE RADAR SENSOR: An active vehicle sensor installed above 

or adjacent (side-mounted) to the roadway that transmits and receives 

energy in the microwave portion of the electromagnetic spectrum. Two 

types of microwave radar sensors exist. The Doppler or continuous wave 

radar is a passage sensor and detects only vehicles moving faster than 

some minimum speed. The FMCW presence radar detects stopped and 

moving vehicles. Models are manufactured for single and multilane 

applications. 

MINIMUM GREEN INTERVAL: The shortest green time of a phase. If a 

time setting control is designated as “minimum green,” the green time 

shall not be less than that setting. 

MINIMUM VEHICLE STANDARD: A test unit that produces the 

minimum change in input for which the sensor system must detect and 

indicate passage or presence. NEMA specifies a Class 1 vehicle (a small 

motorcycle). 

MOTION SENSOR: A sensor that detects the motion of a vehicle passing 

through the detection zone of the sensor at some minimum speed, usually 

2 to 3 mi/h (3.2 to 4.8 km/h). Vehicles traveling slower than the minimum 

speed or stopped in the detection zone are not detected. 

NANOHENRY: One billionth of a henry, from the unit of measurement of 

inductance. 

NEMA: National Electrical Manufacturers Association. 
 Traffic Detector Handbook—2006 

October 2006 Page P-11 Federal Highway Administration

NETWORK: A series of intersecting arterials or streets that are a part of a 

coordinated signal system. 

A. Open Network: A network where the arterials do not intersect more 

than once (i.e., there are no closed loops in the system). 

B. Closed Network: A network that contains closed loops. 

NOMINAL INDUCTANCE: A design value of inductance where the actual 

value can vary from the specified value within a range that permits 

satisfactory equipment operation. 

NONCONFLICTING PHASES: Two or more traffic phases that will not 

cause interfacing traffic movements if they are operated concurrently. 

NON-DIRECTIONAL SENSOR: A sensor capable of being actuated by 

vehicles proceeding in any direction. 

NON-LOCKING MEMORY: A mode of actuated-controller unit operation 

that does not require memory (NEMA). In this mode of operation, the call 

of a vehicle arriving on the red (or yellow) is forgotten or dropped by the 

controller as soon as the vehicle leaves the detection area. 

OCCUPANCY: The percent of time a sensor’s detection zone is occupied. 

Occupancy is a pseudomeasure of density on a roadway. 

OHM: The unit of electrical resistance equal to the resistance through which 

a current of one ampere will flow when there is a potential difference of 1 

volt across it. 

OVER-ROADWAY SENSOR: A sensor that is mounted above the surface of 

the roadway either above the roadway itself or alongside the roadway, 

offset from the nearest traffic lane by some distance. 

PASSAGE DETECTION: The ability of a vehicle sensor to detect the 

passage of a vehicle moving through the detection zone and to ignore the 

presence of a vehicle stopped within the detection zone (NEMA). 

PASSAGE SENSOR: A traffic flow sensor that detects the passage of a 

vehicle moving through the detection zone and ignores the presence of a 

vehicle stopped within the detection zone (NEMA). Also known as a 

motion sensor, dynamic detector or sensor, and movement detector or 

sensor. 

PASSAGE PERIOD: The time allowed for a vehicle to travel at a selected 

speed from the sensor to the nearest point of conflicting traffic (NEMA). 

PASSAGE TIME: The timing interval during the extensible portion that is 

resetable by each electronics unit actuation. The green right-of-way of the 

phase may terminate on expiration of the unit extension time. Also 

known as vehicle interval or preset gap. 

PASSIVE SENSOR: A sensor that transmits no energy of its own, but 

rather receives energy emitted from objects in its detection zone or 

energy transmitted from other sources that is reflected from objects in 

the detection zone. 

PASSIVE ACOUSTIC SENSOR: A sensor that detects acoustic energy or 

audible sounds produced by vehicular traffic from a variety of sources 

within each vehicle and from the interaction of vehicle’s tires with the 

road. Also known as passive acoustic array sensors because they consist 

of arrays of small microphones. 
Appendix P—Glossary 

October 2006 Page P-12 Federal Highway Administration 

PASSIVE INFRARED SENSOR: A sensor utilized in traffic management 

applications, which operates in the far infrared spectrum. It transmits no 

energy of its own. Rather it detects energy from two sources: (1) energy 

emitted from vehicles, road surfaces, and other objects in their field of 

view and (2) energy emitted by the atmosphere and reflected by vehicles, 

road surfaces, or other objects into the sensor aperture. 

PEDESTRIAN ACTUATED CONTROLLER ASSEMBLY: A controller 

assembly in which intervals, such as pedestrian WALK and clearance 

intervals, can be added to or included in the controller cycle by the 

actuation of a pedestrian sensor. 

PEDESTRIAN CLEARANCE INTERVAL: The first clearance interval 

following the pedestrian WALK indication, normally flashing DON’T 

WALK. 

PEDESTRIAN SENSOR: A sensor that is responsive to operation by or the 

presence of a pedestrian (NEMA). This traditionally has been of the 

push-button type, installed near the roadway and operated by hand. 

Preferably it should have some form of pilot light to indicate upon 

actuation that the unit is operating, but this is rarely provided because of 

susceptibility to vandalism. Also, NEMA does not provide an output to 

illuminate this indicator. 

PEDESTRIAN PHASE: A traffic phase allocated to pedestrian traffic, 

which may provide a right-of-way indication either concurrently with one 

or more vehicular phases, or to the exclusion of all vehicular phases. 

PEDESTRIAN-ACTUATED CONTROLLER: See Controller Assembly. 

PHASE: A traffic signal phase has two different meanings in traffic signal 

terminology, as follows: 

A. NEMA: A vehicular phase is a phase that is allocated to one specific 

vehicular traffic movement (e.g., eastbound through traffic as timed 

by a dual ring controller unit). See Conflicting Phases and NonConflicting Phases. 

B. TRADITIONAL: A part of the cycle allocated to any specific traffic 

movement receiving the right-of-way or to any combination of traffic 

movements receiving the right-of-way simultaneously during one or 

more intervals. 

PHASE SEQUENCE: A predetermined order in which the phases of a cycle 

occur. 

POINT DETECTION: The detection of vehicles as they pass a specific point 

on the roadway. Frequently referred to as small area detection. 

POINT SENSOR: A sensor that measures the passage of vehicles past a 

point [i.e., a small area usually not exceeding 6 x 6 ft (1.8 x 1.8 m)]. Also 

referred to as a small area detector or small area sensor. 

POLYCHLOROPRENE: (Chemical name for Neoprene.) Used for jacketing 

wire and cable that will be subject to rough usage, moisture, oil, greases, 

solvents, or chemicals. May also be used as a low-voltage insulating 

material. 

POLYETHYLENE: A family of insulating materials derived from the 

polymerization of ethylene gas. All members of the family are excellent 

dielectrics. Electrically they are superior to other extruded dielectrics in 

use today. They have high insulation resistance, high dielectric strength, 

and good abrasion resistance. Polyethylene is widely used for insulation 

on signal and loop detector wire and cable. It is suitable for direct burial. 
 Traffic Detector Handbook—2006 

October 2006 Page P-13 Federal Highway Administration

POLYOLEFINS: A family of plastics including cross-linked polyethylene 

and various ethylene copolymers, one that shrinks when heat is applied. 

Commonly used in splicing. 

POLYPROPYLENE: A thermoplastic with good electrical characteristics, 

high tensile strength, and resistance to heat. 

POLYSULFONE: A polymer highly resistant to mineral acid, alkali, and 

salt solutions. 

POLYURETHANE: Enamel that has excellent moisture resistance, easily 

soldered, and excellent winding properties. 

POLYVINYL CHLORIDE (PVC): A family of insulating compounds whose 

basic ingredient is either polyvinyl chloride or its copolymer with vinyl 

acetate. Can be either rigid as used in conduit for preformed detector 

loops or flexible as used in duct tubing to encase detector wire. It is also 

widely used as insulation material on wire known as types T and TW in 

detector applications. Known as PVC or Vinyl. 

POWERHEAD: A small 3- x 6-ft (0.9- x 1.8-m) loop installed at the stop line 

end of a long loop in order to improve the detection of small vehicles. It 

usually has multiple turns of wire and may have an angled configuration. 

PRESENCE DETECTION: The ability of a vehicle sensor to detect a 

moving or stopped vehicle in its detection zone. 

PRESENCE SENSOR: A traffic flow sensor that detects the presence of a 

vehicle within its detection zone and holds the call for a specified 

minimum time. The sensor may cover a large area or be a series of small 

sensors wired together in series, parallel, or series/parallel. 

PRESENCE HOLDING TIME: The time that a sensor system will continue 

to indicate the presence of a vehicle over one of its loops without 

adjusting to vehicle movement to a new position. When the adjustment 

occurs, the actuation is terminated. NEMA requires that a Class 2 

vehicle (large motorcycle) be detected for a minimum of 3 minutes. 

PRESENCE LOOP DETECTOR: An inductive-loop detector that is capable 

of detecting the presence of a standing or moving vehicle in any portion of 

the effective loop area (ITE). 

PRESET GAP: See Passage Time. 

PRESSURE DETECTOR OR SENSOR: A sensor that consists of two 

metal plates separated by spacers. When a vehicle tire passes over the 

sensor, it compresses the spacers and allows the two plates to make 

contact, thereby closing an electrical circuit. This type of sensor normally 

produces two or more pulses per vehicle because each axle creates a 

contact. 

PROBE: The type of sensor commonly used with a magnetometer sensor. A 

magnetic detector is also referred to as a probe. 

PROGRESSIVE FLOW: Coordinated movement along an arterial at a given 

speed. 

PULL BOX: A container usually at least 1 cubic foot (0.028 cubic meter) in 

size that is placed underground with a removable cover flush with the 

ground surface. Splices between the loop lead-in wires (or magnetometer 

or magnetic detector cables) and the lead-in cable to the controller 

cabinet are located in the pull box. 
Appendix P—Glossary 

October 2006 Page P-14 Federal Highway Administration 

PULSE MODE: The electronics unit output that produces a short pulse 

(between 100 and 150 ms) when a vehicle enters the sensor detection 

zone. This occurs even though the vehicle remains in the detection zone 

for a longer time. See Electronics Unit Mode. 

Q: See Quality Factor. 

QUADRUPOLE: A loop configuration that adds a longitudinal saw slot 

along the center of the rectangle, so that the wire can be installed in a 

figure-eight pattern, thereby producing four electromagnetic poles 

instead of the normal two. The design improves the sensitivity to small 

vehicles and also minimizes splashover. 

QUALITY FACTOR: A numerical index for rating the quality of a resonant 

circuit. A higher number indicates lower loss and increased detection 

sensitivity in a resonant type sensor system such as is typical of 

inductive-loop detectors. 

QUEUE DETECTOR OR SENSOR: Component of a traffic control system 

that senses the presence (or number) of vehicles waiting in a queue at an 

intersection or on a freeway ramp. 

QUEUE LENGTH: Number of vehicles stopped or slowly moving in a line, 

where the movement of each vehicle is constrained by that of the lead 

vehicle. 

RACK-MOUNTED ELECTRONICS UNITS: Electronics units that have no 

enclosing case and are connected to the sensor system by inserting its 

printed circuit board into a wired receptacle or “rack” located in the 

controller cabinet. Compare with shelf mounted electronics units. 

RADIO-FREQUENCY DETECTOR: A vehicle detector consisting of a loop 

of wire embedded in the roadway that is tuned to receive a preselected 

radio frequency from a transmitter located on a vehicle. 

REACTANCE: The reactance (ohms) of a circuit component is the product of 

the voltage across the component and the sine of the phase angle between 

the voltage and the current divided by the current through the 

component assuming sinusoidal operation. 

RED CLEARANCE INTERVAL: A clearance interval that may follow the 

yellow interval during which both the terminating phase and the next 

right-of-way phase display red (NEMA). 

REJECTION (Adjacent Lane): The ability of a sensor not to output an 

unwanted actuation caused by a vehicle in a lane adjacent to that in 

which the sensor is located. 

RELUCTANCE: The opposition that a magnetic material or magnetic circuit 

offers to the passage of magnetic lines of force. Reluctance is the 

reciprocal of permeability (the ability of a metal to conduct lines of force) 

and permeance (the measure of that conductivity). Therefore, reluctance 

is related to permeability and permeance in much the same way as 

resistance is related to conductance in electrical circuits. 

REPHASE: The process of resetting, after a pulse actuation, to enable 

another pulse actuation should another vehicle enter the detection zone. 

Can also be used as a verb: “The sensor shall rephase 2 seconds after 

initiating an output pulse.” 

RESIN: An organic substance that is a nonconductor of electricity. Resins 

are widely used for insulation and encapsulation. 
 Traffic Detector Handbook—2006 

October 2006 Page P-15 Federal Highway Administration

RESISTANCE: The opposition that a device or material offers to the flow of 

current, equal to the voltage drop across the element divided by the 

current through the element. 

RESONANT FREQUENCY: The oscillation frequency of a loop and its leadin wire, to which the electronics unit must tune. 

RESPONSE TIME: The time a sensor system takes to respond to the 

initiation of a detection by a vehicle. 

REST-IN-RED: A controller operational mode intended to display red to all 

movements, in the absence of any traffic demand. 

RMS: Root-Mean-Square. 

SAMPLING SENSOR: Any type of vehicle sensor used to obtain 

representative traffic flow information (NEMA). 

SCANNING ELECTRONICS UNIT: A multichannel electronics unit in 

which the loop(s) of each channel are energized in sequence, one at a 

time, in quick succession. 

SCATS: The Sydney Coordinated Adaptive Traffic System (SCATS). A realtime system developed by the Roads and Traffic Authority (RTA) of New 

South Wales, Australia. SCATS uses two levels of control. Strategic 

control determines timing plans for areas and sub-areas based on 

average traffic conditions. Tactical control applies at the individual 

intersection. This system is considered to fall under the Traffic 

Responsive category of real time traffic signal system control categories. 

SCOOT: The Split, Cycle and Offset Optimization Technique (SCOOT) is a 

real-time system developed by the Transport and Road Research 

Laboratory (TRRL) in Great Britain. SCOOT computes the cyclic flow 

profile for every traffic link every four seconds. It projects these profiles 

downstream using the TRANSYT dispersion model. 

SDLC: Synchronous Data Link Control. 

SEALANT: The material used in a saw slot of an inductive-loop detector to 

seal the wires in the slot. 

SELF-POWERED VEHICLE DETECTOR (SPVD): One type of 

magnetometer sensor buried in the pavement, which uses a selfcontained battery for power and transmits vehicle flow information to the 

controller without the need for direct connection (lead-in cable). 

SELF-TRACKING ELECTRONICS UNIT (DETECTOR): A loop 

electronics unit, not necessarily self-tuning, which includes circuits that 

compensate for environmental drift. 

SELF-TUNING LOOP ELECTRONICS UNIT (DETECTOR): One that is 

capable of adapting its operation to the resonant frequency of the loop 

and lead-in wire without any manual adjustment required. The term 

applies particularly to the startup of the electronic unit’s operation, upon 

turn-on. Compare Self-Tracking Electronics Unit. 

SEMI-TRAFFIC-ACTUATED CONTROLLER ASSEMBLY: A controller 

assembly for supervising the operation of traffic control signals in which 

means are provided for traffic actuation on one or more but not all 

approaches to the intersection. 

SENSING ZONE: See Detection Zone. 
Appendix P—Glossary 

October 2006 Page P-16 Federal Highway Administration 

SENSITIVITY: As it relates to an inductive-loop system, the change in total 

inductance of a system caused by a minimum-sized vehicle at one loop, 

expressed as a percentage of the total inductance. As it relates to an 

electronics unit, it is the minimum inductance change in percent required 

at the input terminals to the electronics unit that cause it to actuate. 

SENSOR (SYSTEM AND LOCAL): Traffic detection devices that permit 

the system master or a local controller to obtain information about the 

traffic flow characteristics in the detection zone of the sensor. (See 

Detector.) NEMA limits the meaning of “sensor” to the sensing element 

of a detection system. 

SENSOR LOOP: An electrical conductor arranged in a square, rectangular, 

circular, or other configuration to encompass a portion of the roadway. It 

provides a detection zone designed such that the passage or presence of a 

vehicle in the zone causes a decrease in the inductance of the loop that 

can be measured for detection purposes (NEMA). Also known as wire loop 

or inductive-loop detector. 

SENSOR OFFSET: For an over-roadway sensor, the horizontal distance 

between the sensor mounting location and the edge of the closest portion 

of the detection zone. 

SERIES-PARALLEL: Type of electrical interconnection of four 6- x 6-ft (1.8- 

x 1.8-m) loops, usually 9 ft (2.7 m) apart and installed in a line in one 

lane to give a 51 ft (15.5 m) length of detection area. This interconnection 

scheme gives a combined inductance close to the optimum value. 

SENSOR SETBACK: The longitudinal distance between stop line and 

traffic flow sensor. 

SENSOR SYSTEM: The complete sensing and actuation group of 

components, consisting of the electronics unit in the controller, the leadin cable, the lead-in wires, and the sensor. 

SENSOR WEB DENSITY: The density of the configuration of sensors 

required for different kinds of arterial and network traffic control and 

surveillance systems by link and by lane. 

SERVICEABLE CONFLICTING CALL: A call that: 

A. Occurs on a conflicting phase not having the right-of-way at the time 

the call is placed. 

B. Occurs on a conflicting phase that is capable of responding to the call. 

C. When occurring on a conflicting phase operating in an occupancy 

mode, remains present until given its right-of-way (NEMA). 

SHEATH: The outer covering or jacket over the insulated conductors to 

provide mechanical protection for the conductors. 

SHELF-MOUNTED ELECTRONICS UNITS: Electronics units that have 

an enclosing case and, therefore, can be mounted in the cabinet by 

placing them on a shelf. They can stand alone. Compare Rack-Mounted 

Electronics Units. 

SHIELD: A conductive material surrounding the twisted pair(s) of wires in 

the lead-in cable of an inductive-loop detector installation, so that outside 

electrical interference will not induce noise onto the twisted pair(s). 

SHORTED-TURN MODEL: A lossless shorted, single-turn loop with a size 

and shape equal to the perimeter of the vehicle located above the loop at 

the average vehicle undercarriage height. 
 Traffic Detector Handbook—2006 

October 2006 Page P-17 Federal Highway Administration

SIDE-FIRE SENSOR: A vehicle sensor located to one side of the roadway, 

such as on a pole, rather than directly over the roadway. 

SIU: Serial interface unit which provides communication, interface and 

message protocol between the ATC and the Input and Output units but 

does no processing of the data and control messages. 

SMALL-AREA SENSOR: A sensor that detects vehicles at a spot location, 

i.e., a small area usually not exceeding 6 x 6 ft (1.8 x 1.8 m). It may detect 

vehicles in more than one lane. The 6- x 6-ft (1.8- x 1.8-m) inductive-loop 

detector is a prominent example. Ultrasonic and radar devices may 

function as small area sensors with detection areas as long as 20 to 30 ft 

(6 to 9 m). When used in this manner, the length of time the moving 

vehicle is in the detection zone is not needed for the intersection control 

logic. The small area sensor is also referred to as a point sensor or point 

detector. 

SOUND-SENSITIVE VEHICLE SENSOR: A sensor that responds to 

sound waves produced when a vehicle passes through the detection area 

of the sensor. 

SPEED ANALYSIS SYSTEM: A type of hardware assembly composed of 

two loop detectors and auxiliary logic. The two loops are in the same lane, 

a known distance apart. A vehicle passing over the loops produces two 

actuations. The time interval between the first and the second is 

measured, and a speed is computed. 

SPLASHOVER: An unwanted actuation caused by a vehicle in a lane 

adjacent to the lane in which the sensor is located. Often occurs where 

long loops are used. 

SPLICE BOX: See Pull Box. 

SPLIT: A division of the cycle length allocated to each of the various phases 

(normally expressed in percent). 

SPVD: See Self-Powered Vehicle Detector. 

STRETCH DETECTOR: See Extended Call Detector. 

SYSTEM DETECTOR OR SENSOR: Sensors located to provide 

information to central control computers selecting appropriate control 

programs to meet the traffic demands. 

TAPESWITCH: A temporary sensor consisting of two strips of metal 

encased in a flat ribbon that is temporarily affixed to the roadway. When 

a vehicle tire crosses the tapeswitch, the two metal strips make contact, 

thereby closing an electrical circuit. 

TERMINAL: Any fitting used for making a convenient electrical connection. 

TF: The Underwriter’s Laboratories, Inc. (UL) designation for fixture wire, 

solid soft copper conductor, insulated with thermoplastic. 

TFF: Same as TF, except has stranded copper conductor. 

THHN: Building wire, plastic insulated, 90 °C, 600 volt, nylon jacketed. 

THRESHOLD (LOOP): A minimum level of percent change in inductance 

that occurs to produce an actuation. 

THW: Building wire, plastic insulated, heat, flame, and moisture resistant, 

75 °C. 

THWN: Same as THW with overall nylon jacket. 
Appendix P—Glossary 

October 2006 Page P-18 Federal Highway Administration 

TIME HEADWAY: The time separation between vehicles approaching an 

intersection, measured from the front of the lead vehicle to the front of 

the trailing vehicle. 

TIME-BASED COORDINATION: Control systems in which basic 

coordination is provided by a highly accurate real time clock within the 

traffic signal controller. This information is used to determine the local 

signal timing of offset and cycle/split or actuated timing based on the 

Time-of-Day/Day-of-Week. Also called time-based coordinated control. 

TIME-OF-DAY/DAY-OF-WEEK (TOD/DOW): Timing plan selections based 

on the clock and calendar rather than on system traffic information. 

TOD/DOW systems operate with no system sensors (or at a sensor 

density Level of 0 or 0.5). These systems measure no characteristics of 

traffic flow. They may use clocks or wireline or wireless communications 

to provide the timing plan selection. Two categories of traffic control 

systems have predominantly TOD/DOW characteristics. These are the 

time based coordination and the interconnected control categories. In 

addition, isolated intersection control may sometimes use TOD/DOW 

considerations. 

TIMER: Obsolete name. See Controller Unit. 

TOTALLY ISOLATED SYSTEMS: Control systems without either timebased or communications-based connections between intersections. Thus, 

these systems operate each intersection without consideration of the 

operation of adjacent signalized intersections. 

TRAFFIC DETECTOR OR SENSOR: A device by which automotive 

vehicles, light and heavy rail, buses, or pedestrians are enabled to 

register their presence with a traffic-actuated controller. 

TRAFFIC PHASE: Right-of-way and clearance intervals in a cycle assigned 

to any independent movement(s) of traffic. 

TRAFFIC-ACTUATED CONTROLLER ASSEMBLY: A controller 

assembly for supervising the operation of traffic control signals in 

accordance with the varying demands of traffic as registered with the 

controller by traffic flow sensors. 

TRAFFIC-ACTUATED CONTROLLER: See Controller Assembly. 

TRAFFIC ADAPTIVE (CONTROL) SYSTEM: Control systems that 

operate with 2 sensors per lane per link for optimum efficiency (Sensor 

web density level of 3.0). Adaptive systems do not have cycle, split, and 

offsets in the classic sense. They forecast traffic into the near future and 

proactively reoptimize selection, sequence, and duration of phases every 

several seconds. These systems need to measure the flow of individual 

vehicles so that they can predict the future flow of the individual vehicles 

or small packets of vehicles. System response to traffic changes is 

proactive. 
 Traffic Detector Handbook—2006 

October 2006 Page P-19 Federal Highway Administration

TRAFFIC ADJUSTED CONTROL SYSTEM: Traffic Adjusted systems 

operate with few sensors (or at sensor web density levels of 1.0 and 1.5) 

and use fixed timing plans, the selection of which is adjusted by a 

minimum number of system sensors at periodic intervals. The sensors in 

these systems measure system-wide variations in traffic demand using 

either volume plus weighted occupancy or predominant direction of flow. 

They then use pattern matching to choose from among a set of 

preselected plans. This definition builds on the original definition by Kell 

and Fullerton in the Manual of Traffic Signal Design. The first usage of 

this term was by Automatic Signal in describing one of their control 

systems in the 1950s. System response to variations in traffic flow is 

relatively slow. 

TRAFFIC RESPONSIVE CONTROL SYSTEM: Control systems that use 

flexible timing plans in which the offsets, splits, and phase durations can 

be adjusted in reaction to changes in traffic on a nearly cycle-by-cycle 

basis. They contain at least one sensor per lane per link and operate at a 

moderate sensor density (Sensor web density levels of 2.0 and 2.5). Cycle, 

split, and offset are each optimized in the selection of the flexible timing 

plan to be implemented. These systems typically use macroscopic 

measures of traffic flow on individual links such as platoon and other 

characteristics. System response to traffic changes is prompt. 

TRAILING CAR: With a volume density controller, when the time headway 

between two consecutive cars passing over a sensor exceeds the allowable 

gap imposed by the controller, an event called Gap-Out occurs. The first 

of these cars is called the last car and the second is called the trailing car. 

TRANSDUCER: That part of a sensor that transmits and receives or simply 

receives energy from the detection zone and converts the received signal 

into electrical form. 

TRANSMISSION LINE: See lead-in cable. 

TW: The UL designation for thermoplastic insulated wire for use in conduit 

and underground and wet locations. It is a common building wire having 

a soft copper conductor, which may be either solid or stranded. 

TWISTED PAIR: Two insulated conductors twisted together with each end 

marked for identification. 

TWO-COIL MAGNETIC DETECTOR: A magnetic detector with two coils. 

This sensor is capable of serving as a directional sensor. 

TYPE 170 (179) AND 2070 CONTROLLER: One of the two major types of 

traffic signal controllers, the other being NEMA controllers. The traffic 

management function is implemented through standardized hardware 

and specialized software. Inputs from loop detectors, magnetic detectors, 

magnetometers, and a variety of over-roadway sensors may be used to 

provide traffic flow information to these controllers. 

UL: Underwriter’s Laboratories, Inc., a chartered, nonprofit organization 

that maintains and operates laboratories for the examination and testing 

of devices, systems, and materials relative to life, fire and casualty, 

hazards, and crime prevention. 

UL APPROVED: A product that has been tested and approved to 

Underwriter’s Laboratories standards. 
Appendix P—Glossary 

October 2006 Page P-20 Federal Highway Administration 

ULTRASONIC SENSOR: An active vehicle sensor that transmits sound 

energy from a transducer at a high frequency (one that is above the 

upper range of human hearing) and that senses the reflection of the 

energy from a vehicle in its detection zone. 

UNCOORDINATED (SIGNAL) CONTROL: Form of signal control for a 

single signalized intersection that does not have control relationships 

between intersections nor any data flow between intersections. The 

system of intersections is not responsive to traffic in real time. The 

individual isolated intersection may have local actuated control or Time 

of Day/Day of Week changes based on historical traffic flows. Also called 

isolated signal control. See also totally isolated systems.

VAC: Voltage Alternating Current. 

VARIABLE INITIAL INTERVAL: A controller design feature that adjusts 

the duration of initial interval for the number of vehicles in the queue. 

VDC: Voltage Direct Current. 

VEHICLE DETECTOR (SENSOR) SYSTEM: A system for indicating the 

presence or passage of vehicles (NEMA). See Sensor System. 

VEHICLE STANDARD: A test unit that produces a change in the loop 

inductance equivalent to a conventional American sedan. 

VEHICULAR PHASE: A traffic phase allocated to vehicular traffic. 

VIDEO IMAGE PROCESSOR (VIP): A type of passive sensor that 

analyzes a video image of a roadway by digitizing it and subjecting the 

digitized information to algorithms or neural networks that differentiate 

moving objects from background patterns. 

VOLUME DENSITY CONTROLLER: An actuated controller that has a 

gap-reduction factor based on opposing phase vehicle time waiting. 

VOLUME EXTENSION MODE: Operation of a multiple sensor design in 

which the green is extended by heavy traffic operating at a speed below 

the Design Speed. The speed is so low that the extension is attributable 

to the heavy volume. Compare with Volume Density Controller. 

WEIGH-IN-MOTION (WIM) SYSTEM: A system that estimates a moving 

vehicle’s gross weight and the portion of that weight carried by each 

wheel, axle, or axle group, or combination thereof, by measurement and 

analysis of dynamic vehicle tire forces (ASTM E 867 and E 1318). 

WIM: See Weigh-In Motion. 

WIRE GAUGE: See AWG. 

WATER BLOCKED: Impervious to water entrance and migration as a 

water blocked lead-in cable or water blocked splices. 

XHHW: Cross-linked polyethylene insulated wire, rated at 90 o

C in wet 

locations. 

YELLOW CHANGE INTERVAL: The first interval following the green 

right-of-way interval in which the signal indication for that phase is 

yellow (NEMA), indicating the imminent change of right-of-way. 

YIELD COMMAND: See Hold. 

ZONE OF DETECTION: See Detection Zone. 
 Traffic Detector Handbook—2006 

October 2006 Page Q-i Federal Highway Administration 

TABLE OF CONTENTS: APPENDIX Q. 

APPENDIX Q. REGISTERED TRADEMARKS .....................................................Q-1

 Traffic Detector Handbook—2006 

October 2006 Page Q-1 Federal Highway Administration

APPENDIX Q. REGISTERED 

TRADEMARKS 

Trademark name Trademark owner 

3M 3M Company, St. Paul, MN 55144 

AASHTO American Association of State Highway and 

Transportation Officials, Suite 225, 444 N. Capitol 

St., NW, Washington, DC 20001 

Autoscope Image Sensing Systems, St. Paul, MN 55104 

Autoscope Solo Image Sensing Systems, St. Paul, MN 55104 

Autovue Iteris, Inc., Anaheim, CA 92802 

Detector Systems Intersection Development Corporation, Stanton, 

CA 90680 

Eagle Signal Gulf & Western Industries, Inc. New York, NY 

10023 

Econolite Econolite Control Products, Inc., Anaheim, CA 

92806 

Groundhog Nu-Metrics, Uniontown, PA 15401 

Iteris Iteris, Inc., Anaheim, CA, 92802 

Motorola Motorola, Inc., Schaumburg, IL 60196 

NEMA National Electrical Manufacturers Association, 

Rosslyn, VA 22209 

Nestor Traffic 

Systems 

Nestor Traffic Systems, Inc., Providence, RI 02906 

Nu-Metrics Nu-Metrics, Uniontown, PA 15401 

Orincon Lockheed Martin Orincon Defense Corporation, 

San Diego, CA 92037 

Peek Peek Traffic, Inc., Sarasota, FL 34236 

Reno A&E Reno A&E, Reno, NV 89502 

RTMS EIS Electronic Integrated Systems Inc., North 

York, ON, Canada M6A 1Z5 

Safetran Safetran Systems Corporation, Minneapolis, MN 

55421 

SCATS Roads & Traffic Authority of New South Wales, 

Sydney, NSW 2000, Australia 

Smartsensor Wavetronix LLC, Lindon, UT 84042 

SmarTek Systems SmarTek Systems, Inc. Woodbridge, VA 22193 

Sumitomo Sumitomo Heavy Industries, Ltd., Tokyo, Japan 

Traficon Traficon N.V., Bissegem Belgium B-8501 

Vantage Iteris, Inc., Anaheim, CA, 92802 

VideoTrak Peek Traffic, Chicago, IL 60601 

Whelen Whelen Engineering, Chester, CT 06412-0684 

Note: The information in this table was gathered from http://www.uspto.gov/# 

using the link for “Search Trademarks.” 
HRDO-04/10-06(1M)E

Recycled

Recyclable