Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

Traffic Detector Handbook:

Third Edition—Volume I

Publication No. FHWA-HRT-06-108 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 first, Volume I (FHWA-HRT-06-

108), containing Chapters 1 through 4. Volume II (FHWA-HRT-06-139) contains Chapters 5 and 6 

and all Appendixes. 

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-06-108 

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

5. Report Date 

October 2006 

4. Title and Subtitle 

Traffic Detector Handbook: Third Edition—Volume I 

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 first, Volume I (FHWA-HRT-06-108), containing 

Chapters 1 through 4. Volume II (FHWA-HRT-06-139) contains Chapters 5 and 6 and all Appendixes. 

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 

288 

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 1-i Federal Highway Administration 

TABLE OF CONTENTS: CHAPTER 1. 

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 1—Introduction 

October 2006 Page 1-ii Federal Highway Administration 

LIST OF FIGURES: CHAPTER 1. 

Figure 1-1. Growth in highway miles traveled in the United States. 

The 2003–2010 trend is based on linear extrapolation 

of previous years’ data...............................................................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 (Photograph courtesy of Lawrence 

A. Klein). ....................................................................................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 

(Photograph courtesy of Lawrence A. Klein).........................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 (Picture courtesy of EIS, Toronto, Canada). .....1-17

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

traffic lane (Drawing courtesy of Schwartz Electro-Optics, 

now OSI Laserscan, Orlando, FL). .........................................1-18

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

(Photograph courtesy of Schwartz Electro-Optics, now 

OSI Laserscan, Orlando, FL)..................................................1-18

LIST OF TABLES: CHAPTER 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
 Traffic Detector Handbook—2006 

October 2006 Page 1-1 Federal Highway Administration

CHAPTER 1. INTRODUCTION 

This edition of the Traffic Detector Handbook (the Handbook) is an updated 

version of the previous Handbook originally published as Implementation 

Package FHWA-IP-85-1, and supersedes the previous two editions. While the 

basic philosophy of the original document has been retained, the Handbook

has been restructured and revised to update discussions of concepts and 

equipment that reflect the state of the practice, particularly as they relate to 

incorporation of new sensor and controller technologies in traffic 

management applications. 

SCOPE AND OBJECTIVES OF THE HANDBOOK 

The overall objective of the Handbook is to provide a reference to assist the 

practicing engineer and technician in planning, designing, installing, and 

maintaining vehicle sensors that support traffic management on surface 

streets, arterials, and freeways. In accordance with this objective, the 

Handbook: 

• Provides a compendium of existing sensor technology. 

• Facilitates the understanding of basic sensor technology. 

• Aids in the application of new sensor technology. 

• Identifies the best current practices. 

• Serves as a training aid for traffic engineers, technicians, and 

field personnel. 

The National Electrical Manufacturers Association (NEMA) Standards 

define a vehicle detection system as “.... a system for indicating the presence 

or passage of vehicles.” These systems provide traffic flow data for trafficactuated signal control, traffic-responsive signal control, freeway surveillance 

and traffic management, and data collection systems. 

The Traffic Detector Handbook describes the theory of operation, installation, 

and applications of in-roadway and over-roadway sensors. An in-roadway 

sensor is one that is placed as one of the following ways: 

• Embedded in the pavement of the roadway. 

• Embedded in the subgrade of the roadway. 

• Taped or otherwise attached to the surface of the roadway. 

By contrast, an over-roadway sensor is one that is mounted above the surface 

of the roadway in one of the following two ways: 

• Above the roadway itself. 

• Alongside the roadway, offset from the nearest traffic lane by 

some distance. 

The objective of the Traffic 

Detector Handbook is to provide a 

single resource and basic 

reference for the design, 

installation, and maintenance of 

traffic flow sensors. 

A traffic flow sensor is a device that 

indicates the presence or passage 

of vehicles and provides data or 

information that supports traffic 

management applications such as 

signal control, freeway mainline 

and ramp control, incident 

detection, and gathering of vehicle 

volume and classification data to 

meet State and Federal reporting 

requirements. 

In-roadway and over-roadway 

sensor technologies and 

applications are described in this 

Handbook. 
Chapter 1—Introduction 

October 2006 Page 1-2 Federal Highway Administration 

Examples of in-roadway sensors include inductive-loop detectors, which are 

sawcut into the pavement; magnetometers, which may be placed underneath 

a paved roadway or bridge structure; and tape switches, which are mounted 

on the roadway surface. Examples of over-roadway sensors are video image 

processors that utilize cameras mounted on tall poles adjacent to the 

roadway or traffic signal mast arms over the roadway; microwave radar, 

ultrasonic, and passive infrared sensors mounted in a similar manner; and 

laser radar sensors mounted on structures that span the lanes to be 

monitored. Some emerging applications for wide area surveillance envision 

over-roadway sensors mounted on tall buildings and radio towers near the 

roadway and on aerial platforms. 

THE NEED FOR SENSORS IN MODERN TRAFFIC 

MANAGEMENT SYSTEMS 

Maximizing the efficiency and capacity of existing transportation networks is 

vital because of the continued increase in traffic volume and the limited 

construction of new highway facilities in urban, intercity, and rural areas. In 

the United States for example, highway miles traveled increased by 33 

percent while public road mileage increased by less than 2 percent from 1987 

to 1997.(1,2) Figure 1-1 shows the projected growth in U.S. highway demand to 

the year 2010 in terms of highway miles traveled per year.(2,3) The increase in 

demand, relative to the limited construction of new roads, has caused 

recurring congestion in the U.S. and throughout the industrialized world, as 

well as in developing nations. 

2,000

2,100

2,200

2,300

2,400

2,500

2,600

2,700

2,800

2,900

3,000

3,100

3,200

3,300

3,400

3,500

'88 '89 '90 '91 '92 '93 '94 '95 '96 '97 '98 '99 '00 '01 '02 '03 '04 '05 '06 '07 '08 '09 '10

Year

Billions of Miles

Projected

1 mile (mi) = 1.6 kilometers (km) 

Figure 1-1. Growth in highway miles traveled in the United States. The 2003–2010 trend 

is based on linear extrapolation of previous years’ data. 

Figure 1-1 shows the growth 

in U.S. highway demand 

from 2 trillion miles (mi) (3.2 

trillion kilometers (km) in 

1988, to 2.600 trillion mi (4.1 

trillion km) in 1998, to a 

projection of over 3.3 trillion 

miles (5.3 trillion km) by 

2010 in terms of highway 

miles traveled per year.(2,3)

The increase in demand 

relative to capacity has 

caused recurring congestion. 

The annual growth is 

approximately linear. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-3 Federal Highway Administration

Even when additional facilities are built to ease congestion and promote the 

use of multiple occupancy vehicles, the cost is often quite high. For example, 

the freeway-to-freeway high-occupancy vehicle (HOV) bypass structure, 

illustrated in Figure 1-2, costs approximately $150 million (U.S.) to 

construct. Construction and striping of HOV freeway lanes in Los Angeles 

County vary between $400,000 and $750,000 (U.S.) per lane mile ($640,000 

to $1,200,000 per lane km), depending on the freeway configuration. Striping 

only costs $100,000 (U.S.) per lane mile ($160,000 per lane km). The latter 

estimates include costs associated with providing safety for the construction 

workers and building temporary roadbeds to maintain traffic flow during 

construction. 

An alternative to expensive new highway construction is the implementation 

of strategies that promote more efficient utilization of current road, rail, air, 

and water transportation facilities. These strategies are found in Intelligent 

Transportation Systems (ITS) roadway and transit programs that have 

among their goals reducing travel time, easing delay and congestion, 

improving safety, and reducing pollutant emissions. ITS that contain 

electronic surveillance, communications, and traffic analysis and control 

technologies bring benefits to transportation system users and managers. 

Users gain from the information and guidance provided by ITS. 

Transportation managers and agencies profit from improved ability to 

monitor, route, and control traffic flows and disseminate information. 

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

CA–57 and CA–91 freeways in Anaheim, CA (Photograph courtesy of Lawrence A. Klein). 

Figure 1-2 shows a freeway-tofreeway high occupancy vehicle 

(or HOV) bypass lane ramp 

structure under construction at 

intersection of CA–57 and CA–91 

freeways in Anaheim, CA. HOV 

bypass lanes are a way of 

encouraging use of carpooling to 

reduce congestion. 
Chapter 1—Introduction 

October 2006 Page 1-4 Federal Highway Administration 

Millions of research and operations dollars are budgeted for managing traffic 

and alleviating congestion and delay on the Nation’s existing streets and 

freeways. ITS applications for Advanced Traffic Management Systems, 

Advanced Traveler Information Systems, Commercial Vehicle Operations, 

Advanced Vehicle Control Systems, Advanced Public Transit Systems, and 

Archived Data User Services rely on traffic flow sensors to provide vehicle 

detection, incident detection, automatic traveler surveillance, real-time 

traffic adaptive signal control, archival data, and data for traveler, 

commercial, and emergency information services. The success of these 

intelligent transportation systems depends to a large extent on the proper 

design, installation, and maintenance of the sensor component of the overall 

system. Consequently, it is incumbent on the jurisdictions or agencies 

implementing or operating ITS to assure that appropriate attention is 

directed toward this relatively straightforward, but critical system element. 

EVOLUTION OF TRAFFIC FLOW SENSOR 

TECHNOLOGY 

In the 1920s, when manually operated traffic signals were being replaced by 

automatic, pretimed traffic signal control devices, engineers soon realized 

they needed a method to collect the traffic data previously obtained visually 

by the police officer on duty. Among those concerned was Charles Adler, Jr., 

of Baltimore, MD, a railway signal engineer. He developed a sensor that was 

activated when a driver sounded his car horn at an instrumented location. 

This device consisted of a microphone mounted in a small box on a nearby 

utility pole. First installed in 1928 at a Baltimore intersection, Adler’s device 

enabled the first semiactuated signal installation to assign right-of-way by 

means of a vehicle sensor. 

At nearly the same time, Henry A. Haugh, an electrical engineer, developed 

an in-roadway pressure-sensitive sensor, utilizing two metal plates that 

acted as electrical contacts. The wheel pressure of passing vehicles brought 

the plates together. This pressure-sensitive, treadle type sensor proved more 

popular than the horn-activated sensor. In fact, this sensor enjoyed 

widespread use for over 30 years as the primary means of detecting vehicles 

at actuated signals. 

Adler continued his work with sound detectors and in l931 introduced 

another sound detector, which employed hollow steel boxes embedded in the 

intersection approach. These boxes picked up the sound of passing wheels, 

which was transmitted to microphones. 

Mechanical problems with the contact-plate sensor led to the introduction of 

the electro-pneumatic sensor. Although this device found some application, it 

was costly to install, capable of only passage (motion) detection, and its (axle) 

counting accuracy was limited by the generation of air pressure waves and 

capsule contact bounce. 

In retrospect, it seems unfortunate that the treadle detector, which utilized 

the most obvious and most easily detected property of vehicles—their 

weight—could not be economically produced. Snow plows could lift the plate 

from the roadway, resulting in costly repairs. There was also the expense of 

reinstalling the detector after roadway resurfacing. These problems led to the 

search for traffic flow sensors based on more subtle properties such as: 

• Sound (acoustic sensors). 

Charles Adler, Jr., of 

Baltimore, MD, developed 

the first traffic sensor based 

on detecting acoustic energy 

from the vehicle’s horn. 

The first vehicle passage 

sensor used a pressuresensitive device. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-5 Federal Highway Administration

• Opacity (optical and infrared sensors and video image 

processors). 

• Geomagnetism (magnetic sensors, magnetometers). 

• Reflection of transmitted energy (infrared laser radar, 

ultrasonic sensors, and microwave radar sensors). 

• Electromagnetic induction (inductive-loop detectors). 

• Vibration (triboelectric, seismic, and inertia-switch sensors). 

Not all of these concepts have been commercially exploited. Today, the 

inductive-loop detector is, by far, the most widely used sensor in modern 

traffic control systems. Magnetometers, magnetic sensors, video image 

processors, microwave and laser radar sensors, ultrasonic, acoustic, and 

passive infrared sensors are also produced commercially and used for various 

traffic management applications. The optical sensor has found use for 

detecting priority and overheight vehicles. 

NEED FOR SENSOR ALTERNATIVES 

While single inductive-loop detectors give direct information concerning 

vehicle passage and presence, other traffic flow parameters such as density 

and speed must be inferred from algorithms that interpret or analyze the 

measured data. When these parameters are calculated from inductive loop 

data, the values may not have sufficient accuracy for some applications (such 

as rapid freeway incident detection) or the available information may be 

inadequate to support the application (such as calculation of link travel 

time). Furthermore, the operation of inductive-loop detectors is degraded by 

pavement deterioration, improper installation, and weather-related effects. 

Street and utility repair may also impair loop integrity. Thus, a good loop 

installation, acceptance testing, repair, and maintenance program is required 

to maintain the operational status of an inductive-loop-based vehicle 

detection system. 

Evaluations of modern over-roadway sensors show that they provide an 

alternative to inductive-loop detectors. The traffic flow parameters measured 

with over-roadway sensors satisfy the accuracy requirements of many 

current freeway and surface street applications, provided suitable mounting 

is available. The mounting location must provide an unobstructed view of 

vehicles for optimum performance. In general, when sensors are installed 

over the lane of traffic they are intended to monitor, their view and hence 

their data collection ability is not occluded by other vehicles that are present 

within the viewing area of the sensor. Over-roadway sensors that are 

mounted on the side of a roadway and view multiple lanes of traffic at angles 

perpendicular to or at an oblique angle to the flow direction may experience 

two types of data anomalies. The first occurs when tall vehicles block the 

sensor’s view of distant lanes. The occlusion may potentially cause an 

undercount or false average speed measurement. The second anomaly occurs 

when tall vehicles project their image into adjacent lanes. When a sensor is 

sensitive to this effect, it will overcount and again may report a misleading 

average speed. Thus, sensor type, mounting height and location, vehicle mix, 

road configuration, and sensor viewing angles must be analyzed with respect 

to the intended application. Some over-roadway sensors may be more 

susceptible to these anomalies than others. 
Chapter 1—Introduction 

October 2006 Page 1-6 Federal Highway Administration 

Installation and maintenance of over-roadway sensors mounted over the 

traffic lanes they monitor may require lane closure for bucket trucks to be 

parked on the mainline. This can disrupt traffic and pose a safety risk to the 

installers. Traffic should be managed in accordance with “Part 6—Temporary 

Traffic Control,” as found in the Manual on Uniform Traffic Control Devices 

(MUTCD).

(4) 

Most over-roadway sensors have relay or solid state outputs that are 

compatible with systems that accept inductive loop data. Some also have 

serial outputs that directly provide multilane traffic volume, occupancy, 

speed, vehicle length, and classification that are not ordinarily available from 

inductive-loop detectors. As there are presently no generally accepted 

standards for serial data formatting, software code called drivers must be 

written before the serial data stream can be decoded by the field controller or 

central computer at a traffic management center. A standard National 

Transportation Communications for ITS Protocol (NTCIP) protocol is needed 

to ameliorate this situation. 

An emerging potential source of traffic flow data is from cellular telephone 

companies who monitor the transmitting status of telephones that are 

engaged in conversations in support of the wireless enhanced all automatic 

location identification (ALI) directive of the Federal Communications 

Commission (FCC). This directive mandates providing the caller’s location to 

within: 

• 328 feet (ft) (100 meters (m)) for 67 percent of the calls and 984 

ft (300 m) for 95 percent of the calls when network-based 

solutions are implemented. 

• 164 ft (50 m) for 67 percent of the calls and 492 ft (150 m) for 95 

percent of the calls when handset-based solutions are 

implemented. 

The location of these telephones can potentially be made available to traffic 

management agencies to anonymously track vehicles on a noncooperative 

basis. This information can assist in estimating congestion and travel time 

over wide areas, while protecting the identity of the telephone subscriber. 

This method of gathering traffic flow data is outside the scope of the 

handbook and, therefore, will not be discussed further. 

Another unconventional source of traffic monitoring data is from nonstationary and airborne platforms. Information gathered from satellite, 

aircraft, and unmanned aerial vehicles can be used to estimate arterial and 

freeway traffic characteristics over long time scales and large geographic 

areas, including those where data were previously unavailable. The spatial 

coverage provided from air- and satellite-based sensors can potentially 

support the development of new metrics that better represent highway 

utilization and congestion. 

The issue of how to handle 

sensor data more 

intelligently rather than as 

simple contact closures is 

now beginning to be 

addressed by researchers. 

See especially the work with 

NCHRP 3-66 “Traffic Signal 

State Transition Logic Using 

Enhanced Sensor 

Information” and Small 

Business Innovative 

Research Project DTRS57-

03-R-SBIR “Real Time Linux 

Operating System Software 

for Advanced Traffic 

Controller to Host Traffic 

Control Software.” 
 Traffic Detector Handbook—2006 

October 2006 Page 1-7 Federal Highway Administration

Although the installation and maintenance of in-roadway sensors such as 

inductive-loop and magnetic field sensors can disrupt traffic and pose a 

safety risk to the installers, a requirement for in-roadway sensors continues 

for several reasons. These include aesthetic considerations that dictate their 

use for traffic management when over-roadway sensors are excluded from 

consideration, axle counting and weigh-in-motion applications requiring 

sensors (such as pneumatic tubes, fiber-optic, bending plates, piezoelectric, 

pressure sensitive resistance, load cells, and capacitance mats) under or on 

the road surface, cost and safety issues associated with mounting overroadway sensors where existing structures are not available, and policy that 

prohibits over-roadway sensors in certain locations. Newly installed 

inductive-loop detectors may also provide more accurate data than overroadway sensors when they are coupled with advanced electronics units 

available from several manufacturers. Sensors used in weigh-in-motion 

applications are not described in this Handbook, but information concerning 

their operation and installation may be found elsewhere.(5,6) 

SENSOR TECHNOLOGY CHARACTERISTICS 

Table 1-1 compares the strengths and weaknesses of current sensor 

technologies with respect to installation, parameters measured, and 

performance in inclement weather, variable lighting, and changeable traffic 

flow. Most over-roadway sensors are compact and mounted above or to the 

side of the roadway, making installation and maintenance relatively easy. 

Some sensor installation and maintenance applications may require the 

closing of the roadway to normal traffic to ensure the safety of the installer 

and motorist. All the sensors listed operate under day and night conditions.(5) 

Table 1-2 lists the types of data typically available from each sensor 

technology, coverage area, communication bandwidth requirements, and 

purchase costs. 

Several technologies are capable of supporting multiple lane, multiple 

detection zone applications with one or a limited number of units. These 

devices may be cost effective when larger numbers of detection zones are 

needed to implement the traffic management strategy. 

A low to moderate communication bandwidth is indicated if only data and 

control commands are transmitted between the sensor, controller, and traffic 

management center. Larger bandwidth is required if real-time video imagery 

is transmitted at 30 frames/second (s). The requirement for large bandwidth 

communications media such as T1 telephone lines, which support 

transmission rates of 1.544 x 106

 bits/s (baud) at a bandwidth of 125 

Megahertz (MHz), and fiber can be reduced if compressed imagery (e.g., 

transmission rates of 256,000 bits/s at a bandwidth of 20.5 MHz) is suited for 

the application. The required transmission rate increases when large 

numbers of sensors, roadside information devices such as changeable 

message signs and highway advisory radio, signal timing plans, and traveler 

information databases are used to implement traffic management strategies. 

The range of purchase costs for a particular sensor technology reflects cost 

differences among specific sensor models and capabilities. If multiple lanes 

are to be monitored and a sensor is capable of only single lane operation, 

then the sensor cost must be multiplied by the number of monitored lanes. 
Chapter 1—Introduction 

October 2006 Page 1-8 Federal Highway Administration 

Table 1-1. Strengths and weaknesses of commercially available sensor technologies.

Technology Strengths Weaknesses 

Inductive loop • Flexible design to satisfy large variety of 

applications. 

• Mature, well understood technology. 

• Large experience base. 

• Provides basic traffic parameters (e.g., 

volume, presence, occupancy, speed, 

headway, and gap). 

• Insensitive to inclement weather such as 

rain, fog, and snow. 

• Provides best accuracy for count data as 

compared with other commonly used 

techniques. 

• Common standard for obtaining accurate 

occupancy measurements. 

• High frequency excitation models provide 

classification data. 

• Installation requires pavement 

cut. 

• Improper installation decreases 

pavement life. 

• Installation and maintenance 

require lane closure. 

• Wire loops subject to stresses 

of traffic and temperature. 

• Multiple loops usually required to 

monitor a location. 

• Detection accuracy may decrease 

when design requires detection of 

a large variety of vehicle classes. 

Magnetometer 

(two-axis fluxgate 

magnetometer) 

• Less susceptible than loops to stresses of 

traffic. 

• Insensitive to inclement weather such as 

snow, rain, and fog. 

• Some models transmit data over wireless 

radio frequency (RF) link. 

• Installation requires pavement 

cut. 

• Improper installation decreases 

pavement life. 

• Installation and maintenance 

require lane closure. 

• Models with small detection 

zones require multiple units for 

full lane detection. 

Magnetic 

(induction or search 

coil magnetometer) 

• Can be used where loops are not feasible 

(e.g., bridge decks). 

• Some models are installed under roadway 

without need for pavement cuts. However, 

boring under roadway 

is required. 

• Insensitive to inclement weather such as 

snow, rain, and fog. 

• Less susceptible than loops to stresses of 

traffic. 

• Installation requires pavement cut 

or boring under roadway. 

• Cannot detect stopped vehicles 

unless special sensor layouts and 

signal processing software are 

used. 

Microwave 

radar 

• Typically insensitive to inclement weather 

at the relatively short ranges encountered 

in traffic management applications. 

• Direct measurement of speed. 

• Multiple lane operation available. 

• Continuous wave (CW) Doppler 

sensors cannot detect stopped 

vehicles. 

The good performance of inroadway sensors such as 

inductive loops, magnetic, 

and magnetometer sensors 

is based, in part, on their 

close location to the vehicle. 

Thus, they are insensitive to 

inclement weather due to a 

high signal-to-noise ratio. 

Their main disadvantage is 

their in-roadway installation, 

necessitating physical 

changes in the roadway as 

part of the installation 

process. Over-roadway 

sensors often provide data 

not available from inroadway sensors and some 

can monitor multiple lanes 

with one unit. 

Table 1-1 summarizes the 

strengths and 

weaknesses of inductive 

loop, magnetometer, 

microwave radar, active 

infrared, passive infrared, 

ultrasonic, acoustic, and 

video image processor 

sensors. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-9 Federal Highway Administration

Table 1-1. Strengths and weaknesses of commercially available sensor technologies—Continued

Technology Strengths Weaknesses 

Active infrared 

(laser radar) 

• Transmits multiple beams for 

accurate measurement of 

vehicle position, speed, and 

class. 

• Multiple lane operation 

available. 

• Operation may be affected by fog when 

visibility is less than ≈20 feet (ft) (6 m) or 

blowing snow is present. 

• Installation and maintenance, including 

periodic lens cleaning, require lane closure. 

Passive infrared • Multizone passive sensors 

measure speed. 

• Passive sensor may have reduced vehicle 

sensitivity in heavy rain, snow and dense fog. 

• Some models not recommended for 

presence detection. 

Ultrasonic • Multiple lane operation 

available. 

• Capable of overheight vehicle 

detection. 

• Large Japanese experience 

base. 

• Environmental conditions such as 

temperature change and extreme air 

turbulence can affect performance. 

Temperature compensation is built into some 

models. 

• Large pulse repetition periods may degrade 

occupancy measurement on freeways with 

vehicles traveling at moderate to high 

speeds. 

Acoustic • Passive detection. 

• Insensitive to precipitation. 

• Multiple lane operation 

available in some models. 

• Cold temperatures may affect vehicle count 

accuracy. 

• Specific models are not recommended with 

slow-moving vehicles in stop-and-go traffic. 

Video image 

processor 

• Monitors multiple lanes and 

multiple detection zones/lane. 

• Easy to add and modify 

detection zones. 

• Rich array of data available. 

• Provides wide-area detection 

when information gathered at 

one camera location can be 

linked to another. 

• Installation and maintenance, including 

periodic lens cleaning, require lane closure 

when camera is mounted over roadway (lane 

closure may not be required when camera is 

mounted at side of roadway) 

• Performance affected by inclement weather 

such as fog, rain, and snow; vehicle 

shadows; vehicle projection into adjacent 

lanes; occlusion; day-to-night transition; 

vehicle/road contrast; and water, salt grime, 

icicles, and cobwebs on camera lens. 

• Reliable nighttime signal actuation requires 

street lighting 

• Requires 30- to 50-ft (9- to 15-m) camera 

mounting height (in a side-mounting 

configuration) for optimum presence 

detection and speed measurement. 

• Some models susceptible to camera motion 

caused by strong winds or vibration of 

camera mounting structure. 

• Generally cost effective when many detection 

zones within the camera field of view or 

specialized data are required. 
Chapter 1—Introduction 

October 2006 Page 1-10 Federal Highway Administration Sensor purchase costa (each in 1999 U.S. $) Lowi ($500–$800) Moderatei ($900–$6,300) Low to moderatei ($385–$2,000) Low to moderate ($700–$2,000) Moderate to high ($6,500–$3,300) Low to moderate ($700–$1,200) Low to moderate (Pulse model: $600– $1,900) Moderate 

($3,100–$8,100)

Moderate to high 

($5,000–$26,000) 

Communication 

bandwidth 

Low to moderate 

Low Low 

Moderate 

Low to moderate Low to moderate 

Low 

Low to moderate 

Low to highh

Multiple lane, 

multiple detection 

zone data 

9e

9

9g

9

Classification 

9c

9e

9

9

Output 

data 

999

9e

99999

Speed 

9b9b9b

9

9f9f

99

Presence 

99

9d9e

99999

Count 

999999999

a Installation, maintenance, and repair costs must also be included to arrive at the true cost of a sensor solution as discussed in the text. b Speed can be measured by using two sensors a known distance apart or estimated from one sensor, the effective detection zone and vehicle lengths. c With specialized electronics unit containing embedded firmware that classifies vehicles. d With special sensor layouts and signal processing software. e With microwave radar sensors that transmit the proper waveform and have appropriate signal processing. f With multidetection zone passive or active mode infrared sensors. g With models that contain appropriate beamforming and signal processing. h Depends on whether higher-bandwidth raw data, lower-bandwidth processed data, or video imagery is transmitted to the TMC. i Includes underground sensor and local detector or receiver electronics. Electronics options are available to receive multiple sensor, multiple lane data. 

Table 1-2. Traffic output data (typical), communications bandwidth, and cost of commercially available sensors. 

Sensor 

technology 

Inductive loop Magnetometer (two axis fluxgate) Magnetic induction coil Microwave radar Active infrared Passive infrared Ultrasonic Acoustic array Video image processor 

Table 1-2 lists traffic 

flow sensor technologies and 

their capabilities. 

Most measure count, 

presence, and 

occupancy. Some 

single detection zone 

sensors, such as the 

 range-measuring 

ultrasonic sensor and 

some infrared sensors, 

do not measure speed. 

CW Doppler radar 

sensors do not detect 

stopped or slow moving 

vehicles. Magnetometer, 

magnetic, passive 

infrared and soundbased, acoustic sensor 

models do not classify 

vehicles. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-11 Federal Highway Administration

Direct hardware and software purchase costs are not the only costs 

associated with a sensor. Installation, maintenance, and repair should also 

be factored into the sensor selection decision. Installation costs include fully 

burdened costs for technicians to prepare the road surface or subsurface (for 

inductive loops or other surface or subsurface sensors), install the sensor and 

mounting structure (if one is required for over-roadway sensors), purchase 

and install conduit, close traffic lanes, divert traffic, provide safety measures 

where required, and verify proper functioning of the device after installation 

is complete. Environmental concerns may warrant providing for the removal 

of cutting water and debris from the site. Maintenance and repair issues are 

discussed in Chapter 6. 

The technologies listed in Tables 1-1 and 1-2 are mature with respect to 

traffic management applications, although some may not provide the data 

required for a specific application. Some technologies, such as video image 

processing, microwave and laser radars, and inductive-loop detectors, 

continue to evolve by adding capabilities that measure additional traffic 

parameters, track vehicles, improve spatial resolution, or link data from one 

sensor to those from another. 

MODERN VEHICLE SENSORS 

The following discussion provides a broad overview of the operation of inroadway and over-roadway traffic flow sensors most used today. These 

sensors include inductive-loop detectors, magnetometers, video image 

processors, microwave radar sensors, laser radar sensors, passive infrared 

sensors, ultrasonic sensors, and passive acoustic sensors. Typical applications 

include traffic signal control, freeway ramp metering, freeway mainline 

control, incident detection, and gathering of vehicle volume and classification 

data to meet State and Federal reporting requirements. These devices are 

either installed in, below, or above the roadway. Subsequent chapters of the 

Handbook describe the installation and operation of these sensors in more 

detail. 

INDUCTIVE-LOOP DETECTORS 

An inductive-loop detector senses the presence of a conductive metal object 

by inducing currents in the object, which reduce the loop inductance. 

Inductive-loop detectors are installed in the roadway surface. They consist of 

four parts: a wire loop of one or more turns of wire embedded in the roadway 

pavement, a lead-in wire running from the wire loop to a pull box, a lead-in 

cable connecting the lead-in wire at the pull box to the controller, and an 

electronics unit housed in the controller cabinet as shown in Figure 1-3. The 

electronics unit contains an oscillator and amplifiers that excite the 

embedded wire loop. The electronics unit also supports other functions such 

as selection of loop sensitivity and pulse or presence mode operation to detect 

vehicles that pass over the detection zone of the loop. 

Inductive-loop detectors sense the 

presence of a conductive metal 

object by inducing electrical 

currents in the object. The induced 

current decreases the loop 

inductance, which is sensed by the 

inductive-loop electronics unit. The 

electronics unit interprets the 

decreased inductance as a vehicle 

detection and sends an appropriate 

call to the controller. 
Chapter 1—Introduction 

October 2006 Page 1-12 Federal Highway Administration 

Roadway sensor

composed of one

or more turns of wire

Lead-In

Wire

Pull

Box

Lead-In

Cable Electronics Unit

in Controller Cabinet

Splice

Figure 1-3. Inductive-loop detector system. 

When a vehicle passes over the wire loop or is stopped within the area 

enclosed by the loop, it reduces the loop inductance, which unbalances the 

tuned circuit of which the loop is a part. The resulting increase in oscillator 

frequency is detected by the electronics unit and interpreted as a vehicle 

detection by the controller. 

Conventional inductive loops are constructed by cutting a slot in the 

pavement and placing one or more turns of wire in the slot as indicated in 

Figure 1-4. The wire is then covered with sealant. 

The size, shape, and configuration of the loop vary depending upon the 

specific application, ranging from the common 6- x 6-ft (1.8- x 1.8-m) loops, to 

long rectangular loops 6- x 40- to 70-ft (1.8- x 12- to 21-m) for actuated signal 

control. Because of the flexibility of its design, the inductive-loop detector is 

capable of detecting a broad range of vehicles. 

An alternate, more durable construction is to place the turns of wire in a 

plastic conduit just below the pavement surface. Another option is to encase 

the wire in a plastic sleeve before installing the wire loop in the sawcut slot 

in the pavement. A wide variety of loop sizes and shapes are available to 

meet specific needs as described in Chapter 4. 

Controller cabinet

Lead-in conduit

Pullbox

Conduit-to-curb run

Shielded lead-in cable

Splice in pullbox

Twisted wire to suppress

electrical interference Loop wire plan

3 turns

Roadway section

Loop sawcut plan

Roadway centerline

Curb line

Electronics units

(card file format)

12 ft

3 ft 6 ft 3 ft

(1 ft = 0.305 m)

• •

Figure 1-4. Inductive-loop installation example. 

Figure 1-3 illustrates an 

inductive-loop detector 

system consisting of one or 

more turns of wire laid in a 

loop in the roadway, which is 

connected by lead-in wire to 

a pull box located at the side 

of the road. The pull box, in 

turn, is connected by lead-in 

cable to an electronics unit in 

a cabinet. The electronics 

unit energizes the wire loop, 

analyzes the signal, and 

transmits vehicle detection 

information to the controller. 

Figure 1-4 depicts an 

inductive-loop detector 

installation, which contains a 

3-turn 6- x 6-ft (1.8- x 1.8-m) 

square loop and connecting 

wire and cable. The loop is 

centered in a 12-ft (3.7 m) 

roadway, 3 ft (0.9 m) from 

either side. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-13 Federal Highway Administration

MAGNETIC SENSORS 

Magnetic sensors are passive devices that detect the presence of a ferrous 

metal object through the perturbation (known as a magnetic anomaly) they 

cause in the Earth’s magnetic field. Figure 1-5 shows the magnetic anomaly 

created by the magnetic dipoles, i.e., energy fields, on a steel vehicle when it 

enters a magnetometer’s detection zone. The upper part of Figure 1-5 

indicates how the vector addition of the dipole magnetic field to the quiescent 

Earth’s magnetic field produces the magnetic anomaly. The lower portion of 

the figure depicts several dipoles on a vehicle and their effect on compass 

readings and sensor output. 

(a) Magnetic anomaly induced in the Earth’s magnetic field by a magnetic dipole. 

(b) Perturbation of Earth’s magnetic field by a ferrous metal vehicle 

(Source: Nu-Metrics, Vanderbilt, PA). 

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

dipoles in a ferrous metal vehicle. 

Two types of magnetic field sensors are used for traffic flow parameter 

measurement. The first type, the two-axis fluxgate magnetometer, detects 

changes in the vertical and horizontal components of the Earth’s magnetic 

field produced by a ferrous metal vehicle. The two-axis fluxgate 

magnetometer contains two primary windings and two secondary "sense" 

windings on a bobbin surrounding a high permeability soft magnetic material 

core. In response to the magnetic field anomaly, i.e., the magnetic signature 

of a vehicle, the magnetometer’s electronics circuitry measures the output 

voltage generated by the secondary windings. The vehicle detection criterion 

is for the voltage to exceed a predetermined threshold. In the presence or 

stopped vehicle mode of operation, the detection output is maintained until 

the vehicle leaves the detection zone.(5) 

Magnetic sensors are passive 

devices that detect the presence of 

a ferrous metal object through the 

perturbation they cause in the 

Earth’s magnetic field 

Figure 1-5 shows that the magnetic 

dipole produced by the ferrous 

materials in vehicles causes an 

anomaly in the Earth’s magnetic 

field, which can be measured by 

sensors as the vehicle passes over 

them. 
Chapter 1—Introduction 

October 2006 Page 1-14 Federal Highway Administration 

The second type of magnetic field sensor is the magnetic detector, more 

properly referred to as an induction or search coil magnetometer. It detects 

the vehicle signature by measuring the distortion in the magnetic flux lines 

induced by the change in the Earth’s magnetic field produced by a moving 

ferrous metal vehicle. These devices contain a single coil winding on a 

permeable magnetic material rod core. Similar to the fluxgate magnetometer, 

magnetic detectors generate a voltage when a moving ferromagnetic object 

perturbs the Earth’s magnetic field. Induction magnetometers do not detect 

stopped vehicles since they require a vehicle to be moving or otherwise 

changing its signature characteristics with respect to time. However, 

multiple units of some magnetic detectors can be installed and utilized with 

specialized signal processing software to generate vehicle presence data. 

Magnetic detectors are inserted horizontally below the roadway. Since they 

provide only passage data and not occupancy or presence data, their use is 

limited to special applications. 

Another device similar to the magnetic detector is the microloop probe. As a 

vehicle passes over the microloop, the change in inductance is sensed by a 

conventional inductive-loop detector electronics unit. Some models are 

inserted into holes bored into the roadway surface. Other models are inserted 

into sleeves below the road surface using horizontal drilling from the side of 

the road. Often two or more microloop probes are connected in series or with 

conventional wire loops to detect a range of vehicle sizes and obtain required 

lane coverage. One microloop probe model can be connected in rows of three 

to generate signals that detect stopped vehicles. Application-specific software 

from its manufacturer is also needed to enable stopped vehicle detection. 

VIDEO IMAGE PROCESSORS 

Video cameras were introduced to traffic management for roadway 

surveillance based on their ability to transmit closed-circuit television 

imagery to a human operator for interpretation. Present-day traffic mangers 

utilize video image processing to automatically analyze the scene of interest 

and extract information for traffic surveillance and management. A video 

image processor (VIP) system typically consists of one or more cameras, a 

microprocessor-based computer for digitizing and analyzing the imagery, and 

software for interpreting the images and converting them into traffic flow 

data. A VIP can replace several in-ground inductive loops, provide detection 

of vehicles across several lanes, and perhaps lower maintenance costs. Some 

VIP systems process data from more than one camera and further expand the 

area over which data are collected. 

VIPs can classify vehicles by their length (usually three length classification 

ranges are available) and report vehicle presence, volume, lane occupancy, 

and speed for each class and lane. VIPs that track vehicles may also have the 

capability to register turning movements and lane changes. Vehicle density, 

link travel time, and origin-destination pairs are potential traffic parameters 

that can be obtained by analyzing data from a series of image processors 

installed along a section of roadway.(5) The types of information provided by 

VIPs makes suitable for arterial and freeway applications. An example of a 

camera mounted to transmit imagery to a VIP for an arterial traffic signal 

control application is shown in Figure 1-6. 

Video image processors 

(VIPs) detect vehicles and 

provide traffic flow data 

across several lanes and in 

multiple areas in one lane. 

They require a line-of-sight 

view of the area they monitor 

and are subject to occlusion 

and the effects of inclement 

weather. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-15 Federal Highway Administration

VIP systems detect vehicles through the analysis of black and white or color 

imagery gathered by cameras at a section of roadway. Black and white image 

analysis is performed by algorithms that examine the variation of gray levels 

in groups of pixels (picture elements) contained in the video frames. Research 

has been conducted on algorithms that are sensitive to color features, for 

example those that assist in eliminating shadow artifacts or enhance vehicle 

discrimination in inclement weather. Along with vehicle size and class data, 

color fingerprints or signatures have been proposed to determine traffic 

volume, lane changes, turning movements, and link travel time by reidentifying a vehicle or group at a downstream site.(7,8) 

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

to a VIP for arterial signal control (Photograph courtesy of Lawrence A. Klein). 

Algorithms utilized in video image processing are designed to ignore gray 

level or color variations in the stationary image background. The algorithms 

are intended to also ignore variations caused by weather conditions, 

shadows, and daytime or nighttime artifacts, but retain objects identified as 

automobiles, trucks or buses, motorcycles, and bicycles. Traffic flow 

parameters are calculated by analyzing successive video frames. 

MICROWAVE RADAR SENSORS 

Microwave radar was developed for detecting objects in the period before and 

during World War II. Radar is defined as “a device for transmitting 

electromagnetic signals and receiving echoes from objects of interest (i.e., 

targets) within its volume of coverage.”(9) Radar was originally an acronym 

for RAdio Detection And Ranging. 

Figure 1-6 shows a camera in a 

weatherproof housing mounted on 

a vertical pole that extends upright 

from a signal mast arm. 
Chapter 1—Introduction 

October 2006 Page 1-16 Federal Highway Administration 

The term microwave refers to the wavelength of the transmitted energy, 

usually between 0.4 inch and 11.8 inches (1 and 30 centimeters (cm)). This 

corresponds to a frequency range of 1 gigahertz (GHz (109

 Hertz (Hz)) to 30 

GHz 109

 Hz. Microwave sensors designed for traffic data collection in U.S. 

roadside applications are limited by FCC regulations to operating frequency 

intervals near 10.5, 24.0, and 34.0 GHz. The sensor manufacturers satisfy 

these requirements, as well as others that restrict the transmitted power and 

bandwidth. Thus, the end users are not required to possess special licenses or 

test equipment to verify the output frequency or power of the devices. Radars 

at frequencies above 30 GHz operate in the millimeter-wave spectrum since 

the wavelength of the transmitted energy is expressed in terms of 

millimeters (mm). Most commercially available microwave radar sensors 

utilized in roadside applications transmit electromagnetic energy at the Xband frequency of 10.525 GHz. Higher frequencies illuminate smaller ground 

areas with a given size antenna and thus are capable of greater spatial 

resolution. FCC-approved frequencies for vehicle-mounted radars utilized in 

collision avoidance, obstacle detection, and automatic cruise control are 47.5 

to 47.8 GHz and 76 to 77 GHz. 

Figure 1-7 shows the transmission of energy by an overhead-mounted 

microwave radar toward an area of roadway. The beamwidth or area in 

which the radar energy is concentrated is controlled by the size and the 

distribution of energy across the aperture of the antenna. The sensor 

manufacturer usually establishes these design constraints. When a vehicle 

passes through the antenna beam, a portion of the transmitted energy is 

reflected back towards the antenna. The energy then enters a receiver where 

the detection is made and traffic flow data, such as volume, speed, and 

vehicle length, are calculated. 

Vehicle

Path of transmitted and received energy

Microwave

Radar

Antenna

Sign bridge,

overpass, pole,

or mast arm mounting

Reflected signal from vehicle can be used

to determine presence, passage, volume, lane 

occupancy, speed, and vehicle length depending 

on the waveform transmitted by the radar sensor

Controller

cabinet

Power and

data cables

Figure 1-7. Microwave radar operation. 

The radar sensor may be mounted over the middle of a lane to measure 

approaching or departing traffic flow parameters in a single lane, or at the side 

of a roadway to measure traffic parameters across several lanes as shown in 

Figure 1-8. Forward-looking wide beamwidth radars gather data 

representative of traffic flow in one direction over multiple lanes. Forwardlooking narrow beamwidth radars monitor a single lane of traffic flowing in one 

direction. Side-mounted, multiple detection zone radars project their detection 

area (i.e., footprint) perpendicular to the traffic flow direction. These sensors 

provide data corresponding to several lanes of traffic, but generally not as 

accurately as can the same radar mounted in the forward-looking direction. 

Side-mounted, single detection zone radars are typically used to detect vehicle 

presence in one or more lanes at signalized intersections.(5) 

Figure 1-7 shows that a 

microwave radar mounted on 

overhead bridges, poles or 

mast arms transmits signals 

that are reflected off vehicles 

back to the radar sensor. 

The reflected energy is 

analyzed to produce traffic 

flow data. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-17 Federal Highway Administration

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

detection and signal actuation at an intersection (Picture courtesy of EIS, Toronto, Canada). 

Microwave sensors that transmit a continuous wave (CW) Doppler waveform 

detect vehicle passage and provide measurements of vehicle count and speed. 

They cannot detect stopped vehicles. Microwave sensors that transmit a 

frequency modulated continuous wave (FMCW) detect vehicle presence as 

well as vehicle passage. They can detect stopped vehicles and provide 

measurements of lane occupancy, vehicle count, speed, and vehicle length 

grouped into several length bins. 

INFRARED SENSORS 

Active and passive infrared sensors are manufactured for traffic flow 

monitoring applications. Active infrared sensors illuminate detection zones 

with low power infrared energy transmitted by laser diodes operating in the 

near infrared region of the electromagnetic spectrum at 0.85 mm. A portion 

of the transmitted energy is reflected or scattered by vehicles back towards 

the sensor. Although light-emitting diodes may also be utilized as the energy 

source in an active IR sensor, there are currently no commercial models 

marketed in the U.S. that exploit this design. A prototype sensor system 

using modulated light emitting diodes was designed to measure the speed 

and height of high and long trucks entering a curved freeway-to-freeway 

interchange. The diodes operated in the near infrared spectrum at 880 

nanometers (nm). The signal modulation prevented interference from other 

sources of infrared energy, including sunlight. Two transmitter-receiver 

systems measured the vehicle speed and one measured the vehicle height. 

When trucks susceptible to rollover or jackknifing were encountered, flashers 

were activated to warn drivers to reduce speed.(5,10) 

Passive sensors transmit no energy of their own. Rather they detect energy 

from two sources: 

• Energy emitted from vehicles, road surfaces, and other objects 

in their field-of-view. 

• Energy emitted by the atmosphere and reflected by vehicles, 

road surfaces, or other objects into the sensor aperture. 

Figure 1-8 shows that presencedetecting microwave radar sensors 

can detect traffic flow in multiple 

lanes when mounted on poles 

adjacent to pedestrian crosswalks 

or roadway shoulders. 

Microwave radar sensors that 

transmit a frequency modulated 

waveform detect stopped and 

moving vehicles. Microwave radar 

sensors that transmit a continuous 

wave signal constant in frequency 

cannot detect stopped vehicles. 

Passive infrared sensors transmit 

no energy of their own. Rather they 

detect energy emitted from 

roadways and vehicles or energy 

that is reflected from them. 
Chapter 1—Introduction 

October 2006 Page 1-18 Federal Highway Administration 

The energy captured by active and passive infrared sensors is focused by an 

optical system onto an infrared-sensitive material mounted at the focal plane 

of the optics. This material converts the reflected and emitted energy into 

electrical signals. Real-time signal processing is used to analyze the signals 

for the presence of a vehicle. The sensors are mounted overhead to view 

approaching or departing traffic. They can also be mounted in a side-looking 

configuration. Infrared sensors are utilized for signal control; volume, speed, 

and class measurement; detection of pedestrians in crosswalks; and 

transmission of traffic information to motorists. 

LASER RADAR SENSORS 

Laser radars are active sensors in that they transmit energy in the near 

infrared spectrum. Models are available that scan infrared beams over one or 

two lanes or use multiple laser diode sources to emit a number of fixed beams 

that cover the desired lane width. An example of a laser radar beamscanning configuration is shown in Figure 1-9. Laser radars provide vehicle 

presence at traffic signals, volume, speed, length assessment, queue 

measurement, and classification. Multiple units can be installed at the same 

intersection without interference from transmitted or received signals. 

Modern laser sensors produce two- and three-dimensional imagery of 

vehicles suitable for vehicle classification as illustrated in Figure 1-10. Their 

ability to classify 11 types of vehicles has found application on toll roads. 

Scanning beams

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

courtesy of Schwartz Electro-Optics, now OSI Laserscan, Orlando, FL). 

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

of Schwartz Electro-Optics, now OSI Laserscan, Orlando, FL). 

A laser radar is an active 

infrared sensor, since it 

transmits energy in the 

infrared spectrum. 

Figure 1-9 depicts a 

scanning laser radar 

mounted over a lane. This 

mounting configuration 

optimizes the sensor’s view 

of vehicular traffic flow. 

Figure 1-10 illustrates a 

three-dimensional image of a 

van pulling a boat as 

produced by a scanning 

laser radar. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-19 Federal Highway Administration

OTHER SENSOR TECHNOLOGIES 

In addition to the technologies discussed above, others find application to 

traffic management. These include ultrasonic sensors, passive acoustic 

sensors, and devices that use a combination of sensor technologies. These 

devices are described further in Chapter 2. Chapter 2 also discusses the 

operation of passive infrared sensors in more detail. 

DEFINITION OF TERMS 

Traffic sensor literature often uses different terms to describe the same 

traffic flow sensors or their characteristics. To reduce potential confusion, the 

terms utilized in this Handbook are defined below. Words or phrases having 

the same meaning are listed along with the primary word being defined. 

Additional terms are defined in the glossary that appears in Appendix P. 

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 mutual coupling causes 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. 

Detector Electronics Unit (Electronics Unit, Sensor Electronics Unit, 

Amplifier): An electronic device that energizes an inductive-loop 

detector, monitors loop inductance, and responds to a predetermined 

decrease in inductance with outputs that indicate the passage or 

presence of vehicles in the detection zones (NEMA). 

An inductive-loop detector electronics unit is sometimes called an 

amplifier or detector, although it performs other functions as well, 

e.g., sensitivity adjustment, failure indication, and delayed actuation 

of controlled signals. Electronics units are also used with magnetic 

detectors and magnetometers. The electronics unit is typically located 

in a controller cabinet. 

Detection Zone (Area of Detection, Detection Area, Zone of Detection, 

Effective Loop Area, Field of Influence, Field of View, Sensing Zone, 

Footprint): The area of the roadway within which a vehicle is 

detected by a sensor system. 

Inductive-loop Detector (Loop Detector System): A sensor capable of 

detecting vehicle passage and presence. It consists of four parts, 

namely one or more turns of wire embedded in the pavement, a leadin wire running from the wire loop in the pavement to the pull box, a 

lead-in cable spliced to the lead-in wire at the pull box, which 

connects to the controller, and an electronics unit housed in the 

controller. 

Fluxgate Magnetometer: Two-axis fluxgate magnetometers are sensors 

that detect changes in the vertical and horizontal components of the 

Earth’s magnetic field produced by a ferrous metal vehicle. They 

detect moving and stopped vehicles and thus provide passage and 

presence information. 

Although not discussed in this 

chapter, other sensor technologies 

find application in traffic 

management. These include 

ultrasonic sensors, passive 

acoustic sensors, and devices that 

use a combination of sensor 

technologies. 
Chapter 1—Introduction 

October 2006 Page 1-20 Federal Highway Administration 

Large Area Sensor (Area Sensor): For inductive-loop detectors utilized for 

traffic signal actuation, it is an inductive loop or combination of 

inductive loops connected in series, parallel, or series and parallel 

covering an area in the approach to an intersection. Detection area 

varies from 6 x 40 ft (1.8 x 12 m) to 6 x 100 ft (1.8 x 30 m) or larger. 

One of the more common configurations is four 6- x 6-ft (1.8- x 1.8-m) 

loops spaced 9 or 10 ft (2.75 or 3 m) apart for a length of 51 or 54 ft 

(15.5 or 16.5 m). 

Lead-In Cable (Feeder Cable, Home-Run Cable, Transmission Line): The 

electrical cable that is spliced to the lead-in wire in the pull box and 

connects to the input of the inductive-loop detector electronics unit. 

Lead-In Wire: That portion of an inductive-loop wire between the 

physical edge of the loop and the pull box. For a magnetic detector 

and magnetometer, it is the wire that runs from the sensor (probe) to 

the pull box. 

Magnetic Detector (Induction or Search Coil Magnetometer): A passive 

device that detects changes in the Earth’s magnetic field caused by 

the movement of a ferrous-metal vehicle in or near its detection area. 

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. 

Their output is connected to an electronics unit. 

Magnetic Sensor: Passive devices that detect the presence of a ferrous 

metal object through the perturbation (known as a magnetic 

anomaly) it causes in the Earth’s magnetic field. Its output is 

connected to an electronics unit. The two types of magnetic sensors 

are fluxgate magnetometers and induction magnetometers, also 

referred to as magnetic detectors in this Handbook. 

Passage Sensor (Motion Detector, Motion Sensor, Dynamic Detector, 

Movement Detector): 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. 

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. 

Pull Box: (Hand Hole, Junction Box, Junction Well, Splice Box): A 

container usually at least 1 cubic foot (e.g., approximately 1 ft3

 (0.028 

m3

)) in size that is placed underground with a removable cover flush 

with the ground surface. Splices between lead-in cable and loop leadin wire are located here. 

Sensor: A device for indicating the presence or passage of vehicles or 

pedestrians. This general term is usually supplemented with a 

modifier indicating type (e.g., inductive-loop detector, magnetic 

detector, video image processor, microwave sensor, and infrared 

sensor); operation (e.g., point sensor, large area sensor, and presence 

sensor); or function (e.g., calling sensor, extension sensor, and 

classification sensor). 

Sensor Amplifier: A device capable of intensifying the electrical energy or 

signal produced by a sensor. An example is a magnetic detector 

amplifier. An inductive-loop detector electronics unit is sometimes 

called an amplifier, although it performs other functions in addition 

to signal amplification. 
 Traffic Detector Handbook—2006 

October 2006 Page 1-21 Federal Highway Administration

Small Area Sensor (Point 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). 

Splashover: An unwanted actuation caused by a vehicle in a lane 

adjacent to that in which the sensor is located. 

ORGANIZATION OF HANDBOOK 

The Traffic Detector Handbook is structured to parallel the progression of 

decisions, activities, and functions related to the design, installation, and 

maintenance of sensor systems. Chapter 1 addresses the need for sensors as 

an integral part of modern traffic control and management systems and 

discussed the operation of several of the sensor technologies currently 

exploited for traffic management. 

Chapter 2 describes the theory of operation of the inductive-loop detector, 

magnetometer, video image processor, microwave radar sensor, laser radar 

sensor, passive infrared sensor, ultrasonic sensor, passive acoustic sensor, 

and other sensors that utilize a combination of technologies. It addresses the 

needs of traffic and electrical engineers who have responsibility for selecting 

or specifying sensors that meet specific operational requirements. It also 

describes the NEMA Standards and the Type 170 and 2070 Controller 

Specifications. 

Chapter 3 provides an overview of traffic control and management 

applications that rely on vehicle detection and monitoring of environmental 

conditions. It identifies how sensors support these applications. 

The design and operating characteristics of in-roadway sensors such as 

inductive loops, magnetometers, and magnetic detectors for surface street 

and freeway traffic management are discussed in Chapter 4. This topic 

should be of particular interest to traffic engineers who develop plans and 

specifications for local intersections, traffic signal systems, and freeway 

surveillance and control systems. 

Chapter 5 illustrates the installation procedures and best current practices 

for in-roadway and above-roadway sensors. The information is directed 

toward project engineers, contractors, inspectors, field crew supervisors, and 

traffic technicians. 

Chapter 6 describes the broad spectrum of maintenance activities associated 

with in-roadway and over-roadway sensors. It provides management and 

supervising engineers with information needed to identify and resolve 

maintenance-related issues. The chapter contains detailed guidelines for 

maintenance supervisors and technicians to assist in identifying sensor 

failure mechanisms and corresponding corrective actions. 

The appendices provide additional detailed information concerning several of 

the topics discussed in the main body of the Handbook. 

REFERENCES 

1. Transportation Statistics Annual Report 1999, BTS99-03. U.S. 

Department of Transportation, Bureau of Transportation Statistics, 

Washington, DC. 1999. 

2. Davis, S.C., and S.W. Diegel. Transportation Energy Data Book:

Edition 24-2004, Table 8.1, ORNL-6973. Oak Ridge National 

Laboratory, Oak Ridge, TN. December 2004. 
Chapter 1—Introduction 

October 2006 Page 1-22 Federal Highway Administration 

3. Sweedler, B.M. “Toward a Safer Future,” TR News. Transportation 

Research Board, National Research Council, no. 201, 3-6. 

Washington, DC. March–April 1999. 

4. Manual on Uniform Traffic Control Devices. U.S. Department of 

Transportation, Federal Highway Administration, Washington, DC. 

2003. http://mutcd.fhwa.dot.gov/pdfs/2003/pdf-index.htm 

(Accessed Nov. 26, 2004.) 

5. Klein, L.A. Sensor Technologies and Data Requirements for ITS. 

Artech House, Norwood, MA. 2001. 

6. McCall, W., and W. Vodrazka. States’ Successful Practices Weigh-InMotion Handbook. Iowa State University, Center for Transportation 

Research and Education, Ames, IA. December 1997. 

http://www.ctre.iastate.edu/research/wim_pdf/index.htm (Accessed 

Nov. 26, 2004.)

7. Zeng, N., and J.D. Crisman. “Evaluation of Color Categorization for

Representing Vehicle Colors,” Proc. SPIE, vol. 2902. Bellingham, 

WA. 1996. pp. 148–155. 

8. Chachich, A., A. Pau, A. Barber, K. Kennedy, E. Olejniczak, J. 

Hackney, Q. Sun, and E. Mireles. “Traffic Sensor Using a Color 

Vision Method,” Proc. SPIE, vol. 2902. Bellingham, WA. 1996. 

pp. 156–165. 

9. Kurpis, G.P., and C.J. Booth, eds. The New IEEE Standard 

Dictionary of Electrical and Electronics Terms, 5th ed. Institute of 

Electrical and Electronics Engineers, New York, NY. 1993. 

10. Lee, C.E., D.W. Borchardt, and Q. Fei. Truck-Monitoring and 

Warning Systems for Freeway to Freeway Connections, report 

number 2915-1. University of Texas, Texas Transportation Institute 

and Center for Transportation Research, Austin, TX. October 1999. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-i Federal Highway Administration 

TABLE OF CONTENTS: CHAPTER 2. 

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

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 
Chapter 2—Sensor Technology 

October 2006 Page 2-ii Federal Highway Administration 

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

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

 
 Traffic Detector Handbook—2006 

October 2006 Page 2-iii Federal Highway Administration 

LIST OF FIGURES: CHAPTER 2. 

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

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....................................................................................... 2-41

Figure 2-29. Classes available from inductive-loop-classifying sensor...... 2-42
Chapter 2—Sensor Technology 

October 2006 Page 2-iv Federal Highway Administration 

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. ..............................................2-84

Figure 2-63. EFKON Traffic Observation Module.......................................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. Eltec 842 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
 Traffic Detector Handbook—2006 

October 2006 Page 2-v Federal Highway Administration 

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 

(manufactured by Sumitomo Electric, Japan) with separate 

transmitting and receiving transducers, as shown in the 

left portion of the figure. ........................................................ 2-91

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

LIST OF TABLES: CHAPTER 2. 

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

 Traffic Detector Handbook—2006 

October 2006 Page 2-1 Federal Highway Administration 

CHAPTER 2. SENSOR TECHNOLOGY 

The operation of in-roadway and over-roadway sensors is described in this 

chapter. The technologies represented include inductive-loop detectors, 

magnetometers, video image processors, microwave radar sensors (presence 

detecting and Doppler), laser radar sensors, passive infrared sensors, 

ultrasonic sensors, passive acoustic sensors, and devices that utilize a 

combination of technologies. The information is intended to provide the 

practicing traffic engineer and electrical engineer with the knowledge needed 

to select the proper sensor technology for specific applications. 

INDUCTIVE-LOOP DETECTORS 

Since its introduction in the early 1960s, the inductive-loop detector has 

become the most utilized sensor in a traffic management system. The 

principal components of an inductive-loop detector system include: 

• One or more turns of insulated loop wire wound in a shallow slot 

sawed in the pavement. 

• Lead-in cable from the curbside pull box to the intersection controller 

cabinet. 

• Electronics unit housed in a nearby controller cabinet. 

Figure 2-1 displays a notional diagram of an inductive-loop detector system 

and the vehicle and steel reinforcement elements in the roadway with which 

it reacts. 

VehicleInduced

Currents

Inductive

Loop Wire

Pull

Box

Reinforcing

Steel Mesh

or Rebar

Lead-In

Wire

Lead-In

Cable

Electronics Unit

Containing

Tuning Network,

Oscillator, and

Sensitivity Settings

Presence

or Pulse

Output

Controller Cabinet

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

The electronics unit transmits energy into the wire loops at frequencies 

between 10 kHz to 200 kHz, depending on the model. The inductive-loop 

system behaves as a tuned electrical circuit in which the loop wire and leadin cable are the inductive elements. When a vehicle passes over the loop or is 

stopped within the loop, the vehicle induces eddy currents in the wire loops, 

which decrease their inductance. The decreased inductance actuates the 

electronics unit output relay or solid-state optically isolated output, which 

sends a pulse to the controller signifying the passage or presence of a vehicle. 

Vehicles passing over or 

stopped within the 

detection area of an 

inductive-loop detector 

decrease the inductance 

of the loop. The 

electronics unit senses 

this event as a decrease 

in frequency and sends 

a pulse to the controller 

signifying the passage or 

presence of a vehicle. 
Chapter 2—Sensor Technology 

October 2006 Page 2-2 Federal Highway Administration 

The following sections describe inductive-loop system theory, loop 

characteristics, and the electronics unit. 

THEORY OF OPERATION 

The principles of operation of the inductive-loop detector system discussed 

below are common to all of the inductive-loop system designs described in 

Chapter 4. The loop wire and lead-in cable contain a combination of resistance, 

inductance, and capacitance (both wire-to-wire and wire-to-Earth capacitance). 

LOOP WIRE AND CABLE RESISTANCE 

Inductive-loop wire, lead-in wires, and lead-in cables typically use #12, #14, 

or #16 American Wire Gauge (AWG) wire with the low frequency or direct 

current resistance measured in units of ohms (Ω). The wire resistance is 

inversely proportional to the square of the wire diameter and increases as the 

wire diameter decreases. A volt-ohmmeter (VOM) measures direct current 

resistance. The wire resistance to alternating current flow increases as the 

frequency increases because the conducting area of the wire decreases due to 

the nonuniform flux inside the wire. The high frequency resistance cannot be 

measured with a VOM, but can be obtained from a measurement of quality 

factor as defined later in this chapter. 

The loop in the roadway also contains an induced resistance (called the 

ground resistance) caused by transformer coupling between the loop and 

induced currents flowing in the roadway and subgrade materials. Appendix A 

provides a detailed derivation of ground resistance. Table 2-1 contains DC or 

low frequency resistance values for commercially available loop wire and 

lead-in cables. 

Table 2-1. Resistance of cables commonly found in inductive-loop detector systems. 

Manufacturer’s 

wire or cable type 

Function Wire gauge 

(AWG) 

DC resistance 

(Ω/ft) 

9438 Loop wire 14 0.0025 

8718 Lead-in cable 12 0.0019 

8720 Lead-in cable 14 0.0029 

8719 Lead-in cable 16 0.0045 

LOOP INDUCTANCE 

All wire conductors carrying an electrical current produce magnetic flux 

lines, which encircle the current flow that forms them. The magnetic flux 

induces the electrical property called inductance, measured in henrys (H). 

The inductance of the wire is called self-inductance. If the flux from current 

flowing in one wire couples to other wires, the resulting inductance is called 

mutual inductance. 

Figure 2-2 displays the flux around a single turn wire loop. The plane 

containing the flux is normal to the current flow in the wire, where the flux 

direction is determined by the right hand rule. This rule is applied as follows: 

Place the right hand under the wire with fingers curled in the direction of the 

flux lines. The thumb points in the direction of the current flow. All flux lines 

are in the same direction inside the loop. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-3 Federal Highway Administration 

Figure 2-3 illustrates the magnetic flux lines for a solenoid or coil whose length 

is greater than the diameter. The magnetic flux is uniform inside the coil 

except near the ends. The magnetic field for this coil geometry is given by 

 l

NI H = (2-1) 

where 

H = Magnetic field, ampere turns per meter, not to be confused with 

the units of inductance in henrys 

N = Number of turns 

I = Coil current, amperes 

l = Length of coil, meters. 

Current Flow

Figure 2-2. Magnetic flux around loop. The black arrows represent the current flow in the wire 

and the white arrows the induced flux as determined by the right hand rule. 

Figure 2-3. Magnetic flux for solenoid (coil). The black arrows represent the current flow, while the 

circles with the black and “X” centers represent the induced flux flow out of 

and into the plane of the figure, respectively. 

Since the magnetic flux is uniform inside the coil, the flux is given by 

 φ = BA (2-2) 

where 

φ = Magnetic flux, webers 

B = Magnetic flux density, webers per m2

A = Cross sectional area of coil, m2

. 

X X X X X
Chapter 2—Sensor Technology 

October 2006 Page 2-4 Federal Highway Administration 

The magnetic flux density is expressed as 

 φ = BA (2-3) 

where 

μr = Relative permeability of material (1 for air) 

μ0 = 4π x 10-7 henrys per meter. 

The inductance of a coil is defined as 

 I

NBA

I

N L = φ = (2-4) 

where 

L = Inductance, henrys 

N = Number of turns 

I = Coil current, amperes. 

The inductance of a coil, with a length much greater than the coil area to 

ensure uniform magnetic flux inside the coil, is given by 

 

l

N A

I

N HA

L r r 2

0 μ μ0 = μ μ = . (2-5) 

This equation shows that coil inductance is directly proportional to the turns 

squared and the coil area, and inversely proportional to coil length. Although 

the inductance formula as written is not directly applicable to a roadway 

inductive loop, the formula can be modified by a factor F′ to account for the 

nonuniform flux in the roadway inductive loop. Thus 

 l

' 2

0N AF

L μrμ = . (2-6) 

Equation 2-6 is applied to a loop inductance calculation in Appendix B. In 

this case, l is referred to as the “length of the current sheet.” Equation 2-6 

shows that iron, with a relative permeability greater than one, will increase 

the loop inductance. Although the greatest increase in inductance occurs 

when an iron core passes directly through the loop, the iron mass of a vehicle 

engine, transmission, or differential will slightly increase the loop 

inductance. This condition is called the “ferromagnetic effect.” 

FERROMAGNETIC EFFECT AND VEHICLE DETECTION 

However, the ferromagnetic effect produced by the iron mass of the engine, 

transmission, or differential does not create a presence or passage indication 

by the controller. When the heavy ferrous engine enters the inductive loop’s 

detection area, it increases the inductance of the wire loop. This effect occurs 

because the insertion of any iron core into the field of any inductor reduces 

the reluctance (i.e., a term that corresponds to the resistance of a magnetic 

circuit) of the flux path and, therefore, increases the net inductance. 

However, the peripheral metal of the vehicle has an opposite effect on the 

inductance due to eddy currents that are produced. The decrease in 

inductance from the eddy currents more than offsets the increase from the 

ferrous mass of the engine, and the net effect is an overall reduction in the 

inductance of the wire loop. 

The ferromagnetic effect 

increases loop inductance. 

However, vehicle-induced 

eddy currents decrease loop 

inductance even more. 

Therefore, net effect is 

decreased loop inductance 

when a vehicle passes 

through the detection zone of 

an inductive loop. 

A roadway inductive loop 

has a nonuniform flux 

field that produces an 

inductance value given 

by Equation 2-6. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-5 Federal Highway Administration 

LOOP CHARACTERISTICS 

The inductive-loop detector provides a wide range of geometries to the traffic 

engineer for satisfying diverse traffic signal control applications, as discussed 

in Chapter 4. The size and the number of turns of a loop or combination of 

loops, together with the length of the lead-in cable, must produce an 

inductance value that is compatible with the tuning range of the electronics 

unit and with other requirements established by the traffic engineer. NEMA 

standards for inductive-loop detectors (see Appendix J) specify that an 

electronics unit must be capable of operating satisfactorily over an 

inductance range of 50 to 700 microhenrys (μH). Some units tolerate much 

larger inductance values, for example, from several loops wired in series. 

While larger inductance values are technically feasible, NEMA has specified 

a conservative upper limit to promote practices compatible with all existing 

electronics units. 

LOOP CAPACITANCE 

Figure 2-4 depicts the dominant capacitive coupling phenomena that exist 

between (1) loop wires themselves and (2) loop wires and the sidewalls of the 

sawcut slot. The capacitance related to the sawcut slot is directly 

proportional to the dielectric constant of the slot sealing material. Figure 2-5 

is an equivalent electrical circuit representing the inductive-loop wire 

resistance RS, inductance LS, and capacitance CP that are created when a loop 

is installed in roadway pavement. 

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

Rs

L CP s

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

to the sidewalls of a sawcut slot. 

The measurement data in Figure 2-6 show the effect capacitance CP has on 

increasing the inductance at the loop terminals as operating frequency 

increases.(1) If the slot sealing material is hygroscopic (i.e., readily absorbs 

and retains water) or incomplete (i.e., does not fill the slot or encapsulate the 

wires, allowing water to enter the slot and penetrate between the loop wire 

turns), the variation in capacitance, and hence inductance, will be large 

because of the large dielectric constant of water. 
Chapter 2—Sensor Technology 

October 2006 Page 2-6 Federal Highway Administration 

2 connected in parallel

1 single loop

4 loops: 2 in series, paralleled

with another 2 in series

2 connected in series

3 connected in series

4 connected in series

Inductance (μH)

Measurement Frequency (kHz)

0 50 100 150 200 250 300

50

100

150

200

250

300

350

400

450

500

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. 

The plots represent curve fits to the measured data. 

The capacitance change due to water can, therefore, result in unstable 

inductive-loop detector operation. At frequencies of 1 kilohertz (kHz), the 

capacitance effect is insignificant. At frequencies of 10 kHz or greater, the 

capacitance effect is important. When loop inductance is measured at 20 kHz 

or greater, the measurement frequency must be specified since the measured 

inductance is frequency dependent. A large number of turns on large area 

loops further increases the loop capacitance and lowers the self-resonant 

frequency of the loop (i.e., no loop inductance is measured at the loop 

terminals when the loop is self resonant). 

Figure 2-6 also illustrates how different series, parallel, and series-parallel 

configurations of wire loops affect the resultant loop inductance and its rate 

of change with frequency. The effect of the connection method on system 

inductance is discussed further under “Loop System Inductance 

Calculations,” later in this chapter. 

Loop system capacitance 

must be minimized for proper 

operation at frequencies of 

10 kilohertz (kHz) and 

greater. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-7 Federal Highway Administration 

LOOP QUALITY FACTOR Q

The resonant efficiency of a circuit is expressed through the dimensionless 

quality factor Q. If the losses of the inductor are large, Q is low. A perfect 

inductor has no losses; therefore, there is no dissipation of energy within the 

inductor and Q is infinite. 

Total energy loss in a lossy inductor is calculated by modeling the inductor as 

an equivalent lossless inductor in series with a resistor. The quality factor is 

equal to the ratio of the inductive reactance to the resistive loss of the 

inductor. Since inductive reactance is a frequency-dependent quantity, the 

frequency must be specified when measuring quality factor. The formula for 

Q is written as 

S

S

S

S

R

L

R

f L Q ω = π = 2

 (2-7) 

where 

Q = Quality factor 

π = 3.14159 (a constant) 

f = Inductive-loop system excitation frequency, Hz 

LS = Loop series inductance, henrys 

RS = Loop series resistance, ohms 

ω = Radian frequency = 2πf. 

The resonant frequency ω0 of an inductive-loop-equivalent electrical circuit 

represented by Figure 2-5 is given by 

( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

ω =

2

0

0

1 1

1

Q

LSCP

.

(2-8)

From Equation 2-7, 

S

S

L

Q0R

ω0 = . (2-9) 

Therefore, the equation for the loop quality factor Q0 of the resonant circuit 

becomes 

( )

1 1

2 0 = −

P

S

S C

L

R

Q . (2-10) 

The electronics unit adds a load resistance RL in parallel with the capacitor 

CP shown in the inductive-loop-equivalent electrical circuit of Figure 2-5. The 

effect of RL is to reduce the quality factor. The resulting quality factor is 

QP = ω0CPRL (2-11) 

or 

QP CP RP = ω ′ 0 (2-12) 

where R′P is the transformed series resistance in parallel with RL. 

Loop quality factor Q is a measure 

of the losses in an inductive-loop 

detector system. 
Chapter 2—Sensor Technology 

October 2006 Page 2-8 Federal Highway Administration 

The loaded quality factor QL of the circuit in Figure 2-5 with a load resistance 

RL in parallel with the capacitor CP is 

0

0

Q Q

Q Q Q

P

P

L

+ = . (2-13) 

Quality factors of 5 and above are recommended when installing inductiveloop detectors as oscillators in most electronics units will not operate with 

low Q. Moisture in the pavement and subgrade can increase the loop ground 

resistance such that the Q of the inductive-loop system decreases below 5, 

thereby reducing the sensitivity of most inductive-loop electronics units. Loop 

capacitance will also reduce Q. 

The loaded quality factor QL given by Equation 2-13 applies to low loss 

applications, where the quality factor is large and f, LS, and RS can be readily 

measured. Inductive-loop detectors used in roadways, on the other hand, are 

not as adaptable to the above analysis because the inductance is distributed 

over the loop and lead-in cable and is difficult to measure. Calculation of the 

quality factor for roadway loops is further complicated by the larger actual 

resistances of the loop wire and lead-in cable as compared to the series value 

measured with an Ohm-meter. The extra losses are due to the high frequency 

excitation and ground currents in the pavement associated with the loop 

configuration and the roadway environment near the wire. As a result, the Q

of an identical wire configuration will vary from location to location. 

Figure 2-7 illustrates an inductive-loop system quality factor calculation 

using Q0 and QP. Tables 2-2 through 2-4 list calculated quality factors for 

rectangular, quadrupole, and circular inductive loops, respectively, of 1, 2, 3, 

4, and 5 turns. Loops are excited at 20 kHz in these tables, with conductor 

and/or quadrupole lateral spacing of 200 mils. All inductance and quality 

factors are apparent values (i.e., loop capacitance and resistance are 

included). 

Losses caused by the high 

frequency loop excitation 

and ground currents in the 

pavement near the wire 

further reduce the quality 

factor. As a result, the Q of 

an identical wire 

configuration will vary from 

location to location. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-9 Federal Highway Administration 

LOOP SYSTEM QUALITY FACTOR Q CALCULATION

Assumptions: 

Loop Type: 3-turn, 6 x 6 ft (1.8 x 1.8 m) of #14 AWG wire 

Loop Inductance: 74 μH at 20 kHz from Appendix C 

Loop Resistance (in air): 0.0025 Ω/ft (0.0083 Ω/m) from Appendix D 

Lead-in Cable Type: 100 ft (30 m) of Belden 8718 

Lead-in Cable Inductance: 0.20 μH/ft (0.67 μH/m) from Appendix D 

Lead-in Cable Resistance: 0.0031 Ω/ft (0.0103 Ω/m) from Appendix D 

Operating Frequency: 20 kHz 

Total Loop System Series Inductance: 74 μH + 20 μH = 94 μH 

Total Loop System Series Resistance: 0.25 Ω + 0.62 Ω = 0.87 Ω

Note: Wire length for resistance calculation is per wire (i.e., twice the 

cable length) 

Total Inductive-Loop System Capacitance: 

6.74 10 F

(2 20 10 ) (94 10 )

1 1

7

2 3 2 6

0

−

−

= ×

π× × × = ω

=

S

P L

C

Quality Factor of Inductive-Loop System: 

13.54

1

(0.87) (6.74 10 )

94 10 1 1

2 7

6

2 0

=

− × ×

× = − = −

−

P

S

S C

L

R

Q

This value is the unloaded inductive-loop system quality factor 

with 100 ft (30 m) of Belden 8718 #12 AWG lead-in cable. 

Assume that the inductive-loop electronics unit adds a shunt parallel 

resistance of 1,000 ohms. Then 

84.70

(2 20 10 ) (6.74 10 ) 1000 3 7

0

=

= ω = π× × × × × − QP CPRP

Therefore, the total loaded loop system quality factor is 

11.67

84.70 13.54

84.70 13.54

0

0

=

+

× = + = Q QQ Q Q

P

P

L

Figure 2-7. Loop system quality factor sample calculation. 
Chapter 2—Sensor Technology 

October 2006 Page 2-10 Federal Highway Administration 

Table 2-2. Rectangular loop inductance and quality factor parameters at f = 20 kHz.* 

Wire 

gauge 

(AWG) 

1 

Turn 

inductance 

(μH) 

1 Turn 

quality 

factor 

Q

2 

Turn 

inductance 

(μH) 

2 Turn 

quality 

factor 

Q

3 

Turn 

inductance 

(μH) 

3 Turn 

quality 

factor 

Q

4 

turn 

inductance 

(μH) 

4 Turn 

quality 

factor 

Q

5 

Turn 

inductance 

(μH) 

5 Turn 

quality 

factor 

Q

12 10 20 35 30 73 37 123 43 184 47 

14 11 16 36 24 74 30 125 35 186 40 

14** 63 12 89 14 128 18 180 21 243 25 

16 11 12 37 18 75 23 126 28 188 31 

18 11 8 37 13 77 17 127 20 189 23 

*6 x 6 ft (1.8 x 1.8 m) loop. ** With lead-in cable. 

Table 2-3. Quadrupole loop inductance and quality factor parameters at f = 20 kHz. 

Wire 

gauge 

(AWG) 

1 

Turn 

inductance 

(μH) 

1 Turn 

quality 

factor 

Q

2 

Turn 

inductance 

(μH) 

2 Turn 

quality 

factor 

Q

3 

Turn 

inductance 

(μH) 

3 Turn 

quality 

factor 

Q

4 

Turn 

inductance 

(μH) 

4 Turn 

quality 

factor 

Q

5 

Turn 

inductance 

(μH) 

5 Turn 

quality 

factor 

Q

12 17 22 60 33 125 40 211 46 315 50 

14 18 17 61 27 127 33 213 38 317 43 

16 18 13 62 20 129 26 215 30 320 34 

18 19 9 63 14 130 19 217 22 323 25 

Table 2-4. Circular loop inductance and quality factor parameters at f = 20 kHz.* 

Wire 

gauge 

(AWG) 

1 

Turn 

inductance 

(μH) 

1 Turn 

quality 

factor 

Q

2 

Turn 

inductance 

(μH) 

2 Turn 

quality 

factor 

Q

3 

Turn 

inductance 

(μH) 

3 Turn 

quality 

factor 

Q

4 

Turn 

inductance 

(μH) 

4 Turn 

quality 

factor 

Q

5 

Turn 

inductance 

(μH) 

5 Turn 

quality 

factor 

Q

12 10 20 34 31 71 38 120 44 179 49 

14 10 16 35 25 72 32 121 37 181 41 

16 10 12 35 19 73 24 122 29 182 32 

18 11 8 36 13 74 17 123 21 184 24 

*7-ft (2.1-m) diameter loop. 

LOOP LEAD-IN WIRE 

Table 2-5 contains loop-to-pull box lead-in wire inductance, capacitance, and 

resistance values for two common types of wire. 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. The twisting 

reduces crosstalk and noise pickup in the lead-in wire. Most manufacturers 

recommend at least five turns per foot (16.5 turns per meter). The wire twists 

form small loops along the wire, which alternate in winding direction. An 

external magnetic field from noise or crosstalk induces voltages in the small 

loops, which almost cancel, thus reducing interference. The importance of 

twists in the lead-in wire is discussed further in Chapter 5. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-11 Federal Highway Administration 

Table 2-5. Twisted loop lead-in wire specifications. 

Wire 

manufacturer 

and type 

Wire 

insulation 

type 

AWG 

number 

Jacket 

diameter 

(mils) 

Number of 

twists per 

foot 

Inductance 

(μH/ft) 

Capacitance 

(pF/ft) 

Resistance 

(Ω/ft) 

XHHW Cross-linked 

polymer 

14 

stranded 

130 3 to 4 0.24 10 0.006 

Belden 9438 High density 

polyethylene 

14 

stranded 

139 5.5 0.22 10 0.00252 

LEAD-IN CABLE 

Shielded, twisted wire pairs are used for the lead-in cable (home run cable), 

which runs from the pull box to the electronics unit terminals in the 

controller cabinet. The conducting shield reduces interference from external 

electric fields. Lead-in cable inductance, capacitance, and resistance values 

for several types of cable are given in Table 2-6. 

Table 2-6. Commercial lead-in cable specifications. 

Cable 

manufacturer 

and type 

Wire 

insulation 

type 

AWG 

number 

Insulation 

diameter 

(mils) 

Cable 

insulation 

type 

Inductance 

(μH/ft) 

Capacitance 

(pF/ft) 

Resistance 

(Ω/ft) 

Belden 

 8718 

 8720 

 8719 

Polyethylene 

Polyethylene 

Polyethylene 

12 

14 

16 

37 

32 

32 

Vinyl 

Vinyl 

Vinyl 

0.2 

0.2 

0.2 

25 

24 

23 

0.0019 

0.0029 

0.0045 

Clifford 

 IMSA 

 Specification 

 50-2-1984 

Polyethylene 

Polyethylene 

Polyethylene 

12 

14 

16 

30 

30 

30 

Polyethylene 

Polyethylene 

Polyethylene 

0.2 

0.2 

0.2 

25 

24 

23 

0.0016 

0.0025 

0.0040 

The measurements of loop system quality factor (with 100 ft (30 m) of 

shielded lead-in cable connected to a loop) in Appendix D show that little 

benefit is gained from using larger conductor diameters in the shielded leadin cable. For example, the quality factor associated with #14 AWG shielded 

lead-in cable is not substantially reduced by substituting #12 cable. The 

principal loss results from the type of shielding rather than the conductor 

diameter. Table 2-7 illustrates how lead-in cable type and length affect the 

quality factor. 

CALCULATING INDUCTANCE 

Several simplified formulas are available for calculating the approximate 

inductance of an inductive-loop detector. More accurate inductance values 

are obtained from the mutual coupling method discussed in Appendix A. 

The simplified formulas provide acceptable accuracy for the self inductance of 

multiturn, rectangular, quadrupole, and circular loops, which have a large 

area relative to the conductor spacing. The approximations compare 

favorably with a range of measured inductive-loop inductance values. 

Appendix C contains calculated loop inductance values for various size loops 

and shapes (rectangular, quadrupole, and circular). Inductance and quality 

factor for several numbers of turns of wire were calculated using the mutual 

coupling formula discussed later in this chapter. 
Chapter 2—Sensor Technology 

October 2006 Page 2-12 Federal Highway Administration 

Table 2-7. Influence of lead-in cable type and length on Q. 

Turns 

of #14 

AWG 

wire in 

loop 

Leadin 

cable 

type, 

Belden 

Leadin 

cable 

length 

(ft) 

Cable 

wire 

gauge 

(AWG) 

Total 

parallel 

capac. 

(μF) 

Series 

loop 

induct. 

(μH) 

Leadin 

cable 

induct†

(μH) 

Total 

series 

induct. 

(μH) 

Loop 

resist.* 

(Ω) 

Lead-in 

cable 

resist.†,**

(Ω) 

Total 

series 

resist. 

(Ω) 

Loop 

system 

Q

Loop 

system 

loaded 

Q (kΩ) 

3 8718 100 12 0.674 74 20 94 0.25 0.62 0.87 14 12 

3 8720 100 14 0.670 74 21 95 0.25 0.80 1.05 11 10 

3 8719 100 16 0.670 74 21 95 0.25 1.00 1.25 10 9 

4 8718 100 12 0.437 125 20 145 0.33 0.62 0.95 19 14 

4 8720 100 14 0.434 125 21 146 0.33 0.80 1.13 16 13 

4 8719 100 16 0.434 125 21 146 0.33 1.00 1.33 14 11 

5 8718 100 12 0.312 186 20 206 042 0.62 1.04 25 15 

5 8720 100 14 0.306 186 21 207 0.42 0.80 1.22 21 14 

5 8719 100 16 0.306 186 21 207 042 1.00 1.42 18 12 

5 8718 1,000 12 0.172 186 200 386 0.42 6.20 6.62 7 5 

5 8720 1,000 14 0.160 186 210 396 0.42 8.00 8.42 6 5 

5 8719 1,000 16 0.160 186 210 396 0.42 10.00 10.42 5 4 

Loop size is 6 x 6 ft (1.8 x 1.8 m). Excitation frequency is 20 kHz. 

* Measured series resistance of loop 3 ft (0.9 m) above the laboratory floor. 

** 8719 resistance value estimated. 

† Lead-in cable length is 100 ft. 

LOOP SYSTEM INDUCTANCE CALCULATIONS 

Inductance attributed to the lead-in cable is added to wire loop inductance at 

the rate of 21 μH per 100 ft (30 m) of #14 AWG lead-in cable. For example, a 

6- x 6-ft (1.8- x 1.8-m) rectangular loop should have three turns, according to 

Appendix C, and an inductance of 74 μH. If the lead-in cable is 200 feet (61 

m) in length, the total inductance is 

( ) 21 74 42 116 H

100

200 74 ⎟ = + = μ ⎠

⎞ ⎜

⎝

⎛ L = + . (2-14) 

The inductance L of two or more loops connected in series is additive such 

that L = L1 + L2 ± 2M, where L1 and L2 represent the inductance of each of 

the individual series-connected loops, M is the mutual inductance between 

the two loops, and the sign of M is positive if flux is increased by current 

flowing in the same direction in the closest spaced loop wires. 

The mutual inductance is negligible when the loops are separated by a large 

distance. In this case, L = L1 + L2, i.e., the loops are connected in series 

producing maximum loop inductance. 

If the loops are connected in parallel, then the combined inductance is 

calculated as 1/L = 1/L1+1/L2. For example, the combined inductance of two 6- 

x 6-ft (1.8- x 1.8-m) loops of three turns each connected in parallel is given by 

1

L = 1

74

+

1

74 = 274

. (2-15) 

Thus, 2L = 74 μH and L = 37 μH. 

Thus, parallel connection of loops reduces the inductance. Good design 

practice requires that the combined loop inductance be greater than the 

lower limit of 50 μH. Therefore, the parallel connection described above is not 

suitable as a vehicle sensor. 

In some cases, both series and parallel connections of inductive loops are 

desirable. Consider, for example, four 6- x 6-ft (1.8- x 1.8-m) three-turn loops 
 Traffic Detector Handbook—2006 

October 2006 Page 2-13 Federal Highway Administration 

installed 9 ft (2.7 m) apart to provide detection in a left-turn lane. Three 

possible types of connections are shown in Figure 2-8. Connection in series 

produces an inductance of 4 x 74 = 296 μH. Parallel connection produces only 

18.5 μH (4L = 74 μH, L = 18.5 μH). A series-parallel configuration, where the 

upper two loops are connected in series and the bottom two loops are 

connected in series, produces two loop pairs, which are then connected in 

parallel to give a combined inductance of 74 μH. 

74 μH 

74 μH 

74 μH

148 μH 

18.5 μH

Series Series-Parallel Parallel

296 μH

74 μH 74 μH 

74 μH 74 μH 74 μH 

74 μH 74 μH 

74 μH 74 μH 

74 μH 

148 μH 

Figure 2-8. Four 6- x 6-ft (1.8- x 1.8-m) three-turn loops connected 

in series, parallel, and series-parallel. 

NUMBER OF TURNS REQUIRED 

Wire loops should have a sufficient number of turns to provide a nominal 

minimum inductance of 100 μH per loop to ensure stable operation of the 

inductive-loop system. A rule of thumb for the number of turns needed to 

produce an inductance value within the required range is: 

• If the loop perimeter is less than 30 ft (9 m), use three turns of wire. 

• If the loop perimeter is over 30 ft (9 m), use two turns of wire. 

LOOP SENSITIVITY TO AN ELECTRICALLY 

CONDUCTIVE OBJECT 

The current flowing through the loop wire creates a magnetic field around 

the wire as given by Equations 2-1, 2.1 and 2-3. If a vehicle (or any other 

electrically conductive object) enters this magnetic field and the magnetic 

field or a component of the magnetic field is normal to the area of the object, 

eddy currents are induced in the conducting object. The eddy currents 

generate another magnetic field that opposes the magnetic field of the loop, 

causing a decrease in the total magnetic field around the loop. Since the loop 

inductance is proportional to magnetic flux, the loop inductance decreases. 

Loop sensitivity to a conductive object can be tested with a 12 inches (30 cm) 

long wire formed into a circle whose diameter is approximately 4 inches 

(10 cm). The circular loop forms an open electrical circuit when the wire ends 

are held such that they do not touch one another. No actuation should occur 

when the open circular loop is rapidly moved horizontally over the inEddy currents are induced in an 

electrically conductive object, such 

as a metal vehicle, by the magnetic 

field created by the current flowing 

through the wire loop. The eddy 

currents then generate a magnetic 

field that opposes the original 

magnetic field produced by the 

inductive loop. The result is a 

decrease in loop inductance. 
Chapter 2—Sensor Technology 

October 2006 Page 2-14 Federal Highway Administration 

roadway inductive loop. When the ends of the circular loop are made to 

touch, forming a closed circuit before being thrust over the in-roadway loop, 

an actuation will occur because of the flow of eddy currents. This 

demonstrates that it is the shorted turn, and not the wire or vehicle mass, 

which is important in producing the actuation. 

BICYCLE AND MOTORIZED VEHICLE DETECTION 

MODELS 

Figure 2-9 illustrates the detection of a bicycle or motorcycle by an inductive 

loop. These conveyances can be modeled as a vertical conducting object 

relative to the plane of the loop. When the cycle travels along the loop wire, 

eddy currents are induced in the conducting wheel rims and frame. When the 

cycle is directly over the loop wire, coupling between the inductive loop and 

the cycle is maximized. 

Figure 2-9. Bicycle detection showing induced eddy currents. The black arrows represent 

the current flow in the loop wire and the white arrows the induced flux. 

A vehicle undercarriage, on the other hand, is a horizontal target. As shown 

in Figure 2-10, the undercarriage is modeled as a conducting rectangular 

plate, whose width is equal to the width of the vehicle and whose length is 

equal to the length of the vehicle at some average undercarriage height. 

A conducting mesh can be used to approximate the electrical characteristics 

of the continuous plate. When the mesh is symmetrically located over the 

inductive loop to produce maximum sensitivity, all induced internal mesh 

currents cancel. This results in a single induced current flowing around the 

perimeter of the mesh, which is equivalent to a single turn rectangular wire 

loop or shorted turn. The air core transformer on the right of Figure 2-10 

models the coupling between the vehicle undercarriage, as represented by a 

shorted turn of wire, and the inductive-loop wire. 

Maximum vehicle detection sensitivity is produced by a shorted turn at 

minimum distance from the loop wires. Consequently, the ideal inductiveloop detector has a shape that approximates the vehicle’s periphery. That is, 

a 6- x 6-ft (1.8- x 1.8-m) square loop would be preferable to one the size of a 

vehicle’s engine. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-15 Federal Highway Administration 

Because of undercarriage height, high-bed trucks are difficult to detect. 

Detection of these vehicles is maximized when the width of the loop is equal 

to the width of the truck, lane width permitting. The length of the loop 

should not be less than its width to avoid a loss in sensitivity. 

Figure 2-10. Vehicle undercarriage model. The upper part of the figure depicts the vehicle 

undercarriage electrical models and the lower part the inductive-loop wire. 

MUTUAL INDUCTANCE 

Inductive-loop self-inductance is defined using the loop’s magnetic flux. 

When the magnetic flux of a loop couples to a vehicle, the coupled flux is used 

to define mutual inductance. 

Figure 2-10 showed the magnetic coupling between a loop and shorted turn, 

which behaves as an air core transformer. The mutual inductance between 

the primary circuit (i.e., the inductive loop) and secondary circuit (i.e., the 

shorted turn) is given by 

1

2 21

21 I

N M × φ = (2-16) 

where 

M21 = Mutual inductance between circuit one (loop) and 

circuit two (shorted turn), henrys 

N2 = Number of turns (equals 1 for a shorted turn) 

φ21 = Magnetic flux normal to shorted turn area, webers 

I1 = Current flowing in the loop, amperes. 

LOOP SENSITIVITY 

The loop sensitivity SL of an inductive-loop detector is defined as 

L

L

L

L L S

NV

NV V

L

Δ = × − = 100× 100 (2-17) 

Loop sensitivity is equal to the 

change in loop system inductance 

induced by a conductive metal 

object divided by the original 

inductance of the loop system. 
Chapter 2—Sensor Technology 

October 2006 Page 2-16 Federal Highway Administration 

where 

LNV = Inductance in absence of vehicle, henrys 

LV = Inductance with vehicle present, henrys. 

The sensitivity SL for the air core transformer shown in Figure 2-10, 

assuming a quality factor Q greater than 10, is given by 

( )

11 22

2

21 100

L L

M SL = × K = percent (2-18) 

where 

K = Coefficient of coupling 

M21 = Mutual coupling between loop and shorted turn, henrys 

L11 = Self inductance of loop, henrys 

L22 = Self inductance of shorted turn, henrys. 

Simplified expressions for the self inductance and mutual coupling can be 

derived by assuming the effect of vehicle iron is negligible. Then μr = 1 and 

the self inductance of the roadway loop of length l1 is found from Equation 2-

6 as 

( )

1

1

2

0 1

11 l

N A F

L μ= . (2-19) 

The inductance of the shorted turn loop of length l2 is given by 

( )

2

2

2

0 2

22 l

N A F

L μ V = . (2-20) 

The mutual inductance between the shorted turn loop and the roadway loop 

is given by 

21

0 1 1

21 d

N A F

M

μ V = (2-21) 

where 

AV = Area of vehicle undercarriage, (meters)2

d21 = Distance between loop and shorted turn, meters. 

The sensitivity is then expressed as 

2

2

21

1 2 1

A (d ) F

A F

S V

L

l l = (2-22) 

where AV ≤ A. 

Equation 2-22 shows that the sensitivity decreases for loop areas larger than 

the vehicle undercarriage area. The sensitivity decreases as the square of the 

vehicle undercarriage distance from the loop. The sensitivity is independent 

of the number of loop turns; however, pulling the turns apart slightly 

increases sensitivity by increasing l1 at the expense of a deeper sawcut slot in 

the roadway. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-17 Federal Highway Administration 

Appendix E contains more complex formulas for calculating SL for two-turn 

and other multi-turn inductive loops. Comparisons of measured and 

calculated sensitivities are also available in this appendix. 

Figure 2-11 illustrates the variation of loop sensitivity with vehicle 

undercarriage height for 6- x 2-ft (1.8- x 0.6-m), 6- x 4-ft (1.8- x 1.2-m), and 6- 

x 6-ft (1.8- x 1.8-m) three-turn inductive loops. The sensitivity of the 6- x 2-ft 

(1.8- x 0.6-m) loop is small because of its short length l1. 

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

of vehicle undercarriage height. 

Figure 2-12 depicts the decrease of loop sensitivity that occurs when a 200-ft 

(60-m) lead-in cable is added to the loops specified in Figure 2-11. The 6- x 

2-ft (1.8- x 0.6-m) loop will probably double count a high-bed truck under this 

condition. 

Figure 2-13 shows the decrease in loop sensitivity for a vehicle centered in 

two-turn long inductive loops as compared to the sensitivity of three-turn 

loops. Loop sensitivity decreases further when a lead-in cable is added. 

EFFECT OF REINFORCING STEEL 

Figure 2-14 illustrates the reduction in loop sensitivity that occurs when an 

inductive-loop detector is installed over reinforcing steel mesh. The effect of 

the reinforcing steel is modeled as a shorted turn at twice the mesh spacing 

from the loop. The reinforcing steel reduces the magnetic field around the 

loop wire conductors, which causes a decrease in loop inductance and in loop 

sensitivity. Table 2-8 shows the effect on loop inductance when reinforcing 

steel is added to the pavement subsurface. The values are conservative since 

the mesh is assumed to be a perfect conductor. Modern inductive-loop 

detector electronic units are capable of detecting vehicles even though the 

loop wire is laid on the rebar before concrete is poured. 
Chapter 2—Sensor Technology 

October 2006 Page 2-18 Federal Highway Administration 

1 ft = 0.3 m 

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. 

1 ft = 0.3 m 

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

of vehicle undercarriage height. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-19 Federal Highway Administration 

1 ft = 0.3 m 

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

Inductive loops do not function as vehicle sensors when installed above steel 

rebar, whose pieces are connected such that current flows through the rebar. 

This induced current fully or partially cancels the vehicle-induced current in 

the inductive loop. If the rebar spacing is sufficiently large, then the current 

flows may not cancel. Conversely, if the rebar is not shorted together when it 

is installed, it will not support the flow of counter currents that inhibit the 

performance of the inductive loop. 

Table 2-8. Influence of reinforcing steel on loop inductance (μH). 

Number of turns No reinforcing 

steel 

Steel of 1- 

inch diameter 

Steel of 2-inch 

diameter 

Steel of 4-inch 

diameter 

1 11 9 10 10 

2 35 28 31 33 

3 73 56 63 68 

4 121 89 103 112 

5 179 127 151 166 

6 248 167 206 228 

7 325 206 266 298 

1 inch = 2.5 cm 

Epoxy coatings normally placed on rebar are insulating in nature. However, 

the nature of the coating process usually places voids in the coating, which 

allow currents to flow. The number of allowable voids may be specified in the 

construction documents. The counter-current flow may increase in winter 

months in cold climates where salts are placed on the roadway or bridge 

deck. 

LOOP SYSTEM SENSITIVITY 

Loop system sensitivity is defined as the smallest change of inductance at the 
Chapter 2—Sensor Technology 

October 2006 Page 2-20 Federal Highway Administration 

electronics unit terminals that will cause the controller to actuate. This 

sensitivity must be equal to or greater than the threshold for the electronics 

unit. Many states specify that the electronics unit must respond to a 0.02 

percent change in inductance. NEMA Standards (see sSection 15.3.2 of 

Appendix J), recognizing the differences in electronics unit design (∆L/L or 

∆L), specify the sensitivity threshold for three classifications of test vehicles 

when they are centered in a single 6- x 6-ft (1.8- x 1.8-m) three-turn loop with 

100 ft (30.5 m) of lead-in cable. The vehicle classes are: 

• Class 1: 0.13 percent (ΔL/L) or 0.12 μH (ΔL) inductance change 

(small motorcycle). 

• Class 2: 0.32 percent (∆L/L) or 0.3 μH (∆L) inductance change (large 

motorcycle). 

• Class 3: 3.2 percent (∆L/L) or 3.0 μH (∆L) inductance change 

(automobile). 

An inductance in series or parallel with an inductive-loop detector will 

reduce the loop system sensitivity at the input terminals of the electronics 

unit. 

A study performed for the SCANDI project in Detroit found that the duration 

of a call is impacted by the height of the flux field, which, in turn, depends on 

the presence and depth of reinforcing steel and other location-specific 

factors.(2) The study indicated that adjustable, diamond-shaped loops 

compensate for such factors at each location, resulting in a uniform duration 

from loop to loop for a given vehicle at a given speed. 

SENSITIVITY OF TWO SERIES INDUCTORS 

Figure 2-15 illustrates the total inductance calculation for the combination of 

two separate inductive loops connected in series as one equivalent loop. 

(Figure 2-19 illustrates the connection of two loops in this manner.) The 

equivalent total series inductance LTS is 

LTS = LA + LB (2-23) 

where LA and LB are the individual inductance values of the loops. 

LA LTS

LB

SL

A STS

Figure 2-15. Equivalent total inductance from two inductive loops in series. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-21 Federal Highway Administration 

The equivalent total series sensitivity STS is 

A

B

A

TS L

L

L S S

+

=

1

1

 (2-24) 

where 

A SL = Loop sensitivity as vehicle enters Loop A. 

SENSITIVITY OF TWO PARALLEL INDUCTORS 

Figure 2-16 illustrates the sensitivity calculation for two separate inductive 

loops connected in parallel as an equivalent single loop. (Figure 2-21 

illustrates the connection of two loops in this manner.) The equivalent total 

parallel inductance LTP is 

A B

A B

TP L L

L L L

+

× = . (2-25) 

The equivalent total parallel sensitivity STP is 

B

A

A

TP L

L

L S S

+

=

1

1 . (2-26) 

LA LTP

SL STP

LB

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

SINGLE LOOP EXAMPLE 

1. What is the loop sensitivity at the pull box assuming a high-bed vehicle 

(4-ft (1.2-m) undercarriage) passes over the loop? Figure 2-17 illustrates this 

case and gives the lead-in wire lengths. The equivalent electrical circuit is 

shown in Figure 2-18. 
Chapter 2—Sensor Technology 

October 2006 Page 2-22 Federal Highway Administration 

Loop

A

Lane Markers 

Electronics

Unit

Pull

Box

12’ 12’ 12’ 200’

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

L2 L5

L1

Pull Box

Junction

Lead-in

Cable

Lead-in

Wire To Electronics

Unit

Figure 2-18. Equivalent single loop electrical circuit. 

The sensitivity SL for a 4-ft (1.2-m) high undercarriage and a three-turn, 6- x 

6-ft (1.8- x 1.8-m) loop of #14 AWG wire is 0.1 percent from Figure 2-11. The 

twisted loop wires form an approximately 24-ft (7.3-m) lead-in wire to the 

pull box. The inductance per foot for #14 AWG loop wire with 5 twists per 

foot is 0.22 μH/ft (0.7 μH/m). The lead-in inductance LS is 

LS = (0.22 μH/ft)×(24 ft) = 5.3μH . (2-27) 

The self inductance LL of a three-turn, 1.8-1.8-m (6- x 6-ft) loop of #14 AWG 

wire at 20 kHz from Appendix C is 74 μH. Therefore, the sensitivity SP (in 

percent) at the pull box is 

0.093%

74 H

5.3 H 1

0.1%

1

=

μ

μ +

=

+

=

L

S

L

P

L

L

S S . (2-28)

2. What is the inductive-loop system sensitivity at the input terminals of 

the electronics unit with a 200-ft (61-m) length of Type 8720 shielded lead-in 

cable between the pull box and the electronics unit? 

From Table 2-6, the inductance of type 8720 cable is 0.22 μH/ft. The total 

series inductance between the loop and the input terminals of the electronics 

unit is 

= [(0.22)×(24)]+ [(0.22)×(200)] LS (2-29a) 

LS = 5.3μH+ 44 μH = 49.3μH . (2-29b) 
 Traffic Detector Handbook—2006 

October 2006 Page 2-23 Federal Highway Administration 

Then the sensitivity SD at the input terminals of the electronics unit is 

0.060%

74 H

49.3 H 1

0.1%

1

=

μ

μ +

=

+

=

L

S

L

D

L

L

S S . (2-30) 

3. What is the inductive-loop system sensitivity at the input terminals of 

the electronics unit with a 200-ft (61-m) length of Type 8720 shielded lead-in 

cable between the pull box and the electronics unit if a four-turn, 6- x 6-ft 

(1.8- x 1.8-m) loop #14 AWG wire is used? 

The sensitivity SL for a 4-ft (1.2-m) high undercarriage and four-turn, 6- x 6-

ft (1.8- x 1.8-m) loop is 0.1 percent. From Appendix C, the loop self 

inductance is 125 μH at 20 kHz. The series inductance is the same as in the 

previous example. 

Therefore 

0.072%

125 H

49.3 H 1

0.1%

1

=

μ

μ +

=

+

=

L

S

L

D

L

L

S S . (2-31) 

TWO LOOPS IN SERIES EXAMPLE 

1. What is the inductive-loop system sensitivity at the input terminals of 

the electronics unit when a second identical loop is placed in series with the 

loop sensing the vehicle? Figure 2-19 illustrates the loop configuration and 

shows lead-in wire lengths. The series connection is made in the pull box. 

Loop

A 

Lane Markers 

Electronics

Unit

Pull

Box

Loop

B 

12’ 12’ 12’ 200’

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

Figure 2-20 shows the equivalent electrical circuit. The sensing loop is a 6- x 

6-ft (1.8- x 1.8-m), three-turn loop of #14 AWG wire. The self inductance of 

the second loop (i.e., series Loop B) is 74 μH. The lead-in wire inductance for 

Loop B is 

LS = [ ] ( ) 0.22 μH/ft ×(12 ft) = 2.6 μH . (2-32) 

The total series inductance of Loop B and lead-in wire to the pull box is 

= 2.6 μ H+ 74 μ H = 76.6 μ H B LT (2-33) 
Chapter 2—Sensor Technology 

October 2006 Page 2-24 Federal Highway Administration 

and the total series inductance between the two loops and the input 

terminals of the electronics unit is 

S

A

T

B

T

S LT = L + L + L (2-34a) 

= 76.8μH+ 79.3μH+ 47.6μH = 203.7 μH . (2-34b) 

L2 Pull Box

Junction

Lead-in

Cable

To Electronics

Unit

Lead-in Wire 

(First Loop)

L3 L4

Lead-in Wire 

(Second Loop)

First 

Series 

Loop

Second 

Series Loop

L5

L1

Figure 2-20. Equivalent electrical circuit for two loops connected in series 

to a pull box and electronics unit. 

Then 

0.066%

74 H

203.7 H 1

0.1%

1

=

μ

μ +

=

+

=

L

T

L

D

L

L

S S . (2-35) 

TWO LOOPS IN PARALLEL EXAMPLE 

1. What is the loop system sensitivity at the electronics unit terminals with 

two identical loops connected in parallel? Figure 2-21 illustrates the loop 

configuration and shows lead-in wire lengths. The equivalent electrical 

circuit is shown in Figure 2-22. All parameters are the same as in the 

previous series loop example. The total inductance and sensitivity at the 

input to the electronics unit are calculated as 

LTS = L1 + L2 (2-36) 

and 

1

2 1

L

L

S S L

TS

+

= . (2-37) 
 Traffic Detector Handbook—2006 

October 2006 Page 2-25 Federal Highway Administration 

Loop

A 

Lane Markers 

Electronics

Unit

Pull

Box

Loop

B 

12’ 12’ 12’ 200’

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

L2 L5

L1

Pull Box

Junction

Lead-in

Cable

Lead-in

Wire

To Electronics

Unit

SL

Lead-in

Wire

L3

L4

Figure 2-22. Equivalent electrical circuit for two loops connected in parallel 

to a pull box and electronics unit. 

Let 

LA = L1 + L2 (2-38) 

and LB = L3 + L4 . (2-39) 

Then 

( )( )

1 2 3 4

1 2 3 4

L L L L

L L L L

L L

L L L

A B

A B

TP

+ + +

+ × + =

+

× =

 (2-40) 
Chapter 2—Sensor Technology 

October 2006 Page 2-26 Federal Highway Administration 

3 4

1 2 1

1

1

1

L L

L L S

L

L S S

TS

B

A

TP TS

+

+

+

=

+

=

 (2-41) 

and 

S

A B

A B

D TP S L

L L

L L L L L +

+

× = + = (2-42) 

( ) ( ) LS L L L L

L L L L ⎥ +

⎦

⎤ ⎢

⎣

⎡

+ + +

+ × + =

1 2 3 4

1 2 3 4 . (2-43) 

Therefore 

( )( )

1 2 3 4

3 4 1 2 3 4

1 2

1

2 1 1

1

1

1

1

1

L L L L

L L L L L L

L L

L

L LD SD SL

+ + +

+ × +

+

×

+

+

+

×

+

= = . (2-44) 

RESONANT CIRCUIT 

Many self-tuning inductive-loop electronics units use a frequency shift or 

change in period of an oscillator to indicate the passage or presence of a 

vehicle. The frequency of the oscillator is controlled by a parallel resonant 

circuit, sometimes called a tank circuit, composed of the equivalent loop 

system inductance and the tuning capacitance found in the electronics unit. 

The equivalent loop system capacitance also includes capacitive effects due to 

the placement of the loop wires in the sawcut. The associated equivalent 

quality factor accounts for the effect of system resistance losses. If the 

equivalent loop system inductance is too small, the oscillator will not 

oscillate. The manufacturer of the electronics unit specifies the range of loop 

system inductance and minimum loop system quality factor that are 

acceptable. 

The oscillator frequency is calculated as 

( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

π +

=

2

1 2 1

1

D

D D

D

Q

L C

f (2-45) 

where LD, CD, QD are the inductance, capacitance, and quality factor, 

respectively, of the tank circuit. 

Equation 2-45 shows that a decrease in the inductance increases the 

resonant frequency. Furthermore, a quality factor greater than five will have 

negligible effect on the performance of the resonant circuit. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-27 Federal Highway Administration 

TEMPORARY LOOPS 

Several manufacturers and State agencies have sought to develop a durable 

and cost effective temporary loop that satisfies the needs of speed monitoring, 

vehicle counting, vehicle classification, and portable weigh-in-motion (WIM) 

programs. Two types of temporary and portable loop systems are described 

below. 

MAT-TYPE LOOPS 

The mat-type temporary loop consists of a durable rubber mat into which 

multiple turns of wire are embedded. The mats are usually smaller in width 

than the typical 6-ft (1.8-m) inductive loop. Standard sizes vary, ranging from 

4 x 6 ft (1.2 x 1.8 m) to 3 x 6 ft (0.9 x 1.8 m). The mats are positioned in the 

center of the traffic lane with the longer dimension parallel to the flow of 

traffic so that most vehicles straddle the mat, thereby extending the life of 

the mat. A typical installation is shown in Figure 2-23. Nails and washers 

are commonly used to secure the mat to the road surface. A wide 3-inch (7.6-

cm) heavy-duty adhesive tape is applied to prevent the mat edges from 

lifting. The lead-in wires from the mat to the data collection equipment at the 

roadside are encased between two layers of tape. 

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

Some agencies have manufactured this type of sensor in their own shop. 

However, hand-producing these mats was too labor intensive to be costeffective. The mats were reliable, but with heavy truck traffic, some of the 

mats did not last more than a few hours. 

OPEN LOOP CONFIGURATION 

One manufacturer produces a preformed temporary portable loop that is 

4 x 6 ft (1.2 x 1.8 m). The loop is composed of a sandwich of five layers as 

illustrated in Figure 2-24. The bottom layer is a 4-inch (101.6-mm) wide 

paper release sheet, which protects the 2-inch (50.8-mm) wide strip of 

adhesive bituminous rubber compound. Its upper surface is finished with a 

high-density polyethylene film. This padding strip is the bed for three turns 

of #22 AWG loop wire. An identical 2-inch (50.8-mm) padding strip covers the 

loop wires. The top layer is a 4-inch (101.6-mm) wide strip of adhesive 

bituminous compound reinforced with an overlay of woven polypropylene 

mesh. 
Chapter 2—Sensor Technology 

October 2006 Page 2-28 Federal Highway Administration 

The preformed, open loop configuration can be transported to the selected 

location and installed by one man in a few minutes. Installation consists of 

removing the bottom backing, positioning the loop on the roadway, and 

applying sufficient pressure to ensure adhesion. Five feet of protected lead-in 

wire is standard. Other loop dimensions and protected lead-in wire lengths 

are available. 

Another approach to the open loop configuration was developed by the 

Special Study Section of the Nevada Department of Transportation (DOT).(3)

The Nevada DOT previously used a 6- x 6-ft (1.8- x 1.8-m) portable loop 

constructed of three turns of #14 AWG stranded copper wire wrapped with 

black duct tape. Time consuming durability and maintenance issues 

increased as the use of the portable loops increased. This led to the testing of 

a variety of tapes, rubber tubing, and a rubber mat material as replacements 

for the duct tape covering of the original loops. 

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

A bitumen tape manufactured by Polyguard Products was eventually 

selected to enclose the wire loops. It is a fabric-reinforced rubber-like 

material with one adhesive side. The final configuration consists of four turns 

of #14 AWG copper wire, wound in the shop, and taped together for easy 

handling. The loops are encased in two wraps of the Polyguard material and 

installed as shown in Figure 2-25. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-29 Federal Highway Administration 

1 ft = 0.3 m 

1 inch = 2.5 cm 

Figure 2-25. Nevada portable open loop installation. 

A number of tests were conducted to measure the durability and accuracy of 

the loops as compared with conventional loops installed in sawcut slots. 

Other tests compared the 4- x 6-ft (1.2- x 1.8-m) configuration to the 6- x 6-ft 

(1.8- x 1.8-m) configuration. The test loops were installed on a rural two-lane 

FAP roadway with a high percent of multiple unit trucks. Both series of tests 

used the same counter/classifier recorder. 

After almost 5,000 actuations, there was less than 1 percent difference 

between the number of vehicles counted by this type of portable loop and a 

saw-cut installed loop. It was also found that the 4- x 6-ft (1.2- x 1.8-m) loop 

size performed virtually the same as the 6- x 6-ft (1.8- x 1.8-m) size whether 

the loop was in a saw-cut or in the portable form. 

The portable loops were still functioning after a more than yearlong product 

durability test consisting of over a million activations. This evaluation, on 

U.S. 395 between Reno and Carson City, NV, showed the loops to be 

extremely durable and capable of withstanding a wide range of weather 

conditions. The roadway contained an asphalt surface and, after several 

months, the loops became embedded in the pavement, which may have 

contributed to their longevity. On a concrete surface, these loops are expected 

to last well past one-half million activations. The loops have also been used 

with overlays and were able to withstand the heat involved in this process. 

Testing in a semipermanent location enhanced loop longevity, as the loops 

were not subjected to repeated removal and reinstallation. However, other 

loops of the same type have been repeatedly installed without signs of undue 

wear. As a result of these tests and the experience with these loops, the 

Nevada DOT is now using the Polyguard loop in all of their portable loop 

installations. 
Chapter 2—Sensor Technology 

October 2006 Page 2-30 Federal Highway Administration 

ELECTRONICS UNITS 

The electronics unit, which generates the inductive-loop excitation frequency 

and monitors the operation of the inductive-loop system, has changed 

significantly since the 1970s. Early versions of inductive-loop electronic units 

operated at a fixed resonant frequency, using a crystal to stabilize the 

frequency. There were many problems with the crystal electronics units, 

particularly when used with long lead-in cables. 

One of these was resonant-frequency drift due to environmental changes in 

temperature and moisture. These units were removed from service in the 

1970s and were initially replaced with designs that incorporated analog 

phase shifters, which were capable of compensating for (or tracking) drift 

caused by environmental changes. Modern electronics units stabilize the 

oscillation frequency and detect vehicles with configurations that incorporate 

digital frequency shift, digital ratioed frequency shift, digital period shift, 

and digital ratioed period shift designs. The theory of operation of these 

devices is described below. Analog phase shift electronics units are still in 

limited use for vehicle classification. 

ANALOG PHASE-SHIFT ELECTRONICS UNIT 

This device was developed to meet the demands of the European market, 

where bicycles must be detected. Like the crystal model, it operates as a 

phase shift sensor, but uses two variable-frequency oscillators rather than 

one crystal-controlled oscillator. The loop oscillator operates at a frequency 

between 25 and 170 kHz, as determined by the loop and lead-in wire. The 

loop oscillator is coupled to a second internal oscillator such that the initial 

manual tuning procedure brings the two oscillators into synchronization in 

frequency and phase. 

The tuning knob moves a ferrite core back and forth inside an inductor, 

causing the oscillator connected to it to change its frequency (and phase) to 

match that of the loop oscillator. Arrival of a vehicle into the loop decreases 

loop inductance and the loop oscillator attempts to pull out of 

synchronization with its companion oscillator. It is not able to change 

frequency because of a cross-coupling resistor, but a phase shift is developed 

that is the basis for vehicle detection. 

With this design concept, the electronics unit is able to compensate for (or 

track) environmental drift. As the temperature within the controller cabinet 

changes, the two oscillators drift identically. The output of the two oscillators 

are fed to a phase comparison circuit, which develops a DC voltage 

proportional to the amount of shift; thus, the term analog because it uses 

varying voltages rather than digital counts to indicate vehicle passage or 

presence. 

When there are no vehicles within the detection zone, the DC voltage is 

stored and remembered by a memory capacitor. When a vehicle causes a 

change in the phase comparator output, the difference between it and the 

memory capacitor causes a relay to change state. Very slow changes in the 

DC voltage are followed by the memory capacitor, which allows the circuit to 

compensate for drift due to environmental changes. The memory circuit will 

ultimately forget a vehicle parked over the loop and drop that call. Detail on 

compensating for environmental drift is included later under the “Digital 

Frequency Shift Electronics Unit.” 
 Traffic Detector Handbook—2006 

October 2006 Page 2-31 Federal Highway Administration 

OVERVIEW OF DIGITAL ELECTRONICS UNITS 

The stability and added features afforded by electronic digital processing 

have lead most inductive-loop detector electronics unit manufacturers to 

produce digital devices. Digital techniques allow more reliable, accurate, and 

precise measurements than the analog techniques. 

In using digital electronics units, one must be aware of the relation between 

increased sensitivity and the resulting increased response time. A large 

response time can result in a significant error in vehicle velocity 

measurements when two loops are utilized in a speed trap configuration (i.e., 

separated by a known and measured distance). Response times vary with 

electronics unit manufacturers. 

Digital electronics units sense either a change in the frequency or period of a 

waveform. The oscillator frequency or period shift is caused by the decrease 

in loop inductance created when a vehicle is within the loop’s detection zone. 

The oscillator frequency for a quality factor Q of 5 or greater is given by 

fD = 1

2π LDCD

 (2-46) 

where 

fD = Oscillator frequency, Hz 

LD = Total inductance (i.e., loop plus lead-in cable) across 

the input terminals of the electronics unit, henrys 

CD = Total capacitance across the input terminals of the 

electronics unit, henrys. 

The normalized oscillator frequency change due to a normalized change in 

inductance at the input terminals to the electronics unit when the Q is 5 or 

greater is given by 

D

D

D

D

D S

L

L

f

f

2

1

2

1 = − Δ = − Δ

 (2-47) 

where 

ΔfD = Change in oscillator frequency of the electronics unit, Hz 

ΔLD = Change in inductance at the input terminals of the 

electronics unit, henrys 

SD = Sensitivity of the electronics unit to the inductance 

change. 

Vehicle detection by an inductive-loop detector system is primarily induced 

by vehicle proximity to a buried inductive wire loop, which causes a change 

in the loop inductance in the inductance-capacitance (LC) oscillator circuit 

formed by the loop, lead-in cable, and the input capacitor located in the 

electronics unit. Some manufacturers detect vehicles through the percent 

change of loop inductance ∆LL/LL, while others simply use the change in loop 

inductance ∆LL. Neither of these quantities can be measured directly at the 

input terminals to the electronics unit. However, to indicate sensitivity, 

several manufacturers provide frequency meters to measure the resonant 

frequency and the amount of frequency change. 
Chapter 2—Sensor Technology 

October 2006 Page 2-32 Federal Highway Administration 

Experience shows that the percentage change of inductance (∆LL/LL) from an 

unoccupied loop to an occupied loop is extremely repeatable for a given loop 

size and geometry, a given vehicle size and geometry, and a given location of 

the vehicle with respect to the loop. Since parameters such as actual loop 

inductance and loop operating frequency do not affect ∆LL/LL but do affect 

∆LL, the following discussions and computations address the ∆LL/LL concept. 

The term “electronics unit sensitivity,” in the context of this discussion, is 

defined as the value of ∆LL/LL that actuates the electronics unit with smaller 

values, which are interpreted as denoting greater sensitivity. 

For short lead-in cables with negligible series cable inductance 

L

L

L

D

D S

L

L

L

L = Δ = Δ

 (2-48) 

where 

ΔLL = Change in loop inductance when sensing a vehicle, henrys 

LL = Loop inductance, henrys 

SL = Sensitivity of loop to vehicle in detection area. 

The period of the oscillator TD is defined as the inverse of the frequency fD. 

For a Q of 5 or greater, TD is given by 

TD = 1

f

D

= 2π LDCD . (2-49) 

The normalized change in oscillator period caused by a normalized change in 

inductance at the input terminal to the electronics unit when Q is 5 or 

greater is approximately equal to 

ΔTD

TD

= − 1

2

ΔLD

LD

= − 1

2

SD. (2-50) 

The negative sign indicates that the change in period is opposite in direction 

to the change in inductance. 

With the advent of sophisticated digital microprocessors and the availability 

of loop network resonant frequency information at the electronics unit input 

terminals, precise measurements of the following parameters can be obtained 

with relative ease: 

• Frequency shift (∆fD). 

• Ratioed frequency shift (∆fD/fD). 

• Period shift (∆TD). 

• Ratioed period shift (∆TD/TD). 

The four types of digital electronics units, each utilizing one of these 

measurement techniques, are introduced below. Detailed analyses and block 

diagrams of each device are provided in Appendices F through I. 

DIGITAL FREQUENCY SHIFT ELECTRONICS UNIT 

This type of unit is not manufactured. However, the theory and operating 

characteristics associated with this concept are included so that the operation 

of the digital ratioed frequency shift electronics unit may be better understood. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-33 Federal Highway Administration 

The digital processor in a digital shift electronics unit would compare counts 

proportional to the oscillator frequency when a vehicle is present to a 

reference count taken periodically when no vehicles are present. The 

reference count is stored in a memory. During vehicle detection, when the 

count exceeds the reference count by a preset sensitivity threshold count, a 

vehicle call is initiated. 

The sensitivity f

DS of a frequency shift electronics unit is calculated from 

Equation 2-47 as 

D

f D

D f

f SΔ = −2 . (2-51) 

Appendix F shows that 

fc

ft

f D D

f

D N

N

S K L C

2 = = (2-52) 

where 

Nft = Fixed frequency threshold count selected by 

sensitivity switch 

Nfc = Number of oscillator cycles counted by the 

 variable frequency counter 

Kf = Frequency sensitivity constant. 

In the digital frequency shift method, SD is proportional to the square root of 

the LDCD product. Since larger values of SD represent decreased sensitivity, it 

follows that sensitivity decreases in proportion to the square root of the LDCD

product with a ΔfD measurement. Hence, every time the frequency switch is 

changed to a new position (e.g., to avoid crosstalk), the sensitivity would 

change and, if critical, would necessitate a new setting of the sensitivity 

switch. 

Increased lead-in cable length increases the inductance of the lead-in cable 

and, hence, causes some loss of sensitivity. The increased LDCD product would 

cause even more loss of sensitivity. Therefore, this type of measurement does 

not appear to be practical. 

DIGITAL RATIOED FREQUENCY SHIFT ELECTRONICS 

UNIT 

The digital processor in the digital ratioed frequency shift electronics unit 

compares counts proportional to the oscillator frequency when a vehicle is 

present to a reference count taken periodically when no vehicle is present. 

The reference count is stored in memory. When the count during vehicle 

detection exceeds the reference count by a preset sensitivity threshold count, 

a vehicle call is initiated. 

The ratioed frequency shift electronics unit differs from the frequency shift 

unit in that the frequency counter is held approximately constant (as 

explained further in Appendix G). 

The sensitivity f

DS is independent of the inductance LD and the capacitance CD

across the terminals of the electronics unit. The sensitivity is calculated as 
Chapter 2—Sensor Technology 

October 2006 Page 2-34 Federal Highway Administration 

fc

f ft

D N

N

S = 2 (2-53) 

where 

Nft = Fixed frequency threshold count 

Nfc = Count produced by the fixed frequency counter. 

From Appendix G, the measurement response time t

f

 is 

f

D D

ft f

m f S

N

t = 2 (2-54) 

where m is a frequency multiplier. 

The advantage of having the sensitivity independent of the inductance and 

capacitance across the input terminals of the electronics unit is illustrated by 

the following example. This example also applies to the digital ratioed period 

shift electronics unit discussed later. 

Assume four equal size loops, say 6 x 6 ft (1.8 x 1.8 m) with an equal number 

of turns, say three. Connect the loops as shown in Figure 2-8, namely 

• All series (296 μH). 

• Series-parallel (74 μH). 

• All parallel (18.5 μH). 

For simplicity, lead-in cable length is not considered. The sensitivity of the 

ratioed frequency or ratioed period shift electronics unit is identical for the 

above three loop connection configurations. Accordingly, a sensitivity 

threshold sufficient to detect a small motorcycle over one of the four loops 

when wired in series need not be changed when rewired in series-parallel or 

all parallel. 

Although lead-in cable length was not considered above, an extra long lead-in 

cable will produce varying amounts of change in inductance due to 

inductance sharing. The amount of change depends on the length of the leadin cable and the wiring scheme used for the multiple loops. Figure 2-26 

provides an estimate of the inductance change at the electronics unit input 

terminals generated by a small motorcycle traveling over one of the four 

loops.(1) 
 Traffic Detector Handbook—2006 

October 2006 Page 2-35 Federal Highway Administration 

1 ft = 0.3 m 

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. 

DIGITAL PERIOD SHIFT ELECTRONICS UNIT 

The digital period shift concept utilizes the period of the loop oscillator 

frequency, where period is defined as the time required for one full cycle of 

the oscillator frequency. The period is calculated by dividing one by the 

frequency in Hz or equivalently dividing one by the frequency in cycles per 

second. 

Digital period shift electronics units use a reference clock running at 

megahertz (MHz) frequencies, i.e., between 20 and 100 times faster than the 

inductive-loop oscillation frequency, to measure the period of the loop 

oscillation as shown in Figure 2-27. Measurement precision is enhanced 

without sacrificing a great deal of time between measurements by 

determining the time for 32 oscillation cycles for sensitivity one, 64 cycles for 

sensitivity two, and so on. The loop oscillation period is calculated in terms of 

the number n of reference clock cycles contained within the period. Since the 

oscillation frequency increases when a vehicle passes over the loop, the 

period of the oscillation decreases, as it is equal to the inverse of the 

frequency. The reduction in the oscillation period results in a smaller number 

of cycles of the reference clock within the oscillation period. When the 

number of reference cycles is reduced more than a preselected threshold, a 

call is initiated to indicate the presence of a vehicle. 
Chapter 2—Sensor Technology 

October 2006 Page 2-36 Federal Highway Administration 

•

Loop oscillation period

One reference clock 

cycle

Number n of reference clock cycles that corresponds 

to the loop oscillation period 

1 n

• •

Figure 2-27. Measurement of inductive-loop oscillation period by a reference clock (Source: L.A. 

Klein, Sensor Technologies and Data Requirements for ITS (Artech House, Norwood, MA, 2001)). 

A judicious choice of reference clock frequency and threshold value (4 counts 

± 2 counts) makes the digital period shift design practical at any frequency 

encountered in practice. The time to detect is sufficiently short to enable the 

electronics to sequentially scan or operate four small loops, one at a time, 

several times a second. (Multichannel operation is discussed later.) 

The period shift electronics unit is entirely self-tuning on installation and, 

similar to most other designs, is able to track environmental drift. Like the 

digital frequency shift unit, most models stop tracking for a time after a 

vehicle enters the loop, to guarantee that the call placed by a small vehicle is 

held long enough to bring the green to that approach. 

The sensitivity p

DS of a period shift electronics unit is found from Equation 2-

50 as 

D

p D

D T

T S Δ = 2 . (2-55) 

Appendix H shows that 

pc

pt

D D

p p

D N

N

L C

K

S = = 2 (2-56) 

where 

Npt = Fixed frequency threshold count selected by the 

sensitivity switch 

Npc = Count produced by the variable frequency counter 

Kp = Frequency sensitivity constant. 

Sensitivity p

D S is inversely proportional to the square root of the LC product 

with a ∆T measurement. When p

D S takes on small values, the sensitivity is 

increased. Hence, with increased lead-in cable length, part of the loss of 

sensitivity due to the added lead-in cable inductance is automatically 

compensated for by the increase in the LC product. Unfortunately, the 

compensation is not perfect because of the square root relationship. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-37 Federal Highway Administration 

The response time t

p

 of the electronics unit, as derived in Appendix H, is 

p

c D

p pt

f S

N

t = 2 . (2-57) 

Most electronics units use a transformer to connect the external inductiveloop terminals to the internal oscillator. A loosely coupled transformer 

produces a series leakage or swamping inductance. This inductance reduces 

the influence of the lead-in cable on sensitivity at the expense of overall 

sensitivity. 

If a swamping inductance LT is used in the electronics unit, then 

p

c D

D

T

pt

D

T

p

c D

p pt

f S

L

L

N

L

L f S

N

t

⎥

⎦

⎤ ⎢

⎣

⎡

+ +

=

+

=

2 1 1

1

1

2 . (2-58) 

For example, let 

Npt = 4 

LT = 150 μH 

LD = 75 μH 

fD = 2.22 MHz 

p

F S = 0.005%. 

Then 

216 ms

(2.22 10 Hz) (5 10 )

75 H

160 H 2 4 1

6 5 = × × ×

⎥

⎦

⎤ ⎢

⎣

⎡

μ

μ × +

= −

p t . (2-59) 

The percentage error in vehicle speed obtained from a speed trap using two 

inductive loops a known distance apart is given by 

X

T S

S

S Δ × = × Δ 100 (2-60) 

where 

ΔS/S = Error in vehicle speed, percent 

ΔT = Error in measured time, seconds 

X = Distance between leading edges of the loop, distance 

units 

S = Vehicle speed, distance units/second. 
Chapter 2—Sensor Technology 

October 2006 Page 2-38 Federal Highway Administration 

The maximum measured time error in vehicle speed or occupancy 

measurements is attributed to the response time of the electronics unit. The 

speed measurement error caused by finite response times is illustrated by the 

following example. 

Let 

ΔS/S = Unknown error in vehicle speed, percent 

ΔT = 2 x 216 milliseconds (ms) = 432 ms (0.432-second 

error in measured time) 

X = 100-ft (30.5-m) spacing between leading edges of the 

loop 

S = 60 miles per hour (mi/h) = 88 ft/s (96.6 kilometers per hour 

(km/hr) = 2.68 m/s) vehicle speed. 

Then 

0.38 38 percent 30.5

0.432 2.68 100 = = × = × Δ

S

S . (2-61) 

This example indicates that the loop system should be designed so that the 

system sensitivity is as large as possible. By setting the electronics unit to a 

less sensitive range, the response time is decreased, producing a more 

accurate vehicle speed measurement. 

An increase in the electronics unit’s clock frequency from 2.22 MHz to 22.2 

MHz reduces the percentage velocity error from 38 percent to 3.8 percent. 

Many of the newer electronics units use clock frequencies between 20 and 25 

MHz and, thus, are capable of reducing the percent velocity error. 

DIGITAL RATIOED PERIOD SHIFT ELECTRONICS 

UNIT 

The digital processor in this design compares counts proportional to the 

oscillator period when a vehicle is present to a reference count taken 

periodically when no vehicle is present. The reference count is stored in 

memory. When the count during detection is less than the reference count by 

a preset sensitivity threshold count, a vehicle call is initiated. The ratioed 

period shift electronics unit differs from the period shift electronics unit in 

that the threshold count Npt is not fixed. 

The threshold count (see Appendix I) is given by 

T

pc

p T pc

D S

N

S N

S 2

2 = = . (2-62) 

Since the sensitivity of the electronics unit is independent of the count 

measured by the period counter, the sensitivity is also independent of 

frequency. The response time is identical to that of the digital period shift 

electronics unit. 

COMPARISON OF DIGITAL ELECTRONICS UNITS 

Table 2-9 compares the various inductive-loop digital electronics unit 

concepts in terms of sensitivity and response time. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-39 Federal Highway Administration 

Table 2-9. Comparison of sensitivities and response times of digital electronics units. 

Digital 

electronics 

unit type 

Sensitivity (SD) Response time Interpretation 

Frequency 

shift f D D

f SD = K L C t

f

 = frame time Sensitivity decreases when the 

inductance increases from the use of a 

larger loop or longer lead-in cable. 

Response time is fixed by the frame time 

of the electronics unit. 

Ratioed 

frequency shift 

fc

ft f

D N

N

S

2 = f

D D

ft f

mf S

N

t = 2 Sensitivity is independent of the 

inductance value at the input terminals to 

the electronics unit. Response time 

increases directly with an increase in 

inductance at the input terminals to the 

electronics unit and inversely with 

sensitivity of the unit. 

Period shift 

D D

p p

D L C

K

S =

p c D

D

T pt

p

f S

L

L N

t

⎥

⎦

⎤ ⎢

⎣

⎡ +

=

2 1 Sensitivity is increased by increase in 

size of loop and longer lead-in cable. 

However, the effect of a change in 

inductance is mitigated by a swamping 

inductance from a loosely coupled 

transformer. Response time is dependent 

on loop inductance. 

Ratioed period 

shift T

p

DS = 2S p

D

p pt

S

N t 2 = Sensitivity is independent of inductance 

value at the input terminals to the 

electronics unit. Response time is 

dependent on loop inductance. 

MULTICHANNEL DIGITAL MODELS 

Controller cabinet space can be conserved if the electronics unit can operate 

more than one loop. Most digital electronics unit manufacturers offer 

products that can operate four or more loops. Some models address the 

crosstalk problem by providing a frequency-separation switch, while others 

separate the loops by a time-division scanning process. 

One manufacturer’s scanning electronics unit energizes and analyzes each of 

four or more channels sequentially up to 100 times per second. The digital 

period shift electronics unit is inherently fast enough to permit scanning. The 

time to analyze a channel depends on the sensitivity desired, as high 

precision in thresholding demands more time for counting reference pulses. 

For example, if three 150 μH loops were connected and used with 

sensitivities of 1, 4, and 6, and the fourth channel was switched to off, then 

the four channels would require 2.3, 9, 63, and 0.9 ms respectively, for a total 

of 75 ms. Each channel would be energized and analyzed 1,000 ÷ 75 = 13 

times per second. The use of loops with larger inductance values reduces the 

scan rate, as does the selection of the highest sensitivity settings on the unit. 

If more than four similar and nearby loops are involved, the frequency switch 

or the size and/or number of turns in the loops can be varied to provide 

crosstalk protection. Equation 2-40 and Tables 2-2, 2-3, and 2-4 can be used 

to design frequency separations of 7 percent or more. 
Chapter 2—Sensor Technology 

October 2006 Page 2-40 Federal Highway Administration 

Manufacturers utilize higher clock speeds to provide faster scanning rates. 

For example, on the lower sensitivity settings, the sample time is 0.5 ms per 

channel. Therefore, the total time to scan all four channels is 2 ms. When a 

channel is switched off, the scan time for that channel is zero. 

ADDED FEATURES FOR DIGITAL ELECTRONICS 

UNITS 

During the l980s, several refinements were incorporated into inductive-loop 

detector digital electronics units. Recognizing the heavy demand on 

maintenance dollars, some manufacturers added circuitry that reduces the 

frequency of trouble calls to reset units attached to faulty loops. These 

features, intended to reduce maintenance costs and maximize traffic 

performance, include open loop test, automatic reset, and remote reset, as 

discussed below. 

Open Loop Test 

This feature allows the electronics unit to continue to operate an 

intermittently open loop system. A momentary open caused by a broken wire, 

poor splice, or loose connection will be stored in memory. If the connection 

remakes, the unit will promptly retune and continue to operate properly. If 

the open continues, it will result in a constant call. 

When making a service call to the intersection, the technician may observe an 

indicator light that will flash a distinct pattern if an open has occurred. With 

other brands of electronics units, the technician presses the “Open Loop Test” 

button to determine whether an opening has occurred since the last service 

call. The open loop memory can be queried repeatedly as it can be reset only by 

power interruptions (such as removing the module from its card rack and 

reinserting) or by pressing the common reset button on the electronics unit. 

This constitutes a system reset, which will clear the open loop memory. 

Automatic Reset 

Some electronics units can be programmed to generate an internal reset if a 

call (i.e., the output of the electronics unit) exceeds the programmed time. 

The reset is controlled by the termination of the associated phase green. One 

agency claims that this feature reduced their electronics unit maintenance 

costs by 42 percent. 

Remote Reset 

Remote reset allows automatic investigation of suspicious calls generated by 

computer or software-program control systems. A remote master monitoring the 

actuations of each system sensor may suspect that an electronics unit is 

malfunctioning. By asserting the Reset command, the unit can frequently be 

returned to normal operation. The reset causes presence calls to be cleared, but 

it does not clear open loop memory nor does it prevent an open loop from calling. 

If the reset fails to restore normal operation, the fault can be recognized and 

printed out for maintenance attention. An open loop that is constantly calling 

can be taken off line so that it does not falsely influence system operating 

parameters. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-41 Federal Highway Administration 

INDEPENDENT LOOP FAIL OUTPUT 

In addition to the normal output of the electronics unit, a second output for 

loop status is provided on some models. Whenever the loop inductance 

undergoes a step change of ± 25 percent or more, the loop fail output is 

turned on. If the inductance returns to a value less than ± 25 percent of the 

reference, the loop fail output turns off. This enables remote interrogation of 

the loop status. 

Other fault detection algorithms are embedded in the microprocessors found 

in modern controllers. These algorithms output digital codes that identify the 

fault type to a controller, which transmits the information to a central 

location. 

VEHICLE CLASSIFICATION 

Newer inductive-loop detector electronics units and loop configurations are 

capable of vehicle classification. The electronics module shown in Figure 2-28 

uses artificial neural network software to classify the traffic stream into the 

23 categories depicted in Figure 2-29. The first 13 are the standard FHWA 

classes, while the remaining ones represent vehicles with unique 

characteristics.(4) 

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

(Photograph courtesy of Reno A&E, Reno, NV). 

 
Chapter 2—Sensor Technology 

October 2006 Page 2-42 Federal Highway Administration 

Class 1 - Motorcycles Class 2 - Passenger Cars Class 3 - Two Axle, Four Tire

Single Units Class 4 - Buses

Class 5 - Two Axle, Six Tire

Single Units

Class 6 - Three Axle

Single Units

Class 7 - Four or More Axle

Single Units

Class 8 - Four or Less Axle

Single Trailers

Class 9 - Five Axle Single

Trailers

Class 10 - Six or More Axle

Single Trailers

Class 11 - Five or Less Axle

Multi-Trailers

Class 12 - Six Axle Multi-Trailers Class 13 - Seven or More Axle Multi-Trailers

Class 14 - Passenger Car (Class 2)

+Trailer

Class 15 - Class 3 +Trailer Class 16 - Class 5 +Trailer

Class 17 - Class 6 +Trailer Class 18 - Loaded Auto Carrier

Class 19 - Empty Auto Carrier Class 20 - Bobtail Tractor

(Semi Without Any Trailers)

Class 21 - Goose Neck Trailer or Moving Van

Class 22 - 30 ft Buses Class 23 - 20 ft Buses

Figure 2-29. Classes available from inductive-loop classifying sensor (Source: Model 

IVS-2000 Installation and Operation Manual, Rev. 1.53 (Intersection 

Development Corp, Downey, CA, Sept. 1997)). 

Special configurations of inductive loops have been developed to detect axles 

and their relative position in a vehicle. Such systems are used at toll plazas 

to elicit the correct payment for the vehicle class. In the application shown in 

Figure 2-30, an axle loop array is situated between two main loops. Axle 

presence is detected by the axle loop array. The relative position of the axles 

in the vehicle is determined from the signatures provided by the main loops. 

The data obtained are vehicle length, speed, acceleration, vehicle type, 

number of axles, and axle separation. Profile information can also be 

obtained to refine and validate classification in ambiguous cases. This 

electronics unit, as well as the one shown in Figure 2-28, can be used to 

identify transit buses and provide priority treatment at traffic signals. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-43 Federal Highway Administration 

Idris

Loop

1

Idris

Loop

2

Idris

Loop

3

Traffic Flow

Axle Loop

Array

Canopy

Vehicle Enforcement System

Camera/Light

Lane Center Line

Idris installation at toll plaza

Direction

Toll Booth

Camera lens-to-trigger distance

(Application dependent)

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

(Drawing courtesy of Peek Traffic, Inc.–Sarasota, Sarasota, FL). 

NEMA STANDARDS 

As with traffic signal controllers, loop detector electronics units were 

developed and marketed by numerous manufacturers, each using a different 

type of harness connector and detection technique. To overcome subsequent 

interchangeability problems, NEMA developed a set of standards known as 

“Section 7. Inductive-Loop Detectors.” These were released early in 1981. 

This section of the NEMA Standards defined functional standards, physical 

standards, environmental requirements, and interface requirements for 

several inductive-loop electronics unit configurations. 

Section 7 described only the basic functions associated with inductive-loop 

detector electronics units. Users identified the need for additional functions 

for specific locations, particularly delay and extension timing. To cover this 

gap, NEMA developed and in 1983 released “Section 11. Inductive-Loop 

Detectors with Delay and Extension Timing.” This section was basically 

identical to Section 7 with the addition of requirements for the timing of 

delayed call and extended call features. A further revision resulted in a new 

Section 15, which was released February 5, 1987 (a reproduction is provided 

in Appendix J). This new standard combines, updates, and supersedes 

Sections 7 and 11. 

NEMA ELECTRONICS UNIT CONFIGURATIONS 

The NEMA Standards define two basic types of electronics unit 

configurations: shelf mounted and card-rack mounted. Shelf mounted units 

are commonly used in NEMA controllers and are available in both singlechannel and multichannel (two- or four-channel) configurations. Figure 2-31 

shows an example of a shelf-mounted unit, which is powered by the 120-volt 

AC supply in the cabinet. Outputs are generated by electromechanical relays 

or by electrically isolated solid-state circuits. Physical dimensions and 

connector requirements are included in the NEMA Standards in Appendix J. 
Chapter 2—Sensor Technology 

October 2006 Page 2-44 Federal Highway Administration 

Card-rack mounted electronics units, illustrated in Figures 2-32 and 2-33, fit 

into a multiple card rack and operate with external 24-volt DC power 

generated in the rack assembly or elsewhere in the controller cabinet. These 

devices are an effective way to reduce cabinet space requirements where 

large numbers of inductive-loop detector electronics units are needed. 

OUTPUT TYPES 

Electronics units have one of two output types: relay or solid-state optically 

isolated. Relay outputs use electromechanical relays to generate a circuit 

closure and generate a detection call to the controller. Solid-state outputs 

have no moving parts and are, therefore, generally more reliable and more 

accurate in tracking the presence of vehicles. This factor can be important in 

some traffic signal timing operations. 

The relay outputs are designed to fail “on” (contacts closed) when power to 

the electronics unit is interrupted. The solid-state output fails “off” 

(nonconducting) in the same circumstances. Therefore, a relay output may be 

more desirable for use with intersection actuation because a constant-call 

would be safer than a no-call situation. A solid-state output is more desirable 

where accurate presence detection is desired. 

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

NEMA electronics units are freestanding shelf modules.They are 

connected to a NEMA-style 

controller using a cable that 

attaches to circular connectors on 

the electronics unit and the 

controller. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-45 Federal Highway Administration 

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

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

Card-rack mounted 

electronics units contain an 

edge connector. The card 

slides into the rack cage 

where the male edge 

connector fits snuggly into 

the female connector in the 

card cage. 
Chapter 2—Sensor Technology 

October 2006 Page 2-46 Federal Highway Administration 

PRESENCE AND PULSE MODES OF OPERATION 

Two modes of operation are selectable for each electronics unit channel: 

presence and pulse. The presence mode of operation provides a constant 

output while a vehicle is over the loop detection area. 

NEMA Standards require the electronics unit to sustain a presence output 

for a minimum of 3 minutes before tuning out the vehicle. Most units will 

maintain the call for periods up to 10 minutes. This mode is typically used 

with long loop installations on intersection approaches with the controller in 

the nonlocking detection memory mode. 

Nonlocking detection memory is a controller function whereby the controller 

retains a call (vehicle detection) only as long as the presence inductive loop is 

occupied (vehicles are passing over or stopped on the loop). The controller 

drops the call if the calling vehicle leaves the detection area. 

The pulse operating mode generates a short pulse (between 100 and 150 ms) 

each time a vehicle enters the loop detection area. Pulse operation is typically 

used when inductive-loop detectors are located well upstream of the 

intersection with the controller in the locking detection mode. That is, the 

controller does not drop the vehicle call when the calling vehicle leaves the 

detection area. Chapter 4 contains additional information about applications 

of locking and nonlocking controller operation. 

CROSSTALK 

When two loops constructed of the same wire diameter have the same loop 

dimensions, number of turns, and lead-in length, they have the same 

resonant frequency. When these two loops are near each other or when the 

lead-ins from these loops are in close proximity (perhaps running in the same 

conduit), a phenomenon known as “crosstalk” can occur. This effect is caused 

by an electrical coupling between the two loop channels and will often 

manifest itself as brief, false, or erratic actuations when no vehicles are 

present. 

NEMA Standards require inherent, automatic, or manual techniques to be 

utilized to prevent crosstalk. The most common feature is a frequency 

selection switch that varies the operating frequency of the adjacent loop 

channels. 

TIMING FEATURES 

As contained in Section 15 of the NEMA standards (Appendix J), the timing 

features include delay and extension timing. Delay timing can be set from 0 

to 30 seconds, indicating the time that the electronics unit waits, from the 

start of the continuous presence of a vehicle until an output begins, as shown 

in Figure 2-34. The output terminates when the vehicle leaves the detection 

area. If the vehicle leaves the detection area before the delay time has 

expired, no output is generated. Extension time, shown in Figure 2-35, 

defines the amount of time the output is extended after the vehicle leaves the 

detection area and can be set from 0 to 15 seconds. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-47 Federal Highway Administration 

Delay Operation – No Output Occurs

Delay Operation – Output Occurs

Time Set

Time Set

Vehicle present 

Vehicle not present 

Output conducting 

Output not conducting

Vehicle present 

Vehicle not present 

Output conducting 

Output not conducting

Figure 2-34. Delay operation. 

Time Set

Vehicle present 

Vehicle not present 

Output conducting 

Output not conducting

Figure 2-35. Extension operation. 

Timing features can be controlled by external inputs to the electronics unit. 

For units with relay outputs, a delay/extension inhibit input is provided and 

requires 110 volts to activate. Electronics units with solid-state outputs have 

a delay/extension enable input, which requires a low-state DC voltage (0 to 8 

volts). 

A typical delayed call installation might be a semiactuated intersection with 

heavy right-turns-on-red from the side street, which has long loop presence 

detection. Using a relay output electronics unit, the side street green field 

output (110 volts AC) is connected to the delay/extension inhibit input. Thus, 

a delay is timed, allowing right-turns-on-red to be made without 

unnecessarily calling the controller to the side street. (Heavy right-turn 

movements will bring up the green anyway, as the loop will be occupied by 

following vehicles.) However, when the side street has the green, the delay is 

inhibited, permitting normal extensions of the green. 

Conversely, extended call detectors could be used on high-speed approaches 

to an intersection operated by a basic (non-volume-density) actuated 

controller. Using this technique, the apparent zone of detection is extended, 

and different “gap” and “passage” times are created without the volumedensity controls (this does not, however, replace volume-density functions). 

The delay/extension enable input on a solid-state output unit could be tied to 

the controller’s “Phase On” output.(5) 
Chapter 2—Sensor Technology 

October 2006 Page 2-48 Federal Highway Administration 

TYPE 170 CONTROLLER SPECIFICATION 

Simultaneously with the evolution of the NEMA Standards, the States of 

California and New York developed the Type 170 controller specification.(6,7)

As with the NEMA Standards, the development of this new controller was a 

direct response to the problems of noninterchangeability. The Type 170 

system was to be interchangeable between all manufacturers supplying 

equipment for either state. 

Unlike the NEMA Standards, which standardize functions, the Type 170 

Specification standardizes hardware. The Type 170 system stipulates 

cabinet, controller, and all required accessories including sensors. A 170 

controller is shown in Figure 2-36. The California 170 system, shown in 

Figure 2-37, specifies a large, base-mounted cabinet with full component 

layout for 28 two-channel electronics units. The New York system features a 

smaller, pole-mounted cabinet with 14 two-channel electronics units. New 

York subsequently revised its specification to require a new microprocessor. 

This system, denoted as a Type 179, includes the same cabinet and 

electronics units as the original Type 170. 

Figure 2-36. Model 170 controller (Photograph courtesy of Lawrence A. Klein). 

All 170-system modules are shelf or card-rack mounted as the controllers 

are housed in standardized cabinets. The electronics units are mounted on an 

edge-connected, printed circuit board. One card slot in the cabinet will 

support two detector input channels. Cards supporting four detector input 

channels require two card slots. The electronics unit’s front panel has a hand 

pull for insertion and removal from the input file. 

2070 ADVANCED TRANSPORTATION CONTROLLER 

SPECIFICATION 

An Institute of Transportation Engineers (ITE) committee composed of 

members from industry, Federal Highway Administration, various States 

and cities, and NEMA developed the concept for the advanced transportation 

controller (ATC). The development of a functional standard was subsequently 

assigned to a committee composed of affiliates from the American Association 

of State and Highway Transportation Officials (AASHTO), ITE, and NEMA. 

This group was charged with completing a consensus-based standard.(8) 

The advanced transportation controller is designed to control or process data 

from one or more roadside devices. It operates as a general-purpose computer 

with a real-time operating system. One or more applications are stored in a 

FLASH drive. The application software is loaded and launched from the FLASH 
 Traffic Detector Handbook—2006 

October 2006 Page 2-49 Federal Highway Administration 

drive into dynamic random access memory (DRAM). The open architecture 

found in the ATC allows hardware modules and software to be purchased from a 

variety of vendors. A future central processor unit (CPU) option will support a 

daughter-board CPU that incorporates an applications program interface (API), 

allowing use of different microprocessors and real-time operating systems. 

Power Supply

Blank Panel

170 Controller Unit

Input File

Input File

24V, 5A

AC line regulation

AC load regulation

Dual or 4-channel electronics units

(Loop, magnetic, magnetometer)

or up to 14 isolation modules

for each file

Flash relay

Flasher

Duplex receptacle

6 load switches

2 flash transfer relays

210 monitor

7”

7”

7”

10”

19” Rack

6 load switches

2 flash transfer relays

Power Distribution Assembly

51⁄4”

51⁄4”

1 inch = 2.5 cm 

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

The California Department of Transportation (Caltrans) has continued 

development of the ATC and issued standards for the development of a Model 

2070 ATC.(9) 

The 2070 controller contains eight module types as follows: 

• Central processor unit (CPU). 

• Field input/output (I/O) module. 

• Front panel assembly (FPA). 

• Power supply. 

• Chassis. 

• Modem. 

• NEMA interface. 

• Back cover. 
Chapter 2—Sensor Technology 

October 2006 Page 2-50 Federal Highway Administration 

Figure 2-38 shows a 2070 ATC controller. An optional NEMA module may 

be mounted below the main chassis. Table 2-10 describes the options 

available for each of the modules.(9) 

Figure 2-38. Model 2070 controller. The 2070 ATC controller is manufactured by Econolite, 

Eagle, Naztec, Safetran, GDI Communications, and McCain Traffic Supply at present. 

Table 2-10. 2070 ATC module options. 

Module Option Description 

CPU 1A 

1B 

1C 

Two-board CPU using Motorola 68360 microprocessor and MicroWare OS-9 

operating system or PTS real-time Linux operating system. 

One-board CPU using Motorola 68360 microprocessor and MicroWare OS-9 

operating system or PTS real-time Linux operating system. 

Daughter-board CPU using applications program interface (API) to allow use of 

different processors and real-time operating systems. 

Field I/O 

module 

2A 

2B 

Isolates the EIA-485 internal signal line voltages between the CPU module and 

external equipment, and the drive and receive interfaces on external equipment. 

170 cabinet module: Field controller unit (FCU); 64-bit parallel I/O ports; other 

circuit functions (60-Hz square wave line synchronization signal, jumper to activate 

monitoring of the watchdog timer input signal (used with Model 210 cabinet monitor 

unit only), watchdog timer, 1-kHz reference signal, 32-bit millisecond counter, loss 

of communications message, logic switch to disconnect C12S connector upon 

command); EIA-485 serial communications; C1S, C11S, and C12S connectors; 

+12Vdc to +5Vdc power supply; any required software. Directly accommodates 64 

inputs and 64 outputs. 

2070 cabinet module: EIA-485 serial communications, DC power supply, C12S 

connector. Current serial bus #1 address scheme accommodates a maximum of 

120 inputs and 42 outputs. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-51 Federal Highway Administration 

Table 2-10. 2070 ATC module options—Continued

Module Option Description 

Front panel 

assembly 

3A 

3B 

3C 

FPA controller that interfaces with the CPU, 16-key pad for hexadecimal 

alphanumeric entry, 12-key pad for cursor control and action symbol entry, liquid 

crystal backlighted display of 4 lines with 40 characters per line, minimum character 

dimensions of 0.26-inch- (5.00-mm-)wide by 0.4- inch- (10.44-mm)-high, auxiliary 

switch, alarm bell. 

FPA controller that interfaces with the CPU, 16-key pad for hexadecimal 

alphanumeric entry, 12-key pad for cursor control and action symbol entry, liquid 

crystal backlighted display of 8 lines with 40 characters per line, minimum character 

dimensions of 2.65 mm wide by 4.24 mm high, auxiliary switch, alarm bell. 

No display, system serial port. 

Power supply 4A 

4B 

+5VDC (±5%) at 10 A, ±12VDC (±8%) at 0.5 A, 

+12VDC (±8%) at 1.0 A. 

+5VDC (±5%) at 3.5 A, ±12VDC (±8%) at 0.5 A, 

+12VDC (±8%) at 1.0 A. 

Chassis 5A 

5B 

Contains VME cage assembly compatible with 3U boards, metal housing, serial 

motherboard, backplane, power supply module supports, slot card guides, wiring 

harness, cover plates. 

Contains two-board CPU (option 1A) mounting assembly, metal housing, serial 

motherboard, backplane, power supply module supports, slot card guides, wiring 

harness, cover plates. 

Modem 6A 

6B 

Two channels of 300 to 1,200 baud asynchronous EIA-232 serial communications. 

Two channels of 0 to 9,600 baud asynchronous EIA-232 serial communications. 

Modem 7A 

7B 

Two channels of 300 to 1,200 baud asynchronous EIA-485 serial communications. 

Two channels of 0 to 9,600 baud asynchronous EIA-485 serial communications. 

NEMA interface 8 Contains four NEMA connectors and EIA-485 serial communications to interface 

with the 2B field I/O module. Accommodates 118 inputs and 102 outputs. 

Back cover 9 Protects the interface harness connecting the NEMA interface module with the 

2070 controller chassis. 

Five Intelligent Transportation System (ITS) cabinet styles are 

manufactured to house the 2070 ATC controller and associated electronics, 

which includes the cabinet monitoring system, power distribution system, 

control and data communications systems, inductive-loop detector electronics 

assemblies, batteries, and power supplies. The ITS cabinets incorporate three 

serial buses. The first bus supports command and response communications 

needed for real-time control of the modules in the cabinet. A serial interface 

unit (SIU) can be connected to serial bus #1 to accommodate 54 inputs or 

outputs, including detector calls or status reports. Alternatively, each SIU 

can support up to 14 load switches. Serial bus #1 also sends polling 

commands to the emergency cabinet monitor unit (CMU) located in the 

power distribution assembly. The function of the CMU is to query various 

cabinet conditions, such as power supply voltages and line synchronization 

signal, and if warranted, transfer control from the ATC to a safe control 

mode. The second bus supports command and response communications 

between the 2070 controller and the controlled field devices. Serial bus #3 

supports the cabinet emergency system. This system senses and monitors 

load outputs, various operational functions, and the bus controls and 

communications. When an abnormal operational condition is encountered, 

the cabinet emergency system controls cabinet emergency actions, through 

the CMU, and reports the condition to the ATC. 
Chapter 2—Sensor Technology 

October 2006 Page 2-52 Federal Highway Administration 

An advantage of serial communications between the controller and the field 

devices is the simplified wiring and connector utilized in the ITS cabinets as 

contrasted with the 104-pin C1S parallel connector found on the 170 

controller or the three NEMA connectors on TS-1 controllers. The C1S 

connection may still be made in the 2070 using the C1S connector found on 

the front panel of the 2070–2 field input/output (I/O) module. Any required 

NEMA connections may be made through the optional NEMA module, which 

is mounted below the 2070. 

The cabinets, constructed of aluminum sheet, contain front and rear doors. 

Designed to be rainproof, they are ventilated using a thermostatically 

controlled fan. Table 2-11 describes the cabinet styles recommended for 

traffic signal control and traffic management applications. 

Table 2-11. 2070 ATC ITS cabinet options. 

Application Model Description 

Traffic signal control1 340 

342 

346 

4-door cabinet with P-base ground mount 

2-door cabinet with 170-base ground mount 

2-door cabinet with 170-base adapter mount 

Traffic management1

(e.g., ramp metering) 

354 

356 

2-door cabinet with 170-base ground mount 

2-door cabinet with 170-base adapter mount 

1. The cabinets utilized for traffic signal control and traffic management differ in the types and 

numbers of cages, power supplies, power distribution and output assemblies, and serial bus 

harnesses. 

Five models of the 2070 ATC have been designated to incorporate the VME 

cage and cabinet options as shown in Table 2-12. 

Table 2-12. 2070 ATC models and cabinet options. 

Model number Description 

2070 V 

2070 N 

2070 L 

2070 containing a VME cage and mounted in a 170 or TS2 cabinet 

2070 containing a VME cage and mounted in a TS2 cabinet 

2070 without a VME cage and mounted in a 170 or TS2 cabinet 

2070 LC 2070 without a VME cage and mounted in an ITS serial communications 

cabinet 

2070 LCN 2070 without a VME cage and mounted in a TS2 cabinet 

Table 2-13 relates the 2070 ATC module options of Table 2-10 to the model 

and cabinet options of Table 2-12. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-53 Federal Highway Administration 

Table 2-13. 2070 ATC modules corresponding to model and cabinet options. 

Module 

designator 

Module description Model 

2070 V 

Model 

2070 N 

Model 

2070 L 

Model 

2070 LC 

Model 

2070 LCN 

— Unit chassis 1 1 1 1 1 

2070-1A Two-board CPU 1 1 – – – 

2070-1B One-board CPU – – 1 1 or 1 or 

2070-1C Daughter-board CPU – – – 1 1 

2070-2A Field I/O for 170 cabinet 1 – 1 – – 

2070-2B Field I/O for ITS and NEMA cabinets – 1 – 1 or none 1 or none 

2070-3A Front panel 4-line display 1 1 – – – 

2070-3B Front panel 8-line display – – 1 – – 

2070-3C No front panel display – – – 1 1 

2070-4A +5 VDC (±5%) at 10 A power supply 1 1 1 or 1 or 1 or 

2070-4B +5 VDC (±5%) at 3.5 A power supply – – 1 1 1 

2070-5A VME cage assembly 1 1 – – – 

2070-5B Two-board CPU mounting assembly 1 1 – – – 

2070-8 NEMA interface – 1 – – 1 

2070-9 2070 N back cover – 1 – – 1 

MODEL 222/224 INDUCTIVE-LOOP DETECTOR 

ELECTRONICS UNITS 

Chapter 5 of the California Department of Transportation (Caltrans) 

Transportation Electrical Equipment Specifications (TEES), (reproduced in 

Appendix K) provides a general description, functional requirements, and 

electrical requirements for the Model 222 Two-Channel Loop Detector 

Electronics Unit and Model 224 Four-Channel Loop Detector Electronics 

Unit. These were illustrated in Figures 2-32 and 2-33. 

Each channel in an electronics unit has panel-selectable sensitivity settings 

for presence and pulse operation. As with the NEMA standards, the TEES 

requires some mechanism to prevent crosstalk with other modules. It also 

requires that the selected channel not detect moving or stopped vehicles at 

distances of 3 ft (1 m) or more from any loop perimeter. The timing features 

are incorporated into the various Type 170 software programs such as the 

Caltrans Local Intersection Program (LIP). 

SELECTION FACTORS 

Several operational characteristics influence the selection of an electronics unit 

model. These characteristics depend on the application and its requirements. 

Tuning Range 

NEMA requires that an inductive-loop detector electronics unit be capable of 

tuning and operating as specified over a range of inductance from 50 μH to 

700 μH. For most applications, this range is adequate. Some units, however, 

are capable of tuning and operating over a range of 1 μH to 2,000 μH. This 

larger operating range permits extra long lead-in cables and/or several loops 

to be connected in series to one unit. 
Chapter 2—Sensor Technology 

October 2006 Page 2-54 Federal Highway Administration 

Response Time 

The time required for an electronics unit to respond to the arrival and 

departure of a vehicle is critical when the output is used to calculate speed 

and occupancy. Systems for the control of surface street traffic signals and 

monitoring of freeway conditions usually perform these calculations. 

If the time for vehicle call pick-up is close to the time for drop-out, little or no 

bias in the vehicle occupancy time is introduced. If there is a significant 

difference, but the difference is about the same from unit to unit, a correction 

for the bias is easy to apply. 

NEMA specifies that an electronics unit respond to the arrival or departure 

of a small motorcycle into and out of a 6- x 6-ft (1.8- x 1.8-m) loop within 125 

ms. An automobile call must be initiated or terminated within 50 ms. NEMA 

also states that shorter response times might be required for specific 

surveillance applications that involve vehicle speeds in excess of 45 mi/h (72 

km/h). 

Modern traffic management systems often demand small response times. 

Whereas speed and occupancy measurements averaged over many vehicles to 

a marginal accuracy were adequate in the past, current and future 

applications require faster response times with greater traffic flow parameter 

measurement accuracies. Many of the manufacturers are responding to this 

need by supplying new models of electronics units with enhanced 

capabilities. 

Recovery from Sustained Occupancy 

Recovery time can become critical when the electronics unit is operated in 

the presence mode with a long loop, say 6- x 50-ft (1.8- x 15-m), at the stop 

line, or four 6- x 6-ft (1.8- x 1.8-m) loops in a left-turn lane wired in a 

combination of series-parallel. NEMA requires that after a sustained 

occupancy of 5 minutes by any of the three test vehicles, the electronics unit 

shall recover to normal operation with at least 90 percent of the minimum 

specified sensitivity within 1 second after the zone of detection is vacated. If 

an electronics unit does not recover quickly enough, the next vehicle may not 

be detected at all and will be trapped until a new vehicle arrives on the loop. 

Loss of Detection During Saturated Flow 

During peak periods, a long loop or combination of loops may be held in 

detection without a break for an hour or more. In these situations, the 

electronics unit must continue outputting for at least an hour without 

dropping the detection because of an environmental tracking feature or other 

design defect. NEMA does not address this condition. 

Sensitivity with Pavement Overlay 

Depth of loop wires has traditionally been considered to be a critical factor 

when the pavement is overlaid. However, tests conducted in Texas suggest 

that with high sensitivity, proper installation, and calibration, the depth at 

which a loop is buried should have little effect on automobile detection. In 

these tests, a 6- x 6-ft (1.8- x 1.8-m), five-turn loop was buried at a depth of 

18.5 inch (50 cm) and encased in a 1.5-inch (1.2-cm) PVC conduit with no 

filler. There was no appreciable difference in the detection of large cars 

between the near-surface mounted loops and the deeply buried loops. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-55 Federal Highway Administration 

Bicycle detection was only slightly less efficient with deeply buried loops. The 

difference occurred at the medium sensitivity setting. The surface loop 

detected the bicycle 1 ft (0.3 m) outside the loop at the medium setting, while 

the deeply buried loop did not. The rectangular 6- x 6-ft (1.8- x 1.8-m) loop 

did not detect bicycles in the center portion of either the surface mounted or 

deeply buried loop regardless of the electronics unit sensitivity setting. 

Pulse-Mode Reset 

The pulse mode of operation provides an output (100 to 150 ms) that is useful 

in counting vehicles. An electronics unit should be capable of resetting or 

rephasing correctly to avoid either overcounts or undercounts when specific 

conditions occur. NEMA requires only that an electronics unit produce one, 

and only one, output pulse for a test vehicle moving at 10 mi/h (16 km/h) in 

the detection zone of a 6- x 6-ft (1.8- x 1.8-m) loop. 

When vehicles are counted simultaneously in two or more lanes, a separate 

electronics unit and loop are recommended for each lane to guarantee 

accuracy. This approach can be implemented for a small additional cost over 

the use of a single electronics unit and a wide loop. 

OPERATION WITH GROUNDED OR OPEN LOOPS 

Although NEMA does not address this point, some electronics units provide 

failsafe operation with faulty loops, i.e., grounded or open loops. The use of a 

loop isolation transformer permits these models to operate if the loop 

insulation is leaky or even shorted completely to ground at a single location. 

Isolation allows “balanced to ground” operation of the loop circuit. This 

reduces the effect of the loop and lead distributed capacitance and, hence, 

thermal- and moisture-induced changes in the capacitance. The balanced 

operation also minimizes loop circuit coupling from lead-in cables in common 

conduits. If the loop breaks, this design fails in a “safe” way by holding a 

constant call, thus keeping traffic moving and avoiding trapping any 

vehicles. Such operation is, however, very inefficient. 

Lightning Damage and Electrical Interference 

An extensive study into the effects of lightning related electrical surge 

problems was conducted by the Ontario Ministry of Transportation.(10) The 

major conclusions of the study were: 

• Most problems occurred on common grounding systems. 

• Isolated grounding techniques reduced maintenance problems by 

almost 80 percent. 

• If a system using isolated grounding is also upgraded to include both 

primary and secondary surge protection, maintenance problems are 

reduced by more than 90 percent. 

NEMA requires that electronics units withstand the same power-line 

transients specified for controller units. The input terminals to the 

electronics unit must be able to withstand 3,000 volts. The primary to 

secondary insulation of the input (loop-side) transformer protects against 

“common mode” lightning voltages. 
Chapter 2—Sensor Technology 

October 2006 Page 2-56 Federal Highway Administration 

With one electronics unit model, differential lighting protection consists of a 

four-element protective circuit composed of the input resistors, a neon tube, 

transformer leakage inductance, and the diode path across the secondary. 

Differential lightning-induced currents are also potentially damaging. These 

currents are shunted through neon bulbs, limiting the voltage across the 

transformer. 

Specifications for Model 170 controller cabinets require that lightning 

protection be installed within the inductive-loop detector electronics unit. 

The protection allows the electronics unit to withstand the discharge of a 10-

microfarad capacitor charged to ±1,000 volts directly across the electronics 

unit input pins with no loop load present. The protection must also withstand 

the discharge of a 10-microfarad capacitor charged to ±2,000 volts directly 

across either the electronics unit input inductance pins or from either pin to 

earth ground. The electronics unit chassis is grounded and a dummy resistive 

load of 5 ohms is attached to the pins. 

The Model 170 specifications also include provisions for preventing 

interference between channels in a given electronics unit as well as between 

units. The prevention techniques may be either manual or automatic. 

MAGNETIC SENSORS 

Two types of magnetic field sensors are used for traffic flow parameter 

measurement, the two-axis fluxgate magnetometer (that detects stopped and 

moving vehicles) and the magnetic detector, more properly referred to as an 

induction or search coil magnetometer (that typically detects only moving 

vehicles). Magnetic sensors were introduced in the l960s as an alternative to 

the inductive-loop detector for specific applications. A magnetic sensor is 

designed to detect the presence or passage of a vehicle by measuring the 

perturbation in the Earth’s quiescent magnetic field caused by a ferrous 

metal object (e.g., a vehicle) when it enters the detection zone of the sensor. 

An example of a magnetic sensor installation is shown in Figure 2-39. 

Normally 1” deep

Controller

cabinet

Conduit (optional)

1” diameter

or larger hole

Sand or other 

fill

1⁄4” wide sawcut and

holes filled with sealant

From 1 to 

several probes 

per lane

Pull boxes for

splicing probe

cable to lead-in

cable

1 inch = 2.5 cm 

Figure 2-39. Magnetic sensor installation. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-57 Federal Highway Administration 

Early magnetic sensors were utilized to determine if a vehicle had arrived at 

a “point” or small-area location. They actuated controller phases that 

operated in the locking detection memory mode. They were also effective in 

counting vehicles. Modern two-axis fluxgate magnetometers are used for 

vehicle presence detection and counting. Unlike the inductive-loop detector, 

the magnetometer will usually operate on bridge decks where uncoated steel 

is present and cutting the deck pavement for loop installation is not 

permitted. The magnetometer probe and its lead-in wire tend to survive in 

crumbly pavements longer than ordinary loops. Another benefit is that they 

require fewer linear feet of sawcut. 

THEORY OF OPERATION 

Magnetic sensor operation is based on an Earth magnetic field model that 

depicts the Earth as a large bar magnet with lines of flux running from pole 

to pole as illustrated in Figure 2-40. A vertical axis magnetometer requires 

the vertical component of the Earth’s magnetic field to exceed 0.2 Oersteds. 

Therefore, magnetometers that contain only vertical axis sensors cannot be 

used near the Equator, where the magnetic field lines are horizontal. This 

situation is illustrated in Figure 2-41 by the cross-hatched area near the 

equator, which defines the region that is not suitable for vertical axis 

magnetometers. However, modern fluxgate magnetometers are built with 

both horizontal and vertical axis sensors. Therefore, they can operate 

anywhere on the face of the Earth. 

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

Magnetic sensor operation is 

based on detecting the 

concentration of the Earth’s 

magnetic flux above and 

below a ferrous metal 

vehicle. 
Chapter 2—Sensor Technology 

October 2006 Page 2-58 Federal Highway Administration 

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. 

An iron or steel vehicle distorts the magnetic flux lines because ferrous 

materials are more permeable to magnetic flux than air. That is, the flux 

lines prefer to pass through the ferrous vehicle. As the vehicle moves along, it 

is always accompanied by a concentration of flux lines known as its 

“magnetic shadow” as illustrated in Figure 2-42. There is reduced flux to the 

sides of the vehicle and increased flux above and below it. A magnetometer 

installed within the pavement detects the increased flux below the vehicle. 

Modern fluxgate magnetometers 

built with both horizontal and 

vertical axis sensors can operate in 

the equatorial belt region shown in 

Figure 2-41. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-59 Federal Highway Administration 

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

TWO-AXIS FLUXGATE MAGNETOMETERS 

Two-axis fluxgate magnetometers contain sensors that detect both the 

vertical and horizontal components of the Earth’s magnetic field and any 

disturbances to them. One of the secondary windings in a two-axis fluxgate 

magnetometer senses the vertical component of the vehicle signature, while 

the other, offset by 90 degrees, senses the horizontal component of the 

signature. The horizontal axis of the magnetometer is usually aligned with 

the traffic flow direction to provide in-lane presence detection and adjacent 

lane vehicle rejection. Fluxgate magnetometers measure the passage of a 
Chapter 2—Sensor Technology 

October 2006 Page 2-60 Federal Highway Administration 

vehicle when operated in the pulse output mode. In the presence mode, they 

give a continuous output as long as either the horizontal or vertical signature 

exceeds a detection threshold. 

The data provided by fluxgate magnetometers are the same as from inductive 

loops. Typical applications are vehicle presence detection on bridge decks and 

viaducts where inductive loops are disrupted by the steel support structure 

or weaken the existing structure, temporary installations in freeway and 

surface street construction zones, and signal control. 

The infusion of modern digital processing and radio frequency (RF) 

communications technology in the area of magnetic anomaly detection have 

generated new designs, such as the self-powered vehicle detector (SPVD) and 

Groundhog magnetometers, justifying a reassessment of their supplementary 

role in vehicle detection. In addition, an array of magnetometers sharing a 

common signal processor has the potential to locate, track, and classify 

vehicles in a multilane scenario using a row of above-ground sensors.(11) 

MAGNETOMETER SENSOR AND COMPANION 

ELECTRONICS UNIT DESIGN 

Figure 2-43 shows the electrical configuration of the core and primary and 

secondary windings typically found in a magnetometer sensor. The core and 

windings create a small, stable transformer-like element. The two primary 

and two secondary windings are placed over a single strip of Permalloy core 

material, which is treated to produce special satur4able magnetic properties. 

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

Sensor operation is based on generating a second harmonic signal from the 

open saturable core, which is oriented to provide maximum sensitivity to 

disturbances in the Earth’s magnetic field. A triangular wave excitation 

current of suitable frequency (typically 5 kHz) is applied to the two primary 

windings connected in series opposition. The secondary windings, which also 

carry the DC bias to neutralize the quiescent Earth field, are connected in 

series, enhancing the supply of second-harmonic signal to the sensor’s 

electronics unit. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-61 Federal Highway Administration 

A polyurethane casing is often used for abrasion resistance. It also makes the 

probe impervious to moisture and chemically resistant to all normal motor 

vehicle petroleum products. If the pavement is soft, some probes may tilt, 

causing them to change their orientation and lose sensitivity. A length of 

PVC pipe can be used to hold these probes vertical. 

Figure 2-44 contains an electrical circuit diagram of a magnetometer sensor 

and companion electronics unit that performs signal amplification. Provided 

the ambient magnetic field is stable and exceeds about 16 ampere per meter 

(20,000 gamma), the magnetic shadow cast by a vehicle causes a local field 

increase of the order of 20 percent. The switch-like action of the magnetic 

material in the sensor induces a signal change several times this amount. 

This detection principle provides high sensitivity and signal-to-noise ratio. 

Regulated 

DC Bias 

Supply

Input 

Amplifier

Phase

Detector

DC

Amplifier

Controller

Interface

Frequency

Divider (÷2)

Integrator

Roadway Probes

Field Terminals

Reference

Oscillator

Excitation

Driver

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

MAGNETOMETER DETECTION SENSITIVITY 

Since magnetometers are passive devices, they do not transmit an energy 

field. Therefore, a portion of the vehicle must pass over the sensor for it to be 

detected. Consequently, a magnetometer can detect two vehicles separated 

by a distance of 1 foot (0.3 m). This potentially makes the magnetometer as 

accurate as or better than the inductive-loop detector at counting vehicles. 

Conversely, the magnetometer is not a good locator of the perimeter of the 

vehicle, with. There is an uncertainty of about ±1.5 ft (45 cm). A single 

magnetometer is, therefore, seldom used for determining occupancy and 

speed in a traffic management application. Two closely spaced sensors are 

preferred for that function. 
Chapter 2—Sensor Technology 

October 2006 Page 2-62 Federal Highway Administration 

Magnetometers are sensitive enough to detect bicycles passing across a 4-ft 

(1.2-m) span when the electronics unit is connected to two sensor probes 

buried 6 inches (16 cm) deep and spaced 3 ft (0.9 m) apart. Magnetometers 

can hold the presence of a vehicle for a considerable length of time and do not 

exhibit crosstalk interference. Vehicle motion is not required for detection by 

two-axis fluxgate magnetometers. 

One channel of an electronics unit can support as many as 12 seriesconnected sensors. However, the sensitivity is divided among the probes so 

that there is a loss in sensitivity per probe when more than one is used per 

channel. The electronics unit will detect with a vehicle over one out of five 

probes in the series (i.e., 20 percent sensitivity on each probe). 

ELECTRONICS UNIT 

Specific magnetometer electronics units are matched with various sensor 

probes. Some units provide only signal amplification and relay or solid statestate outputs that indicate the passage or presence of a vehicle, while others 

add additional features such as background compensation and sensitivity 

selection. Some include 2 or more independent detection channels, which can 

accept 1 to 12 series-connected sensors on each channel. 

Calibration procedures are utilized to compensate the electronics unit for the 

magnetic environment around the roadway sensors. A check is made prior to 

tuning to assure that no vehicles or movable ferrous equipment are within 

20 ft (6 m) of a sensor. Some magnetometer electronics units include, for each 

channel, a calibration knob that is turned until a pilot light flashes, 

indicating that the ambient magnetic field has been neutralized. Some 

magnetometer sensors do not offer a delayed-call timing feature. 

Four operating modes are typically available. These are selected on the front 

panel of the electronics unit and include: 

• Presence—Output is maintained throughout the time the vehicle is 

over the sensor probe. Hold time is unlimited. 

• Extended Presence—Output is held for a preselected time interval of 

up to 5 seconds after each vehicle departs. 

• Pulse—A single output occurs during the first 40 ms a vehicle 

presence is sensed. A subsequent output cannot occur while that 

presence is maintained. 

• Inhibited Pulse—Subsequent output pulses are inhibited for a 

preselected time up to 5 seconds after each vehicle departs. 

This eliminates multiple pulses from trailers and long vehicles. 

MODEL 227/228 MAGNETOMETER SYSTEM 

Chapter 6 of the Caltrans Type 170 Specification contains specifications for 

the Model 227 magnetometer sensing element and the Model 228 twochannel electronics unit. The electronics unit is defined only for a twochannel version. Each channel operates one to six Model 227 sensors. The 

electronics unit produces an output signal whenever a vehicle passes over 

one or more of the sensors. 

Since a magnetometer measures the passage or presence of a vehicle, two 

modes of operation are provided by the electronics unit: pulse, which gives an 

output closure of 125 ± 25 ms for each vehicle entering the zone of detection, 
 Traffic Detector Handbook—2006 

October 2006 Page 2-63 Federal Highway Administration 

and presence, which gives a continuous output as long as a vehicle occupies 

the detection zone. 

SPVD TWO-AXIS FLUXGATE MAGNETOMETER 

Figure 2-45 shows the self-powered vehicle detector. Applications include 

permanent and temporary installations on freeways and surface streets or 

where mounting under bridges or viaducts is desired. The SPVD-2 

approximates the shape of a 5-inch (13-cm) cube and fits into a cylindrical 

hole of 6-inch (15-cm) diameter and about 7-inch (18-cm) depth. The upper 2 

inches (5 cm) of the hole is filled with cold patch or other sealant that can be 

removed when the battery needs replacing. 

This two-axis fluxgate magnetometer has a self-contained battery and 

transmitter that broadcasts passage or presence information at 47 MHz over 

a 400- to 600-ft (122- to 183-m) range to a receiver that can be located 

remotely in a controller cabinet. The transmitting frequency can be selected 

to operate in a quiet frequency band, i.e., one where other licensed and 

unlicensed devices are not operating in the local region. A direct connection 

(lead-in cable) is not required—a transmitting antenna is built into the 

housing that encloses the magnetometer electronics and battery. When 

operated in the presence mode, presence is maintained with a voltage 

latching circuit for as long as the vehicle is in the detection zone. The 

sensitivity can be adjusted to restrict vehicle detection to the lane in which 

the device is installed. 

SPVD-2 sensor/

transmitter

One-channel

receiver

Type 170 1-2 channel

receiver NEMA TS-1 style 

1-4 channel receiver

Figure 2-45. SPVD-2 magnetometer system (Photograph courtesy 

of Midian Electronics, Tucson, AZ). 

The SPVD is microprocessor controlled and self-calibrating, i.e., it can be 

installed and adapt to a location in either the Northern or Southern 

Hemisphere. The battery life is a function of traffic volume and battery type. 

With an average of 10,000 arrivals and 10,000 departures per day, the 

alkaline battery is quoted by the SPVD manufacturer as lasting 

approximately 4 years. Turning off the departure pulse adds another year or 

two of battery life. A low battery warning is transmitted to the receiver to 

indicate that 5 to 6 months of battery life remain. 
Chapter 2—Sensor Technology 

October 2006 Page 2-64 Federal Highway Administration 

GROUNDHOG MAGNETOMETER 

Figure 2-46 illustrates the Groundhog magnetometer sensors. The G-1 fits 

into a hole of 4.5-inch diameter and 7.5-inch depth (115-mm diameter x 190-

mm depth). The G-2 models require a 6.75-inch diameter hole with 7.5-inch 

depth (172-mm diameter x 190-mm depth). A shorter G-3 model series was 

designed for installation under a bridge deck. These models had the same 

functionality as the G-1 and G-2 models. The G-4 series sensors are designed 

to fit into the G-3 housing, which requires a 6-inch diameter hole of 3.25-inch 

depth (152.4-mm diameter x 82.6-mm depth). 

Buried in the roadway, the G-1 and G-2 sensors transmit their data over the 

908 to 922 MHz spread spectrum band to a local base unit located within 200 

m (656 ft) of the sensor. The G-4 series sensors transmit data using the 2.45 

GHz spread spectrum band. The base unit can be powered from batteries 

recharged by solar energy. 

Groundhog magnetometers incorporate a bridge circuit that is balanced to 

output zero voltage in the absence of a vehicle and a nonzero voltage when a 

vehicle enters the detection zone. The G-1 model provides volume, lane 

occupancy, and road surface temperature. The G-2 adds vehicle speed 

reported in up to 15 bins, vehicle class in terms of vehicle length reported in 

up to 6 bins, and a wet/dry pavement indicator. In addition to the previously 

mentioned traffic parameters, the G-2wx provides chemical analysis for 

measuring the quantity of anti-icing chemicals on the road surface. The G-4C 

provides vehicle count only, while the G-4CS provides vehicle count, speed, 

and classification. The G-4WX adds environmental monitoring of road 

surface temperature from –67 °Fahrenheit (F) to 185 °F (–55 ° Celsius (°C) to 

85 °C) and road surface wet or dry condition to the G-4CS data set. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-65 Federal Highway Administration 

 G-1 

Removable Cap

Dual Silicon

O-Ring

SpreadSpectrum

Dual Sideband

Transmitter

Removable

Electronic

Module

Lithium Battery

Pack > 5 Yr Life

3.6V@60Ah

1/2 Conformal

Patch Antenna

Vertically Polarized

Permanently

Mounted

PVC Casing

Casing Size:

6-in Dia x 6.75 in

(152-mm Dia x 172 mm)

Nitrogen Sealed

Electronic

Compartment

Vehicle

Magnetic Imaging 

Sensor

 G-2 and G-2wx 

Figure 2-46. Groundhog magnetometer sensors (Photographs courtesy 

of Nu-Metrics, Uniontown, PA). 

The G-1 sensor is powered from 2 lithium batteries with a quoted battery life 

of 5 years based on 18,000 cars/day and report intervals of 2 minutes. The G2 requires 4 lithium batteries with a quoted battery life of 5 years based on 

18,000 cars/day and report intervals of 1 minute. The G-4 also operates from 

4 lithium batteries for up to 5 years, depending on annual average daily 

traffic (AADT) and polling interval. 

MAGNETIC DETECTORS 

The magnetic detector is a simple, inexpensive and rugged device that is 

capable of only a pulse output. It can be used for traffic-actuated signal 

control or to simply count vehicles. Some magnetic sensors are installed 

inside a nonferrous conduit by boring under the roadway. Others are 

mounted under bridges or in holes cored into the road surface. 

THEORY OF OPERATION 

Magnetic detectors, i.e., those based on induction or search coil 

magnetometers, also respond to perturbations in the Earth’s magnetic field 

produced by vehicles passing through the detection zone. The axis of the coil 

in all magnetic detectors is installed perpendicular to the traffic flow and has 

an associated spherical detection zone. The distortion and change of the 

Removable Cap

Dual Silicon

O-Ring

SpreadSpectrum

Dual Sideband

Transmitter

Removable

Electronic

Module

Lithium Battery

Pack > 5 Yr Life

3.6V@30Ah

1/2 Conformal

Patch Antenna

Vertically Polarized

Permanently

Mounted

PVC Casing

Casing Size:

4-in Dia x 6.75 in

(102-mm Dia x 172 mm)

Nitrogen Sealed

Electronic

Compartment
Chapter 2—Sensor Technology 

October 2006 Page 2-66 Federal Highway Administration 

magnetic flux lines with respect to time induces a small voltage that is 

amplified in an electronics unit located in the controller cabinet. This signal 

is interpreted by the controller as the passage of a vehicle and produces a call 

if appropriate. 

Magnetic detectors contain a highly permeable magnetic core on which 

several coils are wound, each with a large number of turns of fine wire, and 

connected in series. Disturbed lines of magnetic flux cut the turns of the coil 

and create an output for as long as the vehicle is in motion through the 

detection zone. Minimum speeds of 3 to 10 mi/h (5 to 16 km/h) are required to 

produce an actuation. A single magnetic detector does not detect stopped 

vehicles; therefore, it cannot be used as a presence detector. However, 

multiple units of some devices can be installed and used with specialized 

signal processing to generate vehicle presence. 

Magnetic detectors respond to flux changes in the lane under which they are 

buried and to flux changes in adjacent lanes. However, the signal processing 

is designed to ignore the lower magnitude signals generated in adjacent lanes 

and analyze only the larger signals produced by vehicles in the lane 

containing the sensor. 

These detectors provide volume, occupancy, and speed data based on the 

detection zone size and an assumed vehicle length. The historical criteria for 

their selection are traffic volume accuracy, sensitivity, output data rate, no 

requirement to detect stopped vehicles, and cost. Magnetic detectors are well 

suited for snow-belt States where deteriorated pavement and frost break 

wire loops and where subsurface sensors are desired but the pavement 

cannot be cut.(2) They also perform well in hot climates where asphalt 

pavements can become soft from the sun heat load. 

Magnetic detector models differ in their installation and size. Summary 

specifications of several models are given below. 

MODEL 231/232 MAGNETIC DETECTOR SYSTEM 

Specifications for the Model 231 sensing element and Model 232 two-channel 

electronics unit are found in Chapter 5 of the Caltrans TEES document (see 

Appendix K). 

Each individual sensor channel and its associated magnetic sensing element 

operate independently and produce an output signal when a vehicle passes 

over the embedded sensing element. The solid-state electronics unit, located 

in the controller cabinet, activates the controller by amplifying the voltage 

induced in the sensing element by a passing vehicle. 

The Model 231 sensor, shown in Figure 2-47, is installed by tunneling under 

the roadway 14 to 24 inches (356 to 610 mm) and inserting the sensor into a 

nonferrous plastic or aluminum conduit. In bridge or overpass installation, 

the sensor is fastened to the understructure directly below the center of the 

traffic lane. The sensor has a diameter of 2.25 inches (57 mm) and a length of 

21 inches (533 mm). The speed data, available from the electronics modules, 

are based on a detection zone length of 6 ft (1.8 m) and an assumed vehicle 

length of 18 ft (5.5 m). 
 Traffic Detector Handbook—2006 

October 2006 Page 2-67 Federal Highway Administration 

 

231E Detector Probe 232E Detector Electronics 

Figure 2-47. Magnetic probe detector (Photographs courtesy 

of Safetran Traffic Systems, Colorado Springs, CO). 

FLUSH-MOUNTED MAGNETIC DETECTOR 

Another type of passive magnetic detector is flush-mounted with the surface 

of the roadway. It is approximately 3 x 5 x 20 inches long (76 x 127 x 508 mm) 

encased in a cast aluminum housing. 

MICROLOOP PROBES 

Figure 2-48 depicts other forms of magnetic detectors called microloop 

probes. The Model 701 probe is inserted into 1-inch (25-mm) diameter holes 

bored to a depth of 16 to 24 inches (406 to 610 mm). The Model 702 probe is 

inserted into 3-inch (76-mm) Schedule 80 PVC placed 18 to 24 inches (457 to 

610 mm) below the road surface using horizontal drilling from the side of the 

road. Often two or more microloop probes are connected in series or with 

conventional wire loops to detect a range of vehicle sizes and obtain required 

lane coverage. The Model 702 microloop probe can be connected in rows of 

three to generate signals that detect stopped vehicles. Application-specific 

software from 3M is also needed to enable stopped vehicle detection. 
Chapter 2—Sensor Technology 

October 2006 Page 2-68 Federal Highway Administration 

 

 

Model 701 Model 702 

Figure 2-48. Microloop probes (Photographs courtesy of 3M Company, St. Paul, MN). 

VIDEO IMAGE PROCESSORS 

The use of video and image processing technology as a substitute for 

inductive-loop detectors was first investigated during the mid-1970s. This 

research was funded by the U.S. Federal Highway Administration (FHWA) 

and performed by the Jet Propulsion Laboratory. The project combined 

television camera and video processing technology to identify and track 

vehicles traveling within the camera’s field of view.(12) During the 1970s and 

1980s, parallel efforts were undertaken in Japan, the United Kingdom, 

Germany, Sweden, and France. These investigations addressed the problems 

and limitations of existing roadway sensors in attempting to fulfill 

requirements for state-of-the-art control of traffic and detection of incidents. 

As a follow-on to the initial FHWA project, a video image processor was 

developed by University of Minnesota research personnel.(13) Dubbed the 

Video Detection System (VIDS), it was jointly funded by FHWA, Minnesota 

Department of Transportation, and the University of Minnesota. The sensor 

provided volume and occupancy data equivalent to those from multiple 

inductive-loop detectors. Full bandwidth imagery and the traffic data were 

transmitted to a central location for interpretation and management of 

traffic. 

VIDEO IMAGE PROCESSING VEHICLE DETECTION 

CONCEPTS 

Three classes of VIP systems are now produced or in development: tripline, 

closed-loop tracking, and data association tracking. Tripline systems, as used 

by the sensors in Figures 2-49a, 2-49b, 2-49c, and 2-49d, operate by allowing 

the user to define a limited number of linear detection zones on the roadway 

in the field-of-view of the video camera. When a vehicle crosses one of these 

zones, it is identified by noting changes in the properties of the affected 

pixels relative to their state in the absence of a vehicle. Tripline systems that 

estimate vehicle speed measure the time it takes an identified vehicle to 

traverse a detection zone of known length. The speed is found as the length 

divided by the travel time. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-69 Federal Highway Administration 

Surface-based and grid-based analyses are utilized to detect vehicles in 

tripline VIPs. The surface-based approach identifies edge features, while the 

grid based classifies squares on a fixed grid as containing moving vehicles, 

stopped vehicles, or no vehicles. 

Figure 2-49. Tripline video image processors. 

The advent of VIP tracking sensors has been facilitated by low-cost, highthroughput microprocessors. Closed-loop tracking systems, as used in some 

VIPs, as illustrated in Figure 2-50, detect and continuously track vehicles 

along larger roadway sections. The tracking distance is limited by the field of 

view, mounting height, and resolution of the camera. Multiple detections of 

the vehicle along the track are used to validate the detection and improve 

speed estimates. Once validated, the vehicle is counted and its speed is 

updated by the tracking algorithm.(14) These tracking systems may provide 

additional traffic flow data such as lane-to-lane vehicle movements. Thus, 

they have the potential to transmit information to roadside displays and 

radios to alert drivers to erratic behavior that can lead to an incident. Their 

ability to monitor turning movements on arterials may allow more frequent 

updating of signal timing plans. 

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

Data association tracking systems identify and track a particular vehicle or 

groups of vehicles as they pass through the field of view of the camera. The 

computer identifies vehicles by searching for unique connected areas of 

 (a) Autoscope 2004 (b) Autoscope Solo 

(Photographs courtesy of Econolite Control Products, Anaheim, CA) 

(c) Traficon VIP 3 (Photograph courtesy of 

Traficon, Heule, Belgium)

(d) Iteris Vantage processors 

(Photograph courtesy of Iteris, Anaheim, CA) 
Chapter 2—Sensor Technology 

October 2006 Page 2-70 Federal Highway Administration 

pixels. These areas are then tracked from frame-to-frame to produce tracking 

data for the selected vehicle or vehicle groups.(15–17) Objects are identified by 

markers that are derived from gradients and morphology. Gradient markers 

utilize edges, while morphological markers utilize combinations of features 

and sizes that are recognized as belonging to known vehicles or groups of 

vehicles.(18) Data association tracking can potentially provide link travel time 

and origin-destination pair information by identifying and tracking vehicles 

as they pass from one camera’s field of view to the next camera’s field of view. 

Modern video image processors conserve transmission bandwidth by 

performing signal processing in microprocessors located either in the camera 

(as for example in the Autoscope Solo) or in modules mounted in a controller 

cabinet at the roadside as shown on the upper shelf in Figure 2-51. The data 

are used locally by the controller for signal timing and to characterize the 

level of service on freeways, for example. The data can also be transmitted to 

the traffic operations center over low-bandwidth communications media for 

incident detection and management, database update, and traveler 

information services. 

By multiplexing video images from several cameras on one transmission line 

and sending the video only when requested, operating costs associated with 

leased transmission media are further reduced. By extending the 

identification of vehicles or groups of vehicles from the field of view of one 

camera into that of another, vehicles can be tracked over greater distances to 

compute link travel times and generate origin-destination pair data. The 

output products from the traffic management center can be forwarded to 

other transportation management centers, emergency service providers, and 

information service providers. 

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

(Photograph courtesy of Iteris, Anaheim, CA). 

SIGNAL PROCESSING 

Image formatting and data extraction are performed with firmware that 

allows the algorithms to run in real time. The hardware that digitizes the 

video imagery is commonly implemented on a single formatter card in a 

personal computer architecture or in microprocessors located in the camera 

housing. Once the data are digitized and stored by the formatter, spatial and 
 Traffic Detector Handbook—2006 

October 2006 Page 2-71 Federal Highway Administration 

temporal features are extracted using a series of image processing algorithms 

to differentiate vehicles from roadway or background pixels. 

In the concept illustrated in Figure 2-52, a camera is used to acquire imagery 

of the traffic flow. The images are digitized and stored in memory. A 

detection process establishes one or more thresholds that limit and segregate 

the digitized data passed on to the rest of the image processing algorithms. It 

is undesirable to severely limit the number of potential vehicles during 

detection, for once data are removed they cannot be recovered. Therefore, 

false vehicle detections are permitted at this stage since the declaration of 

actual vehicles is not made at the conclusion of the detection process. Rather 

algorithms that are part of the classification, identification, and tracking 

processes still to come are relied on to eliminate false vehicles and retain the 

real ones.(19) Image segmentation is used to divide the image area into 

smaller regions (often composed of individual vehicles) where features can be 

better recognized. The feature extraction process examines the pixels in the 

regions for preidentified characteristics that are indicative of vehicles. When 

a sufficient number of these characteristics are present and recognized by the 

processing, a vehicle is declared present and its flow parameters are 

calculated. 

Detection

Feature

Extraction

Classification and

Identification Tracking Image

Segmentation

Image Digitization

and Storage

Traffic Under

Observation Camera

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

(Source: L.A. Klein, Sensor Technologies and Data Requirements for ITS

(Artech House, Norwood, MA, 2001)). 

Artificial neural networks are another form of processing used to classify and 

identify vehicles, measure their traffic flow parameters, and detect 

incidents.(20) Features are not explicitly identified and sought when this 

processing approach is used. Rather an electrical network that emulates the 

processing that occurs in the human brain is trained to recognize vehicles. 

The digital imagery is presented to the trained network for vehicle 

classification and identification. 

VIPs with tracking capability use Kalman filtering techniques to update 

vehicle position and velocity estimates.(21) The time trace of the position 

estimates yields a vehicle trajectory. By processing the trajectory data, local 

traffic parameters (e.g., flow and lane change frequency) can be computed. 

These parameters, together with vehicle signature information (e.g., time 

stamp, vehicle type, color, shape, position, and speed), can be communicated 

to the traffic management center.(22) Tracking vehicle subfeatures. such as 

edges, corners, and two-dimensional patterns, rather than entire vehicles, 

has been proposed to make the VIP robust to partial occlusion of vehicles in 

congested traffic. Preliminary results with this technique show insensitivity 

to shadows since shadow subfeatures tend to be unstable over time, 

especially in congestion. Camera mounting position and continued full 

occlusion of smaller vehicles by trucks still deteriorate performance.(23,24)
Chapter 2—Sensor Technology 

October 2006 Page 2-72 Federal Highway Administration 

A signal processing technique implemented by Computer Recognition 

Systems (Wokingham, Berkshire, England; Knoxville, TN) incorporates 

wireframe models composed of line segments to represent vehicles in the 

image. This approach claims to provide more unique and discriminating 

features than other computationally viable techniques. 

The artificial neural network approach is incorporated by Nestor Traffic 

Systems, Inc. (Providence, RI) in their VIP products. An advantage of the 

Nestor implementation is that the camera can be repositioned for data 

acquisition and surveillance.(25) VIPs that utilize tracking offer the ability to 

warn of impending incidents due to abrupt lane changes or weaving, 

calculate link travel times, and determine origin-destination pairs. The 

tracking concept is found in the VideoTrak 905 and 910 by Peek TrafficTransyt, the Traffic Analysis System by Computer Recognition Systems, 

MEDIA4 developed by Citilog (Paris, France), and the IDET-2000 by 

Sumitomo (Japan). 

PERFORMANCE 

VIP signal processing is continually improving its ability to recognize 

artifacts produced by shadows, illumination changes, reflections, inclement 

weather, and camera motion from wind or vehicle-induced vibration. 

However, artifacts persist and the user should evaluate VIP performance 

under the above conditions and other local conditions that may exist. In their 

1998 report to the TRB Freeway Operations Committee, the New York State 

Department of Transportation (NYSDOT) stated that one VIP model had 

difficulty detecting vehicles on a roadway lightly covered with snow in good 

visibility. Another model did not experience this problem. A 2004 evaluation 

of VIP performance by Purdue University described significant false and 

missed vehicle detections as compared with loops, even when the cameras 

were installed at vendor-recommended locations at a well-lighted 

intersection.(24) 

Figure 2-53 illustrates the effect day-to-night illumination change has on VIP 

performance. The VIP cameras viewed downstream traffic from a mast arm 

position approximately 25 ft (7.6 m) above the curb and middle lanes of a 

three-lane roadway. Shortly after 1,900 hours, there are changes in the 

slopes of the VIP vehicle count data due to either degradation in performance 

of the daytime algorithm or the different performance of the nighttime 

algorithm.(11) 
 Traffic Detector Handbook—2006 

October 2006 Page 2-73 Federal Highway Administration 

VIP 2

VIP 3b

VIP 1

VIP 4c

Time of Day

1600 1700 1800 1900 2000 2100 2200

2000

1500

1000

500

0

b Software eventually modified to improve nighttime performance.

c This VIP was not designed to count vehicles. It produced a

presence output every 1 second that indicated if one or more

vehicles had been detected during the previous 1-second interval.

Vehicle Count

Sensor Counts % Differencea

Square ILD 2 1600 0

Round ILD 1 1603 +0.2

Round ILD 2 1608 +0.5

VIP 1 1536 -4.0

VIP 2 1681 +5.1

VIP 3b 1339 -16.3

VIP 4c 1246 -22.1

a Computed with respect to square 

ILD 2

Surface street arterial evaluation site. Warm, dry weather. Curb lane.

Square and

round ILDs

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

(source: L.A. Klein and M.R. Kelley, Detection Technology for IVHS: Final Report, 

Report No. FHWA-RD-95-100 (USDOT, FHWA, Washington, D.C.,1995)). 

Figure 2-54 shows the effects of day-night illumination changes from another 

VIP test.(26) The camera faced upstream traffic, mounted approximately 25 ft 

(7.6 m) above the road surface. The camera was placed on a mast arm 

extension on the left side of a roadway, which contained three through lanes 

and a dedicated right-turn lane. The VIP tracked vehicles and provided flow 

rate, lane speed, queue length, density, number of left and right turning 

vehicles, and approach stops. VIP performance was not optimized because of 

a number of factors: low camera sensitivity that was more problematical at 

low light levels, focal length of lens not matched to the viewing distance, low 

camera resolution, low and offcenter camera mounting height, inadequate 

video signal output from camera to drive both the VIP and video cassette 

recorder, no sun shade, and camera vibration with winds greater than 10 

mi/h (16 km/h). 

Because of these nonideal conditions, a quantitative comparison of the VIPgenerated traffic flow parameters with ground truth is not given here. 

Instead, the focus is on the effect that illumination changes have on VIP 

performance, albeit exaggerated by many of the conditions cited above. The 

effects of low camera sensitivity and inadequate video signal are most 

apparent at dawn, night, and dusk where the false alarm and missed vehicle 

errors (0.15, 0.44, and 0.91, respectively) are largest. These results 

demonstrate the importance of proper camera and lens selection, camera 

mounting, and illumination in maximizing VIP performance. 
Chapter 2—Sensor Technology 

October 2006 Page 2-74 Federal Highway Administration 

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1600

1400

1200

1000

800

600

400

200

0

Number of Vehicles

Error (fraction)

Avg-Day Avg-Night Avg-Dawn Avg-Late p.m. Dusk Total All

Conditions

Error including all false alarms and 

missed vehicles

Ground truth: Total number of vehicles 

during each condition

Lighting Condition

0.19

0.09

0.91

0.44

0.13

540

133

367 367

65

1472

0.15

•

•

• •

•

•

•

Figure 2-54. VIP vehicle count errors under varying illumination conditions. Low camera sensitivity, 

improper focal length of lens, low camera resolution, nonideal camera mounting height, 

inadequate video signal, and lack of sun shade contributed to degraded performance 

(source: Traffic Surveillance and Detection Technology Development: Phase II and 

III Test Results, Troy, MI, JPL Pub. D-15779 (California Institute of Technology, 

Jet Propulsion Laboratory, Pasadena, CA, May 1998)). 

Heavy congestion that degraded early VIPs does not appear to present as 

great a problem to more modern systems. Combined results for clear and 

inclement weather show vehicle volume, speed, and occupancy measurement 

accuracies in excess of 95 percent using a single detection zone and a camera 

mounted at a sufficiently high height. VIPs with single or multiple detection 

zones per lane can be used to monitor traffic on a freeway. For signalized 

intersection control, where vehicle detection accuracies of 100 percent are 

desired, the number of detection zones per lane is increased to between two 

and four, depending on the camera mounting and road geometry. However, 

even with multiple detection zones, vehicle detection accuracy can degrade to 

85 percent or less with side-viewing cameras that are not mounted high 

enough (on the order of 30 ft (9 m) rather than 50 ft (15 m)) or are not 

directly adjacent to the roadway.(27–29) The study that produced these results 

also reported that vehicle detection was sometimes sensitive to vehicle-toroad color contrast. 

Additional merits and issues associated with application of video image 

detection at signalized intersections are reported in a 2004 survey conducted 

by the Urban Transportation Monitor.

(30) The survey contained responses 

from 120 jurisdictions. The survey results were summarized by the Monitor

as follows: 

...the use of video detection at signalized intersections is clearly 

controversial with many respondents having very strong opinions 

(both positive and negative) about its application. For example, 35% 

of the respondents experienced more complaints with the application 

of video detection, 19% indicated they experienced fewer complaints, 

and the balance indicated that the number of complaints have 

remained the same. On the other hand, 66% of respondents indicated 

that they will increase their application of video detection while 12% 

indicated that they will actually decrease their applications. 

These differences in experience and attitude are also reflected in the 

answers to the questions related to the main advantages and 

disadvantages compared to inductive loops. About the same number 
 Traffic Detector Handbook—2006 

October 2006 Page 2-75 Federal Highway Administration 

of positive replies (advantages) and negative replies (disadvantages) 

were received. Most frequent positive replies had to do with the 

ability of video detection to cope with changes in the detection zones 

due to restriping and the most frequent negative replies had to do 

with the inability of video detection to provide adequate results (or 

any results at all) during inclement weather (fog, heavy snow) and 

when the sun shines directly at the video camera. 

What seems to be clear is that agencies who are contemplating the 

use of video detection should approach it carefully as there are many 

pitfalls, as indicated in the survey results. It seems clear that it is 

also important to make sure a vendor is selected that can provide the 

latest improvements in video detection technology. 

MOUNTING AND TRAFFIC VIEWING 

CONSIDERATIONS 

Video image processor cameras can be deployed to view upstream or 

downstream traffic. The primary advantage of upstream viewing is that 

incidents are not blocked by the resultant traffic queues as described in Table 

2-14. However, tall vehicles such as trucks may block the line of sight, and 

headlights may cause blooming of the imagery at night. With upstream 

viewing, headlight beams can be detected as vehicles in adjacent lanes on 

curved road sections. Downstream viewing conceals cameras mounted on 

overpasses so that driver behavior is not altered. Downstream viewing also 

makes vehicle identification easier at night through the information 

available in the taillights and enhances track initiation because vehicles are 

first detected when close to the camera.(31) 

Table 2-14. Performance comparison of a visible spectrum video image processor 

using upstream and downstream viewing. 

Upstream viewing Downstream viewing 

• Headlight blooming, glare on wet pavement, 

headlight beams detected in adjacent lanes on 

curved road sections. 

• More blockage from tall trucks. 

• Traffic incidents not blocked by resulting traffic 

queues. 

• Camera on overpass concealed from drivers. 

• More information from taillights available for braking 

indication, vehicle classification, turning movement 

identification, and tracking. 

• Easier to acquire vehicles that are closer to the camera 

for tracking algorithm implementation. 

Although some manufacturers quote a maximum surveillance range for a 

VIP of ten times the camera mounting height, conservative design procedures 

limit the range to smaller distances because of factors such as road 

configuration (e.g., elevation changes, curvature, and overhead or underpass 

structures), congestion level, vehicle mix, and inclement weather. The impact 

of reduced headway on the effective surveillance range is calculated from the 

distance d (along the roadway from the base of the camera mounting 

structure to the vehicles in question) at which the VIP can distinguish 

between two closely spaced vehicles. The distance d depends explicitly on 

camera mounting height, vehicle separation or gap, and vehicle height, as 

illustrated in Figure 2-55. 
Chapter 2—Sensor Technology 

October 2006 Page 2-76 Federal Highway Administration 

Intervehicle gap (horizontal 

distance projected from the 

intersection of a ray, originating 

at the camera, with the top 

of the front vehicle and the 

bottom of the rear vehicle) 

Camera

mounting

height h

Vehicle

height

Maximum distance d

at which two closely 

spaced vehicles can

be distinguished

θ

Figure 2-55. Distinguishing between two closely spaced vehicles (source: L.A. Klein, Sensor 

Technologies and Data Requirements for ITS (Artech House, Norwood, MA, 2001)). 

If the roadway does not have a grade, d is approximated by 

d = h

height

gap

Veh

Veh

(2-63) 

where: 

 h = camera mounting height 

 Vehgap = intervehicle gap 

 Vehheight = vehicle height. 

Distance d is also implicitly dependent on the pixel size or instantaneous 

field-of-view of the camera, as larger values of d may not be realized without 

a correspondingly small pixel size. Figure 2-56 contains plots of distance d

versus vehicle separation based on Equation 2-63 for vehicle heights of 5 ft 

(1.5 m), representative of a passenger automobile, and 13 ft (4 m), 

representative of a larger commercial vehicle. 

 
 Traffic Detector Handbook—2006 

October 2006 Page 2-77 Federal Highway Administration 

0

250

500

750

1000

1250

1500

1750

2000

2250

2500

0 30 60 90 120 150 180

Vehicle Separation Vehgap (ft)

Distance d at Which Two Vehicles 

Can be Distinguished (ft)

Camera Height = 30 ft

Camera Height = 45 ft

Camera Height = 60 ft

(1 ft = 0.3 m)

Vehicle height = 5 ft

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

0 15 30 45 60 75 90 105 120 135 150 165 180 195

Vehicle Separation Vehgap (ft)

Distance 

d at Which Two Vehicles 

Can be Distinguished (ft)

Camera Height = 30 ft

Camera Height = 45 ft

Camera Height = 60 ft

(1 ft = 0.3 m)

Vehicle height = 13 ft

 1 ft = 0.3 m 

Figure 2-56. Distance d along the roadway at which a VIP can distinguish vehicles: (top) vehicle 

height = 5 ft (1.5 m), (bottom) vehicle height = 13 ft (3.9 m). These values may be further limited 

by road configuration, congestion level, vehicle mix, inclement weather, and pixel size. 

Other factors that affect camera installation include vertical and lateral 

viewing angles, number of lanes observed, stability with respect to wind and 

vibration, and image quality. VIP cameras can be mounted on the side of a 

roadway if the mounting height is high, i.e., 50 ft (15.2 m) or higher. For 

lower mounting heights of 25 to 30 ft (7.6 to 9.1 m), a centralized location 
Chapter 2—Sensor Technology 

October 2006 Page 2-78 Federal Highway Administration 

over the middle of the roadway area of interest is required. However, the 

lower the camera mounting, the greater is the error in vehicle speed 

measurement—the measurement error is proportional to the vehicle height 

divided by the camera mounting height. False detection and missed 

detections also increase at lower camera mounting heights. (See references 

14, 24, and 32–34.)

The number of lanes of imagery analyzed by the VIP is of importance to 

ensure that the required observation and analysis region is supported by the 

VIP. For example, if the VIP provides data from detection areas in three 

lanes, but five must be observed, that particular VIP may not be appropriate 

for the application. 

VIPs that are sensitive to large camera motion may be adversely affected by 

high winds since the processor may assume that the wind-produced changes in 

background pixels correspond to vehicle motion. Software that automatically 

learns the new operating environment and road configuration on which a VIP 

is operating has entered the market place. One such tool developed by Citilog 

advertises that no configuration, calibration, or parameter entry are necessary 

for their software to operate with existing pan, tilt, and zoom cameras for 

detection of stopped vehicles, congestion, and accidents. 

Image quality and interpretation can be affected by cameras that have 

automatic iris and automatic gain control. An automatic iris adjusts the light 

level entering the camera not only when roadway background lighting 

changes, but also when headlights, reflections from windshields and 

bumpers, or white or bright objects are in the field of view. Automatic gain 

control momentarily reduces the sensitivity of the camera to the same 

phenomena. These controls impair the ability of cameras with a low signalto-noise ratio to detect following vehicles when they restrict entering light 

levels.(26) When the camera responds by quickly darkening the entire picture 

and then recovering, some systems interpret this as an extra vehicle. 

In tests conducted by California Polytechnic University at San Luis Obispo 

(Cal Poly SLO), automatic iris and gain controls were disabled in an effort 

not to handicap the detection ability of the VIPs they evaluated.(35) In 

followup tests several years later, Cal Poly SLO found VIPs better able to 

compensate for light level changes when the automatic iris responded slowly 

to variations in light entering the camera.(36) This finding was confirmed by 

the Texas Transportation Institute (TTI), which specifies its VIP cameras to 

have damped iris and automatic gain controls.(32) 

When a traffic management agency wishes to use a single camera to provide 

imagery to a VIP and to obtain video surveillance with pan, tilt, and zoom 

controls, it is necessary to reposition the camera to its calibrated position for 

each VIP application. If the camera is not in the calibrated position, the 

performance of the VIP is degraded. If remote control of cameras and their 

return to calibrated fields of view is not feasible, then separate cameras may 

be required to perform automated traffic data collection and video 

surveillance. Some VIPs can automatically recalibrate the field of view for a 

new camera position using specialized algorithms. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-79 Federal Highway Administration 

VIDEO SURVEILLANCE ON FREEWAYS AND 

ARTERIALS 

Even without its association with VIPs, closed-circuit television (CCTV) has 

become a valuable asset for traffic management. In addition to its primary 

task of incident verification on freeways, CCTV is used for other applications 

in freeway and corridor traffic management. These include: 

• Monitoring traffic movements on the mainline, HOV lanes, and the 

entrance and exit ramps (e.g., driver response to messages on 

changeable message signs, HOV lane use, compliance with ramp 

metering, and verification of incidents). 

• Overlooking surface streets that run parallel to a freeway to verify 

that the local arterial system has unused vehicle capacity before 

implementing freeway diversion and then monitoring system 

performance as diversion occurs. 

• Monitoring the operation of critical signalized intersections and 

evaluating the signal timing and related functions. 

• Verifying message displays on changeable message signs. 

Current CCTV technology allows viewing of 0.25 to 0.5 mi (0.4 to 0.8 km) in 

each direction if the camera mounting, topography, road configuration, and 

weather are ideal. The location for CCTV cameras is dependent on the 

terrain, number of horizontal and vertical curves, desire to monitor weaving 

areas, identification of high-incident locations, and the need to view ramps 

and arterial streets. Each prospective site must be investigated to establish 

the camera range and field of view that will be obtained as a function of 

mounting height and lens selection. 

MICROWAVE RADAR SENSORS 

Two types of microwave radar sensors are used in roadside applications, 

those that transmit continuous wave (CW) Doppler waveforms and those that 

transmit frequency modulated continuous waves (FMCW). The traffic data 

they receive are dependent on the respective shape of the transmitted 

waveform. 

CW DOPPLER RADAR 

Figure 2-57 depicts the constant frequency waveform transmitted by a CW 

Doppler radar. This sensor is also referred to in some literature as a 

microwave or microwave Doppler sensor. The constant frequency signal (with 

respect to time) allows vehicle speed to be measured using the Doppler 

principle. Accordingly, the frequency of the received signal is decreased by a 

vehicle moving away from the radar and increased by a vehicle moving 

toward the radar. Vehicle passage or count is denoted by the presence of the 

frequency shift. Vehicle presence cannot be measured with the constant 

frequency waveform since only moving vehicles are detected. Two microwave 

radars that use the Doppler principle to measure speed are shown in 

Figure 2-58. 
Chapter 2—Sensor Technology 

October 2006 Page 2-80 Federal Highway Administration 

Figure 2-57. Constant frequency waveform. 

Figure 2-58. CW Doppler microwave radars. 

Vehicle speed is proportional to the frequency change between transmitted 

and received signals in Doppler radars. The relation between the transmitted 

frequency f, Doppler frequency fD, and vehicle speed S is expressed as 

fD = 2Sf/c cosθ (2-64) 

where θ is the angle between the direction of propagation of radar energy 

(i.e., the angle that represents the center of the antenna beam) and the 

direction of travel of the vehicle, c is the speed of light (3 x 108

 m/s), frequency 

is in units of Hz, and speed in units of m/s. The received frequency is given by 

f ± fD.

(31)

FMCW RADAR 

The second type of microwave sensor used in traffic management and control 

applications transmits an FMCW waveform in which the transmitted 

frequency is constantly changing with respect to time, as depicted in Figure 

2-59a. The FMCW radar operates as a vehicle presence sensor and can detect 

motionless vehicles. 

TDN-30 Doppler microwave radar. 

(Photograph courtesy of Whelen 

Engineering Company, Chester, CT) 

TC-20 Doppler microwave radar. 

(Photograph courtesy of Microwave 

Sensors, Ann Arbor, MI) 

Doppler frequency shift produced by a

vehicle moving away from the sensor

Transmitted signal

Received signal

Time

Frequency
 Traffic Detector Handbook—2006 

October 2006 Page 2-81 Federal Highway Administration 

(a) Frequency modulated continuous waveform

t1 t2 Tm = 1/fm

Time

ΔF Δf

Frequency

Transmitted

Received

fD

T

(b) Range binned footprints of radar sensors in traffic lanes

Direction of traffic flow

Lane 1

Lane 2

Lane 3 d

Elliptical field of view

or footprint of antenna Microwave radar sensor

Antenna

Range bin 1 Range bin 2

Support structure

(c) Time-phased outputs of range bins

ΔT Time

Time

Output of

range bin 1

Output of

range bin 2

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

speed (Source: L.A. Klein, Sensor Technologies and Data Requirements for ITS 

(Artech House, Norwood, MA, 2001)). 

The range R to the vehicle is proportional to the difference in the frequency 

Δf of the transmitter at the time t1 the signal is transmitted and the time t2 at 

which it is received, as given by 

R = f F m

c f

Δ

Δ

4 (2-65) 

where 

Δf = instantaneous difference in frequency, in Hz, of the 

transmitter at the times the signal is transmitted and received 

ΔF = radio frequency (RF) modulation bandwidth in Hz 

fm = RF modulation frequency in Hz. 

Alternatively, the range may be calculated from t2 – t1 or from the time 

difference T between consecutive peaks in the received and transmitted 

signals as indicated in Figure 2-59a. The range R, in terms of t2 – t1 or T, is 

R = c(t2 – t1)/2 = cT/2 (2-66)
Chapter 2—Sensor Technology 

October 2006 Page 2-82 Federal Highway Administration 

when the transmitter and receiver are collocated. The quantity c is the speed 

of light (3 x 108

 m/s). 

The range resolution ΔR or minimum distance, in meters, resolved by an 

FMCW radar is 

ΔR = F

c

2Δ . (2-67) 

Therefore, if the radar operates in the 10.500 to 10.550 GHz band and the 

bandwidth is limited to 45 MHz (rather than the full 50 MHz) to ensure that 

the field strength is reduced by at least 50 dB outside the band, the range 

resolution is at best 10.8 ft (3.3 m). 

Speed or Doppler resolution ΔfD is given by 

ΔfD = 2/Tm Hz (2-68) 

where Tm = 1/fm is the time for an up and down frequency sweep, as depicted 

in Figure 2-59a. 

The FMCW radar measures vehicle speed by dividing the field of view in the 

direction of vehicle travel into range bins as shown in Figure 2-59b. A range 

bin allows the reflected signal to be partitioned and identified from smaller 

regions on the roadway. Vehicle speed S is calculated from the time 

difference ΔT corresponding to the vehicle arriving at the leading edges of 

two range bins a known distance d apart as illustrated in Figure 2-59c. The 

vehicle speed is given by 

S = 

T

d

Δ (2-69)

where 

d = distance between leading edges of the two range bins 

ΔT = time difference corresponding to the vehicle’s arrival at the 

leading edge of each range bin. 

When mounted in a side-looking configuration, multilane FMCW radar 

sensors can monitor traffic flow in as many as eight lanes. With the sensor 

aligned perpendicular to the traffic flow direction, the range bins are 

automatically or semiautomatically (depending on the sensor model) adjusted 

to overlay a lane on the roadway to enable the gathering of multilane traffic 

flow data. FMCW radars can also use Doppler to calculate the speed of 

moving vehicles. A discussion of this technique is beyond the scope of this 

Handbook, but can be found in References 19 and 37. 

Presence-detecting radars, such as the models illustrated in Figure 2-60, 

control left turn signals, provide real-time data for traffic adaptive signal 

systems, monitor traffic queues, classify vehicles in terms of vehicle length, 

and collect occupancy and speed (multi detection zone models only) data in 

support of freeway incident detection algorithms. CW Doppler radars are 

used to measure vehicular speed on city arterials and freeways, but cannot 

detect stopped vehicles. Multi detection zone microwave presence-detecting 

radars are gaining acceptance in electronic toll collection and automated 

truck weighing applications that require vehicle identification based on 

vehicle length. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-83 Federal Highway Administration 

Figure 2-60. FMCW microwave radars. These are examples of microwave 

radars that detect vehicle presence. 

 LASER RADARS 

Laser radar sensors have two sets of optics. The transmitting optics split the 

pulsed laser diode output into two beams separated by several degrees, as 

displayed in Figure 2-61. The receiving optics has a wider field of view so 

that it can better collect the energy scattered from the vehicles. The multiple 

beams allow laser radars to measure vehicle speed by recording the times at 

which the vehicle enters the detection area of each beam. Since the beams 

are a known distance apart, the speed is easily calculated from the familiar 

speed equals distance over time. 

The laser radar illustrated in Figure 2-62 mounts 20 to 25 ft (6.1 to 7.6 m) 

above the road surface with an incidence angle (i.e., forward tilt) of 5 deg. A 

rotating polygon is utilized to line scan a laser diode rangefinder across the 

traffic lane as shown in Figure 2-61.(38) A sister model scans the laser across 

two lanes.(39) Their ability to classify 11 types of vehicles has found 

application on toll roads. 

RTMS multizone presence-detecting 

microwave radar. (Photograph courtesy 

of Lawrence A. Klein). 

SmartSensor multizone presencedetecting microwave radar. (Photograph 

courtesy of Wavetronix, Provo, UT). 

150LX single zone presence-detecting 

microwave radar. (Photograph courtesy of 

Naztec, Inc., Sugar Land, TX). 

Loren multizone presence-detecting microwave 

radar. (Photograph courtesy of Electronic 

Control Measurement Inc, Manor, TX). 
Chapter 2—Sensor Technology 

October 2006 Page 2-84 Federal Highway Administration 

Figure 2-61. Laser radar beam geometry (drawing courtesy of 

Schwartz Electro-Optics, now OSI Laserscan, Orlando, FL). 

Figure 2-62. Autosense II laser radar sensor (Photograph courtesy of Schwartz 

Electro-Optics, now OSI Laserscan, Orlando, FL). 

Another commercial laser radar for collecting traffic flow data is shown in 

Figure 2-63. This model does not continuously scan the laser beam across a 

traffic lane. Instead it uses six individual pairs of transmitters and receivers 

to produce a beam that traverses the lane. The number of active transmitterreceiver pairs can be specified by the user to control the width of the laser 

scan across a single lane. The effective range of the device is up to 23 ft (7 m). 

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

(Photograph from EFKON product literature). 

Active infrared sensors also communicate traffic information to motorists by 

modulating and coding the infrared beam with the appropriate data. Figure 

2-64 depicts a sensor system used for this application. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-85 Federal Highway Administration 

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

(Photograph courtesy of Sumitomo Electric U.S.A. Inc., Santa Clara, CA). 

PASSIVE INFRARED SENSORS 

Passive sensors transmit no energy of their own. Rather they detect 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. 

The energy captured by passive infrared sensors is focused by an optical 

system onto an infrared-sensitive material mounted at the focal plane of the 

optics. With infrared sensors, the word detector takes on another meaning, 

namely the infrared sensitive element that converts the reflected and emitted 

energy into electrical signals. Real-time signal processing is used to analyze 

the signals for the presence of a vehicle. The sensors are mounted overhead 

to view approaching or departing traffic. They can also be mounted in a sidelooking configuration. Infrared sensors are used for signal control; volume, 

speed, and class measurement; detection of pedestrians in crosswalks; and 

transmission of traffic information to motorists. 

Nonimaging passive infrared sensors used in traffic management 

applications contain one or several (typically not more than five) energysensitive detector elements on the focal plane that gather energy from the 

entire scene. The detector in a nonimaging sensor generally has a large 

instantaneous field of view. Instantaneous field of view is equal to the angle, 

e.g., in the x-y plane, subtended by a pixel. Objects within the scene cannot 

be further divided into subobjects or pixels with this device. 

Passive infrared sensors with a single detection zone measure volume and 

lane occupancy by responding to vehicle passage and presence. Those with 

multiple detection zones can also measure vehicle speed and length (to the 

extent that vehicles are detected in one or more detection zones a known 

distance apart). Imaging sensors, such as modern charge-coupled device 

(CCD) cameras, contain two-dimensional arrays of detectors, each detector 

having a small instantaneous field of view. The two-dimensional array 

gathers energy from the scene over an area corresponding to the field of view 

of the entire array. Imaging sensors display the pixel-resolution details 

characteristic of the imaged area, as illustrated in Figure 2-65.(40) An 
Chapter 2—Sensor Technology 

October 2006 Page 2-86 Federal Highway Administration 

alternative method of gathering two-dimensional images is by scanning onedimensional arrays over the scene of interest. However, this approach is not 

currently applied to passive infrared traffic flow sensors. 

The emitted energy detected by passive sensors is produced by the nonzero 

surface temperature of emissive objects in their field of view. Emission occurs 

at all frequencies by objects not at absolute zero (-459.67 °F or -273.15 °C). If 

the emissivity of the object is perfect, i.e., emissivity = 1, the object is called a 

blackbody. Most objects have emissivities less than 1 and, hence, are termed 

graybodies. Passive sensors can be designed to receive energy at any 

frequency. Cost considerations make the infrared band a good choice for 

vehicle sensors with a limited number of pixels. Passive infrared sensors, 

such as those shown in Figure 2-66, operate in the long-wavelength infrared 

band from 8 to 14 μm and thus minimize the effects of sun glint and changing 

light intensity from cloud movement. Passive vehicle sensors operating at 

microwave frequencies have been evaluated, but their costs are greater.(40,41) 

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

dusk lighting (Source: C.A. MacCarley, B.M. Hemme, and L.A. Klein, Advanced Image 

Sensing Methods for Traffic Surveillance and Detection, Final Report, Document 

No. TEL5859F98 (Rev. 1) (California Polytechnic University, 

San Luis Obispo, CA, Jun. 30, 1998). 
 Traffic Detector Handbook—2006 

October 2006 Page 2-87 Federal Highway Administration 

Siemens Eagle PIR-1 sensor. Performs 

vehicle counting, stop line presence 

detection, occupancy detection, and 

queue detection. (Photograph courtesy 

of Siemens ITS, Austin, TX).

Eltec 842 passive infrared vehicle presence sensor. [Source: L.A. Klein, Sensor Technologies 

and Data Requirements for ITS (Norwood, MA: Artech House, 2001)]. 

ASIM IR 250 series passive infrared sensor. 

This multizone sensor performs vehicle 

counting, speed measurement, classification 

by length, and presence detection. 

(Photograph courtesy of ASIM Technologies, 

Uznach, Switzerland).

Figure 2-66. Passive infrared sensors 

THEORY OF OPERATION 

When a vehicle enters a passive sensor’s field of view, the detected energy 

changes due to the presence of the vehicle. The difference in detected energy 

created by the vehicle is described by radiative transfer theory.(19,31,42) The 

emissivities of the vehicle and road surface in the wavelength region of 

interest are denoted by εV and εR and their surface temperatures in degrees 

Kelvin by TV and TR, respectively, as depicted in Figure 2-67. 

Road surface with emissivity εR

and surface temperature TR

εVTV

(Emissive term)

(1 - εV)Tsky (Reflective term)

θ

Vehicle with emissivity εV

and surface temperature TV

Tsky

Passive sensor

Receiving aperture

Figure 2-67. Emission and reflection of energy by vehicle and road surface. 
Chapter 2—Sensor Technology 

October 2006 Page 2-88 Federal Highway Administration 

The apparent temperature TBV of the vehicle is 

TBV (θ, φ) = εVTV + (1 - εV) Tsky (2-70) 

assuming infrared energy emission by the sensor is negligible. The sky 

temperature Tsky is a function of atmospheric, galactic, and cosmic emission. 

The atmospheric emission in the infrared spectrum is dependent on the 

water, ozone, carbon dioxide, nitrous oxide, and methane concentrations. The 

angles θ and φ are the incident angle with respect to nadir (i.e., directly 

downward) and the angle in the plane of the road surface (the x-y plane), 

respectively. The term εVTV represents the energy emitted from the vehicle, 

referred to as the brightness temperature. The term (1 – εV) Tsky is the portion 

of the sky temperature that is reflected from the metal vehicle into the 

passive sensor. Thus, surfaces that are highly emissive (large ε) have a low 

reflective component (small 1 – ε). 

One can also write an expression similar to Equation 2-70 for the apparent 

temperature of the road surface as 

TBR (θ, φ) = εRTR + (1 – εR) Tsky . (2-71)

The terms in Equation 2-71 have interpretations analogous to those in 

Equation 2-70. By subtracting the apparent temperature of the vehicle from 

that of the road, one gets an expression for the temperature difference ΔTB (θ, 

φ) sensed by the passive infrared sensor when a vehicle passes through its 

field of view. Thus 

ΔTB (θ, φ) = (εR TR – εV TV) + (εV – εR) Tsky . (2-72) 

When TV = TR, then 

ΔTB (θ, φ) = (εR – εV) (TR – Tsky) . (2-73) 

Hence, a vehicle entering the sensor’s field of view generates a signal that is 

proportional to the product of an emissivity difference term and a 

temperature difference term when the surface temperatures of the vehicle 

and road are equal. The emissivity term is equal to the difference between 

the road and the vehicle emissivities. The temperature term is equal to the 

difference between the absolute temperature of the road surface and the 

temperature contributed by atmospheric, cosmic, and galactic emission. On 

overcast, high humidity, and rainy days, the sky temperature is larger than 

on clear days and the signal produced by a passing vehicle decreases. This, in 

itself, should not pose a problem to a properly designed passive infrared 

sensor operating at the longer wavelengths of the infrared spectrum, 

especially at the relatively short operating ranges typical of traffic 

management applications. 

Fields of view can be tailored through the optical design to accommodate 

different requirements such as stopline presence detection and presence 

detection in the approach to an intersection [e.g., a detection zone 68 to 100 ft 

(20.7 to 30.5 m) in advance of the stopline]. A long focal length lens 

eliminates adjacent lane detection when sensing vehicles over 100 ft (30.5 m) 

from the sensor, e.g., as they approach an intersection. 

Multichannel (i.e., incorporating more than one type of sensor technology as 

in Figure 2-74) and multizone (i.e., including more than one detection region) 

passive infrared sensors measure speed and vehicle length as well as the 

more conventional volume and lane occupancy. These models are designed 
 Traffic Detector Handbook—2006 

October 2006 Page 2-89 Federal Highway Administration 

with dynamic and static thermal energy detection zones that provide the 

functionality of two inductive loops. Their footprint configuration is shown in 

Figure 2-68. The time delays between the signals from the three dynamic 

zones are utilized to measure speed. The vehicle presence time from the 

fourth zone is used to calculate the lane occupancy of stationary and moving 

vehicles. 

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

Several disadvantages of infrared sensors are sometimes cited. Glint from 

sunlight may cause unwanted and confusing signals in passive sensors 

operating at near- or midinfrared wavelengths. Atmospheric particulates and 

inclement weather can scatter or absorb energy that would otherwise reach 

the focal plane. The scattering and absorption effects are sensitive to water 

concentrations in fog, haze, rain, and snow as well as to other obscurants 

such as smoke and dust. At the relatively short operating ranges encountered 

by infrared sensors in traffic management applications, these concerns may 

not be significant. However, some performance degradation (i.e., 

undercounting) in heavy rain and snow has been reported.(40) A rule of thumb 

for gauging when an infrared sensor may experience difficulty detecting a 

vehicle is to note if a human observer can see the vehicle under the same 

circumstances. If the observer can see the vehicle, there is a high probability 

the infrared sensor will detect the vehicle. 

ULTRASONIC SENSORS

Ultrasonic sensors transmit pressure waves of sound energy at a frequency 

between 25 and 50 kHz, which are above the human audible range. Most 

ultrasonic sensors, such as the model shown in Figure 2-69, operate with 

pulse waveforms and provide vehicle count, presence, and occupancy 

information. Pulse-shape waveforms measure distances to the road surface 

and vehicle surface by detecting the portion of the transmitted energy that is 

reflected towards the sensor from an area defined by the transmitter’s 

beamwidth. When a distance other than that to the background road surface 

is measured, the sensor interprets that measurement as the presence of a 

vehicle. The received ultrasonic energy is converted into electrical energy. 

This energy is then analyzed by signal processing electronics that is either 

collocated with the transducer or placed in a roadside controller. 
Chapter 2—Sensor Technology 

October 2006 Page 2-90 Federal Highway Administration 

Figure 2-69. TC-30C ultrasonic range-measuring sensor (Photograph courtesy 

of Microwave Sensors, Ann Arbor, MI). 

Pulsed energy transmitted at two known and closely spaced incident angles 

allows vehicular speed to be calculated by recording the time at which the 

vehicle crosses each beam. Since the beams are a known distance apart, the 

speed can be calculated as beam separation distance divided by the time to 

traverse the beams. The preferred mounting configurations for rangemeasuring, pulsed ultrasonic sensors are at nadir, looking from an overhead 

position, and side viewing, as shown in Figure 2-70. 

Constant frequency ultrasonic sensors that measure speed using the Doppler 

principle are also manufactured. However, these are more expensive than 

pulsed models. The speed-measuring Doppler ultrasonic sensor is designed to 

interface with the highway infrastructure in Japan. It is mounted overhead 

facing approaching traffic at a 45-deg incidence angle. It has two 

transducers, one for transmitting and one for receiving, as illustrated in 

Figure 2-71. The Doppler ultrasonic sensor detects the passage of a vehicle by 

a shift in the frequency of the received signal. Vehicle speed can be calculated 

from the pulse width of an internal signal generated by the sensor’s 

electronics that is proportional to the speed of the detected vehicle. 

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

(Illustration courtesy of Microwave Sensors, Ann Arbor, MI). 
 Traffic Detector Handbook—2006 

October 2006 Page 2-91 Federal Highway Administration 

Figure 2-71. Speed-measuring RDU-101 Doppler ultrasonic sensor (manufactured by Sumitomo 

Electric, Japan) with separate transmitting and receiving transducers, as shown in the left portion of 

the figure. The cabinet on the right contains the electronic systems needed to supply power and 

control signals to the transducers, receive the data, and interface with the highway infrastructure. 

The range-measuring ultrasonic sensor transmits a series of pulses of 

width Tp (typical values are between 0.02 and 2.5 ms) and repetition 

period T0 (time between bursts of pulses), typically 33 to 170 ms, as 

illustrated in Figure 2-72. The sensor measures the time for the pulse 

to arrive at the vehicle and return to the transmitter. The receiver is 

gated on and off with a user-adjustable interval that differentiates 

between pulses reflected from the road surface and those reflected 

from vehicles. The detection gates of various models are adjusted to 

detect objects at distances greater than approximately 0.5 to 0.9 m 

above the road surface. This is achieved by closing the detection gate 

several milliseconds before the reflected signal from the road surface 

arrives at the sensor. 

Automatic pulse repetition frequency control reduces effects of multiple 

reflections and improves the detection of high-speed vehicles. This control 

makes the pulse repetition period as short as possible by transmitting the 

next pulse immediately after the reflected signal from the road is received.(43)

A hold time Th (composite values from manufacturers range from 115 ms to 

10 s) is built into the sensors to enhance presence detection. 

Ultrasonic sensors are widely used in Japan in keeping with the government 

policy that discourages pavement cutting on existing highways. In Tokyo, 

ultrasonic sensors are a major component of the traffic control system. A 

central computer monitors traffic signals and vehicle motion, resets timing 

patterns, activates motorist information displays, and relays real-time 

information to motorists and police. The presence or range-measuring type of 

sensor is used more extensively than the Doppler type. The Japanese 

applications appear to be the most extensive use of ultrasonic sensors. 

Temperature change and extreme air turbulence may affect the performance 

of ultrasonic sensors. Temperature compensation is built into some models. 
Chapter 2—Sensor Technology 

October 2006 Page 2-92 Federal Highway Administration 

Large pulse repetition periods may degrade occupancy measurement on 

freeways with vehicles traveling at moderate to high speeds as an 

insufficient number of pulses is transmitted and reflected from the vehicle 

while in the sensor’s detection zone. 

Transmitted

ultrasonic pulses

Detection gate

Reflected pulses

Hold time for

vehicle presence

From

vehicle From road surface

TO

Tp

(Application dependent)

Th

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

PASSIVE ACOUSTIC ARRAY SENSORS 

Acoustic sensors measure vehicle passage, presence, and speed by detecting 

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. When a vehicle passes through the detection zone, an increase in sound 

energy is recognized by the signal processing algorithm and a vehicle presence 

signal is generated. When the vehicle leaves the detection zone, the sound 

energy level drops below the detection threshold, and the vehicle presence 

signal is terminated. Sounds from locations outside the detection zone are 

attenuated. 

Single lane and multiple lane models of acoustic sensors are marketed. Both use 

a two-dimensional array of microphones to detect the sounds produced by 

approaching vehicles. 

SINGLE LANE MODEL OPERATION 

The SmartSonic acoustic sensor, shown in the upper part of Figure 2-73, 

detects vehicles by measuring the time delay between the arrival of sound at 

the upper and lower microphones, which are arranged in a vertical and 

horizontal line through the center of the aperture. The time delay changes as 

the vehicle approaches the array. When the vehicle is inside the detection zone, 

the sound arrives almost instantaneously at the upper and lower microphones. 

When the vehicle is outside the detection zone, sound reception at the upper 

microphone is delayed by the intermicrophone distance. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-93 Federal Highway Administration 

SmartSonic acoustic sensor (Photograph courtesy of IRD, Saskatoon, SK, Canada). 

SAS-1 acoustic sensor (Photograph courtesy of SmarTek Systems, Woodbridge, VA). 

Figure 2-73. Acoustic array sensors. 

The size and shape of the detection zone are determined by the aperture size, 

processing frequency band, and installation geometry of the acoustic array. 

The SmartSonic sensor is tuned to a center frequency of 9 kHz with a 2 kHz 

bandwidth. Preferred mounting is at 10 to 30 deg from nadir with a detection 

range of 20 to 35 ft (6 to 11 m). 

The speed of a detected vehicle is determined with an algorithm that assumes 

an average vehicle length. Vehicle presence detection is 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. When the 

optional acoustic sensor controller board is installed in a NEMA or 170 cardfile, 

two detection zones can be used in a speed trap mode to measure vehicle speed. 

The speed trap activates relay outputs that simulate two inductive loops 

connected to a NEMA or 170 controller. The SmartSonic is recommended for 

data collection applications on bridges and other roads where nonintrusive 

sensors are required, providing either slow moving vehicles (speeds < 20 mi/h 

(32 km/h)) in stop-and-go traffic flow or free-flow traffic is present. The sensor 

is not recommended where a mix of stop-and-go and free-flow traffic occurs, 

such as on a freeway with the potential for congestion, because the vehicle 

detection algorithm cannot switch between these two flow conditions fast 

enough to detect the onset of the change in flow. 
Chapter 2—Sensor Technology 

October 2006 Page 2-94 Federal Highway Administration 

MULTIPLE LANE MODEL OPERATION 

The SAS-1 acoustic sensor in the lower part of Figure 2-73 utilizes a fully 

populated microphone array and adaptive spatial processing to form multiple 

detection zones. The SAS-1 is designed to monitor up to 5 lanes when 

mounted in a side-looking configuration. During setup, the detection zones 

are steered to positions that correspond to the monitored traffic lanes. The 

detection zones are self-normalized and polled for vehicles every 8 ms. 

Detection zones are adjustable to 6 ft (1.8 m) or 12 ft (3.6 m) in the direction 

of traffic flow and have user-specified values in the cross-lane direction. 

Acoustic frequencies between 8 and 15 kHz are processed by this sensor, 

which accommodates mounting heights of 20 to 40 ft (6 to 12 m). The output 

data are volume, lane occupancy, and average speed for each monitored lane 

over a user-specified period (e.g., 20 s, 30 s, 1 minute). Vehicle presence is 

provided by an optional relay interface. 

SENSOR COMBINATIONS 

Figure 2-74 illustrates sensors that combine passive infrared presence 

detection with ultrasound or CW Doppler microwave radar.(44) The passive 

infrared-ultrasonic combination, shown in the upper portion of the figure, 

provides enhanced accuracy for presence and queue detection, vehicle 

counting, and height and distance discrimination. 

The passive infrared-CW Doppler radar sensor, in the lower portion of the 

figure, is designed for presence and queue detection, vehicle counting, speed 

measurement, and length classification. It relies on the radar to measure 

high to medium vehicle speeds and the passive infrared to measure vehicle 

count and presence. At medium speeds, the multiple detection zone passive 

infrared automatically calibrates its speed measurements against the 

radar’s. This calibration permits the infrared to measure slow vehicle speeds 

and detect stopped vehicles. 
 Traffic Detector Handbook—2006 

October 2006 Page 2-95 Federal Highway Administration 

ASIM DT 272 Passive infrared-ultrasonic sensor. 

ASIM DT 281 Passive infrared-CW Doppler microwave radar sensor.

Figure 2-74. Passive infrared combination sensors (Photographs courtesy 

of ASIM Technologies, Uznach, Switzerland). 

REFERENCES 

1. 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, Washington, 

DC. U.S. Department of Commerce, National Technical Information 

Service, PB-263 948, Washington, DC, March 1976. 

2. Gordon, R.L., R.A. Reiss, H. Haenel, E.R. Case, R.L. French, A. 

Mohaddes, and R. Wolcott. Traffic Control Systems Handbook, 

FHWA-SA-95-032. U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC, February 1996. 

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19. Klein, L.A. Millimeter-Wave and Infrared Multisensor Design and 

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 Traffic Detector Handbook—2006 

October 2006 Page 3-i Federal Highway Administration 

TABLE OF CONTENTS: CHAPTER 3. 

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 

Timing Plan Implementation ....................................................................3-10

Control of Groups of Intersections Using Online Generated 

Timing Plans.............................................................................................3-10 

Traffic Adjusted Control and UTCS 1.5-Generation Systems .................3-10

Second-Generation Traffic Signal Control Systems ................................3-11

Third-Generation Traffic Signal Control Systems...................................3-11

Traffic Responsive Control and Centralized Systems ..............................3-12

Traffic Adaptive Control and Distributed Systems...................................3-13

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
Chapter 3—Sensor Applications 

October 2006 Page 3-ii Federal Highway Administration 

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

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

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
 Traffic Detector Handbook—2006 

October 2006 Page 3-iii Federal Highway Administration 

LIST OF FIGURES: CHAPTER 3. 

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 Francisco-Oakland 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 inductive-loop 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 3—Sensor Applications 

October 2006 Page 3-iv Federal Highway Administration 

LIST OF TABLES: CHAPTER 3. 

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
 Traffic Detector Handbook—2006 

October 2006 Page 3-1 Federal Highway Administration

CHAPTER 3. SENSOR 

APPLICATIONS 

Sensor applications to traffic control and management continue to evolve and 

escalate. Originally utilized for signalized intersection control, sensors are 

now used to supply real-time data for traffic adaptive signal control and 

mitigating recurring and nonrecurring congestion on freeways. Many 

advances in traffic control system technology during the past decade have 

been supported by the evolution of microprocessors and other electronics 

components. The relative ease with which research and widespread user 

knowledge can be retrieved has also assisted agencies in selecting 

appropriate sensor technologies and deployment configurations to satisfy 

their operational needs. The Internet with its convenient access to public 

and private libraries that contain evaluation reports of sensor performance 

and traffic management strategies enables the rapid sharing of test and 

operational experiences. 

Traffic control concepts that rely on traffic flow data from sensors are 

common to central business district, arterial, and freeway traffic 

management strategies. The data needs of the applications can generally be 

satisfied by one or more sensor technologies. This chapter presents an 

overview of several of these traffic management strategies so that the traffic 

engineer or other practitioner can better understand their detection 

requirements. Additional information on how to use traffic sensors in traffic 

signal control systems can be found in two recommended references: Manual 

of Traffic Signal Design(1) and Traffic Control System Handbook.(2) The 

Freeway Management and Operations Handbook(3) may be consulted for 

information concerning freeway operation and appropriate freeway traffic 

management strategies. 

PRESENCE AND PASSAGE SENSORS 

Most vehicle sensors in use today monitor the movement of vehicles past a 

given point on the road. The data acquired are transmitted to a traffic signal 

controller, traffic counter, or other device. The controller or counter processes 

some data locally, while others are transmitted to a central computer or 

display monitor, in the case of camera imagery, at a traffic management 

center. 

A sensor that detects the passage of vehicles in a specified direction may be 

used to issue a warning to alert the driver of a vehicle traveling in the 

opposite or forbidden direction. A presence sensor is generally used at 

locations where vehicle speeds are less than approximately 5 mi/h (8 km/h) or 

where stopped vehicle detection is required. A passage (motion-detecting) 

sensor will record the passage of a vehicle in the detection zone as long as the 

vehicle is moving more than 3 to 5 mi/h (5 to 8 km/h). 

When vehicles are forced to stop or move very slowly when approaching a 

traffic-actuated signalized intersection, it is desirable to use presence 

detection to ensure that stopped vehicles waiting in the detection zone are 

detected. Early inductive-loop electronics units operating in the presence 

mode did not have a directional feature and could not differentiate between 

vehicles entering or leaving the detection zone. Some newer units contain 

this feature. Presence sensors that use over-roadway technologies such as 

Presence sensors detect slow 

moving and stopped vehicles. 

Passage sensors detect vehicles 

moving faster than 3 to 5 mi/h (5 to 

8 km/h).
Chapter 3—Sensor Applications 

October 2006 Page 3-2 Federal Highway Administration 

video image processing incorporate directional features. At intersections, 

there may be less need for a directional feature, as the call is not normally 

retained in the controller once the vehicle leaves the detection zone. Passage 

sensors will not retain the call either once the vehicle leaves the detection 

zone. The difference is in controller operation (i.e., whether the controller is 

in locking or nonlocking mode). 

Passage sensors may be used to count vehicles in individual lanes, multiple 

lanes where traffic flows in a particular direction, or all lanes where traffic 

flows in both directions. Passage sensors also measure vehicle speed and 

volume when speeds are greater than 3 to 5 mi/h (5 to 8 km/h). 

Sensors based on video image processing, microwave radar, laser radar, 

passive infrared, ultrasound, passive acoustic, magnetometer, and inductiveloop sensor technologies discussed in Chapter 2 detect either vehicle presence 

or passage, depending on their particular design or the selected operating 

mode. 

Modern traffic sensors can record traffic flow parameters by direction and by 

lane. Accurate arterial vehicle count and speed data are obtained by locating 

the sensors far enough in advance of the signalized intersection so that traffic 

does not back up to the detection zone. If traffic backs up to the sensor, only 

volume is measured. 

Magnetometers can be used to count vehicles, provided the detection zone is 

placed in the desired portion of the lane of interest. Since magnetometer 

detection zones are generally less than 3 ft (1 m) in diameter, multiple units 

may be needed to detect vehicles over the entire lane width. 

A properly functioning inductive-loop detector is an excellent sensor for 

detecting vehicle presence, providing it is properly installed and maintained. 

Loop size can be varied to accommodate different applications. For small area 

detection, the conventional loop and two-axis fluxgate magnetometer may be 

interchanged. 

Some techniques for high-speed intersection signal control utilize 

conventional loops or magnetometer sensors with electronics units that are 

programmed for normal outputs. Other designs use electronic units that 

extend or hold the call of the vehicle after it leaves the detection zone 

(extended call detectors or extended call sensors). Another plan incorporates 

electronic units that delay an output until the detection zone has been 

occupied for a preset time (delayed call detectors or delayed call sensors). 

These are discussed in Chapter 4. 

Over-roadway sensors are becoming more popular as sources of real-time 

data for signal control and freeway traffic management. This is because of 

their ability to provide multiple lane data from a single sensor, reduced 

maintenance, increased safety to installation personnel, data types not 

available from loops or magnetometers, and competitive purchase and 

installation costs. 

SPEED MONITORING WITH INDUCTIVE-LOOP AND 

OTHER POINT SENSORS 

A 1980 study suggested that the optimum characteristics of a sensor used for 

vehicle speed measurement were:(4) 

• Self-tuning electronics to reduce drift. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-3 Federal Highway Administration

• Short response time from intercept to output. 

• High sensitivity without appreciable time delay. 

• Consistency of vehicle location at beginning and end of detection, 

independent of vehicle speed or length of lead-in cable. 

When using two loops to measure speed, the loops should be large enough to 

sense high-body vehicles and provide a sharply defined wave front output as 

the vehicle passes over the loop. Any time differences in the detection of 

different vehicle types passing over the loops should be minimized. The loops 

should be spaced sufficiently far apart so that any difference in the time of 

intercept of the two inductive-loop detectors is small as compared to the 

transit time from the first loop to the second loop. 

A rule of thumb for loop design states that the height of the magnetic field 

intercepted by the vehicle is two-thirds the distance of the shorter loop 

dimension. Therefore, a 6- x 6-ft (1.8- x 1.8-m) loop has intercepts of 

approximately 4 ft (1.2 m), as does a 6- x 100-ft (1.8- x 30.5-m) loop. Both 5-ft 

(1.5-m) wide and 6-ft (1.8 m) wide loops have proven effective at consistently 

detecting high-body vehicles. The choice depends on lane width. A spacing of 

at least 2.5 ft (0.8 m) should be allowed from the centerline to the edge of the 

loop to avoid actuation by traffic in adjacent lanes. In a 12-ft (3.6-m) lane, the 

6-ft (1.8-m) loop should be used to ensure no counts are missed. 

The spacing between loops for speed measurements is often specified as 

16 ft (4.9 m) between the leading edges of two 6-ft (1.8-m) loops as shown in 

Figure 3-1. This sensor layout also applies to any pair of point sensors that 

might be used in a dual sensor speed-trap configuration. The sensitivity of 

the electronics unit connected to each loop must be the same. If not, the 

vehicle position with respect to the loop’s leading edge, which induces the 

critical change in loop inductance needed to activate the electronics unit and 

which is proportional to the sensitivity, will vary from loop to loop, thereby 

introducing a measurement error. In the late 1980s, very fast response times 

at sensitivity levels appropriate for roadway vehicles were made possible by 

new electronic component technology. 

6’

6’

6’

6’

16’ Edge of Pavement

Direction

of Travel

12’

Lane

3’

Lane markings

1 ft = 0.3 m 

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

placed a known distance apart. 

TRAFFIC MANAGEMENT CONCEPTS FOR FREEWAYS 

Freeways were originally conceived as limited access, free-flowing facilities 

with little need of traffic control. Rapid growth in freeway traffic volume and 

resulting congestion have led to development of freeway surveillance and 
Chapter 3—Sensor Applications 

October 2006 Page 3-4 Federal Highway Administration 

control systems. These systems employ techniques such as ramp control, 

mainline control, corridor control, and traveler information systems to 

mitigate freeway congestion. Strategies used today to assist in the 

management of freeways include: 

• Restricted entry (ramp closure, ramp metering). 

• Priority treatment HOV and bus priority lanes). 

• Surveillance. 

• Incident management (detection, identification, and response). 

• Display and broadcast of advisory information (speed, travel time, 

route guidance, diversion). 

Each city street arterial and freeway traffic management concept has its own 

set of operating parameters and components, including sensors, as discussed 

below in the sections on signal control concepts for city streets, freeway 

surveillance and control, and coordinated operation of freeways and surface 

arterials. 

SIGNAL CONTROL CONCEPTS FOR CITY STREETS 

Traffic signal control concepts for city street intersections may be grouped 

into strategies for individual intersections and strategies for groups of 

intersections as follows: 

• Strategies for individual intersections: 

o Pretimed. 

o Traffic actuated. 

• Strategies for groups of intersections: 

o Uncoordinated control: Traffic flow is controlled without 

considering the operation of adjacent traffic signals. 

o Time-based coordinated control (TBC): TBC systems have 

no system sensors. These provide signal progression that allows 

platoons of vehicles to proceed along arterial routes without 

stopping. It can also provide area-wide control to minimize total 

delay and number of stops over an entire network. Control 

information on the time-of-day/day-of-week (TOD/DOW) plan is 

provided by means of a real time clock. Most modern controllers 

have this capability built in. 

o Interconnected control: 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 TOD/DOW. The operator can select 

timing and download timing plans and changes as well as 

monitor and record system status. System response to traffic 

changes is measured in weeks or months. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-5 Federal Highway Administration

o Traffic adjusted control: 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 of 15 minutes or more. 

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 precalculated plans. This 

definition builds on the original definition by Kell and Fullerton 

in the Manual of Traffic Signal Design,(1) which in turn built on 

that of Automatic Signal. System response to traffic changes is 

relatively slow. 

o Traffic responsive control: Systems operate with at least one 

sensor per link up to one sensor per lane per link. They use 

flexible timing plans in which the offsets, splits, and phase 

durations can be promptly changed in reaction to changes in 

traffic on a nearly cycle-by-cycle basis. Cycle, split, and offset are 

each optimized in the selection or calculation 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. 

o Traffic adaptive control: Systems operate with two sensors per 

lane per link for optimum efficiency. 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 

measure the flow of individual vehicles to predict the future flow 

of the individual vehicles. System response to traffic changes is 

proactive. 

• Strategies for groups of intersections can be implemented through 

the selection of timing plans from among a library of prestored fixed 

timing plans that best match current traffic flow conditions, from 

fixed or flexible timing plans generated online in real time based on 

current traffic flow. Flexible timing plans are updated incrementally 

once each cycle. 

o Open network arterial control: Primary consideration is the 

progressive flow of traffic along the arterial by operating as a 

system. 

o Closed network control: Applied to a group of adjacent signalized 

intersections whose signal operations are coordinated, e.g., the 

control of signals in a central business district (CBD). 

o Areawide system control: Treats all of the traffic signals within 

an area, a city, or a metropolitan area as a total system. 

Individual signals within an area may be controlled by isolated, 

open network, or closed network concepts. 

Signal control concepts for special functions include: 

• Preemption and priority vehicle signal control: Preempts or alters a 

normal signal sequence for the movement of emergency and transit 

vehicles, respectively. 

Sensor web density levels and their 

relation to traffic control systems 

are described in 

Appendix L. 
Chapter 3—Sensor Applications 

October 2006 Page 3-6 Federal Highway Administration 

• Driver alerts and warnings: Lighted signs that alert drivers to a red 

signal ahead and warn of excess vehicle speed when approaching 

sharp curves or other roadway conditions that require reduced speed 

or other caution. 

• Pedestrian signal actuation: Devices that request the pedestrian 

crossing phase and alert drivers to pedestrians in a crosswalk. 

STRATEGIES FOR LOCAL INTERSECTION SIGNAL 

CONTROL 

Local intersection control is the strategy residing in the local controller that 

manages traffic flow independently of other traffic signals. Two types of local 

control exist, pretimed and actuated.(2,5) The type of control selected is 

frequently subject to local policy and practice. If the local control is run 

independently of strategies for other intersections, the system control is 

called “uncoordinated control.” Since each signal in uncoordinated control or 

isolated intersection control operates independently, offset is not a controlled 

parameter when isolated intersection control is implemented. 

Pretimed Control 

Sensors are not required for pretimed control when right-of-way is assigned 

based on a predetermined fixed time duration, as determined from historic 

data, for all signal display intervals. Therefore, pretimed control is generally 

inefficient for controlling intersections that undergo changes in demand. 

Pretimed control may be used in conjunction with traffic adjusted or traffic 

responsive timing plan selection where closely spaced signals dictate fixed 

offsets, such as with diamond interchanges or central business districts. 

Actuated Control 

Actuated control utilizes sensors to provide data to a local traffic signal 

controller as illustrated in Figure 3-2. Sensors are typically located at stoplines 

(A), upstream of the stopline (B), left turn lanes (C), and at positions to detect 

emergency (D) and transit vehicles (E). Inductive-loop detectors are the most 

common sensor used for this application, although multiple-lane, multipledetection zone sensors such as the video image processor or true presence 

microwave radar may have merit for this type of signal control. The suitability 

of over-roadway sensors for a specific application should be evaluated through 

field testing by the responsible agency to ensure that the required calls are 

provided reliably. The information gathered by the sensors can be processed as 

indicated in Figure 3-3 or in another manner, depending on the particular traffic 

management requirements and strategies. 

Actuated control can be semiactuated or fully actuated. In semiactuated 

control, the major street operates in a nonactuated mode such that green is 

always present unless a minor street actuation is received. Therefore, sensors 

are required only for the minor cross-street phases. In the absence of crossstreet demand, semiactuated signals are recalled to the major street phase. 

Semiactuated operation is appropriate when vehicles on the minor streets 

approach the intersections in a random manner, that is, where platoons 

(groups of closely spaced vehicles traveling at the same speed) cannot be 

sustained. Such a condition is likely where there are long distances between 

signalized intersections, unpredictable or relatively low minor-street volumes 

(e.g., less than 20 percent of volumes on the major street), and a large 

proportion of turning movements. 

Types of local intersection 

signal control include 

pretimed control and 

actuated control. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-7 Federal Highway Administration

E

A

D

C

B

Inductive

Loop

Detector

Signal Phase Diagram

Traffic

Signal

Controller

1 234

5 678

Multiple lane, multiple

detection zone nonintrusive sensors are 

replacement candidates for 

inductive loop detectors 

Figure 3-2. Isolated intersection control. The letters represent data sources 

that influence signal timing as explained in the text. 

Detector Inputs:

Stop Bar

Detection

Advance

Detection

(High Speed

Approaches)

Left Turn

Phase Detection

Special Vehicle Detection:

Emergency

Vehicles

Transit

Vehicles

Presence

Presence

Presence

Volume

Headway

Presence

Local Intersection

Controller

Right Turn

Screening

Gap Acceptance

Logic

Added Green

Per Actuation

Left Turn Phase

Emergency Vehicle

Preemption Logic

Bus Preemption

Logic

Phase Sequencing

and Control Logic

Signal

Conflict

Monitoring

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

intersection control. 

Fully actuated control operates with traffic detection on all approaches to the 

intersection for all signal phases. It is the most widely applied control 

strategy for isolated intersections. Because the cycle length varies from cycle 

to cycle, it can be utilized at street intersections with sporadic and varying 

traffic distribution. 

Volume-density control, a variant of actuated control, provides a complex set 

of criteria for allocating green time (“added initial” and “time waiting-gap 

reduction”). This mode can be utilized with both semiactuated and fully 

actuated signals. It normally operates on a continuously variable cycle length 

and requires accurate traffic flow data to accommodate changing conditions 

in a timely manner. Although the time waiting gap reduction can be utilized 

with presence sensors, passage sensors are normally installed far in advance 

In Figure 3-2, letter A indicates 

placement of stop bar detection 

sensors that are used to detect 

vehicles that turn right and thus 

avoid the need to call the green. 

Letter B indicates advance 

detection sensors used for 

measuring headways for gap 

acceptance logic, red light running 

minimization, and measuring 

volumes for added green per 

phase. Letter C indicates left turn 

phase detection sensors that 

measure presence and sometimes 

queue length of vehicles needing to 

call left turn phase logic. Letter D 

indicates emergency vehicle 

sensors that detect the approach of 

emergency vehicles and invoke 

emergency preemption logic. Letter 

E represents transit vehicle 

sensors, which invoke transit 

vehicle priority logic. All of the 

sensors provide traffic flow data 

used to actuate appropriate phases 

of isolated intersection signal 

control. The data enter a traffic 

signal controller located at one of 

the corners of the intersection. The 

controller uses logic processing, 

illustrated in Figure 3-3, to display 

the correct indications on the traffic 

signals and pedestrian signals. 
Chapter 3—Sensor Applications 

October 2006 Page 3-8 Federal Highway Administration 

of the intersection, e.g., from 200 to 600 ft (60 to 180 m) before the 

intersection, depending on approach speed. 

Figure 3-4 shows a typical detection zone placement for fully actuated 

intersection control with advance sensors only on the primary phases. The 

sensors near the stopline detect vehicles that would otherwise be trapped in 

the crosswalks or in front of the stopline. Passage and advance sensors are 

normally located in accordance with estimated approach speeds to place a 

call to the controller until the phase is serviced. 

The controller provides memory latching when passage sensors are used. 

Passage sensors generate a pulsed output (generally between 100 and 

150 ms) whenever a vehicle enters the sensor’s detection zone.(2) 

Advance detection

Left turn detection

Stopline/queue

clearing detection

Cross street

Arterial

street

Right turn

detection

Cross street

detection

Detection zone

4

7 6

1

3 8

2

5

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

represent a four-phase signal sequence consisting of traffic movements 

1 and 5, 2 and 6, 4 and 8, and 3 and 7. 

STRATEGIES FOR CONTROL OF GROUPS OF 

INTERCONNECTED INTERSECTIONS 

Interconnected intersection control provides signal progression that allows 

platoons of vehicles to proceed along arterial routes without stopping. It also 

imparts area-wide control to minimize total delay and number of stops over 

an entire network. This form of control is effective when traffic moves in 

platoons and their arrival time can be predicted at downstream intersections. 

Interconnected intersection control can function in a variety of ways. The 

first category selects from among a library of prestored signal timing plans 

based on TOD/DOW(classic pretimed). The prestored plans are generated 

offline from average or historical data. The second uses timing plans that 

best match current traffic flow conditions which may be generated online (see 
 Traffic Detector Handbook—2006 

October 2006 Page 3-9 Federal Highway Administration

1.5 GENERATION and SECOND GENERATION sections). The third uses 

flexible signal timing plans that are generated online in real time based on 

current traffic flow conditions based on centralized control with fixed offsets. 

These traffic responsive may update their flexible timing plans incrementally 

at each signal cycle. The fourth uses continuous adjustments to intersection 

timing with continuous communications between adjacent intersections and 

decentralized control. (See section titled Traffic Adaptive Control and 

Distributed Systems). These categories are detailed in Table 3-1. 

Sensor Data 

Sensors are utilized in interconnected intersection signal control to gather 

traffic flow data for signal timing plan selection or real-time calculation and to 

support critical intersection control. The signal timing selection process is 

similar for arterial and network systems. The operation of signal timing plans 

is determined by the roadway configuration and the goals of the corresponding 

plans. Figure 3-5 shows examples of surface street traffic signal configurations 

found in arterial open network systems and closed network systems typified by 

central business districts. An open network typically has coordination timing 

constraints on only two of the approaches to the signal. This is typical of 

arterial and multiple arterial systems. A closed network typically has 

constraints on all approaches to each intersection. This is typical of central 

business district systems and multiple parallel arterial systems. 

Arterial Systems 

Arterial signal control systems are often implemented using an open network. 

The goals of arterial system timing plans are to provide arterial progression in 

the direction that carries heavier traffic volumes, to maximize arterial capacity, 

and to minimize arterial delay. Cycle length, split, and offset timing plan 

parameters are varied to reflect the current traffic conditions. The maximum 

phase times for minor cross streets in arterial systems are often controlled by 

pedestrian crossing time requirements. Detection of cross-street demand was 

discussed earlier as a part of isolated intersection control. 

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

typical of those found in interconnected intersection control. 

Open Network System Closed Network System

Fully Actuated Traffic Signal

Semiactuated Traffic Signal 

Pretimed Signal
Chapter 3—Sensor Applications 

October 2006 Page 3-10 Federal Highway Administration 

Interconnected Control and Classic Pretimed Network 

Systems 

These network systems typify the era before urban traffic control system 

(UTCS) software was available. Two types of network systems were used: a 

closed-grid roadway configuration typical of urban central business districts 

and an area-wide system that controlled all or a major portion of the signals 

in a city or metropolitan area. The goals of network system timing plans were 

essentially the same as those for arterial systems. 

Many urban jurisdictions installed pretimed and semiactuated traffic signals 

for a closed network. Vehicle detection sensors were used at the semiactuated 

intersection. As with the arterial system, cycle length, split, and offset varied 

from plan to plan. The plans could be computed offline and implemented on a 

TOD/DOW basis as in UTCS 1st-generation software; computed offline and 

implemented on a traffic-adjusted basis as in UTCS 1.5-generation software; 

or computed online in real time as with 2nd-generation software. In all cases, 

the plans were pretimed with sensor inputs used to generate the data needed 

to calculate the pretimed plans. 

Timing Plan Implementation 

Timing plans were normally implemented by dividing a system into a 

number of sections, each of which had homogeneous traffic conditions. 

System sensors, assigned to each section, provided the information required 

for plan selection from a database library of applicable timing plans. 

Control of Groups of Intersections Using Online 

Generated Timing Plans 

The intersection control strategies described in previous sections are typical of 

those employed in pre-UTCS systems using uncoordinated control, time base 

coordinated control or interconnected control systems (see definitions in this 

chapter under “Signal Control Concepts for City Streets”). The UTCS project 

began the large-scale use of sensors and computers for controlling traffic signal 

systems in the United States. This project drove the initial U.S. development of 

traffic adjusted control and traffic responsive control, which led to traffic 

adaptive control. 

Traffic Adjusted Control and UTCS 1.5-Generation 

Systems 

First-generation interconnected traffic signal control systems are 

characterized by the TOD/DOW selection of a timing plan from a set of 

timing plans, which are computed offline. Traffic adjusted control such as 

UTCS 1.5-generation added the capability for timing plans to be selected 

based on a combination of volume V and weighted occupancy O sensor data, 

referred to as VPLUSKO (i.e., volume plus weighted occupancy), where the K

represents the weighting factor. Figure 3-6 illustrates the 1.5-generation 

UTCS timing plan selection process. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-11 Federal Highway Administration

• Comparison function

• Antihunting test

Smoothed

data

Optimum

timing plan

parameters

V + KO

Timing

plan

database

Historic

data

file

Valid

data

range

Sensor

error 

checking

Timing

plan

selection

Timing

plan

generation

Sensor data

needed for 

UTCS are

volume V and

occupancy O

Optimum

timing

plan

Traffic

Management

Center

Figure 3-6. UTCS timing plan selection procedure. 

Each timing plan has a VPLUSKO value corresponding to each of the sensors 

in the system. The sensor outputs are compared to the VPLUSKO values from 

each plan, and the plan that most closely matches the sensor outputs is 

selected. If a potential timing plan is found to be more favorable than the 

current plan, then the new plan is subjected to an antihunting test. The 

purpose of the antihunting test is to verify that the new plan is sufficiently 

better (by a predefined amount) to warrant implementation. This prevents 

needless transitions between timing plans that have similar benefits. The 

comparison operation is repeated at user-selected intervals, e.g., 4 to 15 

minutes. 

However, 1.5-generation systems do not address all the drawbacks of preUTCS signal control such as unexpected traffic flow scenarios (e.g., from 

unanticipated incidents). Traffic adjusted control (such as UTCS 1.5) 

responds successfully to precalculated traffic demands such as those 

produced by large vehicle flows from parking lot exits at sports venues. 

Second-Generation Traffic Signal Control Systems 

Second-generation UTCS was a first attempt at real-time, online computing of 

optimized splits and offsets, while keeping cycle length fixed within variable 

groups of intersections. Trials with this technique demonstrated some 

reduction in vehicle-minutes of travel time (with respect to the base system) on 

the arterial, but increased travel time in the network as a whole.(7) More 

advanced strategies are found in third-generation traffic signal control 

systems. These traffic systems support the online generation and 

implementation of signal timing parameters derived from real-time sensor data 

and modeling, prediction, and optimization techniques. 

Third-Generation Traffic Signal Control Systems 

Traffic responsive and traffic adaptive systems can overcome several 

limitations of signal control systems that rely solely on prestored timing plans. 

For example, prestored timing plans developed offline are best suited for traffic 

flow on a normal day or for events that produce predictable traffic patterns. 

Their major disadvantage is that they are developed from specific traffic flow 
Chapter 3—Sensor Applications 

October 2006 Page 3-12 Federal Highway Administration 

scenarios and, therefore, cannot respond to situations that are significantly 

different from those used to generate them. Their major advantage is that they 

can be implemented at the beginning of a preplanned traffic event or incident 

such as the exiting of a large number of vehicles from a sports venue parking 

facility. Furthermore, data collection and manpower costs limit the ability of 

many traffic management organizations to maintain timing plans that are 

representative of current traffic volumes and patterns. Traffic responsive and 

traffic daptive systems attempt to overcome these limitations by providing 

signal timing that more quickly responds to real-time traffic flow sensor data 

than traditional traffic adjusted or TOD/DOW systems. 

Traffic responsive and traffic adaptive systems generally require a greater 

number of sensors than conventional first-generation systems and usually 

require extensive initial calibration and validation.(8) Therefore, total system 

life-cycle costs, including software licensing, purchase of local controllers and 

central computers, and ongoing operating and field maintenance costs, are 

often compared to expected benefits when evaluating traffic signal operating 

strategies. Nevertheless, continued advancements in sensor and computer 

system technology plus improving traffic control algorithms are making traffic 

responsive and traffic adaptive systems increasingly attractive as compared to 

conventional systems when traffic volumes and roadway network design 

warrant their use. The ability to adapt to changes in traffic flow patterns over 

long-term intervals (i.e., to respond to aging of prestored timing plans) 

frequently makes traffic responsive systems cost effective.(9-26) 

Centralized traffic responsive and distributed traffic adaptive system concepts 

have been developed for signal control. Typical of the traffic responsive concept 

are SCOOT and SCATS. 

Traffic Responsive Control and Centralized Systems 

SCOOT continuously measures traffic demand on most approaches to 

intersections in the network and optimizes signal cycle lengths, splits, and 

offsets to minimize delay and stops. SCOOT sensors are located upstream from 

the signal stopline, approximately 15 meters downstream of the adjacent 

upstream intersection. Timing changes per cycle are small to avoid major 

disruptions to traffic flow, but frequent enough to allow rapid response to 

changing traffic conditions. The prototype SCOOT systems reduced peak period 

average delay at traffic signals by approximately 11 percent as compared to 

fixed signal control plans generated by TRANSYT. Off-peak delay was reduced 

by an average of 16 percent.(10,16) 

Similar to SCOOT, SCATS adjusts cycle time, splits, and offsets in response to 

real-time traffic demand and system capacity. The principal goal of SCATS is 

to minimize overall stops and delay when traffic demand is less than system 

capacity. When demand approaches system capacity, SCATS maximizes 

throughput and controls queue formation. SCATS sensors are installed in each 

lane immediately in advance of the stopline to collect volume and occupancy 

data during the green of the approach. Studies have found that the reduction of 

stopped and approach delay for the main approaches to intersections is greater 

during low-volume time periods than during peak volume periods.(27-29) Sensor 

configurations for SCOOT and SCATS are discussed in Appendix L. 

SCOOT and SCATS have been 

called both traffic responsive and 

traffic adaptive. This edition of the 

Traffic Detector Handbook uses the 

definitions in Appendix L, 

“Classification of Sensor Systems 

by Sensor Density Level,” and 

Appendix P, “Glossary,” and 

classifies them as traffic responsive 

systems. 

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. 

SCATS: The Sydney Coordinated 

Adaptive Traffic System (SCATS). 

A real-time system developed by 

the Roads and Traffic Authority 

(RTA) of New South Wales, 

Australia. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-13 Federal Highway Administration

Traffic Adaptive Control and Distributed Systems 

Traffic adaptive systems are typified by the adaptive control system (ACS), 

which uses OPAC algorithms in one rendering and RHODES algorithms in 

another. These distributed systems are executed at the local intersection using 

an advanced traffic controller such as the 2070. They employ a rolling horizon, 

which recalculates predictions and optimizations every second for several 

seconds. Sensors placed upstream of the stopline give at least 10 to 15 seconds 

of travel time until the upstream vehicle platoon reaches the downstream 

intersections.(2) Sensor configurations for ACS are discussed in Appendix L. An 

implementation of ACS with the OPAC algorithms on Reston Parkway in 

Virginia resulted in a 5 to 8 percent reduction in stops and delay over an 

optimized actuated signal control system.(30) In simulations conducted as part 

of the RT-TRACS program (now called ACS), RHODES was found to increase 

throughput and reduce delay on the test network and, hence, performed better 

than the fixed-timing plan generated from TRANSYT-7F.(31-33) The advent of 

sensors that can be used to measure turning movements and travel times on 

links will expand the application of algorithms used in traffic adaptive 

systems. See Table 3-1 for more detail on differences between traffic responsive 

and traffic adaptive control. 

Review of strategies for control of groups of intersections 

Table 3-1 summarizes the characteristics of different categories of control 

strategies for groups of intersections. The key difference is how rapidly each 

control strategy can make changes in response to variations in traffic demand. 

From this criterion flows differences in the data requirements and data flow 

rate needed to implement the strategies. This in turn enables different timing 

features to be changed at different frequencies in each category. 

The key differences between 

interconnected control, traffic 

adjusted control, traffic 

responsive control and traffic 

adaptive control are shown 

in Table 3-1. 

Wireless communications 

may be substituted for 

wireline for most 

communications speeds. 
Chapter 3—Sensor Applications 

October 2006 Page 3-14 Federal Highway Administration 

Table 3-1. Categories of traffic signal systems and their characteristics or requirements.

Category 

title to right 

characteristic below 

Uncoordinated control Time-based 

coordinated 

control 

(TBC) and 

interconnected 

control 

categories 

Traffic adjusted 

control 

Traffic 

responsive 

control 

Traffic 

adaptive control 

Response to 

variations in 

traffic 

demand 

Very slow 

reactive 

response 

based on 

historic traffic 

flows, prompt 

local actuated 

response 

possible, no 

current 

system 

response 

Very slow 

reactive 

response based 

on historic 

traffic flows, 

prompt local 

actuated 

response 

possible, no 

current system 

response 

Slow reactive 

response based 

on overall 

network trend—

volume plus 

weighted 

occupancy or 

general direction 

of flow 

Prompt reactive 

response based 

on changes in 

platoons 

Very rapid 

predictive 

proactive 

response based 

on predicted 

movements of 

individual 

vehicles or small 

packets of 

vehicles 

Frequency 

of change in 

control plan 

No changes 

related to 

adjacent 

intersections 

Plans for 

peaks, midday 

and evening off 

peaks, and 

weekends 

Minimum of 15 

minutes, usually 

several times a 

rush period 

Minimum of 15 

minutes with 

adjustments 

possible once 

per cycle 

No overall timing 

plan as 

continuous 

adjustments are 

made to all 

parameters. 

What timing 

features are 

changed 

No changes 

related to 

adjacent 

Intersections 

Timing for 

individual 

intersections 

may change 

at preset 

TOD/DOW 

Total timing 

plan may be 

changed out at 

preset 

TOD/DOW—

Fixed plan with 

splits, offsets 

cycle length, 

etc., unchanged 

during period 

Total timing plan 

changed out—

Fixed plan with 

splits, offsets, 

cycle length, etc., 

unchanged 

during period; 

variable TOD 

when plan is 

changed 

Total timing plan 

changed out—

Flexible plan 

with cycle, splits, 

offsets, and 

actuated settings 

changeable once 

per cycle; 

variable TOD 

plan changes 

Phase selection, 

sequence, and 

durations 

adjusted every 

time step; cycle 

and offset not 

required; splits 

are dynamic as 

above 

Data 

communications 

requirements 

None Not required 

for TBC; low 

speed for 

interconnected 

real-time comm 

not required 

(frequently 

twisted pair) 

Low speed, real 

time 

communica-tion 

may be needed; 

twisted pair or 

better preferred 

Low speed but 

faster is better; 

real-time 

communication 

is needed 

Fast and real 

time 

System

sensor 

requirements 

No system

sensors 

required 

No system

sensors 

required 

Minimal, one 

system sensor 

per intersection 

on average

Moderate, 

minimum of 1 per 

approach, 

preferred 1 per 

lane per approach 

High, minimum 

of 1 per lane per 

approach, 

RHODES 2 per 

lane per 

approach 

All of the categories and 

characteristics are for 

systems and not for the 

individual intersection. 

This row describes the 

system response and not 

any local traffic actuated 

response. 

In some interconnected 

systems, operators can 

select plans in real time. In 

all traffic adjusted and traffic 

responsive systems, plans 

may be selected in real time. 

In most traffic adjusted and 

all traffic responsive 

systems, operator may 

select and download 

changes in signal settings 

and plans in real time. 

Fixed timing plans—cycle, 

split, offset, and actuated 

settings are 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. 

Wireless communications 

may be substituted for 

wireline for most 

communications speeds. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-15 Federal Highway Administration

Table 3-1. Categories of traffic signal systems and their characteristics or requirements—Continued

Category 

title to right 

characteristic relow 

Uncoordinated control TBC and 

interconnected 

control 

categories 

Traffic adjusted 

control 

Traffic responsive 

control 

Traffic adaptive 

control 

Local intersection 

controller 

and 

coresident 

local 

computer 

requirements 

For new 

installations 

NEMA/170 

and no 

coresident 

computer 

For new 

installations 

NEMA/170 and 

no coresident 

computer 

NEMA/170 and 

no coresident 

computer 

Moderate—

NEMA/170/ATC 

and no coresident 

computer for 

local control 

algorithms 

High, ATC 

preferred, 

coprocessor 

card or separate 

computer box 

for running local 

algorithms 

Central 

computer or 

system 

master 

requirements 

None TBC None—

interconnect 

minimal, vendor 

system master 

or PC level 

Moderate, 

vendor system 

master or PC 

level 

High end system 

master or PC 

required— 

much central 

processing 

Medium most 

processing is 

local 

Maintenance of 

database, 

communications, 

sensors and 

system 

master 

Low cost 

—local 

settings, 

local sensors, 

traffic studies 

Low cost 

—add setting 

coordination 

settings, some 

communications traffic 

studies for them 

Moderate cost 

—add to 

interconnected 

more database, 

communications and 

minimal system 

sensors 

Medium cost —

add more system 

sensors, central 

database, 

communications 

but fewer traffic 

studies 

High cost 

—add more 

system sensors, 

communications, less 

central database 

Installation Low cost Low cost Moderate cost Medium cost High cost 

System 

design and 

operation 

Requires 

lowest skill 

level and 

staffing 

Requires 

moderate skill 

level and 

staffing 

Complex and 

requires good 

skill level and 

staffing 

More complex 

and requires high 

skill level and 

staffing 

More complex 

and requires 

high skill level 

and staffing 

SYSTEMS ENGINEERING PROCESS IN DESIGN OF 

TRAFFIC SIGNAL SYSTEMS 

A discussion of the systems engineering process involved in designing traffic 

signal systems is found in NCHRP Synthesis 307.(8) The major steps involved 

in the systems engineering process as applied to traffic signal systems begin 

with the identification of requirements for: 

• Function. 

• System design. 

• Project management. 

• Operations. 

• Logistics. 

• Evaluation. 

Once requirements are established, plans and methods to implement them and 

evaluate system performance are developed. 

The Traffic Control Systems Handbook and the Traffic Detector Handbook

describe the types of coordinated traffic signal systems through a category of 

characteristics as listed in Table 3-1. The number of sensors required for each 

level are described in more detail in Appendix L. 
Chapter 3—Sensor Applications 

October 2006 Page 3-16 Federal Highway Administration 

FREEWAY SURVEILLANCE AND CONTROL 

Sensors are used in freeway surveillance and control to detect recurring and 

nonrecurring congestion and to assist in mitigating their effects. Recurring

congestion occurs when both the location and time of congestion are 

predictable such as during weekday peak travel periods. Nonrecurring 

congestion is caused by random, temporary incidents such as stalled vehicles, 

accidents, spilled loads, or other unpredictable events. 

Recurring congestion develops when traffic demand exceeds freeway 

capacity. Recurring congestion may be reduced by decreasing peak period 

demand through techniques such as entrance ramp metering, mainline 

metering, freeway-to-freeway connection control, corridor management, 

control of the number of occupants needed to access high-occupancy vehicle 

lanes, congestion-based pricing for use of toll facilities, and advanced traveler 

information systems that inform motorists of congestion ahead and perhaps 

of alternate routes. Sensors play a major role in alleviating recurring 

congestion, particularly in entrance ramp metering. 

Nonrecurring congestion is more difficult to manage because of its 

unpredictability. Detecting the incident and removing its cause as quickly as 

possible minimize the effects of the nonrecurring events. 

A variety of information gathering techniques are used for incident detection, 

including periodic sensor placement along freeways, closed-circuit television, 

aerial surveillance, emergency call boxes, freeway patrols, and cellular 

telephone calls from motorists. Nonimaging mainline sensors are not as 

effective for incident identification, but are often used to detect the beginning 

of congestion at off peak hours, which usually indicates some type of incident 

has occurred. They can also be used to determine the extent of the incident’s 

impact. Automatic incident detection algorithms face a more difficult task 

during peak traffic hours because the characteristics of nonrecurring 

congestion caused by an incident are often difficult to differentiate from those 

of recurring congestion. 

The following sections further explore techniques that utilize sensors for 

managing traffic on freeways. 

FREEWAY INCIDENT DETECTION 

Fifty to sixty percent of the delay on urban freeways is associated with 

incidents, rather than with recurring congestion due to capacity 

shortfall.(34,35) Often recurring and nonrecurring congestion occur 

simultaneously during peak periods. In fact, since most algorithms detect 

incidents by measuring perturbations in traffic flow, automatic incident 

detection techniques must distinguish between shock waves formed by 

recurring peak-period congestion and those formed by incidents during 

similar periods. An incident can be defined as any anomaly that disrupts the 

smooth flow of traffic. Debris on the road, flooding, cargo spills, vehicle 

collisions or accidents, special events such as ball games and festivals, and 

highway work are examples of incidents. 

Freeway surveillance and 

control are used to mitigate 

recurring and nonrecurring 

congestion. Common 

techniques applied for this 

application include entrance 

ramp metering, mainline 

metering, freeway-to-freeway 

metering, corridor 

management, high 

occupancy vehicle lanes, 

congestion-based pricing for 

toll facilities, advanced 

traveler information systems, 

and automatic incident 

detection. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-17 Federal Highway Administration

Traffic Flow Characteristics During an Incident 

Traffic flow characteristics during a freeway incident can be characterized in 

terms of the four flow regions illustrated in Figure 3-7.(36) Flow region A is far 

enough upstream of the incident so that traffic moves at normal speeds with 

normal density. Flow region B is the area located directly behind the incident 

where vehicles are queuing if traffic demand exceeds the restricted capacity 

caused by the incident. In this region, characterized by the upstream 

propagation of a shock wave, speeds are generally lower, and a greater 

vehicle density may exist. Flow region C, also shown in Figure 3-8, is the 

region directly downstream from the incident where traffic is flowing at a 

metered rate, or incident flow rate, due to the restricted capacity caused by 

the incident. Depending on the extent of the capacity reduction, traffic 

density in region C can be lower than normal, while the corresponding traffic 

speed is generally higher than normal. Flow region D is far enough 

downstream from the incident such that traffic in D flows at normal density 

and speed, as in region A. 

Incident clearing and the return of traffic flow to normal occur in several 

phases. These are detection, verification and identification, response, 

removal, and recovery. Detection determines that something extraordinary 

has occurred. Then the location, types of vehicles, presence of hazardous 

materials, and possible injuries related to the incident have to be verified and 

identified. This step facilitates timely dispatching of appropriate personnel 

and equipment to remove affected vehicles and people from the site. 

AB CD

Incident Location

Upstream Downstream

Shock Wave

Density Speed Region Direction

Normal Congestion Metered Normal 

Figure 3-7. Traffic flow characteristics during an incident (source: Intelligent Vehicle Highway 

Systems: The State of the Art, prepared for Massachusetts Department of Highways 

(JHK and Associates, New York, NY, March 1993)). 
Chapter 3—Sensor Applications 

October 2006 Page 3-18 Federal Highway Administration 

Automatic incident detection identifies 

anomalies in traffic flow and distinguishes

between recurring congestion flow patterns

and non-recurring congestion due to incidents

Upstream Sensor Station

Moderate to high volume

High occupancy

Erratic or low vehicle speed

Downstream Sensor Station

Low volume

Low occupancy

Higher vehicle speed

Figure 3-8. Freeway incident detection scenario. 

Detection and verification are the areas in which improved sensor and 

communications technology can be of most benefit to incident clearing. 

Closed circuit television, roving highway and freeway service patrols, aerial 

surveillance, roadside emergency call boxes, reports from fleet operators, and 

some repeated number of cellular telephone calls to 911 or the traffic 

management center are methods used to verify the occurrence of an incident. 

Timeliness is the key to incident clearing and the minimization of congested 

flow conditions. Rapid detection and verification allow faster and perhaps 

less complex response options and more effective aid to victims. Quick 

response and the proper equipment permit rapid removal of affected vehicles 

and debris, and reduced delay and exposure to secondary accidents at the 

scene and at the end of any queues that form. When an incident blocks a 

traffic lane, the flow is choked and slowed, resulting in a traffic queue 

upstream of the incident. The queue and vehicle-hours of delay continue to 

build until the incident is cleared and normal traffic flow is restored. If the 

normal flow of traffic into the incident site is reduced by diversion onto 

alternate routes, then the vehicle-hours of delay are minimized. If normal 

traffic flow is not diverted, then additional vehicle-hours of delay are 

realized.(35) 

ITS technology, with its emphasis on real-time operation and rapid 

communication, can assist in reducing the delay and crash costs associated 

with incidents. However, without the capability for rapid response and the 

cooperation and coordination among personnel in the responsible agencies 

(e.g., highway patrol, tow truck operators, fire, hazardous material team, 

emergency medical services, local police, traffic and transit management), 

the potential benefits of these high technology systems may not be realized. 

FREEWAY METERING 

Several forms of freeway metering exist. The most common is onramp 

metering, which restricts freeway demand at limited access onramps in an 

effort to prevent breakdown of flow on the mainline. Freeway-to-freeway 

connector metering, also used to control mainline demand, is designed so that 

queues at the metering signals do not compromise safety on the high-speed 

roadways. Mainline metering is applied at selected locations, such as bridges 

and tunnels, to maximize traffic flow through these facilities. It is usually 

implemented where there is sufficient storage capacity (such as at a toll 

plaza) and in conjunction with HOV lanes that bypass the queues upstream 

of the signals. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-19 Federal Highway Administration

Ramp closure, a technique that does not require sensors on the ramp or 

mainline, can be used to restrict the number of vehicles entering a freeway. 

The more modern approach is to use sensors to monitor mainline congestion 

and transmit that information to ramp signals, which moderate the number 

of vehicles entering the freeway mainline. 

Ramp Closure 

Closing an entrance ramp during peak period is a positive technique for 

limiting the number of vehicles entering a congested freeway. It is, however, 

the most restrictive and least popular with the public. If applied in an 

inappropriate situation, it could result in underutilization of the freeway and 

the overloading of alternative routes. Ramp closures are effective where the 

entrance ramp introduces serious weaving or merging problems under 

congested conditions. 

Manually placed barriers (including law enforcement vehicles), automated 

barriers, and signing are used for ramp closures. Sensors are not required 

except, perhaps, during the changeover operation of automated barriers. 

Ramp Metering 

The most common technique for addressing recurring congestion on freeways 

is ramp metering. It limits the rate at which vehicles enter the freeway's 

mainline so that the downstream mainline capacity is not exceeded. Ramp 

metering redistributes the freeway demand over space and time. Excess 

demand is either stored on the ramp or diverted. The diverted vehicles may 

choose less traveled alternate routes, or their occupants may select another 

mode of transportation. Metering rates range from a minimum of 180 to 240 

vehicles per hour (v/h) to a practical maximum of 750 to 900 v/h. 

Ramp meters assist in dispersing platoons of vehicles that are released from 

nearby signalized intersections. By releasing a limited number of vehicles 

into the mainline traffic stream, turbulence is reduced in the merge zone. 

This leads to a reduction in sideswipe and rear-end accidents, which are 

associated with stop-and-go traffic flow. Maximum mainline flow rates can be 

achieved by controlling ramp flow rates such that freeway traffic moves at or 

near optimum speed throughout the network. 

Management of nonrecurring congestion created by freeway incidents is a 

secondary benefit associated with ramp metering. Once an incident is 

detected, ramp metering can potentially reduce the number of vehicles 

impacted by the incident. For example, meters upstream of a detected 

incident can be adjusted to allow fewer vehicles to enter the affected facility, 

potentially diverting some trips to onramps downstream of the incident 

location. Conversely, the downstream ramps can operate with relaxed 

metering rates in order to accommodate the increased demand. This incident 

management strategy works well during peak periods, especially when 

integrated with a smart corridor operation. Incidents occurring during offpeak hours may be mitigated with a strategy that closes upstream onramps 

rather than with metering because of the reduced onramp volumes. 

A seminal study that evaluated the benefits of ramp metering was conducted 

by the Minnesota Department of Transportation (MnDOT) in accordance 

with a bill passed in the year 2000 session by the Minnesota Legislature.(37)

The bill required MnDOT to examine the effectiveness of ramp meters in the 

Minneapolis-St. Paul, MN, region by conducting a shutdown study before the 
Chapter 3—Sensor Applications 

October 2006 Page 3-20 Federal Highway Administration 

next legislative session. The goal was to evaluate and report any relevant 

fact comparisons or statistics concerning traffic flow and safety impacts 

associated with deactivating system ramp meters for a predetermined 

amount of time. The study, completed at a cost of $651,600, occurred in the 

fall of 2000, with the results presented to the Legislature and the public in 

early 2001. 

A summary of the conclusions reached by MnDOT concerning the annual 

benefits of ramp metering is as follows: 

• Traffic volumes and throughput: After the meters were turned off, 

traffic volume decreased by an average of 9 percent on freeways, with no 

significant traffic volume change on parallel arterials included in the 

study. Also, during peak traffic conditions, freeway mainline throughput 

declined by an average of 14 percent in the “without meters” condition. 

• Travel time: Without meters, the decline in travel speeds on freeway 

facilities more than offset the elimination of ramp delays. This gives 

an annual system-wide savings of 25,121 hours of travel time with 

ramp meters. 

• Travel time reliability: Without ramp metering, freeway travel time 

was almost twice as unpredictable as with ramp metering. The ramp 

metering system reduced unexpected delay by 2.6 million hours. 

• Safety: In the absence of metering and after accounting for seasonal 

variations, peak period crashes on previously metered freeways and 

ramps increased by 26 percent. Ramp metering reduced annual 

crashes by 1,041 or approximately 4 crashes per day. 

• Emissions: Ramp metering reduced net annual emissions by 1,160 tons. 

• Fuel consumption: Ramp metering increased annual fuel 

consumption by 5.5 million gallons, based on a simple straight-line 

estimation technique that does not address the tempering of flow, 

typically because of ramp metering, by smoothing the travel speed 

variability (less acceleration and deceleration). This was the only 

criteria category that was degraded by ramp metering. However, four 

other regions of the country examined for fuel consumption impacts 

of ramp metering as part of the MnDOT study showed fuel savings 

ranging from about 6 percent to 13 percent. 

• Benefit/Cost Analysis: Ramp metering produces an annual savings of 

approximately $40 million to the Twin Cities traveling public. The 

benefits of ramp metering outweigh the costs by a significant margin 

and result in a net benefit of $32 to $37 million per year. The 

benefit/cost ratio indicates that benefits are approximately 5 times 

the cost of the entire congestion management system and more than 

15 times the cost of the ramp metering system alone. 

Figure 3-9 depicts a conceptual freeway ramp meter installation. The sensors 

on the mainline serve a dual purpose: adjustment of the ramp-metering rate 

in response to real-time demand and collection of historical volume and 

occupancy data. A demand sensor on the ramp indicates the arrival of a 

vehicle at the stopline and the commensurate start of the metering cycle. 

Demand at the stopline is typically required before the ramp signal is 

allowed to turn green. A passage sensor detects when the vehicle passes the 

stopline and returns the ramp signal to red for the next vehicle. The passage 

sensor can also be used to monitor meter violations (i.e., drivers who ignore 
 Traffic Detector Handbook—2006 

October 2006 Page 3-21 Federal Highway Administration

the red stop signal) and provide historical data about the violation rate at 

each ramp. 

Mainline sensors

Freeway mainline

Demand

sensor 

Queue sensor

Passage sensor

Controller

Advanced sign to warn

that metering is 'on'

Frontage road or surface street

Stopline

Ramp metering 

signals

•

•

•

•

•

•

•

•

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

Ramps that contain two metered lanes or one metered and one unmetered 

HOV lane add a count sensor after the passage sensor to obtain the total 

count of vehicles entering the mainline. The queue sensor is used at locations 

where ramp backup impacts surface street operation. A high occupancy rate 

over a queue sensor can signal the ramp metering logic to increase vehicle 

passage or, in special situations, to suspend metering to prevent vehicles 

from backing onto feeder roads.(2) The advanced warning sign alerts drivers 

to the operational status of metering activity. Where ramp geometrics are 

poor, a merge sensor may be used as a feedback mechanism to prevent 

additional vehicles from proceeding down the ramp if a vehicle is stopped in 

the freeway merge area. 

Current ramp metering strategies include:(2) 

• Pretimed metering. 

• Local traffic responsive control. 

• Coordinated or global traffic responsive control. 

Pretimed Ramp Metering 

The traffic signal in a pretimed metering system operates with a constant 

cycle using a metering rate calculated from historical data. Metering and 

signage can accommodate single vehicle entry or platoon entry onto the 

mainline. The major operational advantage of pretimed metering is the 

regularity of the rate that is easily accommodated by drivers. The principal 

drawback of pretimed metering is its inherent insensitivity to changes in 

traffic conditions. Pretimed metering is often implemented as an initial 

operating strategy until traffic responsive control can be initiated. The 

sensors needed for pretimed metering are a queue sensor, demand sensor, 

and passage sensor. 
Chapter 3—Sensor Applications 

October 2006 Page 3-22 Federal Highway Administration 

The method for calculating pretimed metering rates depends on the primary 

purpose of the metering, namely to minimize congestion or to improve safety. 

Minimizing congestion through pretimed metering—Freeway congestion 

reduction is implemented by choosing a metering rate approximately equal to 

the difference between upstream freeway demand and the downstream 

freeway capacity. Metering rates that address congestion reduction are based 

on: 

• Ramp storage capacity. 

• Availability of alternative routes for diverted traffic. 

• Conditions at other ramps upstream and downstream of the 

controlled ramp. 

Improving safety through pretimed metering—Pretimed metering that 

improves merging safety allows each vehicle enough time to enter the 

mainline before the following vehicle enters the merging area. This prevents 

rear-end and lane-changing collisions by breaking up platoons of vehicles 

that compete for gaps in the freeway traffic stream. Metering rates that 

primarily treat safety depend on the distance from the stopline to the 

merging point, ramp geometry, and vehicle type. 

Local Traffic Responsive Metering 

Local responsive metering uses real-time mainline traffic flow information 

near the ramp or just downstream of the ramp to determine metering rates. 

The traffic flow parameters assist in evaluating freeway operation with 

respect to upstream demand and downstream capacity and in determining 

the maximum number of ramp vehicles permitted to enter the freeway 

without causing congestion. Traffic flow parameter measurements such as 

occupancy and volume are often smoothed to filter short-term random 

fluctuations by calculating running averages over a 5-minute period. Some 

facilities include sensors to determine traffic composition and weather to 

account for the effects of these factors. The sensors needed for pretimed 

metering are a queue sensor, demand sensor, passage sensor, merge sensor, 

and mainline sensors. 

Advantages and disadvantages—The advantage of local traffic responsive 

metering over pretimed metering is the ability of the metering rate to 

respond to short-term variations in traffic demand or to reduced capacity 

caused by incidents downstream of the ramp. Results with traffic responsive 

metering are 5 to 10 percent better than pretimed metering in terms of 

reduced overall delay. A best case example of traffic responsive ramp 

metering benefits is from a ramp control experiment in Los Angeles, CA, that 

produced a 100 percent increase in average speed from 25 to 52 mi/h (40 to 84 

km/h), a 20 percent decrease in ramp wait time, and a 3 percent increase in 

freeway volumes.(38) The main disadvantage of local traffic responsive control 

strategies is that mitigation must wait until the congestion reaches the local 

section controlled by the ramp. Table 3-2 contains a recommended range of 

metering rates based on mainline occupancy as the controlling 

parameter.(39,40) 
 Traffic Detector Handbook—2006 

October 2006 Page 3-23 Federal Highway Administration

Table 3-2. Local responsive ramp metering rates based on mainline occupancy. 

Occupancy 

(percent) 

Metering rate 

vehicles per 

minute (v/min)a

 Occupancy 

(percent) 

Metering rate 

(v/min)b

<10 12 <20 12+ 

11–16 10 20–22 10 

17–22 8 22–25 8 

23–28 6 25–27 6 

29–34 4 27 or higher 4 

>34 3 

a Blumentritt, C.W., et al. Guidelines for Selection of Ramp Control Systems, NCHRP Report 

232. National Research Council, Transportation Research Board, Washington, DC, May 1981. b McDermott, J.M., S.J. Kolenko, and R.J. Wojcik. Chicago Area Expressway Surveillance and 

Control: Final Report, Report Number FHWA-IL-ES-27. Illinois Department of Transportation, 

Oak Park, IL, 1979. 

Improving safety—The impact of local traffic responsive ramp metering on 

the crash rate was studied in an early operational test in Chicago, IL. The 

metering rate was controlled by stream measurements of lane occupancy in 

one of the center lanes near the metering device, plus measurements of 

occupancy at the nearest downstream bottleneck. The prevailing metering 

rate was the most restrictive calculated from local or freeway segment 

conditions. An analysis of merges that occurred during peak traffic flow 

periods showed an 11.6 percent reduction in all traffic conflicts when the 

metering rate was 12 or more v/min. Metering at a more restrictive rate was 

predicted to reduce conflicts further.(40) Although strategies were developed 

for controlling metering rates based on mainline flow rate, ramp flow rate, 

and occupancy measurements, the advantages of these techniques were 

outweighed by operational and maintenance disadvantages created by the 

need for more sensors and more precise and accurate data. 

Implementation—Local traffic responsive metering algorithms may be 

implemented with techniques other than lookup tables. Some algorithms 

attempt to maintain mainline occupancy at a preselected value through a 

recursive process. Asservissement LINéaire d’Entrée Autoroutière 

(ALINEA), is such an algorithm that uses feedback to control mainline 

occupancy by adjusting the metering rate R (v/min) at time t+1 based on its 

value at time t according to(41-43) 

 (3-1) 

where 

R|t = metering rate at time t in v/min 

KR = rate adjustment parameter in v/min 

Oˆ = nominal or target occupancy 

O|t+1 = measured mainline occupancy at time t+1. 

Papageorgiou uses a default value of 1.17 for KR, while Mahmassani reports 

a value of 0.32. Typical values for Oˆ are 0.17 to 0.30. If the calculated value 

for R violates physical or operational constraints, it is suitably adjusted to a 

feasible value. The metering rate at any time may be constrained by 

maximum and minimum values to suit the traffic management objectives. 

R|t+1 = R|t + K [Oˆ – O|t+1] R
Chapter 3—Sensor Applications 

October 2006 Page 3-24 Federal Highway Administration 

The default values of KR are for illustration only and are not meant to imply 

values suitable for any or all applications. 

In the Netherlands, fuzzy logic control (FLC) is used on the A12 freeway 

between The Hague and Utrecht.(44) The algorithm restricts the metering 

rate when the downstream speed is less than the upstream speed. FLC 

produced 35 percent faster travel times and a 5 to 6 percent greater 

bottleneck capacity than two other controllers over a 6.8-mi (11-km) freeway 

section. 

The Washington State Department of Transportation (WSDOT) operated two 

ramp metering algorithms in the greater Seattle area, the local metering 

algorithm and the bottleneck algorithm.(45) The local metering algorithm uses 

linear interpolation between measured mainline occupancy and appropriate 

metering rates for that occupancy. The bottleneck algorithm reduces the 

number of vehicles entering the freeway by the number of vehicles stored in a 

downstream bottleneck section. Both of these algorithms use queue overrides 

to flush the ramp queue when it becomes excessive. A fuzzy logic algorithm 

(FLA) is replacing the older algorithms to meter traffic on more than 100 

ramps on Interstates 5, 405, and 90 and on State Route 520. Congestion at 

the I–90 study site was 8.2 percent lower with the FLA than with the local 

metering algorithm, while throughput was 4.9 percent greater. Some ramp 

queues decreased, while other increased slightly. However, the ramps had 

sufficient storage space, and given the mainline benefits, slightly longer 

ramp queues were acceptable. Results at the more congested I–405 test site 

were mixed. Mainline congestion increased by 1.2 percent with the FLA, 

while throughput increased slightly by 0.8 percent (as compared to the 

bottleneck algorithm). However, the fuzzy logic algorithm significantly 

reduced the time each ramp was congested by an average of 26.5 minutes.(46)

A fuzzy logic control algorithm is under investigation to simultaneously 

control multiple ramps along a freeway section.(47,48) 

Local traffic responsive metering, as used by Caltrans in Orange County, CA, 

establishes a floor below which the rate cannot fall. The local responsive 

ramp-metering algorithm that resides in the 170 controllers at the onramps 

operates as follows. First, a TOD rate is programmed into the 170 controller. 

This rate may be: alternatively(1) manually entered by a traffic engineer into 

the 170 controller in the field, (2) sent by a traffic management center 

operator to the 170 in the field, or (3) sent by area engineers from their office 

to the 170 in the field. Second, critical values for the average 3-min mainline 

volume/lane (currently 75 vehicles) and the average 1-min mainline 

occupancy/lane (currently 20 percent) are entered into the 170 controller. 

Third, a local responsive rate RLR, based on the actual mainline volume, for 

the number of vehicles released by the ramp is calculated as: 

( - )( )/3 veh/min RLR = VC VM NRL (3-2) 

where 

VC = critical value of mainline traffic volume per lane for a 3-min period 

VM = real-time mainline traffic volume per lane over 3 min 

NRL = number of ramp lanes 

and the factor of 3 converts the 3-min values into 1-min values. 

Fourth, if the locally measured mainline volume is less than the critical 

value, then the local responsive meter rate is used, provided RLR is greater 
 Traffic Detector Handbook—2006 

October 2006 Page 3-25 Federal Highway Administration

than the value in the TOD table. If the TOD rate is larger, then it is used. If 

the locally measured mainline volume is greater than the critical value, then 

the TOD rate is also used. Other versions of a local responsive rampmetering algorithm can compare the actual mainline occupancy with the 

critical occupancy value and adjust the meter rate accordingly.(49) 

Coordinated Traffic Responsive Control 

In this strategy, traffic flow data are analyzed at a central traffic 

management center that simultaneously adjusts the metering rates at 

several ramps. The metering rates are found from the analysis of the demand 

and capacity of an entire freeway section rather than traffic conditions in the 

immediate vicinity of individual ramps. A conceptual representation of a 

coordinated traffic responsive ramp control system is shown in Figure 3-10. 

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

A linear programming model is often used to calculate sets of integrated 

metering rates for each ramp based on the expected range of capacity and 

demand. The appropriate metering rate at each ramp is then selected from 

these precomputed sets, based on real-time measurement of freeway 

conditions.(38) In another approach, the temporal and spatial characteristics 

of the traffic flow pattern on the freeway and affected surface streets are 

used as inputs to a metering rate optimization algorithm.(50) 

In the United States, applications of coordinated ramp metering are found in 

Long Island, NY, Seattle, WA, Denver, CO, and Los Angeles, CA. The Los 

Angeles system wide adaptive ramp metering (SWARM) model implemented 

by Caltrans can activate one of three algorithms in an attempt to avoid 

breakdown in the flow-density regime.(51) The first algorithm, SWARM 1, 

adjusts rates at ramps upstream of a bottleneck in an attempt to maintain an 

acceptable level of service in the region of the bottleneck. The second and 

third algorithms, SWARM 2 and 3, are local responsive in nature. SWARM 2 
Chapter 3—Sensor Applications 

October 2006 Page 3-26 Federal Highway Administration 

is headway based, and SWARM 3 is density based. SWARM 1 estimates 

freeway conditions 10 to 15 min into the future using a Kalman filter. 

As depicted in Figure 3-11, the SWARM 1 algorithm clamps down on the 

metering rates at time t0 to prevent a bottleneck from occurring later at time 

t1. The algorithm establishes the extent of the flow rate reduction propagated 

upstream through user-selected factors that control the propagation rate of 

the flow restriction. Data from either several links or the entire freeway can 

be used to establish the metering rate. When a queue override condition is in 

force, the metering goes to the maximum rate, rather than continuous green, 

to maintain platoon dispersion. 

Density

Time

Saturation

Density

reduction

required

t

0 t1

Corrective trend that SWARM 1

algorithm attempts to meet at time t

1

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

In Europe, two coordinated ramp metering approaches have been developed 

as part of DRIVE projects: the Metaline strategy and the Sirtaki strategy. 

The Metaline strategy is based on a linear quadratic integral control law that 

minimizes the deviation of selected bottleneck densities from their desired 

values by modifying the onramp flows. The Sirtaki strategy is built around 

the SIMAUT simulation model. SIMAUT incorporates coordinated ramp 

metering strategies based on demand-responsive control.(52) In case of an 

incident, control strategies can be simulated and analysis aided by the 

model's ability to reconstruct the current traffic stream in real time. 

A summary of ramp metering rates provided by different metering methods 

is found in Table 3-3 in order of metering method preference.(3) 
 Traffic Detector Handbook—2006 

October 2006 Page 3-27 Federal Highway Administration

Table 3-3. Ramp metering rates by metering strategy. 

Ramp metering 

method 

Number of 

metered 

lanes 

Approximate metering 

rate, v/h 

Comments 

Single vehicle 

entry per green 

interval 

1 240–900 (54) • Full stop at meter usually not 

achieved at 900 v/h metering rate. 

Tandem metering 

with single vehicle 

entry per green 

interval per lane 

2 400–1700 • Applies when required metering 

rate exceeds 900 v/h. 

• Requires two lanes for vehicle 

storage. 

• Vehicles may be released from 

each lane simultaneously or 

sequentially. 

• Requires sufficient distance beyond 

metering signal for vehicles to 

achieve tandem configuration 

before merging with freeway traffic. 

Platoon metering 

with single lane 

multiple vehicle 

entry per green 

interval 

1 240–1100 (54) • Platoon lengths permit passage of 

1 to 3 vehicles per green interval. 

• Principally used to increase 

metered volumes when geometrics 

do not permit more than one 

metered lane. 

• May require changeable sign 

indicating permitted number of 

vehicles in green interval. 

• Manual on Uniform Traffic Control 

Devices requires yellow interval 

after green. 

Mainline Metering 

Mainline metering is one form of freeway mainline traffic management that 

also makes use of driver information systems, variable-speed control, lane 

closure, and reversible lane control. Individually or in combination, these 

control techniques are gaining acceptance at many highly congested freeway 

locations throughout the Nation. The role of sensors in mainline control is 

dependent on the selected operational strategy and its data requirements. 

When mainline metering is applied, signals on the freeway mainline control 

flow rates in a manner similar to ramp metering. Mainline metering 

manages traffic demand at a mainline control point to maintain a desired 

level of service on the freeway downstream of the control location. 

The desired level of service for mainline metering is selected to achieve one 

or several of the following objectives: 

• Flow maximization through a downstream bottleneck. 

• High level of service downstream of the control point. 

• Distribution of total delay on the freeway system more equitably. 

• Diversion of traffic to other routes or modes. 

• Increase in the overall safety of the facility. 

Mainline metering is found on the westbound lanes crossing the San Francisco-
Chapter 3—Sensor Applications 

October 2006 Page 3-28 Federal Highway Administration 

Oakland Bay Bridge, as illustrated in Figure 3-12. The signal bridge used for 

metering up to16 lanes of traffic is located about 800 ft (244 m) downstream 

from the toll plaza, which contains 20 toll booths and 2 bypass lanes (used for 

carpools during peak traffic periods and buses all the time). During peak 

periods, two of the toll booth lanes convert into additional HOV lanes that 

bypass the metering. The lanes from the remaining toll booths are reduced to 

12 metered lanes at the signal bridge prior to merging onto the 5-lane Bay 

Bridge. During offpeak periods and weekends, 14 lanes are metered. The 

metering produces a more flexible system that accommodates a mix of bus, 

carpool, and single passenger vehicles, allowing the bridge to operate at peak 

efficiency. 

Figure 3-12. Mainline metering configuration on I–80 westbound crossing the San FranciscoOakland Bay Bridge (Photograph courtesy of Caltrans District 4, Oakland, CA). 

Traffic accidents were reduced by 15 percent after the metering system was 

installed, with as many as 500 additional vehicles per hour crossing the 

bridge during peak periods. Fuzzy logic was utilized to control the queue 

length at the metering stations.(53) A new and enhanced fuzzy control system 

was developed to function with fewer sensor stations, lane occupancy data 

rather than station occupancy data, and new data validation algorithms. As 

reported by Caltrans (Sean Coughlin, Caltrans District 4, August 24, 1999), 

fuzzy control will be activated again once the original infrared sensors in 

each lane (set approximately 600 ft (183 m) apart) are replaced by dual 

magnetic sensors (approximately 1200 ft (366 m) apart). 

Another application of mainline metering has operated since 1978 on Route 

94 upstream of its junction with Route 125 in San Diego, CA. Ramps on 

Route 125 are metered, but the ramps on Route 94 are not. Mainline lanes of 

Route 94 are metered to offset the unbalanced upstream control. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-29 Federal Highway Administration

A gantry, which contains a separate signal head for each of the three lanes 

(two conventional lanes and one HOV lane), is mounted over the roadway. 

During normal system operation from 6 to 9 a.m., each signal displays a 

green ball to allow one vehicle at a time to proceed in that lane. A passage 

sensor is located immediately downstream of the stop line. As the vehicle is 

detected, the signal turns red (no yellow interval is used during the metering 

operation, only during startup). The signal then remains red for the 

remainder of the metering cycle, which is set to achieve the desired flow rate. 

Extensive signing is placed in advance of the metering station to warn 

freeway traffic of the unusual event that they are about to encounter. 

The success of the Route 94/125 mainline metering has encouraged the use of 

three other mainline metering stations on San Diego area freeways. One of 

these, located in El Cajon, is shown in Figure 3-13. Three lanes of 

southbound State Route 67 are metered as they join westbound Interstate 8. 

An internally illuminated sign displays the message “PREPARE TO STOP” 

during the metering operation. The metering signals are 12-inch (300-mm) 

standard 3-section heads centered on each lane and mounted on a 50-ft 

(15-m) mast arm. 

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

In Tokyo and Osaka, Japan, mainline metering has been implemented by 

regulating the number of toll booths that are open at any given time on the 

mainline of the expressways. Traffic entering the expressway system via the 

mainline is controlled based on expressway demand and downstream 

capacity. Such control is rather coarse, however, and might not be 

appropriate for all bottleneck conditions. 

Freeway-to-Freeway Metering 

A third technique for managing recurring congestion is to meter freeway-tofreeway connector ramps. Experiences in Minneapolis, MN, and San Jose, 

San Diego, Los Angeles, and Orange County, CA, indicate that significant 

benefits can be achieved with connector metering under conditions similar to 

those associated with ramp metering. Freeway connectors often have perlane flow rates greater than 900 v/h (the maximum possible with single entry 

metering). Metering rates exceeding this figure are achieved by two-lane 

metering or possibly platoon metering. Such configurations work best when 

there is an added lane downstream from the onramp. 
Chapter 3—Sensor Applications 

October 2006 Page 3-30 Federal Highway Administration 

Figure 3-14 illustrates connector metering from the eastbound I–105 

Freeway onto the southbound I–605 Freeway in Norwalk, CA. Meters on the 

two I–105 lanes allow three vehicles per lane during each green cycle to enter 

the four lanes of the I–605 mainline. One of the two I–105 ramp lanes 

continues as a fifth mainline lane.(49) 

Storage capacity is another issue that affects the metering of freeway-tofreeway connectors. As with onramps, queues are not allowed to extend 

upstream where they might interfere with other freeway movements. In 

addition, end-of-queue protection must be provided with automatic warning 

signs to prevent rear-end collisions. 

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

in Norwalk, CA (Photograph courtesy of Lawrence A. Klein). 

 SPEED MONITORING AND DRIVER NOTIFICATION AT 

CRITICAL FREEWAY SEGMENTS 

Safety problems occur when the design speed for certain curves is below that 

of other portions of the freeway. To lessen the crash potential at such 

locations, a speed measurement system that incorporates a flashing display 

to alert the driver to an unsafe speed can be used. Thus, if a vehicle is 

traveling faster than the desired or safe speed, a flashing sign or signal is 

activated to advise the driver to reduce the speed of the vehicle, as shown in 

Figure 3-15. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-31 Federal Highway Administration

SLOW DOWN

WHEN

FLASHING

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

One system that was evaluated used loops spaced 16 ft (4.8 m) apart to 

measure speed.(54) A display was attached to a bridge structure downstream 

of the loop to alert the driver. If the vehicle was detected traveling 62 mi/h 

(100 km/h) or less, only the speed was displayed. If the vehicle was traveling 

faster, an additional SLOW DOWN message was displayed along with the 

speed. The study concluded that a speed detection system was effective in 

inducing drivers to reduce vehicle speed. 

Unfortunately, at one such installation, drivers were observed deliberately 

accelerating to see how high a reading they could achieve on the speed sign. 

A potential solution to this problem would be to display speed values only up 

to the existing speed limit. Any speed above the limit would receive the 

message SLOW DOWN or YOU ARE EXCEEDING SPEED LIMIT. 

Another speed measuring sensor was designed using modulated light 

emitting diodes. It was deployed to measure the speed and height of high and 

long trucks entering a curved freeway-to-freeway interchange. The diodes 

operated in the near-infrared spectrum at 880 nanometers. The signal 

modulation prevented interference from other sources of infrared energy, 

including sunlight. Two transmitter-receiver systems measured the vehicle 

speed and one measured the vehicle height. When trucks susceptible to 

rollover or jackknifing were encountered, flashers were activated to warn 

drivers to reduce speed.(55) 

COORDINATED OPERATION OF FREEWAYS AND 

SURFACE ARTERIALS 

Corridor control applications that coordinate traffic flow on freeways and 

major surface arterials require improved monitoring of traffic to support 

faster incident detection, quicker prediction and notification of congested 

locations, ramp queue detection, and motorist information services. The 

strategies below assist in optimizing the design of corridor control systems 

and maximizing their utilization: 

• Coordination of actions between local agencies by sharing 

information and providing a unified decision support mechanism. 

• Balancing traffic flow between freeways and arterials to minimize the 

effects of capacity-reducing incidents and maximize the use of 

existing roadway capacity. 

• Providing motorists with access to current traffic information that 

assists them in planning their routes and avoiding congested areas. 

The I–10 Smart Corridor Program in Los Angeles, CA, applies this 

congestion management approach. The service region consists of the 
Chapter 3—Sensor Applications 

October 2006 Page 3-32 Federal Highway Administration 

freeways and arterial surface streets in a geographic area along a 14-mile 

segment of the I–10 (Santa Monica) Freeway. Building on the existing 

infrastructure, it adds new or modifies existing capabilities, resources, and 

policies to provide interagency coordination that maximizes agency 

effectiveness. An expert system helps automate responses to congestion along 

the corridor. Fourteen changeable message signs (also referred to as variable 

message signs), 24 trailblazer signs, 350 real-time traffic controlled signals, 

45 ramp meter stations, and about 3,000 inductive-loop detectors are used in 

the corridor. 

The Information for Motorists (INFORM) corridor in the New York City 

metropolitan area is another example of the corridor approach to traffic 

management. INFORM incorporates 136 mi (219 km) of roadway, consisting 

of two freeways (the Long Island Expressway-I–495 and the Northern State 

Parkway-Grand Central Parkway combinations) and a number of parallel 

and crossing arterial streets. The corridor extends east from the Borough of 

Queens in New York City, through Nassau County and into Suffolk County. 

This corridor uses 80 changeable message signs, 112 real-time controlled 

signals, 70 ramp meter stations, and about 2,400 inductive-loop detectors. 

TRAFFIC DATA COLLECTION 

Data collection is an essential part of any traffic engineering, planning, or 

operational activity. Advanced traffic management and traveler information 

services are supported by the collection of real-time traffic flow information 

on highway segments and surface street networks, especially during peak 

traffic flow periods. 

Advanced traffic management and traveler information systems require realtime, online traffic data to effectively: 

• Operate traffic adjusted, traffic responsive and traffic adaptive signal 

systems for surface streets and highways. 

• Determine location and extent of highway congestion. 

• Regulate congestion pricing. 

• Operate automatic hazard warning devices. 

• Detect and verify traffic incidents. 

• Develop a historical traffic flow database to support planning and 

evaluation. 

• Provide traveler information and traffic advisories. 

Traffic management data requirements are dependent on the application, 

whether it is to support real-time operational strategies, offline planning and 

administration, computation of measures of effectiveness, compilation of 

related statistics, verification of proper sensor operation, or research. The 

following sections discuss the traffic management functions supported by 

real-time and offline data acquisition and analysis and measures of 

effectiveness that require data collection to evaluate the performance of a 

traffic management strategy. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-33 Federal Highway Administration

DATA FOR REAL-TIME TRAFFIC OPERATIONS 

Table 3-4 lists typical demand and capacity management services and 

strategies that support real-time traffic operations. Demand management is 

typically advisory, while capacity management involves enforceable controls. 

Each strategy has its individual data requirements, including data type and 

format, measurement and update rates, precision, and data transmission 

bandwidth. For example, automatic incident detection associated with realtime freeway traffic management requires, as a minimum, traffic volume, 

occupancy, and speed data updated in 20- to 30-second intervals. Volume and 

occupancy provide measures of congestion and alert operations personnel to 

incidents. Speed data provide estimates of travel delay and level of service 

and are inputs to incident detection algorithms. 

Environmental sensors are needed to supply data about current weather 

conditions and information that leads to warnings concerning blowing sand, 

roadway icing, or the presence of other hazards. This information can 

augment and assist in the interpretation of volume, occupancy, and speed 

data. Travelers may report some incidents over cellular or roadside 

telephones, while others are deduced from the volume, occupancy, and speed 

data provided by sensors. Once an incident is verified, complementary data 

from patrol vehicles or surveillance cameras are needed to determine 

incident severity and the resources required to clear the incident. Cameras, 

in turn, require the transmission of control signals for pan, tilt, zoom, and 

focus and the return of video imagery to the operations center.(49) 

Traveler information services provided by kiosks, internet web sites, and 

roadside devices require the transmission of data that reflect the real time 

operational state of the highways, including location of incidents, predicted 

travel times, suggested alternate routes and transportation modes, and 

maintenance, construction, and special event locations and impacts. Often 

interagency and public-private partnership agreements are required to 

implement these strategies. 

Table 3-4. Strategies supporting real-time traffic management.(49)

Service Strategy 

Demand management 

Traffic demand management High occupancy vehicle lanes, ramps, and connectors 

Toll road congestion pricing (public and private) 

Electronic road pricing (ERP) for entry into central business 

district during congested periods 

Diversion to surface streets 

Alternative transportation mode identification 

Fees for single occupant use of high occupancy vehicle lanes 

Parking advisories 

Motorist and traveler information Changeable message sign activation and updating 

Highway advisory radio operation 

Kiosk maintenance 

Web site maintenance 

Personal information access device data transmission 

Information service provider data exchange 

Interagency coordination 
Chapter 3—Sensor Applications 

October 2006 Page 3-34 Federal Highway Administration 

Table 3-4. Strategies supporting real-time traffic management(49) —Continued. 

Service Strategy 

Capacity management 

Traffic signal control Adaptive signal control algorithms that calculate and adjust cycle 

length, split, offset, and green band interval (bandwidth) in real 

time 

Left turn lanes activated by vehicle detection 

Automated red light enforcement 

Metering Ramp metering 

Mainline metering 

Freeway-to-freeway metering 

Incident management Automatic incident detection 

Incident verification 

Incident removal 

End of queue warning 

Alternative route diversion 

Onramp closure and metering 

Incident response teams for large or severe incidents 

Interagency coordination 

Hazard warning Rain and fog detection and motorist alert 

Ice and snow detection and motorist alert 

Dangerous curve, merge area, high wind, and rough pavement 

warnings 

Construction and maintenance lane closures and alerts 

End of queue detection 

Overheight and overweight detection and alert 

Commercial vehicle operations Data exchange in support of electronic clearance, credentialing, 

and weigh-in-motion 

Pollutant emissions enforcement Monitoring of carbon monoxide, hydrocarbons, and nitrogen 

oxides emitted at the tailpipe and evaporated 

Mandating use of alternate transportation modes 

Strategies for real-time arterial signal control generally u travel times from 

an upstream sensor to the stopline at the intersection, startup or lag time for 

vehicles discharging from the stopline, and discharge rates for vehicles at the 

stopline and in queues between an upstream sensor and the stopline. The 

types of data are gathered and the time interval over which the data is 

gathered depends on the category of traffic control system strategy being 

utilized. 

Metering is performed through fixed time-of-day plans or by using sensors 

that monitor mainline traffic flow and adjust the ramp or feeder highway 

metering rates accordingly. In the latter application, the metering rate may 

be responsive to local mainline flow (e.g., flow upstream of the ramp) or to 

flow over a wider area or section of the freeway where metering among 

several ramps is coordinated. 

Automated hazard warning requires sensors to detect inclement weather, 

road condition, or oversize or overweight vehicle. The information is then 

conveyed to the motorist through mobile or fixed changeable message signs, 

highway advisory radio, normal commercial radio, or special radios that 
 Traffic Detector Handbook—2006 

October 2006 Page 3-35 Federal Highway Administration

allow sideband reception of traffic information. Commercial vehicles can 

receive warnings concerning overweight status through transducers mounted 

in the cab when weigh-in-motion sensors are installed. Large, lighted, and 

bright roadside signs are needed to warn drivers of fog. These signs must be 

repeated at frequent intervals to alert the motorist to the continued danger 

and reduced speed limits.(56,57)

Sensors used for electronic clearance and credentialing and weigh-in-motion 

enhance commercial vehicle operations. Analysis of transportation vehicle 

emissions also relies on sensors to gather the needed pollutant data. 

DATA FOR OFFLINE TRAFFIC PLANNING AND 

ADMINISTRATION 

Much of the information collected to execute the real-time functions of a 

traffic management system serves a secondary role as a valuable data 

resource for operations analyses and reviews, new construction and safety 

planning, highway research, and other administrative and planning services. 

The offline applications support a broad spectrum of users and uses, each 

requiring specific data, accuracy, precision, and spatial and temporal 

sampling. In recognition of the importance of archived data, an Archived 

Data User Service (ADUS) has been added to the U.S. National ITS 

Architecture and a five-year plan developed to support ADUS 

implementation.(58) Table 3-5 describes the types of archived data generally of 

interest in offline applications.(49) 

Table 3-5. Data supporting offline traffic planning and administration.(49)

Function/use Data 

Traffic census volumes Vehicle volumes by variable time intervals 

Vehicle classification counts 

Average speed by time periods 

High occupancy vehicle priority entry and facility usage 

Ramp usage by variable time intervals 

Vehicles by person occupancy 

Person and vehicle miles of travel 

Traffic congestion Average speed or travel time by time-of-day periods 

Congestion delay measurement 

Crash and incident events Crashes, types, trends, and rates 

Incidents, types, trends, and rates 

Commercial vehicle crash and incident locations and causes 

Event planning, construction, 

and maintenance lane closures 

Traffic volumes, time distribution, congestion levels 

Volume/capacity relationships 

Lane closure statistics (spatial and temporal) 

Volumes from parking facilities and their impact on local arterials and 

freeways 

Equivalent single axle loads 

Weigh-in-motion is the focus of two 

separate FHWA documents: 

McCall, Bill; Vodrazka, Walter C. 

Jr., States' Successful Practices 

Weigh-In-Motion Handbook, 

Dec. 15, 1997, available from the 

National Transportation Library, 

http://ntl.bts.gov/card_view.cfm?do

cid=6243, 

http://ntl.bts.gov/lib/6000/6200/624

3/wim.pdf 

Data Collection Guide for SPS 

WIM Sites, August 31, 2001, 

www.tfhrc.gov/pavement/ltpp/spstr

affic/tmg.pdf 
Chapter 3—Sensor Applications 

October 2006 Page 3-36 Federal Highway Administration 

Table 3-5. Data supporting offline traffic planning and administration(49) —Continued

Function/use Data 

Resource allocation and policy 

analyses 

Spatial and temporal traffic activity levels 

Spatial and temporal resource deployment 

Measures of effectiveness to support cost effectiveness analyses 

Origin-destination (O-D) data to support trend and policy option studies 

Electronic fare, transit automatic vehicle identification, and O-D data for 

intermodal transportation and land use analyses 

Incident statistics by type, location, time, season, weather 

Flow rate, density, truck use, speed, O-D, etc. data for research 

Pollutant emission concentrations by vehicle type, time-of-day, flow 

rate, speed, and vehicle miles traveled for air quality analyses 

High occupancy vehicle and transit usage to support effectiveness 

analyses 

DATA FOR EVALUATING MEASURES OF 

EFFECTIVENESS 

The strategies used by a traffic management system should be evaluated 

periodically to assess the degree to which the system objectives are satisfied. 

Evaluations are also important to ensure that the needs of the stakeholders 

(e.g., the system operators and agencies that respond to real-time incidents 

and congestion, those involved with planning for future improvements, users 

of historical data, travelers, and allied agencies and private partners) are 

supported. The evaluation criteria are usually expressed in terms of 

measures of effectiveness. Many measures of effectiveness (MOEs) are based 

on sensor surveillance data collected by the traffic management system. 

System-Wide MOEs 

A number of measures have traditionally been used to quantify the 

performance of freeway systems. These include:(59-62) 

• Total travel time in vehicle hours traveled (VHT): The product of the 

total number of vehicles using the roadway or roadway segment 

during a given time period multiplied by the average travel time of 

the vehicles. 

• Total vehicle miles traveled (VMT): The product of the total number 

of vehicles using the roadway or roadway segment during a given 

time period multiplied by the average trip length of the vehicles. 

• VMT/VHT: The average travel speed throughout the network. 

• Vehicle delay: Excess travel time as compared to the free-flow travel 

time. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-37 Federal Highway Administration

• Total minute-miles of congestion: Extent of freeway congestion in 

both time and space. This MOE was developed by the Chicago Area 

Expressway Surveillance Project to quantitatively measure freeway 

congestion.(40) Congestion is defined as a 5-min lane occupancy of 30 

percent or more measured at a mainline sensor. When this condition 

occurs, the number of minute-miles of congestion at the mainline 

sensor is equal to the product of the minutes of congestion multiplied 

by half the distance between adjacent sensors upstream and 

downstream of the mainline sensor at which congestion is measured. 

Minute-miles of congestion is a summary measure that accounts for 

all the variables contributing to the traffic flow condition, e.g., wet 

pavement, accidents, disabled vehicles, and construction. Analysis of 

this congestion measure in the Chicago area has shown correlation 

with total travel time. A completely accurate sensor network is not 

required to implement this MOE. 

• Emissions: Usually the concentrations of three pollutants targeted by 

the Clean Air Act Amendments of 1990 are calculated in the United 

States. These are carbon monoxide (CO), hydrocarbons (HC), and 

nitrogen oxides (NOx) emitted at the tailpipe and through 

evaporative emissions in units of grams per mile and grams per hour. 

• Fuel consumption: Gallons (or liters) of fuel per VMT by link and 

aggregated link data to obtain total gallons (or liters) of fuel 

consumed per day. 

• Crash statistics: Crash rates, types of crashes (fatalities, injuries, rear 

end, sideswipe, property damage only, etc.), correlation with 

congestion level, and trends of these statistics can be used to 

document improvements in motorist safety. 

DETECTION OF PRIORITY VEHICLES 

Priority vehicles may either be emergency vehicles such as fire engines, 

ambulance, and police cars, or mass transit vehicles such as buses and 

trolleys. Emergency vehicles preempt the right-of-way by interrupting the 

normal phase sequence of the signals along the route to allow for their safe 

and rapid passage. After a preset time period, the signal returns to normal 

operation. With railroad preemption, train predictors (sensors provided by 

the railroad) detect the approach of a train and trigger a reaction, which first 

clears the track area and then allows nonconflicting green phases to operate 

during the passage of the train. Transit vehicles receive priority treatment by 

extending the green signal phase or changing the signal to green as soon as 

possible to expedite movement of the vehicle along its route. Transit priority 

is used less frequently than preemption for emergency vehicles, primarily 

because of concerns over disruption to surrounding traffic.(63) The response to 

different types of priority vehicles can be coordinated among several agencies 

and jurisdictions to provide a seamless travel corridor. In urban and rural 

traffic, travel time savings of 10 to 30 seconds and savings greater than 30 

seconds per intersection, respectively, were reported when an emergency 

vehicle preempted a red signal during an ITS field operational test (FOT).(64) 

Some emergency systems preempt the right-of-way without using sensors to 

detect emergency vehicles along the route. In these systems, a green 

indication is displayed at all signalized intersections along the selected route 

traveled by the emergency vehicle. The right-of-way is assigned by activating 

a switch from a central location, such as a fire station. A number of centrally 

Detection of priority vehicles is 

associated with signal preemption 

and signal priority. Preemption 

interrupts or alters the normal 

signal sequence at an intersection 

in deference to a special situation, 

such as the passage of a train, a 

bridge opening, or the granting of 

the right of way to an emergency 

vehicle such as a fire engine, 

ambulance, or police car. Signal 

priority holds the green longer than 

normal or allows the signal to 

change to green as soon as 

possible in order to provide priority 

treatment for transit vehicles. 
Chapter 3—Sensor Applications 

October 2006 Page 3-38 Federal Highway Administration 

controlled computerized signal systems preprogram the emergency 

progression, which leads to more efficient traffic flow than the manual 

switch-activation technique. 

There are two broad classes of priority vehicle detection systems. The first 

class uses transmitters or beacons mounted on the vehicle to identify it to 

receivers located near the controlled signal. Some transit vehicles transmit a 

signal to a central station, which in turn sends a priority command to the 

signal. The first class of priority vehicle detection systems includes highintensity light beacons, RF beacons, infrared beacons, and inductive loops 

operating in conjunction with vehicle-mounted identification devices. The 

second class of priority vehicle detection systems does not require a beacon or 

transmitter on the vehicle. These sensors identify the priority vehicles 

through information they receive from the vehicle signature produced by the 

sensor. Included in this class of sensors are inductive loops that classify 

vehicles using algorithms that are incorporated into the detector electronics, 

laser radar sensors, and sound detection systems. 

HIGH-INTENSITY LIGHT BEACON 

One of the first priority systems developed is the high-intensity light 

emitter/receiver system, which flashed light on and off at a high frequency 

coded to identify different vehicle types.(65) The receiver is able to distinguish 

between emergency vehicles coded for command preemption and transit 

vehicles that receive only priority treatment. Each intersection to be 

preempted is equipped with one or more optical receivers, depending on the 

number of approaches that provide preemption. As the emergency vehicle 

with the light emitter approaches the intersection, the appropriate 

directional receiver senses the coded light, triggering the preemption 

circuitry in the intersection controller. When the light from an emergency 

vehicle is detected, a signal is conveyed to a phase selector (connected to the 

controller) or directly to the controller. The status of the controller is 

examined and the controller either holds the green interval for the 

emergency vehicle or terminates the green on the opposing street(s), thus 

transferring the green to the direction of travel of the emergency vehicle. 

Priority vehicle operation is similar, except that the control function is not 

preempted. If the signal is green, the controller attempts to hold the green 

long enough for the priority vehicle to enter. If the light is not green, the 

controller attempts to activate the green phase as quickly as possible. Newer 

transit priority systems, which are centralized, first transmit the request for 

priority from the vehicle to a centralized transit management center and 

then from the center to the affected intersection. The components of the highintensity light beacon system are similar to those shown below for the 

infrared beacon. 

RADIO BEACON 

Another early signal priority system was the radio beacon. Each emergency 

or transit vehicle is equipped with a RF transmitter and each controlled 

intersection with a receiver. Since the transmitter in this system broadcasts 

the RF frequencies in all directions, it must also incorporate a direction code 

or provide limited preemption such as having all signals flash red. 

Furthermore, the detection range must be limited to avoid preempting 

nearby signals unnecessarily. 

Detection systems used to 

detect priority vehicles either 

use transmitters mounted on 

the vehicles or roadsidemounted sensors that 

identify the priority vehicle by 

its signature (i.e., the 

information they receive by 

regularly scanning the 

roadway for the presence of 

these vehicles). 
 Traffic Detector Handbook—2006 

October 2006 Page 3-39 Federal Highway Administration

Another form of RF priority system is found on the buses in Aalborg, 

Denmark.(66) These vehicles are equipped with a driver’s console, computer, 

global positioning system (GPS), and data radio. The driver inputs pertinent 

details about the current trip and an identifying journey number. The console 

informs the driver whether the vehicle is ahead or behind schedule, at which 

point the driver inputs the journey time in plus or minus form on the console 

display. Priority request information is transmitted by the vehicle to a 

centrally located base station when the vehicle crosses a report line. The base 

station can analyze all priority requests received and, if necessary, decide 

among conflicting requests before relaying the priority command to the 

affected signal. The base station also downloads GPS correction data every 

10 to 20 seconds and new schedules and report lines as needed. 

The GPS approach has become increasing popular in the United States as 

well. Communication techniques include transmission of priority requests 

from the bus to the control center by digital channels on the bus radio system 

and transmission of the signal priority command from the control center to 

the intersection using spread spectrum radio or optical links. 

INFRARED BEACON 

The infrared priority system relies on beacons or transmitters mounted on 

vehicles. As the vehicle approaches the intersection, it saturates the 

intersection with an encoded, infrared signal that is received, decoded, and 

validated to give the requesting vehicle priority. The type of signal priority is 

programmed into the electronics unit located in the controller cabinet. Figure 

3-16 shows an example of beacon, receiver, and electronics units. 

 

Figure 3-16. Infrared beacon priority system (Courtesy of 3M Company, St. Paul, MN). 

INDUCTIVE LOOP WITH VEHICLE-MOUNTED 

TRANSMITTER 

Inductive-loop detectors, in conjunction with electronics units designed to 

receive dozens of different codes from transmitters mounted on the underside 

of the vehicles, allow discrimination among emergency and transit vehicles. 

The transmitter in Figure 3-17 is disk shaped and is attached to the vehicle 

with a threaded shaft that extends from the disk. Its continuously transmitted 

programmed code is detected by any standard loop in the roadway. A special 

digital electronics unit interprets the code and identifies the vehicle or vehicle 

type. The electronics unit also provides conventional vehicle detection data. In 

addition to preemptive and priority control, the vehicle-mounted transmitter 

can also be used to override gate control, recognize vehicles at control positions 

or gasoline pumps, and correlate transit bus passage with schedules. 
Chapter 3—Sensor Applications 

October 2006 Page 3-40 Federal Highway Administration 

INDUCTIVE LOOP WITH VEHICLE CLASSIFICATION 

ALGORITHM 

Another type of digital inductive-loop detector electronics unit eliminates the 

need for mounting a special transmitter on the vehicle. This system consists 

of a conventional loop buried in the roadway and a special digital electronics 

unit that transmits a high frequency excitation signal to the loop and 

receives a unique waveform (also termed the “signature” or “footprint”) 

corresponding to each vehicle class it detects. When a waveform similar to 

that expected for a priority vehicle is identified, priority signal treatment is 

provided. The system can also be used to classify vehicles as described in 

Chapter 2.(67) 

Figure 3-17. Transmitter mounted under a vehicle identifies it 

to a subsurface inductive-loop detector. 

Its application to bus detection is illustrated in Figure 3-18. The digitized 

signature is input to a microprocessor that seeks and compares preidentified 

features to those of known bus profiles stored in memory. If a bus is 

identified, an output is generated to request priority treatment. Typical 

signatures for various classes of vehicles are shown in Figure 3-19. 

Digitized Presence

Signature

Inductive Loop

In Roadway

Inductive Loop

Electronics Unit

Vehicle Presence

Waveform

(Analog)

Digital

Output Port

Output Indicating

Bus Detection

Digital

Input Port

Central

Processing

Signal Processing Unit

Algorithm and 

Stored Vehicle 

Features in RAM

Microprocessor

 Figure 3-18. Inductive-loop detection system for transit vehicles. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-41 Federal Highway Administration

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

signals from specialized electronics units. 

LASER RADAR 

Laser radar sensors that scan one or more traffic lanes are capable of 

supplying unique vehicle profiles. The vehicle’s profile is classified by an 

algorithm incorporated into the sensor or stored in an auxiliary computer 

connected to the sensor through a serial interface. The traffic signal priority 

is adjusted according to the vehicle class. Although these sensors have the 

potential to differentiate between emergency and transit vehicles, this 

application of the sensor has not been demonstrated. 

SOUND DETECTION SYSTEMS 

Sound detection systems, such as that illustrated in Figure 3-20, detect the 

sound energy emitted by sirens on emergency vehicles. Optional ultrasonic 

emitters are available to identify transit vehicles. A set of phased-array 

microphones, mounted on the mast arm, provides directionality to the source 

of the sound. A light can also be mounted to give visual verification that the 

signal priority request has been received. 

Microphones (x4) Traffic Signals

(x4)

Traffic Signal

Controller

Data

SONEM 2000

Controller Card

AC Driver

Card

Microphone

Interface Card

Siren

Visual Verification

Light (x4)

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

(Courtesy of Sonic Systems, Vancouver, British Columbia, Canada). 
Chapter 3—Sensor Applications 

October 2006 Page 3-42 Federal Highway Administration 

PEDESTRIAN DETECTION AND SIGNAL ACTUATION 

The applications described above involved the detection of vehicles. 

However, properly timed actuated signal control also requires the detection 

of pedestrians. Unlike vehicles, pedestrians do not induce changes in the 

Earth’s magnetic field, cause inductance variations, or otherwise produce 

signals that are easily detected by other sensors (an exception might be video 

detection, although this application has not been exploited to date). 

Moreover, pedestrians cannot be depended on to follow a specific path toward 

their intended destination, nor can they be expected to take a specific action 

to make their presence known to the signal controller. 

The push-button sensor, shown in Figure 3-21, is the most common form of 

pedestrian detection. The microswitch contact closure created when a 

pedestrian pushes the button causes a low-voltage current to flow to the 

controller and registers a “demand” for pedestrian service. 

The weak link in the manually operated push-button system is the 

pedestrian, as it requires the pedestrian to be proactive in registering a 

demand. Unfortunately, many pedestrians do not make the necessary effort 

and, in those cases, are likely to cross the intersection illegally or unsafely. 

These pedestrians do not realize that pushing the button will extend their 

green time as well as service their needs faster. 

Where two buttons for crossing in different directions are located on the same 

support, each button’s relationship to the corresponding crosswalk must be 

clearly and unequivocally indicated. Although the Manual on Uniform 

Traffic Control Devices (MUTCD(68) specifies a series of standard signs for use 

with pedestrian push-button devices, many agencies augment these standard 

signs with additional instructional signs. 

Figure 3-21. Manually operated pedestrian push button. 

Active response pedestrian push buttons are common in the United States 

and Europe. When activated, they turn on a small light (usually green) or a 

small sign that flashes the message “WAIT PLEASE” or “WAIT FOR WALK.” 

An example from England is shown in Figure 3-22. Such a response is a 

Pedestrian detection is often 

a major consideration when 

developing traffic signal 

timing plans. Manually 

operated and automatic 

pedestrian sensors may be 

installed for this application. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-43 Federal Highway Administration

confirmation of the pedestrian’s call for service similar to that of a lighted 

elevator button. This response to a call for a walk message appears to 

alleviate pedestrian anxiety from the lack of feedback from conventional 

manually operated push buttons and promotes understanding of the 

pedestrian phasing of the traffic signal. 

Figure 3-22. Interactive pedestrian push button. 

Another responsive crosswalk device is shown in Figure 3-23. Although not 

presently in the MUTCD, the device is increasing in popularity as it displays 

the time remaining for the pedestrian to cross the street. 

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

When visually impaired pedestrians use push buttons, some type of guiding 

device can be used to enable the pedestrian to locate the button. Texturing 

the concrete on the sidewalk approaching the push button, audible locators, 

or handrails can be used for this purpose. 

Audible pedestrian signals are devices that emit buzzing, whistling, beeping, 

or chirping sounds that are correlated with the visual WALK/DON’T WALK 

signs used by sighted individuals. Intermittent pulses in the frequency range 

of 300–1,000 Hz (with 750 Hz being optimal) are the most effective sounds for 

the human ear to localize and do not require a high volume level to be 

effective. The devices may be either pedestrian activated or automatic. 

Pedestrian-activated signals are connected to the pedestrian call (push) 

button, which is connected to the signal controller. Automated signals are 
Chapter 3—Sensor Applications 

October 2006 Page 3-44 Federal Highway Administration 

activated by the cycle change at pretimed intersections. Many of the 

automatic devices, such as that in Figure 3-24, emit different sounds to 

indicate which direction to cross and how much time is available for crossing. 

The two most popular audible pedestrian signals used in the United States 

emit either a buzzer or a birdcall sound. The audible signal most frequently 

used in the western United States emits a "peep peep" tone for the east-west 

direction and a “cuckoo” tone for the north-south crossings. Audible signals 

indicate only that the WALK indication is displayed, not that the intersection 

is clear. The devices are meant to complement rather than be a substitute for 

a visually impaired person's orientation and mobility skills. 

Figure 3-24. Audible pedestrian signal device.

Several innovative pedestrian signal alternatives have been tested to detect 

the presence of pedestrians and provide safer crossing intervals. Instead of 

traditional pedestrian pushbutton devices, which require actuation, the new 

signal installations use pressure-sensitive mats or infrared sensors (mounted 

on poles) to detect the presence of pedestrians waiting to cross the street. 

After being detected, a pedestrian is given a white light, indicating that a 

WALK signal will follow. If pedestrians wait until the WALK signal is 

displayed, a second infrared sensor monitors their progress within the 

crosswalk and holds the pedestrian clearance interval (and red signal for 

motor vehicles) until the pedestrian reaches the other side of the street. 

Thus, slower pedestrians (e.g., elderly, wheelchair users) are given extended 

walking intervals. If the pedestrian steps off the curb too soon or leaves the 

crossing area, the request for a WALK signal is no longer valid, and NO 

WALK interval is given. 

Other systems that alert drivers to the presence of pedestrians are 

illustrated in Figure 3-25. The pedestrian crosswalk lights in Figure 3-25a 

only flash when activated by the pedestrian. Therefore, the motorist receives 

real-time information indicating that a pedestrian is in the vicinity of the 

crosswalk. Figure 3-25b shows automatic detection of pedestrians at 

curbside and within an intersection using microwave radar sensors that 

activate a pedestrian call feature. At the same time, slower pedestrians 

detected within the on-street detection zones receive more time to cross the 

street. The detection of motion in the target area (for example, curbside) has 

the same effect as a pedestrian pressing a call button. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-45 Federal Highway Administration

(a) Crosswalk lighting (b) Microwave radar sensing 

Figure 3-25. Pedestrian crossing safety devices.

DRIVER WARNING FOR RED SIGNAL AHEAD 

Advance driver alerts to a red signal ahead are often required when the 

geometrics of intersections do not permit the signal to be seen in time for 

drivers to react. In addition, there may be sight-distance restrictions due to 

overhead obstructions such as bridges and large trees, which cannot be 

removed. Intersections located on a downgrade may increase the actual 

required stopping distance. 

Other potentially hazardous situations that involve traffic signals occur 

where signals are difficult to see at vertical curves that hide the queue at a 

signal, while not obstructing the signal itself. Another poor signal visibility 

situation arises in areas subject to dense ground fogs that reduce signal 

visibility below normal minimums. 

To alleviate these dangerous situations and modify driver behavior, 

additional information may be provided to the driver and not be limited to a 

simple fixed sign that warns of a potential RED signal ahead. This type of 

tailored response is provided through changeable message advance warning 

signs. These signs have the capability to alter the displayed information 

based on whether the driver should slow or proceed. The warning sign shown 

in Figure 3-26 is used to alert approaching drivers that a hidden signal 

around a curve is red and that they must stop. 

The criteria for a changeable message sign used in a red signal warning 

system include: 

• A sign mounting adjacent to the roadway or overhead (at least 

17 ft (5.2 m) above the pavement); the lettering should be at 

least 12 inches (30 cm) high with 12-inch (300-mm) alternating 

beacons. 

• A legend that reads “PREPARE TO STOP WHEN FLASHING.” 

Curves, hills, inclement weather, 

and natural and man-made visual 

obstructions often require advance 

warning signs to be installed to 

alert drivers to a red signal that lies 

ahead. Such a warning provides 

the extra time needed to bring the 

vehicle to safe stop. 
Chapter 3—Sensor Applications 

October 2006 Page 3-46 Federal Highway Administration 

Figure 3-26. PREPARE TO STOP lighted sign. 

• A warning sign message and beacons that begin to flash 50 to 

60 times per minute just prior to the signal display of the 

yellow change indication, so that the driver will observe the 

signal display just after it has turned yellow. The last car 

passage feature of some of the early density controllers can be 

used for this purpose. 

• Continued flashing of the sign except when the approach signal 

is green. 

• Additional control logic to ensure safety; that is, the sign must go to 

flash if the signal fails, is in conflict, or is placed in a manual or red 

flashing mode. 

When warranted, this type of flashing sign provides an effective solution to 

the problem of rear-end accidents. When actuated in such a manner that it 

provides the approaching drivers with accurate information as to whether 

they will have to stop for the signal, it is a beneficial addition to the signal 

installation. 

TRAFFIC COUNTING AND VEHICLE CLASSIFICATION 

In many locations, inductive-loop detectors and over-roadway sensors have 

replaced pneumatic tubes for counting vehicular traffic. Since the tubes are 

laid over the roadway, they are especially vulnerable to the wear and tear of 

passing traffic. The buried loops or over-roadway sensors provide a better 

means of counting. Moreover, should the loop or over-roadway sensor fail, all 

counting would stop. On the other hand, the tube tends to distort counts 

prior to failure. Another favorable aspect is that the loop and over-roadway 

sensors give only a single output for most vehicles, regardless of the number 

of axles, thus providing a more accurate count. 

In addition to conventional traffic counts, a growing number of computerized 

signal systems are using loop and over-roadway sensors to provide volume 

data. The data collected by the sensors are output directly to the system 

computers, which control signal timing throughout the system. 

Roadway mounted sensors 

are often used for vehicle 

counting and classification, 

over-height detection, and 

road surface and weather 

reporting. Both in-roadway 

and over-roadway sensors 

are used in these 

applications. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-47 Federal Highway Administration

COUNTS BY LOOP INDUCTIVE DETECTORS 

It is not appropriate to generate multiple-lane vehicle counts by extending a 

single loop across several lanes. If a second vehicle in a different lane moves 

over part of a loop before the first vehicle has left its part of the loop, only one 

continuous output will be registered. This could lead to significant 

undercounting. 

When lane discipline is good (i.e., traffic stays in its own lane), a separate 

loop should be placed in each lane. However, in some cases lane discipline is 

poor (i.e., traffic is continually changing lanes) and lanes are wide. If count 

accuracy is important for these instances, then an additional loop could be 

placed between the lanes, as shown in Figure 3-27. This depicts the threeloop layout for a two-lane roadway. 

A

B

C

5’

1 ft = 0.3 m 

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

Ideally, the loops should be placed according to the following constraints: 

• The widest vehicle will not straddle more than two loops. 

• The narrowest vehicle will not pass between any two loops. 

• Two side-by-side vehicles must cross three loops. 

The logic to be inserted into the vehicle count model is described as follows:(69) 

• The operation of loops A or B or C produces one count immediately. 

• Operation of loop A and C together or A, B, and C together produces 

one count and a second count after a short delay. 

• The operation of single loops or adjacent pairs (A and B or B and C) 

produces only one count, but the operation of A and C together 

produces the first count followed by a second count a short time later. 

COUNTS USING LONG LOOPS 

Electronics units provide normal inductive-loop detector operation with 

timing functions (if desired) and also a secondary output of one pulse per 

vehicle in a single long loop or in a series of sequential short loops. Count 

accuracy for a single loop of any size is greater than 95 percent for properly 

functioning loop systems. Accuracy for the four loops in series configuration 

(discussed in Chapter 4) is lower because of the complexities associated with 

the analysis of its count output values. 
Chapter 3—Sensor Applications 

October 2006 Page 3-48 Federal Highway Administration 

DIRECTIONAL DETECTION USING INDUCTIVE LOOPS 

When it becomes necessary to distinguish between the directions of travel 

(such as where two directions of traffic must use the same roadway facility as 

on a reversible lane), two loops, two electronics unit channels, and directional 

logic can be used as illustrated in Figure 3-28. With this system, separate 

counts are accumulated for each direction of travel. An alternative approach 

is to activate the appropriate loops and reversible lane control signals 

according to time-of-day programming. 

Direction

of Travel

5’

5’

6”

6”

1 ft = 0.3 m 

1 in = 2.5 cm 

Figure 3-28. Directional detection using point sensors. 

VEHICLE CLASSIFICATION SENSORS 

As part of the traffic counting process, many agencies wish to obtain vehicle 

counts by class of vehicle. There are several vehicle classification counters 

commercially available. Many of these devices use loops and axle sensors to 

obtain the information required to classify vehicles as was discussed in 

Chapter 2. Other approaches use laser radar sensors to obtain vehicle class 

data. Many video image processors and presence-detecting microwave radar 

sensors can classify vehicles into three to five user-selected length bins. 

OVERHEIGHT SENSORS 

Overheight sensors are deployed at approaches to tunnel portals, bridge 

underpasses, restricted height parking structures, and at other covered 

structures such as those found at weigh stations and toll facilities. Two types 

of sensor technologies are used for overheight detection. The first is an 

optical system that detects overheight vehicles when they interrupt or break 

the transmission of the beam from transmitter to receiver as depicted in 

Figure 3-29. The second, a laser radar, functions in much the same manner, 

except that it transmits longer wavelength energy in the near-infrared 

spectrum. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-49 Federal Highway Administration

Figure 3-29. Overheight sensor configuration (Picture courtesy 

of MBB SensTech, Munich, Germany). 

WEATHER SENSORS 

A variety of data can be gathered from weather information systems 

mounted on or near the roadway, including wind speed and direction, air 

temperature and humidity, precipitation type and rate, visibility, roadway 

surface temperature, roadway wet/dry status, and road surface chemical 

analysis for determining whether application of ice-reducing chemicals is 

warranted. Many of these systems are solar powered and can transmit data 

over wireless communications links. Specialized forward-scatter sensors are 

available for measuring visibility through fog and using the information to 

control brightly lighted signs that warn drivers of the reduced visibility 

conditions ahead.(57) Weather monitoring systems, such as the one in Figure 

3-30, integrate many sensors to provide specific data for an application. Still 

other systems provide computer interfaces that display video and data on a 

monitor as illustrated in Figure 3-31. 

In an evaluation of research on road weather observations and predictions, 

the Desert Research Institute (DRI) and the Nevada Department of 

Transportation (NDOT) concluded that Road Weather Information Systems 

(RWIS) provide data that improve weather forecasts, but that challenges 

remain in forecasting when ice will form on roads.(70) In tests of models that 

predict ice formation as a function of meteorological parameters, DRI found a 

large variation in the minimum pavement temperature from 19 °F to 10 °F 

(−7 to −12 °C). Minimum pavement temperature appeared to be most 

sensitive to air temperature changes, total cloud cover, and precipitation. 

Inaccuracies in air temperature of 34 and 36 °F (1 and 2 °C) caused a change 

in minimum pavement temperature of more than 33 and 34 °F (0.5 and 1 °C), 

respectively. Significant underestimation of precipitation changed minimum 

pavement temperature by about 36 °F (2 °C). Inaccurate cloud cover 

predictions caused changes in minimum pavement temperature of more than 

37 °F (3 °C). 

Environmental observation 

systems are beneficial for tracking 

the location and movement of 

fronts and storm cells and 

providing needed information to 

traffic management agencies. In 

order for roadway weather sensors 

to obtain accurate data, they must 

be calibrated regularly. Such 

calibrated sensor data could then 

also be used to support the needs 

of the National Weather Service 

and improve the accuracy of 

pavement temperature forecast 

models. 
Chapter 3—Sensor Applications 

October 2006 Page 3-50 Federal Highway Administration 

Figure 3-30. Integrated weather information system 

(Picture courtesy of Nu-metrics, Uniontown, PA). 

Figure 3-31. Weather imagery and data display (Picture courtesy 

of System Innovations, Fredericksburg, VA). 

Based on a 2004 National Academy of Sciences report that described the 

need for a robust, integrated weather observational and data management 

system, FHWA responded with an initiative called Clarus—the Nationwide 

Surface Transportation Weather Observing and Forecasting System.(71,72) The 

initiative’s overall objective is to reduce the impact of adverse weather for all 

road and transit users and operators. The projected six-year Clarus program 

has been structured to include: 
 Traffic Detector Handbook—2006 

October 2006 Page 3-51 Federal Highway Administration

• Developing partnerships between the surface transportation and 

weather communities to leverage and share resources for both 

research and operations. 

• Strengthening of ties among Federal agencies with similar objectives 

such as FHWA and the National Oceanic and Atmospheric 

Administration (NOAA). 

• Demonstrating a framework to collect the Nation’s current and future 

surface transportation weather and road condition observations, and 

provide quality assured data as input to advanced weather models, as 

the basis for informative value-added products that can contribute to 

a safer and more efficient surface transportation system. 

• Establishing an instrumented corridor test bed to host new cuttingedge technologies for fixed, mobile, and remote sensing. 

• Establishing an Interagency Coordination Committee. 

VEHICLE-MOUNTED SENSORS THAT ENHANCE SAFE 

OPERATION 

Table 3-6 gives examples of vehicle-mounted sensors that support various 

collision and hazard avoidance strategies and prevent specific types of 

crashes.(73) Headway detection advises the driver of an imminent crash with 

an obstacle or to maintain a safe distance when following another vehicle. 

Proximity detection gives the driver information about adjacent lane vehicles 

or obstacles while backing up. The lane position monitor advises the driver if 

the vehicle is drifting out of its travel lane. In-vehicle signing conveys posted 

or dynamic information to the driver from the traffic management 

infrastructure. A gap acceptance aid advises the driver when it is safe to 

cross or turn at intersections. Vision enhancement gives the driver a clearer 

image of the roadway and environment ahead during reduced visibility 

conditions such as heavy rain, fog, and nighttime. 

However, sensors are not the only component of a hazard warning system. 

Human response time, the devices used to transmit the sensor information to 

the driver, and the dynamics of the encounter with a particular hazard are 

other factors that must be analyzed in designing a hazard warning system. A 

systems analysis approach reveals that uncertainty in human response time 

is the major factor affecting performance of an alerting system.(74) Thus, a 

system designed to protect slowly responding drivers may have a high 

unnecessary alert rate for rapidly responding drivers. Likewise, systems 

designed to have few unnecessary alerts also have relatively high crash 

rates. The way to combat this tradeoff is to decrease the response time 

uncertainty through driver training, improved display and audio alerts, or 

tuning each system to the driver using it. Although the systems methodology 

has been successful in modeling the performance of relatively simple systems 

such as a rear-end collision alert system, its extension to other applications 

requires more detailed dynamic models, probability distributions for a 

collision given an alert (obtained through Monte Carlo simulation), and 

inclusion of representative hazard variables such as driving speed and road 

condition. 

The requirement for increased 

vehicle safety on arterials and 

freeways has led to the 

development of vehicle-mounted 

sensors that support advanced 

cruise control functions such as 

headway and proximity detection, 

and issue warnings when lane 

divider markings are crossed. 
Chapter 3—Sensor Applications 

October 2006 Page 3-52 Federal Highway Administration 

Table 3-6. Collision avoidance strategies and preventable crash types 

that are supported by vehicle-mounted sensors. 

Strategy Vehicle-mounted sensor Preventable crash 

Headway detection Millimeter-wave radar, laser radar, 

video 

Rear-end 

Proximity detection Millimeter-wave radar, ultrasonic Backing, lane change and merge 

Lane position monitor Laser radar, video, passive reflected 

light detector 

Road departure, opposite direction 

In-vehicle signing Video, infrared communications, 

microwave transponder 

Road departure, opposite direction, 

intersection, and crossing path 

Gap acceptance aid Video Intersection and crossing path 

Vision enhancement Millimeter-wave radar, passive far 

infrared, intensified CCD cameras 

Reduced visibility 

IN-VEHICLE SENSORS FOR DISTANCE WARNING, 

CRUISE CONTROL, AUTOMATIC HIGHWAY SYSTEM 

FUNCTIONS, AND PRECRASH DETECTION 

Automobile, truck, and bus use of radar sensors is projected to grow from 

worldwide revenues of $35 million (U.S.) in 1998 to between $300 and $500 

million (U.S.) by 2003.(75) Automotive distance warning systems are a large 

part of this market, including front, side, and back radar to monitor 

obstacles. These systems calculate the distance from objects and their speed, 

if they are moving, and alert drivers if they are too close to another object or 

vehicle. Table 3-7 lists composite requirements for automotive radar 

distance warning systems.(76-78) 

Radars designed for advanced adaptive cruise control provide the input to 

systems that automatically reduce vehicle speed and apply the brakes as the 

separation between approaching vehicles diminishes. Other smart cruise 

systems rely on highway-based sensors and appropriate in-vehicle systems to 

inform the driver of the presence and location of roadway obstacles, vehicles 

in an intersection, road surface condition, and pedestrians crossing streets.(79) 

A still more futuristic look at automatic vehicle control and guidance includes 

automatic highway systems. Here sensors are required to: 

• Validate proper operation of the automotive systems (e.g., braking, 

steering, throttling, and communications). 

• Support automatic guidance (lateral and longitudinal), acceleration, 

and deceleration of the vehicle on the automated highway. 

• Provide coupling (and uncoupling) of the vehicle into (and out of) a 

platoon. 

• Detect and report the presence of obstacles on the highway.(80,81) 
 Traffic Detector Handbook—2006 

October 2006 Page 3-53 Federal Highway Administration

Table 3-7. Automotive radar requirements for distance warning systems. 

Parameter Dimensioning criteria Value 

Center frequency Compatible with government frequency 

allocation for the application 

24 or 76 to 77 GHz 

Waveform Capable of detecting presence and measuring 

the relative speed of vehicles within the range 

measurement interval 

Frequency modulated, 

continuous wave 

(FMCW) 

Azimuth field-of-view Large enough to detect a vehicles at sufficient 

range when the following vehicle is on a 

curved road section 

±8.5 degrees 

Antenna horizontal 

beamwidth 

Obtain a sufficiently high probability 

that all relevant vehicles will be resolved 

from each other 

1.5 degrees 

Range measurement 

interval 

Sufficient to include the distance to a car 

stopping ahead to the distance needed to 

reduce speed comfortably when a car is 

overtaken 

1 to 200 m 

(3.3 to 656 ft) 

Relative velocity of 

vehicles 

Ability to measure relative speed of 

approaching and departing vehicles 

-50 to +100 m/s 

(-112 to +224 mi/h) 

Update rate Fast enough to match the vehicle longitudinal 

control servo bandwidth 

with the reaction times 

10 to 20 Hz 

Mounting position Obtain view of critical areas with radar 

waveform able to penetrate vehicle body 

materials, such as by placing antenna behind a 

plastic bumper or plastic grill 

⎯

Radar sensors operating over shorter distances are finding applications in 

park distance control and airbag precrash detection, especially for side 

airbags. The frequencies being considered for these radars are 24 and 77 

GHz. Laser radars can also potentially be used for the precrash detection 

application. Ultrasonic sensors are available from original equipment and 

aftermarket manufacturers for park distance control and obstacle detection. 

IN-VEHICLE OPTICAL SENSORS FOR PASSENGER 

DETECTION, VISION ENHANCEMENT, AND LANE 

CONTROL 

In-vehicle sensing functions are likely to be performed by optical sensors, 

either in the visible or infrared wavelength bands. Three-dimensional camera 

chips for passenger detection are in development for this purpose. In-vehicle 

sensing is proposed to control inflation of the smart airbag based on the 

shape and position of the occupant on each seat. Occupant data could also 

support automatic validation of high occupancy vehicle lane use and 

calculation of fees for occupant-based services. 

Night vision systems that utilize uncooled infrared detectors such as barium 

strontium titanate are scheduled for insertion into high-end passenger 

vehicles beginning with year 2000 models.(82) Lane tracking systems, such as 

Autovue shown in Figure 3-32, aid drivers of trucks and passenger vehicles 

in staying within prescribed lanes. This in-vehicle camera detects painted 

lines or other markers at the lane edges and sounds an alarm to alert the 
Chapter 3—Sensor Applications 

October 2006 Page 3-54 Federal Highway Administration 

driver when the vehicle deviates from the center of the lane, but before the 

vehicle crosses the lane marker.

Another lane deviation sensing system utilize a laser diode to scan the 

roadway for paint that serves as a lane demarcator. In addition to the small 

glass spheres embedded in the paint, an external coating of spheres is often 

applied to a painted stripe to enhance the retroreflectivity of the marking. 

Since the road surface generally has low reflectivity, the energy 

backscattered from the paint and road surface is different and detectable by 

a scanning laser sensor.(83) 

Figure 3-32. Autovue lane tracking sensor (Photograph courtesy of Iteris, Inc., Anaheim, CA). 

A third method proposed for lane position monitoring utilizes a passive 

sensor, available light, and current road striping practice. The sensor 

consists of a cylindrical lens and a silicon crystal detector. Figure 3-33 shows 

the voltage generated as a prototype sensor scans across the lane centerline. 

The voltage is proportional to the location of the line’s centroid.(84) The scan 

voltage is monotonic, smooth, and fairly linear in the central 50 percent of 

the plot. In this example, the detector output voltage is nonzero when the 

lane is physically centered on the detector. This indicates that overall 

background light falling on the detector is nonuniform, in this case produced 

by external sources in the sensor’s field of view. Baffling is under 

consideration to eliminate the voltage offset. 

95%

26%

34.6 cm 163.1 cm 197.7 cm

1.03 V

1.72 V

1.36 V at scan center

•

•

•

1 cm = 0.4 inch 

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

Voltage is proportional to 

centerline location in the 

sensor’s field of view. The 

contrast between the lane 

lines and the background in 

reflected sunlight was 0.57. 

The scan length was 

approximately 64 inches 

(163 cm) and the sensor 

declination was 21 degrees. 
 Traffic Detector Handbook—2006 

October 2006 Page 3-55 Federal Highway Administration

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 Traffic Detector Handbook—2006 

October 2006 Page 4-i Federal Highway Administration 

TABLE OF CONTENTS: CHAPTER 4. 

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

Green Extension System............................................................................4-32

Extended-Call Detection System ...............................................................4-34

Multiple-Point Detection Methods ............................................................4-34

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

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
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-ii Federal Highway Administration 

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
 Traffic Detector Handbook—2006 

October 2006 Page 4-iii Federal Highway Administration 

LIST OF FIGURES: CHAPTER 4. 

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

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 datagathering 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
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-iv Federal Highway Administration 

LIST OF TABLES: CHAPTER 4. 

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 ft.......................................................................................4-31

Table 4-9. Stopping and clearance distances for intersection width 

of 23 m. .....................................................................................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
 Traffic Detector Handbook—2006 

October 2006 Page 4-1 Federal Highway Administration

CHAPTER 4. IN-ROADWAY 

SENSOR DESIGN

This chapter discusses the design of vehicle detection systems that use 

inductive-loop detectors, magnetometers, and magnetic detectors for surface 

street and freeway traffic management, with primary emphasis on inductive 

loops. The guidelines provided should serve traffic engineers in developing 

plans and specifications for vehicle detection at local intersections, 

detectorization for traffic signal systems, and freeway surveillance and 

control. Above-roadway sensors, such as those described in chapter 2, may 

also be an option for many of the applications discussed in this chapter. 

When an in-roadway sensor is required for vehicle detection in a particular 

application, the first step in the design process is to determine what type of 

in-roadway sensor will supply the needed data or provide the required 

functionality. Some agencies have predetermined policies or standard plans 

that assist in the sensor selection process. To be effective, the standard plans 

should be updated to reflect current state of the practice in sensor and data 

transmission technology. Where standard plans are not appropriate or are 

not available, the following criteria may be applied as guidelines for inroadway sensor selection. 

SELECTION CRITERIA 

Sensor selection for a specific traffic management application depends on the 

data parameters, data accuracies, spatial resolution, detection area, 

appropriate data transmission media, location-specific installation 

requirements, initial sensor cost, and the acceptability of the maintenance 

burden it will impose. These criteria, separately and in combination, should 

be assessed as part of the sensor selection process. 

SELECTION BASED ON SENSOR TECHNOLOGY 

Sensor technology and operating theory described in chapter 2 indicate that 

the principal in-roadway sensors (inductive-loop, presence-detecting 

magnetometers, and passage-detecting magnetometers (also referred to as 

magnetic detectors)) are suitable for some applications, but unsuitable for 

others. For example, magnetometers cannot be used with NEMA controllers 

where delayed-call capability is required, since this feature is not available in 

magnetometers. In Type 170 controllers, the controller performs the timing 

operation; therefore, magnetometers can be used with 170 and the newer 

2070 controllers. Search coil magnetometers must be excluded for operations 

requiring presence detection because they are only capable of detecting a 

moving vehicle (passage detection). 

Inductive-loop detectors are not always appropriate for some traffic 

signalization applications. For example, long loops are not suitable for 

detection of oversaturated flow or long queues. Loops can be utilized for 

freeway surveillance and control if the area of the induced magnetic field is 

tailored to provide vehicle detection in the lane of interest and the time 

required for output, pick-up, and drop-out are predictable. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-2 Federal Highway Administration 

SELECTION BASED ON APPLICATION 

Applying lessons learned from the extensive application of in-roadway 

sensors further narrows sensor selection. In theory, both the inductive-loop 

detector and one or more presence-detecting magnetometers are suitable for 

large area detection on an approach to a signalized intersection. The loop 

detector, however, is usually less expensive. Conversely, for an approach 

where it is not important to screen out false calls for the green (i.e., right 

turns on red) and only rudimentary traffic detection is needed, any of the 

three in-roadway sensor types may be installed. In this particular 

application, the passage-detecting magnetometer might be favored because of 

its ruggedness and low cost/useful life ratio. 

SELECTION BASED ON EASE OF INSTALLATION 

In the 1960s-1970s, it was common for large cities such as New York, NY, or 

Atlanta, GA, to select microwave radar or ultrasonic sensors because they 

could be installed on a suitable pole already in place and not disrupt traffic or 

break the pavement. A crew could install one of these sensors in about 45 

minutes. Recent advances in over-roadway sensor design have encouraged a 

new interest in these sensors and other over-roadway sensor technologies 

such as video image processors and laser radars. 

Today, agencies often look favorably on eliminating a saw cut or replacing it 

with a drilled hole. The pervasiveness of deteriorating pavements has 

produced more interest in installing preformed loops, microloops, or 

pavement slabs with sensors already in place. Another approach to 

eliminating saw cuts is to install preformed loops or prewound loops in 

conjunction with repaving. Chapter 5 describes some of the approaches that 

simplify installation of in-roadway sensors. 

SELECTION BASED ON EASE OF MAINTENANCE 

Most traffic engineering agencies are aware of the cost differences in 

maintaining sensors based on different operating technologies. For example, 

the passage-detecting magnetometer detector, with its limited application, 

has managed to retain some popularity largely because of its ruggedness and 

long life with minimum maintenance. The improved reliability and 

functionality in inductive-loop detector electronics units have shifted the 

main sources of loop detector failure to the wire loop in the pavement, its 

connection to the lead-in cable, and loop installation issues. The requirement 

for more rugged loop detector installations has highlighted the need for 

reduced maintenance costs in terms of frequency of failures and resulting 

maintenance calls. Chapter 6 discusses these issues further. 

DESIGN CONSIDERATIONS 

Sensor selection for vehicle detection at intersections is a function of the 

types of timing intervals generated by the controller and the corresponding 

data needed to compute the intervals. Therefore, the timing interval types 

should be selected early in the traffic signal control design process. Similar 

considerations apply to the design of freeway signal control strategies. The 

following discussion defines various timing parameters, effect of short-loop 

and long-loop configurations, and detection alternatives for low-speed and 

high-speed approaches. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-3 Federal Highway Administration

TIMING PARAMETERS 

An actuated phase normally has three timing parameters in addition to the 

yellow change and all-red clearance intervals. These are the minimum green 

(also known as initial interval), the passage time (also called the vehicle 

interval, extension interval, or unit extension), and the maximum interval.(1)

Relationships among the intervals are shown in Figure 4-1. These intervals 

are timed depending on the type and configuration of the sensor installation 

found at the intersection. 

*

Total Green Period

Minimum

Green Period 

Actuations

Time

* * * * *

*

* * *

Detector actuation on phase with right-of-way 

Detector actuation on a conflicting phase 

Unexpired portions of vehicle intervals

*

*

Legend

Extension Period

Maximum Period

Figure 4-1. Actuated controller green phase intervals. 

 Minimum Green Interval 

The majority of early vehicle sensors were point detectors consisting of 

treadles or pressure plates in the roadway. Today's 6- x 6-ft (1.8- x 1.8-m) 

inductive-loop detector also performs as a point detector. With point 

detection, the minimum green interval is set to allow vehicles stopped 

between the detection point and the stopline to start and move into the 

intersection. Table 4-1 shows the minimum green interval for various 

distances from the stopline. The data are based on an average vehicle 

headway distance of 20 ft (6 m) and the average times for vehicles from a 

queue to enter the intersection. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-4 Federal Highway Administration 

Table 4-1. Minimum green interval. 

Distance between stopline and sensor Minimum green time 

feet meters seconds 

0–40 0–12.2 8 

41–60 12.5–18.3 10 

61–80 18.6–24.4 12 

81–100 24.7–30.5 14 

101–120 30.8–36.6 16 

A different timing approach is used to establish minimum green intervals for 

presence detection. When long loops (or a series of short loops) that terminate 

at the stopline are used, the initial interval is set as small as zero seconds to 

achieve “snappy” response for left-turn phases or possibly longer for through 

movements where drivers may expect a longer green. If the loop ends some 

distance from the stopline, this distance is used to calculate the interval time 

in the same way as for point detection. 

Passage Time Interval 

The passage time interval is calculated such that a vehicle can travel from 

the sensor to the intersection. This is particularly important where point 

detection is used and the sensors are located some distance from the stopline. 

The passage time also defines the maximum apparent time gap between 

vehicle actuations that can occur without losing the green indication to a call 

waiting. As long as the interval between vehicle actuations is shorter than 

the passage time, the green will be retained on that phase (subject to the 

maximum green interval described in the following section). 

The apparent time gap for a single stream of vehicles passing over an 

inductive-loop detector is somewhat shorter than the actual time gap. A 

vehicle activates the loop upon entry over the loop, and the loop does not 

deactivate until the vehicle leaves the loop. The true time gap is reduced by 

the time it takes the vehicle to traverse the loop. 

Another factor in determining an appropriate vehicle interval is the number 

of approach lanes containing sensors. Inductive-loop detectors for the same 

phase and function installed in adjacent lanes are often connected to the 

electronics unit by a single lead-in cable. This may introduce errors into the 

number of vehicles reported. 

NEMA controllers identified as “advanced design,” “exceeds,” or “beyond” 

have a predefined minimum green time. If no further actuations occur, the 

minimum green is the total green. If there are further actuations, the 

passage time interval extends the green until a gap exceeds the passage time 

or the maximum green time is reached. The relationship of these times was 

illustrated in Figure 4-1. 

When long loops are used in approaches, especially in left turn bays, the 

minimum green and the passage time intervals are generally set to zero or 

near zero. The long loop operates in the presence mode and the controller 

continuously extends the green as long as the loop is occupied. The critical 

gap is not a preset value, but a space gap equal to the length of the loop. 

Thus, the gap length is equivalent to no following vehicle entering the loop 

prior to the departure of the previous vehicle. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-5 Federal Highway Administration

When a series of short loops is utilized, it acts as a single long loop, provided 

the space between loops is less than a vehicle length. If the spacing is greater 

than a vehicle length, a short vehicle interval can be used to provide the 

same effect as a single long loop. 

Maximum Green Interval 

The maximum green interval limits the time a phase can hold the green. The 

maximum green begins timing at the first call received from an opposing (or 

conflicting) phase. Ordinarily, maximum intervals for through movements 

are set between 30 and 60 seconds. When the signal is properly timed with 

appropriately short passage times (i.e., vehicle intervals), the maximum 

interval will not consistently time out unless the intersection is badly 

overloaded. Some actuated controllers are capable of providing two maximum 

intervals per phase. This allows longer maximums to be programmed during 

peak periods (or shorter maximums for selected phases) when very heavy 

traffic flows are expected on the major street. 

Volume-Density Mode 

Volume-density phases have more timing parameters than a standard 

actuated phase. For this type of operation, sensors are generally placed 

further back from the intersection, particularly on high-speed approaches. 

The minimum green interval can be increased to provide longer initial green 

times for those instances when the minimum green is not adequate to serve 

the actual traffic present. The length of the initial green interval is governed 

by three factors—minimum green, seconds per actuation, and maximum 

initial—as described by the timing procedure illustrated in Figure 4-2. The 

variable initial interval is described by NEMA Standards in the following 

paragraph:(2) 

In addition to MINIMUM GREEN, PASSAGE TIME, and 

MAXIMUM GREEN timing functions, phases provided with 

VOLUME DENSITY operation shall include VARIABLE 

INITIAL timings and GAP REDUCTION timings. The effect on 

the INITIAL timing shall be to increase the timing in a manner 

dependent upon the number of vehicle actuations stored on this 

phase while its signal is displaying YELLOW or RED. The effect 

on the extensible portion shall be to reduce the allowable gap 

between successive vehicle actuations by decreasing the 

extension time in a manner dependent upon the time waiting of 

vehicles on an opposing RED phase. 

In volume-density phases, the extended green time (passage time) created by 

each new actuation after the initial green time has elapsed is normally based 

on the time required to travel from the sensor to the stopline. Because this 

distance can be relatively long, the passage time can be more than the 

desired allowable gap. The NEMA gap reduction procedure addresses this 

situation. It defines four time settings—time before reduction, passage time, 

minimum gap, and time to reduce, as illustrated in Figure 4-3. The time 

before reduction begins when there is a call on a conflicting phase. Once the 

time before reduction has expired, the allowable gap reduces linearly until 

the minimum gap is reached at the end of the time-to-reduce interval. The 

maximum green extension and yellow change and all-red clearance intervals 

are predetermined (precalculated) and programmed into the controller. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-6 Federal Highway Administration 

Maximum initial

Beginning of green

Seconds per actuation

Vehicle Actuations on Yellow or Red

Initial Timing (seconds)

Minimum green

Added initial range

^ ^ ^ ^^ ^ ^ ^ ^ ^^ ^

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

Passage 

time

Gap Timing (seconds)

Minimum

gap 

Time before reduction

Time to reduce

Green Time (seconds)

Serviceable conflicting call

Start of green phase

Figure 4-3. NEMA gap reduction procedure. 

LOW-SPEED APPROACHES 

Approaches with speeds of less than 35 mi/h (55 km/h) are considered lowspeed approaches. The sensor design for a given approach depends on 

whether the controller phase has been set for locking or nonlocking detection 

memory. This is also referred to as memory ON or memory OFF. The locking 

feature means that a vehicle call for the green is remembered or held by the 

controller until the call has been satisfied by the display of the green 

indication, even if the calling vehicle has left the detection area (e.g., right 

turn on red). In the nonlocking mode, the controller drops a waiting call as 

soon as the vehicle leaves the detection area. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-7 Federal Highway Administration

Locking Memory with Point Detection 

Locking detection memory is associated with the use of small-area point 

sensors such as a 6- x 6-ft (1.8- x 1.8-m) inductive loop and is frequently 

referred to as conventional control. The minimum green interval (or initial 

interval) is preset to provide sufficient time to clear a standing queue 

between the sensor and the stopline. The passage time or unit extension sets 

a common value for both the allowable gap to hold the green and the travel 

time from sensor to stopline. 

Since the allowable gap is usually 3 or 4 seconds, the sensor might be ideally 

located 3 or 4 seconds of travel time upstream from the intersection. This 

sensor position would appear to be the most efficient for accurately timing 

the end of green after passage of the last vehicle of a queue. However, a long 

minimum green (assured green) is created at approaches with speeds greater 

than 25 to 30 mi/h (40 to 50 km/h) because of the longer sensor setback. 

Therefore, the principle is amended to locate sensors 3 to 4 seconds of travel 

time from, but not more than 170 ft (52 m), from the stopline. Some agencies 

limit this distance to 120 ft (37 m). Table 4-2 displays the application of this 

principle to determining sensor location and associated timing parameters as 

a function of vehicle approach speed. 

Table 4-2. Inductive-loop detector location and timing parameters. 

Approach speed Detector setback from stopline to leading 

edge of inductive-loop detector 

Minimum 

green time 

Passage 

time 

mi/h km/h feet meters seconds seconds 

15 24 40 12 9 3.0 

20 32 60 18 11 3.0 

25 40 80 24 12 3.0 

30 48 100 30 13 3.5 

35 56 135 41 14 3.5 

40 64 170 52 16 3.5 

45+ 72+ Volume-density or multiple inductive-loop detectors recommended. 

Note: Volume-density could be considered at speeds of 35 mi/h (56 km/h) or above. 

The advantage of this single sensor approach is that the cost of installation is 

minimized. However, this type of control does not screen out false calls for 

green as occurs with right turns on red. 

Nonlocking Memory with Larger-Area Presence 

Detection 

Nonlocking detection memory is used with larger-area sensors such as a 6- x 

50-ft (1.8- x 15.2-m) loop. Often called loop occupancy control, the technique 

requires a nominal loop configuration as shown in Figure 4-4. The vehicle 

presence information provided in the area near the intersection allows the 

elimination of many of the false calls for green and thus avoids unnecessary 

green signals in approaches with no waiting vehicles. The disadvantages of 

this traffic control method are the higher initial cost of sensor installation 

and the higher replacement and maintenance costs associated with larger 

inductive-loop detectors. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-8 Federal Highway Administration 

Nonlocking detection memory is particularly appropriate in left-turn lanes 

with separate signal control for the left turns. The green arrow is terminated 

as soon as the turning vehicle clears the loop. In addition, a call placed 

during the yellow change interval by a vehicle that clears on the yellow does 

not bring back the green to an empty approach. Another potential advantage 

occurs when the left turn is permitted. In this case, the left turn is allowed to 

filter across oncoming traffic on the circular green shown to the through 

movement. Accordingly, left turns may be serviced during the through phase 

and, therefore, do not require the display of a left arrow. 

75'

100'

75'

100'

1 ft = 0.3 m 

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

The left turn bay may use an electronics unit operating in a delayed-call 

mode, which is designed to send an output to the controller only if a vehicle is 

continuously detected beyond a preset period (e.g., 5 seconds). This allows the 

electronics unit and controller to ignore vehicles that are in transit over the 

loop if oncoming traffic is light enough to allow a left turn without the 

protective left-turn arrow. If oncoming traffic is so heavy that left turning 

vehicles queue up over the loop, then the green arrow is called. This type of 

controller operation is often used with lagging left-turn phasing. 

A similar operation occurs on a single lane approach from the cross street 

where a right-turning vehicle approaches on the red. Again, the use of a 

delayed call electronics unit will avoid calling the green to the side street if 

the right turn on red can be made during the delay time set on the electronics 

unit. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-9 Federal Highway Administration

Loop-occupancy control is also utilized for through lane control on low-speed 

approaches. The technique minimizes delay by allowing short passage times 

(unit extensions) in the range of 0 to 1.5 seconds. The length of the detection 

zone obviously depends on the approach speed and the controller unit time 

settings. Figure 4-5 gives the length of the long-loop presence detector for 

various passage time settings on the controller for approach speeds less than 

30 mi/h (50 km/h). The figure is based on a desired allowable gap of 3 seconds 

and an average vehicle length of 18 ft (5.5 m). The formula for loop length is 

 (4-1a) 

 (4-1b) 

where 

L = length of detection area, ft (m) 

S = approach speed, mi/h (km/h) 

PT = passage time (unit extension),seconds. 

1 mi/h = 1.6 km/h 

1 ft = 0.3 m 

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

High-Speed Approaches 

Approaches with speeds in excess of 35 mi/h (56 km/h) are considered highspeed approaches. Several problems associated with high-speed approaches 

require special consideration in setting signal timing intervals. For example, 

it may be difficult for the driver to decide whether to stop or proceed when 

approaching a yellow change indication. An abrupt stop may result in a rearend collision, while the decision to proceed through the intersection may 

cause a right-angle accident or a traffic violation. 

The portion of the roadway in advance of the intersection in which the driver 

is indecisive (as to stopping or proceeding into and through the intersection 

at the onset of the yellow change interval) is called the dilemma zone. Some 

researchers have defined the dilemma zone as that area of the approach 

between a point where 90 percent of the drivers will stop on yellow and a 

point where 90 percent of the drivers will go (i.e., 10 percent will stop).(3,4)

Table 4-3 shows these boundaries for various speeds. 

120 

100 

80

60 

40 

20 

0 

5 10 15 20 25 30 

Approach Speed, S (mi/h)

Length of Detection Area, L (ft)

0

.5

1.0

1.5

Passage Time (seconds)

120 

100 

80

60 

40 

20 

0 

5 10 15 20 25 30 

Approach Speed, S (mi/h)

Length of Detection Area, L (ft)

0

.5

1.0

1.5

Passage Time (seconds)

L = 1.47 S (3 – PT) – 18 in English units

L = 0.277 S (3 – PT) – 5.5 in metric units
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-10 Federal Highway Administration 

Table 4-3. Dilemma zone boundaries. 

Approach speed Distance from intersection for 

90% probability of stopping 

Distance from intersection for 

10% probability of stopping 

mi/h km/h feet meters feet meters 

35 56 254 77 102 31 

40 64 284 87 122 37 

45 72 327 100 152 46 

50 80 353 108 172 52 

55 88 386 118 234 71 

Figure 4-6 illustrates the dilemma zone for a vehicle approaching an 

intersection at 40 mi/h (64 km/h). To minimize the untimely display of yellow 

and thus the creation of a dilemma zone problem, a number of techniques 

have been devised for controllers with locking and nonlocking detection 

memory, basic actuated and volume-density controller circuitry, and various 

inductive-loop detector configurations. 

The most straightforward conventional design for a high-speed approach 

utilizes a controller with a volume-density mode. This type of actuated 

operation can count waiting vehicles beyond the first because of the added 

initial feature. It also has a timing adjustment to reduce the allowable gap 

based on the time vehicles have waited on the red on a conflicting phase. 

More efficient operation can be achieved with the volume-density mode than 

with fully actuated control because of the added initial and timing 

adjustment features and because detection is farther back on the approach— 

400 ft (120 m) is typical. A calling sensor near the stopline that operates only 

when the phase is red or yellow often supplements high-speed approach 

detection. This calling sensor is disabled when the signal turns green so that 

it cannot extend the green time inappropriately. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-11 Federal Highway Administration 40 mi/h Dilemma Zone 162' 122' 284'

1 ft = 0.3 m 

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

Fully-actuated controllers utilizing an extended-call sensor just upstream of 

the dilemma zone have been in service since at least 1982. Designs for highspeed approaches using nonlocking detection memory include a long loop at 

the stopline as well as one or more small loops upstream. The long loop 

improves the controller’s knowledge of traffic at the stopline, but tends to 

increase the allowable gap. 

Inductive-loop designs for both normal fully actuated and volume-density 

controllers are available. Additional information concerning these solutions 

to the dilemma zone problem is presented later in this chapter. 

Rural High-Speed Roadways 

For isolated high-speed rural intersections, a “PREPARE TO STOP” 

extinguishable message sign is frequently deployed when the signal site 

undergoes periods of poor visibility caused by dense ground fog, orientation of 

the sun, or geometry that prevents signal visibility far enough in advance to 

ensure safety. These situations require vehicle sensors further in advance of 

the intersection than normal. 

Using the last car passage feature of some density controllers, the gap in the 

traffic flow can be identified to allow the last car in the platoon to pass 

through the signal and presumably give the next vehicle sufficient time to 

stop. The message sign, such as the one depicted in Figure 4-7, would flash 

PREPARE TO STOP at the appropriate time, but would be blank or 

unreadable at other times. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-12 Federal Highway Administration 

When the controller is informed of a gap in the traffic, it does not change the 

signal until a preset time has elapsed to allow the last car to clear the 

intersection. The PREPARE TO STOP is illuminated when the gap is 

selected, so that the next vehicle following the platoon will see the sign. 

Thus, the driver will know he will be required to stop even though the signal 

ahead is still green. 

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

A typical inductive-loop detector and sign placement layout for a PREPARE 

TO STOP installation is shown in Figure 4-8. A display used by some 

jurisdictions consists of flashing beacons together with a diamond or 

rectangular sign with the message PREPARE TO STOP WHEN FLASHING. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-13 Federal Highway Administration

200'

PREPARE

TO STOP 

600' Flashing

beacons 

Inductive loop

detectors

1 ft = 0.3 m 

Figure 4-8. PREPARE TO STOP inductive-loop detector system. 

REST-IN-RED SPEED CONTROL 

The commuting driver frequently uses residential roadways as a time-saving 

route for reaching destinations, particularly when the residential street is 

parallel to a congested arterial. Residents generally perceive the added 

commuter traffic as a threat to the safety of children, pets, and their ability 

to safely exit their driveways, particularly where speeding occurs. 

To control traffic speeds in residential areas, traffic engineers have used a 

variety of traffic control devices such as stop signs, warning signs, speed 

bumps, and coordinated traffic signals with vigorous enforcement of posted 

speed limits. While these measures are often successful, there are drawbacks 

associated with their use. 

For example, the use of unwarranted stop signs to control vehicular speeds 

imposes delay penalties on all drivers, and it only affects speeds within 200 ft 

(60 m) of the stop sign. Unwarranted stop signs may also encourage drivers 

to ignore the stop sign, which is even more dangerous. 

One approach for slowing speeding vehicles is to install two speed-measuring 

inductive-loop detectors (or other sensor types) approximately 180 ft (55 m) 

in advance of the intersection. The advance loops measure the speed (through 

measurement of the elapsed time to travel between the loops) of an 

approaching vehicle and register a call on the controller only if the vehicle is 

traveling at or below the speed limit. Assuming the signal is resting in fourway red and the vehicle is not speeding, a green is displayed and the driver 

may proceed through the intersection without being delayed. If, on the other 

hand, the approaching vehicle is exceeding the speed limit, no call is placed 

and the driver must slow down until he reaches a loop near the intersection. 

A call will then be placed and the green interval activated. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-14 Federal Highway Administration 

Consequently, the timing cycle to initiate a call begins when a vehicle crosses 

the first advance loop. If the vehicle speed is low enough, the predetermined 

interval will time out before the vehicle reaches the second loop. Then the 

call request will be passed to the controller. When the vehicle reaches the 

second loop, the timing device is reset, and any call being held at the timing 

device is cancelled. Thus, a vehicle exceeding the speed limit is never 

detected by the advance loop, and each succeeding vehicle is timed 

independently. This method is simple, economical, adjustable, and not 

dependent on vehicle size or length. 

The spacing between the initiating and resetting advance loops is 

approximately 1 second of travel distance at the speed limit. The distance 

from the advanced loop to the first intersection loop is predetermined from 

the lowest travel speed to be accommodated (normally 20 mi/h (32 km/h)) and 

the maximum desirable passage time interval (4 seconds). A comfortable 

reaction time and the stopping distance at the design speed determine the 

minimum distance from a stopline to the advance detector loop. 

INDUCTIVE-LOOP DETECTOR DESIGN ALTERNATIVES 

Small-area (for point or passage detection) and larger-area (for area or 

presence detection) inductive-loop detector design is described in this section. 

Passage-detecting search coil magnetometers (magnetic detectors) can only 

be used for point detection because they are sensitive to vehicles in small 

areas and require vehicle motion (passage) for activation. Presence-detecting 

two-axis fluxgate magnetometers are also point detectors, but can be used as 

area detectors by using multiple sensors to cover a larger area. 

Typical design configurations for sensor locations in through lanes and in 

left-turn lanes are also presented. Both simple and complex designs are 

described along with the type of controller operation, if appropriate. 

Treatments to alleviate the dilemma zone problem are also discussed. 

SMALL-AREA DETECTION 

Small-area detection is commonly implemented with a single short inductive 

loop. Although the literature defines short loops as being up to 20 ft (6.1 m) 

in length, by far the most common short-loop application is the 6- x 6-ft (1.8- 

x 1.8-m) loop in a 12-ft (3.6-m) lane. For narrower lanes, 5- x 5-ft (1.5- x 1.5-

m) loops should be used to avoid adjacent lane pickup (splashover). Smaller 

loops are not recommended in areas where high-bed vehicles must be 

continuously detected. 

The short loop is intended to detect a vehicle upstream of the stopline. When 

a vehicle passes over the loop, a call is output by the electronics unit to the 

controller. Timing of the green interval is commonly based on preset 

controller settings, not by the length of time the detection area is occupied by 

vehicles approaching the intersection. In most cases, the controller operates 

with locking detection memory circuits to insure calling the appropriate 

phase. 

Short-loop detectors may be used in a variety of ways and may be located at 

varying distances from the stopline, depending on the operational 

requirements. A typical application may consist of one or more short loops 

near the stopline on the actuated approach of a low-speed intersection. 

Another typical application is to space a number of these loops well back of 

the stopline to act as extension sensors for higher-speed approaches. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-15 Federal Highway Administration

Loop shapes were the subject of a great amount of research during the l970s 

and 1980s. Subsequently, many loop configurations were designed to detect 

the various sizes and shapes of vehicles that travel on the Nation’s roadways, 

from bicycles and motorcycles to high-bed trailer trucks, while avoiding 

detection of vehicles in adjacent lanes. Each design purports to have 

advantages over other designs. Examples of short loop shapes are illustrated 

in Figure 4-9. Some of these configurations are common, while others are 

found at either a site-specific location or when particular types of vehicles 

require detection. 

Rectangle 

Parallelogram 

or 

Skewed

Square

Diamond

Circle

Chevron

Modified 

Chevron

Quadrupole

Triangle

Diamond

Hexagon

Octagon

Diamond

Figure 4-9. Small loop shapes. 

A number of agencies and universities have conducted tests to determine an 

optimum loop shape.(5,6,7) These projects typically involved installing several 

different loop shapes and then testing and comparing the sensitivity of the 

loops in detecting several types of test vehicles. None of these projects tested 

all of the loop designs currently in use. In some cases, one loop design would 

test better when compared to one or two different loop designs. In most 

instances, the difference in sensitivity among loops was not significant, given 

the state of the art in electronics units. It is therefore difficult to cite one 

particular design as superior to all others. However, it is generally accepted 

that some loop designs are better suited than others for detecting small 

vehicles or high-bed trucks, as discussed in later sections. 

Many States specify the acceptable loop shapes for use in their jurisdiction. 

An example is Caltrans’s specified shapes shown in Figure 4-10. In this 

particular example, each unique shape is given a letter designation (e.g., type 

A is the conventional 6- x 6-ft (1.8- x 1.8-m) loop). 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-16 Federal Highway Administration 

LARGER-AREA DETECTION 

Larger-area detection normally contains a detection zone covering an area of 

at least 20 ft (6 m) or more in a traffic lane. It is primarily used for presence 

detection because the detection zone registers the presence of a vehicle as 

long as the zone is occupied. This concept originally used a single loop 

encompassing the entire detection zone. However, the long loop, as a single 

entity, is being supplanted by a sequence of short loops, which emulate the 

long loop. In this Handbook, the term long loop means either a single long 

loop or multiple short loops functioning as a single long loop. 

TYPE B

Symbol

Winding Detail Sawcut Detail

Direction of Travel

S

F

7'

5'-8

'

Loop C

1'

S

S

Symbol

Winding Detail Sawcut Detail

6"

6"

TYPE A

Symbol

Winding Detail 

(One turn)

Sawcut 

Detail

TYPE C

3' 3'

Direction of Travel

30"

Winding Detail Sawcut Detail 30" 15"

15"

27"

27"

TYPE D

Symbol

Direction of Travel

S

F

F

F

Variable

6'

Winding Detail Sawcut Detail

TYPE Q

S

F

Symbol

Direction of Travel

6'

3' 3'

1 ft = 0.3 m 

Figure 4-10. Caltrans-specified loop shapes. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-17 Federal Highway Administration

Long Loops 

Figure 4-11 illustrates the traditional long loop (i.e., a single loop 6 ft (1.8 m) 

wide by 20 to 80 ft (6 to 24 meters) long or longer) and other long-loop 

shapes. These long loops generally have only one or two turns of wire. If the 

rectangular, powerhead or trapezoidal loop needs to reliably detect all 

roadway vehicles, the sensitivity level must be set high which, in turn, causes 

detection of adjacent lane vehicles (splashover). The quadrupole loop is an 

appropriate design to eliminate this problem. However, due to its limited 

field height, it may have difficulty in continuously detecting high-bed 

vehicles. Quadrupoles are excellent wheel and axle sensors. The lengths 

associated with long loops increase the vulnerability to failure caused by 

pavement cracks and joint movement. In response to these problems, many 

agencies are installing sequential short loops. 

The long loop normally provides input to the controller for operating in the 

loop occupancy mode. In this control mode, the minimum green interval (or 

initial interval in older controllers) is set to zero or near zero, and the 

passage time or vehicle interval is set to a small value, as was shown in 

Figure 4-5. When the green interval appears for the subject phase, it 

remains green as long as the loop is occupied (subject, of course, to the 

maximum green). As soon as the inductive-loop detector is cleared, the 

passage time is measured and, if no further actuation occurs during the 

passage time, the yellow change interval appears. 

Rectangle 

6' x 20' to

6' x 80'

Quadrupole Trapezoid

Parallelogram

with powerhead

1 ft = 0.3 m 

Figure 4-11. Long loop shapes 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-18 Federal Highway Administration 

The effective time gap is equal to the travel time required to traverse the 

length of the loop plus one vehicle length plus the passage time. Thus, the 

length of the loop is a critical measure for providing appropriate operation. 

The length must be sufficient for a following car to come to a stop if the 

yellow interval appears just before the following vehicle reaches the loop or, 

conversely, to allow the vehicle to proceed through the intersection on the 

yellow. 

If heavy trucks are included in the traffic stream, there may be a start-up 

issue if a long queue exists. Passenger cars in front of the truck may 

accelerate and clear the inductive-loop detector before the truck can 

accelerate and reach the detector. 

One researcher examined the relationship between inductive-loop detector 

length and the time settings of vehicle interval and maximum green for 

intersections where vehicle approach speeds were less than 35 mi/h 

(56 km/h).(8) The purpose of the study was to determine the optimal 

combinations of loop length, vehicle interval, and maximum green for a wide 

range of flow conditions (i.e., flow rate per lane, distribution of traffic among 

lanes, and temporal variations in flow rates). Both two- and four-phase 

operation of presence mode control were analyzed for each flow pattern. 

Optimal vehicle intervals are a function of inductive-loop detector length and 

flow rate. The study suggests that for loops 30 ft (9 m) long, the use of 

2-second vehicle intervals can lead to the best signal performance over a wide 

range of operating conditions. For 50-ft (15-m) loops, 1-second vehicle 

intervals are desirable under a variety of flow conditions. When loops 80 ft 

(25 m) long are used, 0-second vehicle intervals can minimize delays. Longer 

vehicle intervals for such loop lengths are not desirable unless the combined 

critical flow at an intersection exceeds 1,400 v/h. 

The study concluded that maximum green for presence-mode control is 

generally longer than optimal green durations for pretimed control. Flow 

patterns characterized by a larger concentration of traffic in short periods of 

time need longer maximum greens. The optimal maximum greens for hourly 

flow patterns with a peaking factor of 1.0 (a uniform flow rate) are about 10 

seconds longer than the corresponding optimal pretimed greens. With a 

peaking factor of 0.85 (a larger concentration of traffic in a short period), the 

optimal maximum greens are approximately 80 percent longer than the 

corresponding optimal pretimed greens. 

The study further concluded that loops 80 ft (25 m) long could consistently 

produce the best signal performance. However, 65-ft (20-m) loops gave 

comparable performance when the combined critical flow was less than 1,100 

v/h. When the combined critical flow was less than 900 v/h, 50-ft (15-m) loops 

rather than 80-ft (25-m) loops incurred a delay of up to 2 seconds per vehicle. 

For a combined critical flow of less than 600 v/h, 30-ft (9-m) loops may be 

used instead of 80-ft (25-m) loops without incurring undue delays. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-19 Federal Highway Administration

Sequential Short Loops 

Use of sequential short loops to emulate a long loop is the preferred 

treatment in many agencies. The advantages of this configuration result 

primarily from fewer failures because of the loop’s shorter length. Thus, they 

are less vulnerable to problems caused by crossing pavement cracks and 

joints and to adjacent lane pickup (splashover). Long loops are more subject 

to adjacent lane splashover since the entire length of the vehicle is exposed to 

the side of the long loop (approximately 17 ft (5 m)) as compared to less than 

a third of the vehicle length of about 6 ft (1.8 m) for a short loop. The short 

loops also provide superior detection of small vehicles, as explained later. 

Sequential short loops usually consist of four 6- x 6-ft (1.8- x 1.8-m) square or 

diamond loops separated by 9 or 10 ft (2.7 or 3.0 m). This configuration is 

equivalent to a 51- or 54-ft- (15.3- or 16.2-m-) long loop. Figure 4-12 shows 

various configurations of sequential loops used by Caltrans. This standard 

employs loop-type designations defined in Figure 4-10. 

1 ft = 0.3 m 

Figure 4-12. Caltrans standard sequential inductive-loop configurations. 

Figure 4-13 demonstrates a different spacing pattern the Pennsylvania DOT 

(PennDOT) uses for installations of sequential short loops. This spacing 

normally requires a passage time or vehicle interval greater than zero to 

provide proper signal operation. Other spacing configurations are used by 

agencies around the Nation. 

 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-20 Federal Highway Administration 

1 ft = 0.3 m 

Figure 4-13. PennDOT short inductive-loop configurations. 

Wide Loops 

Some agencies use wide loops to cover wide lane or multiple lane approaches. 

These loops are normally 6 ft (1.8 m) in length in the direction of traffic flow 

and up to 46 ft (14 m) in width for a four-lane approach. The basic loop 

configuration for a wide lane is shown in Figure 4-14. The number of turns of 

wire varies according to the number of lanes covered. Table 4-4 gives the 

number of turns and the dimensions for the loop. Wide loops are generally 

not recommended nor are permitted by many agencies because they are 

subject to more frequent failure from crossing pavement joints and fractures. 

A failure anywhere on the perimeter takes the entire loop out of operation, 

which in turn removes all detection capability for that approach. Separate 

loops in each lane are less susceptible to failure and, even if a failure in one 

loop occurs, the remaining loops can provide approach detection. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-21 Federal Highway Administration

1 inch = 2.54 cm 

Figure 4-14. Wide inductive-loop detector layout. 

Table 4-4. Wide inductive-loop detector dimensions. 

Number 

of lanes 

Total approach 

width of lanes 

Loop width Loop length Turns 

of wire 

 feet meters feet meters feet meters 

1 12–18 3.7–5.6 6–12 1.8–3.7 6 1.8 4 

2 19–32 5.7–9.8 13–26 3.7–8.1 6 1.8 3 

3 33–45 9.9–13.6 27–39 8.2–12.1 6 1.8 2 

4 46–52 13.7–15.8 40–46 12.2–13.7 6 1.8 1 

Large loops of up to 30 ft (9 m) in width and 50 to 60 ft (15 to 18 m) in length 

have been installed to extend green time when occupancy increases to a 

saturation level in a given direction. The inductive-loop detector electronics 

unit is adjusted to be sensitive to more than some specified number of 

vehicles in the loop. Thus, the electronics unit only responds to a saturated 

condition. No additional green extension is given to the approach unless 

there is congestion. If no extensions are present (i.e., there is no saturation or 

congestion), the opposing street green receives the excess time. This 

application cannot be used for call initiation and is intended for use only in 

locations where unpredictable and extreme fluctuations of traffic are present, 

such as shopping center exits, some freeway exits onto main street flow, and 

industrial plant parking lot exits. 

LEFT-TURN LANE DETECTION 

Vehicle sensors in left-turn lanes can affect the capacity of an intersection by 

reducing unnecessary green time and left turn arrow indications. When the 

last vehicle proceeds a block or so past the signal before the conflicting phase 

begins, the travel time represents lost green time, which could more 

appropriately be used to increase the green time available to other phases. 

The design of left-turn detection is generally based on the premises discussed 

below. 

From 3 to 5 seconds are normally required at the start of the green indication 

for the first vehicle in a queue to start up, with an average headway of 2 to 3 

seconds between following vehicles. Longer startup and headway times are 

accounted for by providing an appropriate loop length to maintain the green for 

these slower vehicles. Moreover, trucks and other slow vehicles may require a 

still longer startup time, which frequently results in a three or four car-length 

gap ahead of them. Loop length also needs to account for these gaps. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-22 Federal Highway Administration 

Because green time is based on vehicular demand, only a short green time is 

needed for one or two vehicles. For example, a rapidly starting single vehicle 

can clear the turn lane with a green time of less than 5 seconds. A driver of a 

following vehicle just entering the left turn lane may be confused by the short 

green. The length of the inductive loop should allow the following car to reach 

the loop in time to enter the intersection on the green indication or brake to a 

stop. This length is based on the equation for maximum deceleration rates, 

which indicates that a vehicle traveling at 30 mi/h (48 km/h) can stop in 83 ft 

(25 m). 

To accommodate these conditions, a loop length of 80 ft (24 m) from the stopline 

and a controller passage time of 1 second are frequently used. Adding more 

passage time on the controller compensates for passage over shorter loops. 

Controller passage time is the time a controller holds the green after actuation. 

A passage time of 1.0 second permits most motorists to almost complete their 

turning radius before the onset of the yellow change interval is displayed. 

Another problem occurs when vehicles are permitted to turn on the circular 

green (green ball) indication. Drivers will usually proceed past the stopline 

and wait for a gap in the opposing traffic. If a gap does not occur or a vehicle 

ahead prevents the turn, the driver may be left stranded beyond a detection 

zone that ends at the stopline. In this case, the controller may skip the turn 

arrow in the next cycle because the vehicle is positioned ahead of the sensor’s 

detection zone. Some agencies think it a good practice to extend the loop 

beyond the stopline 1 to 6 ft (0.3 to 2 m) to prevent this situation. 

Small-vehicle (e.g., motorcycle) detection using short inductive loops requires 

a high sensitivity setting on the electronics unit. However, high sensitivity 

will frequently cause detection of vehicles in adjacent lanes (splashover). 

Many agencies use quadrupole loops to avoid splashover. As the quadrupole 

requires an additional sawcut equal to the length of the loop, it is desirable to 

limit quadrupole installation to the area near the stopline. Quadrupole 

design is discussed further under the topic of “Detection of Small Vehicles.” 

Figure 4-15 illustrates a minimum-length quadrupole left-turn loop designed 

by the Illinois DOT (IDOT) using the procedure below:(6) 

• Locate the stopline in relation to cross-street turning radius. 

• Measure back 80 ft (24 m) from the stopline to establish the 

back loop. 

• Measure 50 ft (15 m) toward the stopline to establish front edge 

of the back loop. 

• Allow 2-ft (0.6-m) gaps between loops and measure 28 ft (8.4 m) 

to the stopline. 

• The front of the front loop should be within 13 ft (4 m) of the 

edge of the cross street traffic lane. 

• Design the front loop as a quadrupole to detect small vehicles. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-23 Federal Highway Administration

3'

80'

Quadrupole loop

6'

3'

50'

2'

28'

Length varies

13'

1 ft = 0.3 m 

Figure 4-15. Left turn detection inductive-loop configuration used by IDOT. 

When the left-turn demand requires 150 ft (45 m) or more of storage length 

or when higher approach speed requires long deceleration lanes, the loop 

layout should include an advanced detection loop. The advanced vehicle 

sensor with a call extension feature will extend the effective detection zone to 

accommodate heavy traffic volumes or high-speed traffic. 

THROUGH-LANE DETECTION 

Detection of vehicles in through lanes depends on the approach speed and the 

type of controller operation being used. Single-point detection, long-loop 

occupancy detection, a combination of long and short loops, or a sequence of 

short loops can be deployed. Each of these is discussed below. 

Single-Point Detection 

This is the simplest type of vehicle detection used for actuated controllers. It 

is primarily found on low-speed approaches where speeds are less than 35 

mi/h (56 km/h). It may also be used on side-street approaches, with another 

form of detection on the major street. 

A point vehicle sensor (e.g., a 6- x 6-ft (1.8- x 1.8-m) inductive loop) is located 

2 to 4 seconds of travel time in advance of the stopline. As this is the only 

sensor in the approach lane, controller timing must be appropriately set to 

use the information. The actual distance divided by 25 ft (7.6 m) (which 

approximates the length of a vehicle plus the space between vehicles) 

indicates the number of vehicles that can be queued between an inductiveloop detector and the stopline when the light turns green. This number 

establishes the minimum green interval for the controller. The distance 

between the inductive loop and the stopline divided by the 15th percentile 

speed provides a good initial estimate of the passage time. The passage time 

is also the allowable gap that causes the green indication to drop. If this 

setting is too long or short to be acceptable as an allowable gap, the position 

of the loop should be moved to ensure an appropriate gap size. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-24 Federal Highway Administration 

Long-Loop-Occupancy Detection 

Loop-occupancy detection is generally used on low-speed approaches. It 

normally consists of a single loop 50 ft (15 m) or more in length or a sequence 

of short loops (usually four) located immediately upstream of the stopline. 

Loop-occupancy timing is used on the controller as described earlier. This 

type of operation is most effective when speeds are 25 mi/h (40 km/h) or less. 

Even at this speed, there is some potential for the signal turning yellow just 

before an approaching vehicle reaches the loop. In this case, the vehicle will 

probably cross the intersection on the yellow indication. 

As speeds increase, the detection zone must lengthen to accommodate the 

increased stopping distance. One jurisdiction uses the combination of 

approach speeds and loop lengths shown in Table 4-5. These loop lengths 

appear to be excessively long, resulting in long minimum gaps. 

The 120-ft (37-m) detection area is measured from the stopline and consists of 

two 56-ft (17-m) loops. Where greater detection areas are required, either 

additional long loops or small loops may be used. If additional small loops are 

chosen, they must be connected to separate electronics units with the extension 

time programmed into the unit. The long loops are set to presence mode. 

Table 4-5. Loop lengths for long-loop-occupancy detection. 

Speed Loop length 

mi/h km/h feet meters 

30 48 120 37 

35 56 160 49 

40 64 200 61 

45 72 250 76 

High-Speed Point Detection 

For high-speed approaches (those with speeds greater than 48 km/h (30 

mi/h)), detection becomes more complex. Volume-density control is one 

technique used that relies on the controller functions rather than extensive 

detectorization. Normally only one sensor is installed in each lane. This point 

sensor is usually placed at least 5 seconds and as much as 8 to 10 seconds 

from the stopline, which is more than the 2 to 4 seconds of travel time 

required for normal actuated operation. 

The sensor is active at all times rather than just during the green interval. 

During the red interval, each actuation increments the variable initial timing 

period. Once the variable initial timing period exceeds the minimum green, 

each additional actuation adds an additional user specified time increment to 

the initial interval. If the variable initial timing does not exceed the initial 

interval, then that duration is used instead for the first portion of the green 

interval. During the green interval, the sensor is used to extend the green. At 

first the extension is equal to the passage time, but after a conflicting phase 

has registered a call, the extension is reduced, eventually reaching a 

minimum gap. 

This type of signal control has the potential for trapping a vehicle in the 

dilemma zone at the onset of yellow. The following section discusses dilemma 

zones and describes how multiple sensors are used to alleviate the dilemma 

zone problem. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-25 Federal Highway Administration

DETECTION FOR DILEMMA ZONES 

Signalized intersections where speeds of approaching traffic are greater than 

30 mi/h (48 km/h) have long been of concern to designers and operations and 

safety engineers. Drivers approaching at these higher speeds are frequently 

confronted with a dilemma—whether to stop or proceed through the 

intersection at the onset of the yellow change interval. The placement of 

sensors to ameliorate this problem has received serious consideration and 

research. This section defines the many variables that affect the dilemma 

zone problem and describes several sensor placement schemes that have 

proven effective. 

Definition of the Dilemma Zone Problem 

When a vehicle traveling at a constant speed S approaches an intersection 

and is positioned at distance X from the intersection at the beginning of the 

yellow change interval, the driver is faced with a decision. He may decelerate 

and stop the vehicle before entering the intersection or continue and enter 

the intersection, accelerating if necessary before the red interval begins. In 

some States, the driver is required to clear the intersection before the red 

appears. Depending on the distance from the intersection and the speed of 

travel, drivers may not be certain that they can stop in time, or they may be 

unsure that they can clear the intersection before conflicting vehicles enter. 

This creates the dilemma. Some drivers will opt to stop, while others may 

accelerate and continue through the intersection. 

If the choice is to stop, the driver will decelerate after a short perception and 

reaction time. The distance the vehicle travels after the beginning of the 

yellow change indication includes the distance traveled during the perception 

and reaction time t and the distance traveled during deceleration. The 

inequality that must be maintained to ensure a safe and complete stop is 

given by 

d

S X St

2

2

≥ + (4-2) 

where 

X = distance from stopline at start of yellow interval, 

ft (m) 

S = approach speed, ft/s (feet per second) (m/s 

(meter per second) 

t = perception and reaction time (typically, 1 

second) 

d = constant deceleration rate, ft/s2 (m/s2). 

Safety and comfort require that the vehicle’s deceleration rate not exceed 

one-third to one-half the acceleration of gravity. Using d* to represent a 

critical deceleration rate under prevailing roadway conditions, a stopping 

distance Xs may be defined as 

2 *

2

d

S X St s ≥ + (4-3) 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-26 Federal Highway Administration 

where Xs is the minimum distance from the stopline in which the vehicle can 

come to a complete stop after the beginning of the yellow interval. Thus, if a 

vehicle is closer to the stopline than Xs when the yellow begins, the driver 

will be unable to stop safely or comfortably before the intersection. The area 

between the stopline and Xs is an area in which drivers should not be 

expected to stop or cannot stop as shown in the upper portion of Figure 4-16. 

Cannot Go

Cannot Stop

Xs

Xc

Figure 4-16. “Cannot Stop” and “Cannot Go” regions. 

If the driver decides to accelerate and pass through the intersection, a 

clearance distance Xc must be maintained as defined by the inequality: 

( )( ) ( )

2

2 a Y R t Xc W L S Y R + − ≤ − + + + + (4-4) 

where 

Xc = clearance distance, ft (m) 

t = perception and reaction time (typically 1 second) 

a = acceleration rate, ft/s2 (m/s2) 

Y = yellow change interval, seconds 

R = red clearance interval, seconds 

W = effective width of intersection, ft (m) 

L = length of vehicle, ft (m) 

S = approach speed, ft/s (m/s) 

W +L = correction to stopbar distance to bring the 

 rear of the car beyond the intersection. 

Equation 4-4 is based on the 

general relationship that 

distance traveled Xc = X0 + St 

+ 0.5at2

, where X0 is some 

initial distance. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-27 Federal Highway Administration

Implementing Equation 4-4 permits the driver to completely clear the 

intersection before the appearance of the red signal. Many traffic engineers 

do not believe that a driver must clear the intersection on the yellow. In fact, 

most State vehicle codes do not require the vehicle to clear the intersection 

prior to the onset of the red indication, but merely to have entered prior to 

the red. If this interpretation applies, engineers may eliminate the (W + L) 

term from the equation or may use 0.5 or 0.25 of the value of this term. In 

some cases, the red clearance interval is increased to assure clearance when 

needed rather than include the (W + L) term in the equation. 

The constant acceleration rate a available to the driver in Equation 4-4 may 

be estimated from Gazi’s equation as 

a = 4.9 − 0.213S in metric units (4-5a) 

or 

a = 16.0 − 0.213S in English units. (4-5b) 

These equations show that larger acceleration rates can be attained when a 

vehicle is traveling at lower speeds. Clearance distance Xc is the maximum 

distance from the stopline at which a vehicle can clear the intersection as 

defined by 

( )( ) ( )

2

2 a Y R t Xc W L S Y R + − = − + + + + . (4-6) 

Since Xc is the maximum distance upstream of the stopline from which a 

vehicle can clear the intersection during the yellow interval, any vehicle 

positioned at a point beyond Xc (i.e., further upstream) would not be expected 

to clear the intersection during the yellow interval, and is thus in a region in 

which the driver “cannot go” without violating the red indication (see lower 

portion of Figure 4-16). 

As both Xs and Xc are measured distances from the stopline, the relationship 

of these two quantities is defined by one of the following conditions: 

• Xs > Xc.

• Xs = Xc.

• Xs < Xc. 

When Xs > Xc, the dilemma zone is the overlapping area in which a vehicle 

can neither stop nor go if faced with a yellow indication as indicated in 

Figure 4-17. In this case, the driver of a vehicle within the dilemma zone at 

the onset of yellow has to accelerate or decelerate at an unsafe rate and 

consequently is vulnerable to a right-angle or rear-end accident. 

When Xs = Xc, Figure 4-18 shows that the dilemma zone and its associated 

problems disappear. A driver in the “cannot go” region is able to stop safely, 

whereas a driver in the “cannot stop” region can successfully accelerate 

through the intersection. 

Finally, when Xs < Xc, a driver in the area between Xs and Xc may either stop 

or go safely. Therefore, this region is considered an optional zone as depicted 

in Figure 4-19. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-28 Federal Highway Administration 

This relatively simply analysis indicates that a dilemma zone is only formed 

when Xs > Xc. Equation 4-3 shows that Xs is a function of speed, perception 

and reaction time, and deceleration rate, while Equation 4-4 shows that Xc is 

a function of speed, perception and reaction time, acceleration rate, yellow 

and red interval times, and effective width of the intersection. 

Tables 4-6 (English units) and 4-7 (metric units) contain stopping distance Xs

and clearance distance Xc for deceleration rates of 10 and 16 ft/s2

 (3.0 and 4.9 

m/s2

) for an intersection width of 48 ft (15 m). Tables 4-8 (English units) and 

4-9 (metric units) repeat the stopping and clearance distances for the same 

deceleration rates for an intersection width of 76 ft (23 m). 

Dilemma

Zone

Cannot Stop

Cannot Go

Xs

Xc

Figure 4-17. Dilemma zone (Xs > Xc). 

Cannot Stop

Cannot Go

Xs = Xc

Cannot Stop

Cannot Go

Xs = Xc

Figure 4-18. Dilemma zone removal (Xs = Xc). 
 Traffic Detector Handbook—2006 

October 2006 Page 4-29 Federal Highway Administration

Xs

Cannot Stop

Cannot Go

Optional

Zone

Xc

Xs

Cannot Stop

Cannot Go

Optional

Zone

Xc

Figure 4-19. Optional zone creation (Xs < Xc). 

Tables 4-16 through 4-19 are based on the following assumptions: 

• All distances are measured from the stopline in the upstream 

direction. 

• Vehicle length is 20 ft (6 m). 

• Acceleration rate a = 16.0 – 0.213 x speed in units of ft/sec2 or 

a = 4.9 – 0.213 x speed in units of m/s2. 

• Perception plus reaction time equals 1 second. 

• Red clearance time equals 0 second. 

Several general conclusions from the analysis of the dilemma zone problem 

are: 

• The dilemma zone size increases as speed increases for a given yellow 

interval. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-30 Federal Highway Administration 

Table 4-6. Stopping and clearance distances for intersection width of 48 feet. 

Decel. rate Speed Stopping 

distance 

Clearance distance (ft) for yellow 

interval equal to: 

ft/sec2 mi/h feet 3 sec 4 sec 5 sec 

 20 73 39.5 93.2 156.6 

 25 104 58.3 115.5 180.8 

 30 141 77.2 137.8 205.0 

 35 184 96.1 160.1 229.1 

10 40 232 115.0 182.4 253.3 

 45 285 133.8 204.7 277.5 

 50 344 152.7 227.0 301.7 

 55 408 174.0 254.6 335.3 

 60 477 196.0 284.0 372.0 

 20 56 39.5 93.2 156.6 

 25 79 58.3 115.5 180.8 

 30 105 77.2 137.8 205.0 

 35 134 96.1 160.1 229.1 

16 40 167 115.0 182.4 253.3 

 45 203 133.8 204.7 277.5 

 50 242 152.7 227.0 301.7 

 55 285 174.0 254.6 335.3 

 60 331 196.0 284.0 372.0 

Table 4-7. Stopping and clearance distances for intersection width of 15 meters. 

Decel. 

rate 

Speed Stopping 

distance 

Clearance distance (m) for yellow 

interval equal to: 

m/sec2 km/h meters 3 sec 4 sec 5 sec 

 20 73 10.3 26.4 45.7 

 25 104 13.8 30.6 50.2 

 30 141 17.4 34.8 54.8 

 35 184 24.6 43.3 64.0 

3 40 232 31.7 51.7 73.1 

 45 285 38.9 60.2 82.3 

 50 344 42.4 64.4 86.9 

 55 408 46.0 68.6 91.4 

 60 477 54.0 79.0 104.0 

 20 56 10.3 26.4 45.7 

 25 79 13.8 30.6 50.2 

 30 105 17.4 34.8 54.8 

 35 134 24.6 43.3 64.0 

5 40 167 31.7 51.7 73.1 

 45 203 38.9 60.2 82.3 

 50 242 42.4 64.4 86.9 

 55 285 46.0 68.6 91.4 

 60 331 54.0 79.0 104.0 
 Traffic Detector Handbook—2006 

October 2006 Page 4-31 Federal Highway Administration

Table 4-8. Stopping and clearance distances for intersection width of 76 feet. 

Decel. rate Speed Stopping 

distance 

Clearance distance (ft) for yellow 

interval equal to: 

ft/sec2 mi/h feet 3 sec 4 sec 5 sec 

 20 73 11.5 65.2 128.7 

 25 104 30.4 87.5 152.9 

 30 141 49.3 109.8 177.0 

 35 184 68.1 132.1 201.2 

10 40 232 87.0 154.4 225.4 

 45 285 105.9 176.7 249.5 

 50 344 124.8 199.0 273.7 

 55 408 146.0 226.7 307.3 

 60 477 168.0 256.0 344.0 

 20 56 11.5 65.2 128.7 

 25 79 30.4 87.5 152.9 

 30 105 49.3 109.8 177.0 

 35 134 68.1 132.1 201.2 

16 40 167 87.0 154.4 225.4 

 45 203 105.9 176.7 249.5 

 50 242 124.8 199.0 273.7 

 55 285 146.0 226.7 307.3 

 60 331 168.0 256.0 344.0 

Table 4-9. Stopping and clearance distances for intersection width of 23 meters. 

Decel. rate Speed Stopping 

distance 

Clearance distance (m) for yellow 

interval equal to: 

m/sec2 km/h Meters 3 sec 4 sec 5 sec 

 30 19.9 2.3 18.4 37.7 

 35 25.5 5.8 22.6 42.2 

 40 31.7 9.4 26.8 46.8 

 50 46.0 16.6 35.3 56.0 

3 60 63.0 23.7 43.7 65.1 

 70 82.5 30.9 52.2 74.3 

 75 93.2 34.4 56.4 78.9 

 80 104.5 38.0 60.6 83.4 

 90 129.2 46.0 71.0 96.0 

 30 15.3 2.3 18.4 37.7 

 35 19.2 5.8 22.6 42.2 

 40 23.5 9.4 26.8 46.8 

 50 33.2 16.6 35.3 56.0 

5 60 44.4 23.7 43.7 65.1 

 70 57.3 30.9 52.2 74.3 

 75 64.2 34.4 56.4 78.9 

 80 71.6 38.0 60.6 83.4 

 90 87.5 46.0 71.0 96.0 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-32 Federal Highway Administration 

• For a given speed and yellow interval, increases in the 

deceleration or acceleration rate cause a reduction in the length 

of the dilemma zone. 

• Increases in the effective width of the intersection directly 

increase the length of the dilemma zone if the total width of the 

intersection is included in the calculation. 

In pretimed signal control, the appropriate strategies for controlling the 

dilemma zone problem consist of providing a consistent yellow change 

interval and incorporating an appropriate red clearance interval. This 

strategy will, however, increase vehicular delay. 

In actuated signal controlled intersections, the most appropriate strategy for 

resolving the dilemma zone problem involves sensor placement before, 

within, and after the dilemma zone in such a way as to reduce the probability 

of entrapment of a vehicle in the dilemma zone at the onset of the yellow 

interval. The various methods of sensor placement for dilemma zones are 

discussed below. 

Multiple-Point Detection 

The dilemma zone problem can be ameliorated by the strategic placement of 

multiple sensors at high-speed approaches to intersections controlled by 

actuated controllers. The mitigation methods described below assume the use 

of inductive-loop detector systems operating in the presence mode. As 

inventive as the procedures are, vehicles will still get caught in the dilemma 

zone because of maximum greens, force-offs, etc. Consequently, adequate 

change intervals (yellow and all-red displays) must be provided to ensure 

motorist safety. 

The three sensor placement strategies in general use for multiple-point 

detection are: 

• Green extension systems (for semiactuated controllers). 

• Extended call detection systems (for basic controllers). 

• Multiple point detection systems. 

Green Extension System 

A green extension system consists of an assembly of extended call sensors 

and auxiliary logic, which enable vehicles to be detected before entering the 

dilemma zone and provide the controller with data to extend the green until 

the vehicles clear the dilemma zone.(9) The logic monitors the signal display, 

enables or disables selected call sensors, and holds the controller in green. 

Although two loops are normally employed, three may be used at high-speed 

intersections. Figure 4-20 shows loop placement for vehicles traveling at the 

85th percentile speed.(10) 
 Traffic Detector Handbook—2006 

October 2006 Page 4-33 Federal Highway Administration

Figure 4-20. Green extension system using two inductive-loop detectors. 

The appropriate distances for placing the loops are calculated using 

f

S D St

30

1.47

2

= + (4-7) 

⎟

⎠

⎞ ⎜

⎝

⎛ = + 1

30 2 1.47 S D S (4-8) 

D1 = D − D2 (4-9) 

where 

S = 85th percentile speed, mi/h (km/h) 

t = perception and reaction time, seconds 

f = coefficient of friction 

D = stopping distance, ft (m) 

D2 = clearing distance, ft (m) 

D1 = separation between loops, ft (m). 

With the loops positioned as shown, a vehicle passing over loop S1 actuates 

an electronic timer, which extends the green for the vehicle to reach loop S2 

in time T1. Similarly, when the vehicle passes over loop S2, a second timer 

maintains the green while the vehicle proceeds toward the intersection. This 

design does not insure that vehicles traveling at speeds less than the 85th

percentile speed would not be trapped in the dilemma zone. 

Dilemma

Zone S2 S1

Stopline

1

D

2

T

T

1

T

D

2

D

Dilemma

Zone S2 S1

Stopline

1

D

2

T

T

1

T

D

2

D
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-34 Federal Highway Administration 

Extended-Call Detection System 

This concept uses a 70-ft (21-m) presence loop extending upstream from the 

stopline and a small extended call loop 250 to 500 ft (75 to 150 m) upstream 

of the stopline, as shown in Figure 4-21. The magnitude of D is calculated 

from Equation 4-7, using the speed limit or the 85th percentile speed. D2 is 

set equal to 70 ft (21 m). Time T1 is calculated as D1 (found from Equation 4-

9) divided by a lower-limit approach speed, which is generally assumed equal 

to the 15th percentile speed. This time is programmed into the extended-call 

electronics unit. The controller is operated in the loop occupancy control 

mode. 

Proper design of an extended-call detection system ensures that the last 

vehicle and those vehicles traveling below the speed limit are not trapped in 

the dilemma zone. A trailing vehicle may be trapped at the end of the 

maximum-extension limit (maximum green time after an actuation on an 

opposing phase). The 70-ft (21-m) presence loop guarantees that stopped 

vehicles queued behind the stopline will move forward and enter the 

intersection without triggering a premature gap out. 

Figure 4-21. Extended call inductive-loop detector system. 

Multiple-Point Detection Methods 

The green extension and the extended-call detection systems used two 

or (at most) three sensors. These techniques can be used effectively at 

intersections with relatively low speeds. However, when speeds are 

high, the dilemma zone becomes longer, and more sensors are needed 

to accommodate the large range of approach speeds that are generated. 

Three techniques are commonly used for determining the placement of 

the required sensors. These methods are identified by the developer, 

agency, or organization that pioneered the technique. 

Beierle Method: This method (originated by Harvey Beierle of the Texas 

DOT(11) (TxDOT) uses a 1-second vehicle interval setting on a controller 

operating with locking detection memory. The sensors are 6- x 6-ft (1.8- x 1.8-

m) presence mode inductive loops. 

1

D

2T 1 T

D

2 D = 70'

S2 S1

1

D

2T 1 T

D

2 D = 70'

S2 S1
 Traffic Detector Handbook—2006 

October 2006 Page 4-35 Federal Highway Administration

The outermost sensor upstream of the intersection is placed at a safe 

stopping distance from the intersection for highest normal approach speed. 

Safe stopping distances are based on a 1-second perception and reaction time 

plus braking distances resulting from coefficients of friction between 0.41 and 

0.54 for speeds between 55 and 20 mi/h (90 and 30 km/h). The next sensor is 

tentatively located at a safe stopping distance for a vehicle traveling 10 mi/h 

(16 km/h) less than that assumed for the first sensor. If the travel time 

between the two sensors is greater than 1 second, the downstream sensor is 

relocated to allow the vehicle to reach the second sensor within the 1-second 

vehicle interval set on the controller. 

This location procedure is repeated for each successive sensor until the last 

loop is within 75 ft (23 m) of the stopline, each time subtracting 10 mi/h (16 

km/h) from the maximum considered speed. The minimum assured green 

time is set on the controller to permit vehicles stopped between the last 

sensor and the intersection to enter the intersection. 

TxDOT examined a modification to the Beierle procedure, which uses 

American Association of State Highway and Transportation Officials 

(AASHTO) stopping distance criteria. Figure 4-22 illustrates the sensor 

spacing for speeds common to Texas. 

1 ft =0.3 ft 

1 mi/h = 1.6 km/h 

Figure 4-22. Multiple inductive-loop detector placement (TxDOT) 

for alleviating the effects of dilemma zones. 

Winston-Salem Method: The second method of multiple sensor placement was 

developed by Donald Holloman for that agency.(9) It is basically the same as 

the Beierle Method. The Winston-Salem Method differs by using a slightly 

shorter stopping distance for the outermost and innermost sensor and 

incorporates speeds up to 60 mi/h (97 km/h). 50 mi/h 45 mi/h 60 mi/h 55 mi/h

Stopline Width of loop assumed

to be 6 ft (1.8 m) 

55'

110' 180'

370' 270'

85' 140' 220'

85' 140' 220'

55'

110' 180' 270'

320'

50 mi/h 45 mi/h 60 mi/h 55 mi/h

Stopline Width of loop assumed

to be 6 ft (1.8 m) 

55'

110' 180'

370' 270'

85' 140' 220'

85' 140' 220'

55'

110' 180' 270'

320'
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-36 Federal Highway Administration 

SSITE Method: The third method was developed by the Southern Section of 

the Institute of Transportation Engineers (SSITE).(12,13) It uses an iterative 

process and engineering judgment in locating the sensors. The outermost 

loop is positioned to provide safe stopping distance, as determined by data 

collected by the Southern Section. The differences with respect to the other 

multiple sensor methods are: 

• Spacing between successive loops is 2 seconds. 

• Innermost loop is located at the stopline. 

• Allowable gap is set between 5 and 7 seconds, which is greater than in 

the other methods. 

Tables 4-10 and 4-11 summarize the characteristics of the extended call and 

multiple point detection methods with respect to the number of loops and 

their spacing. Since the length of the dilemma zone becomes larger as speed 

increases, more sensors are required to track the vehicle through the 

dilemma zone. Moreover, larger spacing between sensors implies detection of 

only larger vehicle intervals and less efficient controller operation. 

In general, it appears that the multiple point detection methods are more 

appropriate for use with high mean approach speed and high flow variability. 

Table 4-10. Inductive-loop detector placement in extension or extended-call systems 

used to ameliorate effects of dilemma zones. 

Green extension systems for the 

semiactuated controllers method 

Extended-call detection 

systems for basic 

controllers method 

Parameter Value Value 

Controller memory Nonlocking Nonlocking 

Sensor type Presence Presence 

Speed range S = 85th percentile S = 85th percentile 

Outermost loopa

f

S

D St 30 1.47

2

= + f

S

D St 30 1.47

2

= +

Innermost loop = + 1 30 1.47 1

S

D S 0 

Spacing between loopsb 2sec 1 > S

D - D 2 sec

70 > Slow limit

D -

Number of Loops 2 (or 3) 2 

Allowable gap 5 to 6 Seconds 5 to 6 Seconds 

a. Distance D is measured from the stopline. 

b. Slow limit = Low speed limit; for example, 15th percentile speed.
 Traffic Detector Handbook—2006 

October 2006 Page 4-37 Federal Highway Administration

Table 4-11. Inductive-loop detector placement in multiple-point detection systems 

used to ameliorate effects of dilemma zones. 

 Beierle Method Winston-Salem 

Method SSITE Method 

Controller memory Locking Nonlocking Nonlocking 

Sensor type Presence Presence Presence 

Speed range S ≤ 50 S ≤ 60 S ≤ 60 

Outermost loopa Use stopping distance 

from Beierle, Ref. 11 

Use stopping 

distance from 

Sackman, Ref. 9 

Use Southern 

Section ITE Report, 

Ref. 12 

Innermost loop Within 75 ft (23 m) of 

approach stopline 86 ft (26 m) 00 

Spacing between loopsb 1 second 1 second 2 seconds 

Number of Loops - 1

10

S - 2

10

S

≤ 6 

Allowable gap 2 to 5 seconds 2 to 5 seconds 5 to 7 seconds 

a. Distance D is measured from the stopline. 

b. Slow limit = Low speed limit; for example, use 15th percentile speed. 

c. S 10 represents the integer part of S/10; for example 37 10 =3.7 =3

Dilemma Zone Design Options 

To simplify their sensor design process, PennDOT defined seven basic design 

options together with an evaluation of the characteristics of each option. The 

following excerpt describes these options from the PennDOT Traffic Signal 

Design Handbook.

(14) 

Option 1: Consists of a long loop 6 x 50 ft (1.8 x 15 m) maximum for 

each approach lane. This enables individual lane detection in the 

presence mode. Although it requires more loop wire than the other 

options, its initial cost is the lowest as less lead wire and fewer pull 

boxes are required. Construction cost is lowest of all options. The 

disadvantages include: all detection for a lane is lost should the loop 

break and long loops are the least sensitive of all loop configurations. 

When the sensitivity is increased, the loop becomes more susceptible 

to detecting vehicles in adjacent lanes. 

Option 2: Consists of sequential short loops for individual lane 

detection in either the pulse or presence mode. They may be wired 

either in series or parallel; however, best results are achieved when 

alternate loops are paired and wired in parallel to separate input 

channels. There is an added safety feature inherent to this option in 

that should one loop fail, detection is not completely lost. Although 

the initial cost is higher than that for long loops, maintenance is 

easier, as only a small loop need be replaced in case of damage. 

Like the long loop, the short loops are susceptible to detecting 

vehicles in adjacent lanes; however, they are more sensitive and are 

better suited for sensing small vehicles. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-38 Federal Highway Administration 

Option 3: Consists of a long quadrupole loop for each approach lane. 

Its operation is identical to that of Option 1. The major advantages of 

Option 3 are increased sensitivity for detecting bicycles and small 

motorcycles, coupled with its ability to reject detection of vehicles in 

adjacent lanes. Like Option 1, detection for an entire lane is lost if 

the loop be severed. Construction cost is approximately 20 percent 

greater than for Option 1. 

Option 4: Consists of one short loop per lane located in advance of the 

intersection based on normal approach speeds. This option, which 

operates in pulse mode only, is best suited for providing extension 

intervals on roads with higher travel speeds. 

The loops installed for Option 4 can also be used for individual lane 

counting and gap determination. There are two disadvantages with 

this option. First, should the loop fail, detectorization for the 

approach is lost. Second, since there are no loops near the stopline, 

any vehicle entering the approach from a driveway between the loop 

and the stopline is not detected and has to wait for another vehicle to 

place a call for the necessary phase unless calling detectors are 

installed. 

Option 5: Basically the same as Option 4, except a single wide loop is 

used for multilane detection instead of individual lane sensors. 

Construction is less expensive than Option 4; however, should 

breakage occur, detection on that approach is completely lost. 

Option 6: Consists of a single short loop per approach lane for use 

where a driveway is located between the intersection and the area of 

detection for Options 4 or 5. Traffic generated by the driveway is 

unable to actuate its phase without the additional loops placed near 

the stopline. A 6-ft x 6-ft (1.8-m x 1.8-m) calling inductive loop is used 

in these cases. 

SPECIAL FUNCTION APPLICATIONS 

Several applications require inductive-loop detector systems to perform 

unique functions. These include small-vehicle and bicycle detection, detection 

of long vehicles or large trucks, queue detection at freeway off-ramps, vehicle 

counting, and special safety applications to prevent accidents and reduce 

speeds. 

Summaries of various loop shapes and operating characteristics are given 

below in terms of the functional requirements determined by the operating 

agency. These requirements are typically based on the type of application 

(e.g., intersection control and freeway surveillance and control), traffic or 

vehicle mix, climate, and other site-specific conditions. 

DETECTION OF SMALL VEHICLES 

Increased fuel costs tend to accelerate the proclivity for smaller, fuelconserving vehicles. These range from small compact automobiles to l00-cc 

motorcycles, mopeds, and lightweight bicycles. The increasing number of 

these small vehicles and their behavior patterns often necessitates their 

detection with existing standard inductive-loop detector configurations. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-39 Federal Highway Administration

A presence sensor should be able to detect a small motorcycle and hold its 

call until the display of a green signal. If the sensor drops a call prematurely, 

the motorcyclist could be trapped on the red phase. The required hold time 

should at least match the shortest cycle time observed at the intersection. 

The NEMA Standards (see appendix J) specify a minimum hold period of 3 

minutes. 

Calls may be dropped prematurely in some older inductive-loop electronics 

units that include the ability to compensate for environmental drift, 

primarily due to changes in temperature and moisture. This circuitry will 

frequently neutralize a weak detection from a small vehicle within a period of 

less than a minute. Newer electronics units do not have this problem and all 

meet the NEMA Standards and the Type 170 Specification, which both 

require a minimum hold time of 3 minutes. 

In California and other temperate areas, the bicycle has become a common 

mode of transportation. As such, properly signalized intersection operation 

and safety require detection of bicycles. The inherent problems associated 

with bicycle detection include: 

• Locating the loop on the street to assure the rider will be within 

the detection zone. A separate bike lane is ideal, but not always 

possible. 

• Sequencing the traffic signal to accommodate a detected 

bicycle. This cannot be done with some control techniques. 

• Providing sufficient signal timing to avoid trapping the bicyclist 

in the intersection. This also can be a problem with some 

intersection designs. 

In response to these problems, it has been suggested that the inductive- loop 

electronics unit have extension timing and delay features. In such a system, 

one loop is located about 100 ft (30 m) from the stopline, and the second loop 

is located at the stopline. When a bicycle is detected at the first loop, the 

extension time is provided to hold the green to allow the bicycle to reach the 

loop at the stopline. 

When the detection is made at the stopline, extension time is provided to 

allow the bicycle to move far enough into the intersection to safely clear 

before the end of the yellow indication. If the detection occurs when the light 

is red, the minimum timing feature assures that when the light turns green, 

the minimum green time will allow safe crossing of the intersection. This 

type of operation works best in a bike lane. The loop in the bike lane with a 

standard electronics unit could be wired to call the pedestrian timing, which 

would allow adequate time for the cyclist to cross the intersection. 

A delay feature is used where vehicles merge into the bike lane to turn right. 

The detection is not immediately registered so that the vehicle may complete 

a right turn without creating delay for other traffic including bicycles. 

A number of factors determine the most effective inductive-loop configuration 

needed to satisfy policies that dictate detection of small vehicles, including 

bicycles. Important criteria include shape of the loop, width of the lane, and 

loop placement within that lane. Loop configurations that enhance the ability 

to detect small vehicles are described below. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-40 Federal Highway Administration 

Multiple, Interconnected Small Loops 

A configuration frequently used for the detection of small vehicles is 

multiple, interconnected small loops or, as it is often referred to, sequential 

short loops. The sensitivity can be better controlled with the multiple loops 

than with the conventional single long loop, whose sensitivity must be set so 

high for small-vehicle detection that false calls from adjacent lanes 

(splashover) result. With the small loops, total-loop inductance very close to 

optimum can be achieved by connecting the loops in series. In the past, 

series/parallel connections were mandated; however, newer electronics units 

have obviated this need. Moreover, the small vehicle will be detected when it 

reaches the first small loop rather than at the stopline, as is the case with the 

long loop with a powerhead described later. Also, should one loop fail, some 

detection capability still remains in that lane. 

Quadrupole-Loop Configuration 

The quadrupole-loop configuration was first used in the early 1970s. As 

shown in Figure 4-23, this configuration adds a longitudinal sawcut in the 

center of the lane. The loops are wired in a figure-eight pattern so that the 

center wires have current flowing in the same direction. Their fields reinforce 

each other, improving the capability to detect small vehicles. The center 

wires counteract the fields of the outer wires, which have their current 

flowing in the opposite direction from the center wires. The influence of the 

outer fields is diminished, thereby reducing the possibility of splashover. 

Figure 4-23. Quadrupole loop configuration. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-41 Federal Highway Administration

The single-wire configuration (“1-2-1” with one layer in the perimeter slots 

and two layers in the center slot), shown in Figure 4-23, is used for the 

detection of automobiles, trucks, and the larger motorcycles. A double layer 

design (“2-4-2”) is recommended for detecting small motorcycles and bicycles. 

Some agencies wind the 2-4-2 as two, 2-turn loops rather than the 

traditional, figure-eight winding pattern. No definitive tests were found to 

favor either method. 

Loop placement in the lane is another important consideration. Installation 

in the center of the lane may fail to detect the small motorcycle if the travel 

path is outside the quadrupole field. For example, motorcyclists waiting to 

turn left will usually stop on the left side of the lane and thus may be outside 

of the quadrupole field. Where detection is required for a left-turn lane, it is 

recommended that the left edge of the quadrupole be located no further than 

2 ft (61 cm) from the left edge of the left-turn lane. 

The quadrupole is used as a short or long loop, a single element, and in 

combinations such as series or series/parallel. The series/parallel 

configuration is used effectively across the country not only to detect small 

vehicles, but also to eliminate the problem of adjacent-lane detection 

(splashover) in high-sensitivity inductive-loop systems. 

Short quadrupoles (those less than 30 ft (9 m)) tend to lose high-bed sections 

of trucks. Since the quadrupole is really two loops whose field detection 

height is approximately two-thirds of the short leg of the loop, the 

approximate detection height is 2 ft (0.6 m). This detection height is based on 

the dimension of two 3-ft- (0.9-m-) wide side-by-side quadrupole loops. With 

longer loops (those greater than 30 ft (9 m)), there are always one or more 

wheels or axles over the loop. 

It is generally agreed that the 6-ft (1.8-m) quadrupole loop detects bicycles 

better than most other loop configurations. The major problem with this 

configuration for small-bicycle detection is the need for the bicyclists to ride 

close enough to a wire within the quadrupole to be detected. A number of 

unique pavement markings and signs have been developed to assist this 

application. The pavement marking identifies the location of the wire and the 

sign explains the markings to the cyclist. 

Figure 4-24 illustrates a marking system used in Clarke County, GA. A 

pattern of 4- x 18-inch (10- x 46-cm) white stripes is placed 18 inches (46 cm) 

apart, starting at the stopline. The length is kept short since only one 

actuation is necessary to call the green indication. 

In the City of San Luis Obispo, CA, an aggressive public information 

program was mounted to inform bicyclists that they could cause the red light 

to turn green by traveling over the bicycle-shaped pavement marking shown 

in Figure 4-25. These markings were painted on all appropriate through, 

right-, and left-turn lanes at all signalized intersections that contained loop 

sensors. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-42 Federal Highway Administration 

Sign

22"

GREEN

CYCLE STOP

ON LINE FOR

Major Street

Gutter

Curb

Sidewalk

Minor Street

Existing 

Stop 

Bar

18"

10'10"

6' x 50' 

Quadrupole 

Loop 18" 18"

New 4" Lines 

(White)

1 ft =0.3 m 

1 inch = 2.5 cm 

Figure 4-24. Bicycle sensor sign and markings used in Clarke County, GA. 1'4" 1'8" 1'8" 5'0" 1'8"

1 ft =0.3 m 

1 inch = 2.5 cm 

Figure 4-25. Special bicycle pavement marking used in San Luis Obispo, CA. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-43 Federal Highway Administration

Chevron-Loop Configuration 

Small vehicles can be detected with the chevron configuration shown in 

Figure 4-26. It consists of one or more four-turn parallelogram loop(s) with 

the short section in the direction of traffic and the long section at an angle of 

30° with the short section. The long sides of the loop sections are 27.5 inches 

(70 cm) apart. Adjacent ends of successive loop sections may be in a single 

slot or separated by 2 ft (61 cm). This alignment allows a vehicle to cut the 

lines of flux more efficiently.(15) 

Loop sections are wound alternately clockwise and counterclockwise so that 

currents in adjacent loop ends are always in the same direction. Successive 

sections wound in the same direction would produce dead paths where the 

sections are joined. 

Figure 4-26. Chevron-loop configuration. 

Long Loop with Powerhead 

Frequently, a small motorcycle or bicycle will not produce a sufficient shift of 

inductance in a one-turn 6 x 20 ft (1.8 x 6.1 m) or longer loop. Some 

inductive-loop electronics units will detect these small vehicles with two or 

more turns of wire although there are dead areas in the center of the loop. 

One approach is to use a small powerhead at the stopline with the long loop. 

This configuration is shown in the upper portion of Figure 4-27. The standard 

powerhead can be improved by angling the transverse wires as shown in the 

lower portion of Figure 4-27. The angling will cause the small vehicle to cut 

the lines of flux more efficiently, thereby increasing the signal by as much as 

25 percent. The disadvantage is that the vehicle may not stop on the 

powerhead unless the detection area is clearly indicated by paint or signs. 

Some traffic engineers have concerns about liability because small vehicles 

are not detected throughout the detection zone. Although these vehicles are 

sensed at the stopline, the controller operation is based on the detection of 

vehicles in the approach. Other engineers feel that the important 

consideration is to detect the small vehicles to ensure they receive the green 

signal, not to ensure that the signal operation is optimized for these vehicles. 

They do not believe that any liability is incurred. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-44 Federal Highway Administration 

Use of sequential short loops rather than a single long loop avoids 

nondetection of small vehicles anywhere in the detection zone. Also, modern 

electronics units sense small vehicles without the resultant splashover that 

occurs when using the single long loop with high-sensitivity settings. These 

two factors have reduced the need for the powerhead design. 

1 ft =0.3 m 

Figure 4-27. Long loop with powerhead. 

Inductive-Loop Bicycle Detectors 

Caltrans developed a Type D loop configuration to better detect bicycles.(16) 

This configuration, shown in Figure 4-28, is a palm-shaped loop that fits into 

a 6-ft (1.8-m) square. The loop has three turns of wire when a single Type D 

loop is connected to an electronics unit channel, and five turns if one Type D 

loop is connected in series with three 6- x 6-ft (1.8- x 1.8-m) loops on an 

electronics unit channel (see Type 5DA or 5DQ installations in Figure 4-12). 

This configuration requires that the wire be protected where it bends around 

the acute corners of the cut. Drilling a hole in the corner or chipping out the 

inner angle to provide a radius will prevent kinking of the wire. This loop 

may be used in either vehicular or bicycle lanes. 

Direction of Travel

30"

Winding Detail Sawcut Detail 30" 15"

15"

27"

27"

1 inch = 2.5 cm 

Figure 4-28. Type D loop configuration (Caltrans). 
 Traffic Detector Handbook—2006 

October 2006 Page 4-45 Federal Highway Administration

Figure 4-29. Type D loop installation (Photograph courtesy Darcy Bullock, 

School of Civil Engineering, Purdue University, West Lafayette, IN). 

In some applications, it is desirable to detect the presence of a bicycle across 

a greater portion of a full-width traffic lane. Figure 4-30 shows one loop 

configuration suitable for this purpose, an 8-ft (2.4-m) square with three 

diagonal saw cuts traversing the square. Two layers of wire are used and are 

wound so that the current flow is in the same direction for both layers. This 

results in four layers of wire in each diagonal. The acute-angle corners are 

rounded to prevent damage to the wires. This is a special configuration that 

is installed in areas of heavy bicycle traffic such as near the University of 

California at Davis. 

Figure 4-31 illustrates another method of reliable bicycle detection with two 

quadrupole loops placed side by side within the traveled area of a bike lane. 

An adjustable timed-call extension is generated to hold the call to the 

controller long enough for the bicycle to clear the intersection when operating 

in a loop occupancy mode. The 4.5- by 6-ft (1.4- by 1.8-m) configuration 

provides assurance of detecting all bicycles and complete adjacent lane 

rejection. 

Figure 4-29 shows a Type D loop 

installation for detecting bicycles on 

a sidewalk or bicycle path. The 

wheel on the pole is used to push 

the wire into the sawcut and then to 

push foam backer firmly on top of 

the wire. There is debate about 

placing foam over the entire wire or 

simply using short foam pieces 

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. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-46 Federal Highway Administration 

Figure 4-30. Wide coverage bicycle loop. 

1 ft =0.3 m 

1 inch = 2.5 cm 

Figure 4-31. Bicycle lane loop layout with side-by-side quadrupoles. 

DETECTION OF LONG, HIGH-BED VEHICLES 

It is generally advisable to allow long, high bed vehicles (e.g., tandem trucks, 

semitrailer trucks, and cars pulling trailers) to travel through the 

intersection without stopping. There are three good reasons why long 

vehicles should not be stopped: 

• Jackknifing of truck-trailers tends to occur under heavy 

braking conditions. 

• After stopping, a large vehicle requires a longer startup time 

delaying following traffic. 

• Increased noise and air pollution are associated with heavy 

truck startup. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-47 Federal Highway Administration

A detection alternative for trucks consists of two loops spaced 30 ft (9 m) 

apart and located 302 ft (91 m) from the stopline.(17) This is the distance a 

loaded semitrailer vehicle traveling 45 mi/h (72 km/h) needs to stop safely. 

The sensor logic requires that the second loop be activated before the first 

loop is vacated. 

The Detroit, MI, freeway program required that all vehicles be detected as a 

single entity, including high-bed trucks, semitrailers, and tanker trucks, as 

well as conventional vehicles. Their test of three-turn 6- x 6-ft (1.8- x 1.8-m) 

loops did not adequately satisfy their requirement. After numerous trials, the 

agency determined that a seven-turn 6- x 6-ft (1.8- x 1.8-m) loop rotated 45 

degrees to form a diamond shape was superior particularly in its accuracy in 

detecting trucks. The diamond shape was further refined by carefully 

adjusting the angles of the diamond to avoid splashover.(25) 

One manufacturer states that reliable truck detection from loop 

configuration change is not due to the diamond shape of the loop as 

suggested above. Rather it is due to the increased number of turns, which 

increases the inductance of the loop. The amount of signal received by the 

electronics unit is dependent on the loop inductance to lead-in cable 

inductance ratio. When the loop inductance to lead-in cable inductance ratio 

equals one, then the amount of change seen by the electronics units is onehalf the change occurring in the loop. By increasing the loop inductance (by 

increasing the number of turns to seven), Detroit has greatly increased the 

amount of change received at the electronics unit, thereby resulting in more 

reliable detection of high-bed trucks. 

QUEUE DETECTION 

Freeways that operate under congested conditions are periodically likely to 

experience heavy volumes on off-ramps. When these off-ramps terminate at a 

signalized intersection, backups can extend to the freeway lanes, causing 

even more congestion. Depending upon agency policy, it may be desirable to 

detect and discharge such queues before they become a freeway problem. 

One solution uses an actuated controller with a queue-discharge system 

consisting of a queue sensor with a preset delay time.(13) As shown in Figure 

4-32, the queue-detection loop is located at a strategic position at the 

upstream end of the off-ramp. A timer starts when a vehicle enters the 

detection zone of the queue sensor and resets to zero when the vehicle exits 

the zone of detection. If the system counts a predetermined number of 

seconds, i.e., the preset delay time, the electronics unit's normal output relay 

is energized. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-48 Federal Highway Administration 

Figure 4-32. Queue discharge system. 

If the queue of vehicles waiting at the red signal stopline extends upstream 

to the queue sensor, a vehicle will be located over the loop longer than the 

selected delay time. When the delay timer reaches the preset time, the sensor 

logic issues a signal to discharge the queue. The green signal remains on 

until all vehicles are moving with gaps longer than the loop itself. 

The inductive loop must be long enough to span the distance between 

standing vehicles. Concomitantly, it must be shorter than the shortest gap in 

moving traffic so that the breaks between moving vehicles will cause the 

delay timer to reset. This latter consideration can be critical when a 

inductive-loop queue-sensor covers two or more lanes. A loop length of 30 ft 

(9 m) will generally satisfy this criterion. 

INDUCTIVE-LOOP DETECTOR DESIGN FOR TRAFFIC 

SIGNAL CONTROL SYSTEMS 

The sensor requirements for area-wide traffic signal control systems are 

dependent on the type of control implemented. TOD control does not require 

detectorization as it operates as a time clock. First-generation traffic 

responsive control and other advanced control strategies require vehicle 

sensors capable of early identification of traffic trends within a system. The 

sensors must provide an early indication of a peak period for the beginning of 

heavy traffic. Thus, the sensors must be placed on heavily traveled links with 

traffic patterns representative of the significant flows within a section of the 

area-wide system. 

The application of sensors to various arterial and freeway traffic 

management techniques was explored in chapter 3. Additional sensor design 

considerations for traffic-responsive traffic control systems are discussed 

below. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-49 Federal Highway Administration

Accuracy Goals 

When sensor systems are used in computerized traffic-control systems, the 

particular traffic management application determines the traffic flow 

parameters to be collected and their accuracy. As discussed in chapter 3, 

volume and occupancy can be measured effectively. Other measured 

parameters may only provide approximations to the required variables, 

depending on how the data are gathered and subsequently processed. In 

estimating link-specific volumes, three components of error limit the 

measurement accuracy of each of the control variables. These are: 

• A measurement error in the data on which the predictor 

operates. 

• A prediction error in estimating the underlying mean. 

• A component reflecting the randomness of traffic. 

The data error can be expressed as X percent probability that the error is 

within Y percent. A normal distribution is assumed for the mean value of a 

large sample. The count error for first generation UTCS critical intersection 

control was plus or minus 3 vehicles 90 percent of the time.(18) 

Several filtering and smoothing techniques may be applied to the data to 

calculate the values needed to produce measures of effectiveness that relate 

to traffic flow. A filtering equation determines the difference between the 

previous time period smoothed value (such as volume) and the latest 

unsmoothed value. The difference is used to update the value of the 

smoothed data. 

Another error is introduced for vehicle presence because the controller does 

not generally observe vehicle presence continuously, but instead samples. 

This results in an error in occupancy and speed computations. There will 

always be a sampling error that increases as vehicle speed increases. This 

error cannot be eliminated but can be decreased by increasing the sampling 

rate. The Computerized Signal Systems Student Workbook(19) describes how to 

compute the percent sampling rate and how to implement filtering and 

smoothing techniques. 

Link Selection for Critical Intersection Control (CIC) 

The location of sensors in a traffic-control system is a three-step process. 

First, links are selected for sensor installation. The lateral and longitudinal 

sensor placements are then determined. The link selection must consider 

each function in the sequence, beginning with intersections that are 

candidates for CIC. Candidate intersections are those that could take 

advantage of variable split, but which operate in an unsaturated condition. 

When installing sensors in an intersection to operate under CIC, all 

approaches served by phases with variable green times must be detectorized. 

The step-by-step process for determining CIC candidates is found in the 

Locating Detectors for Advanced Traffic Control Strategies Handbook.

(20) 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-50 Federal Highway Administration 

The need for measuring general traffic trends will probably be satisfied by 

the CIC sensors in area-wide surveillance systems. If this type of 

detectorization is not installed, sensors should be located on major collectors 

or distributors to the network to obtain system trends with a minimum of 

instrumentation. Another group of candidates for this type of detectorization 

are entrances and exits of large parking facilities that would have significant 

effects on local traffic conditions. 

The location of sensors to evaluate system operation is, to a large extent, 

dependent upon the degree of accuracy required by the evaluation. If a 

general evaluation of changes in system operations is desired, the sensor 

placement for traffic responsive operation would be adequate. If a more 

detailed evaluation is required such that speed and delays can be 

determined, it is necessary to increase the number of sensors within the 

system. The cost of this amount of detectorization may suggest that other 

techniques be employed. 

Lane Selection 

Observations have shown that a single vehicle sensor in the lane carrying the 

maximum through volume will be the most representative of the traffic to 

which the signal must respond. Moreover, the signal should be timed for 

critical lane volumes. However, it is not possible to derive reliable total 

volumes from a single sensor. Yet multiple inductive-loop detectors located in 

noncritical lanes may introduce errors that exceed the value of the data they 

provide as they can also measure parking vehicles, turning vehicles, etc., 

unless individual lane detection is used. Thus, the most reliable data are 

derived from the critical lane. The critical lane (defined as the one carrying 

the greatest volume) is usually easy to identify by observing the lengths of 

queues at the intersection. 

Sensors should be installed in multiple lanes at locations where the critical 

lane changes with time of day, as should the corresponding TOD factors used 

in the software to select the inductive loop currently measuring the volume 

in the critical lane. 

Traffic volumes should be measured on each lane of an intersection 

approach.(21) Average lane volumes per cycle need to be computed and 

compared with the tentative critical lane. Four conditions require 

engineering judgment pertaining to project priorities and knowledge of 

individual link traffic. These conditions are: 

• Approaches where one lane is always critical. 

• Approaches where the critical lane shifts between two lanes, 

but the difference in volume is not great or the shift is infrequent. 

• Approaches where shifts in critical lanes are significant. 

• Approaches exhibiting specific critical lanes during peak hours, 

but, for various reasons, erratic shifts in critical lanes occur 

during nonpeak periods. 

It is unusual for more than two lanes to require detectorization, and most 

often, only one lane will need a sensor. After the critical lanes have been 

identified for the links to be detectorized, it is necessary to determine the 

longitudinal placement of the sensors on the links. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-51 Federal Highway Administration

Longitudinal Placement 

There are two guidelines for the longitudinal placement of sensors for signalsystem control. One relates to the upstream intersection, and the other refers 

to the downstream intersection. From the upstream intersection, a sensor 

should be downstream from the zone of acceleration of vehicles entering the 

link. A distance of approximately 230 ft (69 m) is recommended. 

From the downstream intersection, the sensor should be upstream from the 

point beyond which standing queues of vehicles do not usually extend. 

Although this distance is a function of the cycle length, split, and offset, it is 

recommended that values of 200 to 250 ft (60 to 75 m) be used in urban grid 

areas and values of 300 to 350 ft (90 to 105 m) be used in suburban arterial 

systems. When both criteria cannot be met, the criterion based on typical 

queue size is considered the more critical. 

An additional issue of longitudinal-sensor placement involves the location of 

traffic sinks and sources (e.g., parking facilities). Sensor placement research 

has shown that a sink or source has minimal effect on traffic measured in the 

critical lane when the facility is operated as a sink such as a parking garage 

during the morning peak. Turns into the garage are made from the curb lane, 

which is not usually the critical lane. 

However, during the evening peak hour, when the garage functions as a 

source, there are measurable effects on the critical lane. An evaluation of the 

paths of vehicles entering the lane from a source showed that most vehicles 

wait for a sufficient gap to enter the specific lane within the link. It is 

suggested that a critical lane sensor be located at least 50 ft (15 m) 

downstream from the source, provided that the downstream intersection 

criterion is not violated. In general, unless the source contributes more than 

40 vehicles per hour to the critical lane, the effects of a source on the link 

demand are not significant. 

Summary of System Sensor Location Considerations 

Sensor location information developed in earlier tasks assists in selecting 

links to be detectorized, lanes in which the sensor should be placed, and an 

approximate sensor location with respect to the upstream and downstream 

intersections. With this information indicated on a map, a field visit should 

be conducted for each link. A walk-through by the designer will permit the 

selection of final locations with consideration given to access to the control 

equipment, special driveway problems, or other roadway or parking 

conditions. Each sensor location must pass a reasonableness test as well as 

the analytical test. 

Several general guidelines are suggested concerning the field location of 

individual inductive loops: 

• A loop should be located in the center of the traffic flow, not 

necessarily in the center of the marked lane. The center of the 

traffic flow can usually be identified by the oil markings or tire 

tracks on the pavement. 

• The loop should be located in the areas of stable traffic flow. 

Sections of a link with excessive weaving or heavily impacted 

by entering and exiting driveways should be avoided. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-52 Federal Highway Administration 

• Where a major driveway is located within a link, the loop 

should be located at least 50 ft (15 m) downstream from the 

driveway, provided that the loop is at least 200 ft (60 m) 

upstream of the stopline. 

• Inductive loops should not be located within 10 ft (3 m) of any 

manhole, water valve, or other appurtenance located within the 

roadway. This distance is required to permit sufficient 

clearance for work on the manhole without disturbing the loop. 

In summary, the final decision concerning the location of sensors for 

advanced traffic-control strategies is a blend of analytical procedures coupled 

with engineering judgment. Not all links can be instrumented to yield input 

measures within the accuracy required by the algorithms. Short links and 

links with extremely poor lane discipline are typical of those that are not 

compatible with accurate instrumentation. 

INDUCTIVE-LOOP DETECTOR ELECTRONICS UNITS 

The operational characteristics of various types of modern inductive-loop 

electronics units are discussed in detail in chapter 2. Requirements for 

inductive-loop electronics devices are included in the NEMA Standards and 

the Type 170 Specifications (see appendixes I and J). The NEMA Standards 

define a series of self-contained loop electronics units designed for shelf 

mounting. It also describes a card-type electronics unit designed to insert 

into a multicard housing rack. The revised NEMA Standards (TS-2) 

emphasize card-rack mounted units. The Type 170 Specifications define cardtype modules only, which are designed for insertion into the input file of the 

cabinet system. 

NEMA electronics units are available with one, two, or four independent 

inductive-loop detector channels per unit. Some agencies insist on using only 

single channel electronics units because they believe that failures in the unit 

can be more easily and inexpensively corrected by replacing a single channel 

unit rather than replacing a multiple channel unit for a single channel’s 

failure. Current reliability of electronics units makes this argument obsolete. 

The rack-mounted modules used in the Type 170 system contain either two 

or four independent channels. 

NEMA Standards also define optional timing features that allow for the 

delay or extension of the electronics unit output. In the delayed-call mode, 

the electronics unit will wait for a user-defined period after a vehicle enters 

the detection area before it starts the output signal. In the extended-call 

mode, the unit will extend the output after the vehicle leaves the detection 

area. The Type 170 electronics unit does not provide this capability, as 

extension is normally performed by software in the controller unit itself. 

MAGNETOMETER CONFIGURATIONS 

A magnetometer sensor system, such as the one shown earlier in Figure 2-39, 

consists of one or more magnetometer sensors, the magnetometer electronics 

unit, and the lead-in cable between the sensor and the electronics unit. The 

sensor probe installation is illustrated in Figure 4-33. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-53 Federal Highway Administration

Cover with disk,

notched for cable. 

Caulk to prevent 

absorption of epoxy 

by sand fill.

Magnetometer

sensing element.

Install level. 

Compact

sand fill 

Pavement

Epoxy fill

Drill hole with

diameter equal to 

that of sensing 

element + 1/8". 

Depth as recommended

by manufacturer.

Treated base

Figure 4-33. Conceptual magnetometer sensor installation. 

Since the magnetometer is a passive device, there is no transmitted field or 

transmitted detection area. Modern magnetometer sensors detect changes in 

the horizontal and vertical components of the Earth’s magnetic field caused 

by the passage or presence of a vehicle and, hence, can be used for either 

passage or presence detection as discussed in chapter 3. 

Because the probes are buried in a drilled hole approximately 18 inches 

(0.5 m) below the surface, they find application in the northeastern United 

States, where the pavement deteriorates more rapidly due to thermal 

expansion and contraction and suffers damage from snow-removal 

equipment. They are also used in areas where loops cannot be cut in the road 

surface, such as on steel bridge decks. 

The configuration for a magnetometer installation is dependent on a number 

of factors that must often be traded against one another when designing an 

optimum configuration for a specific location. The following section discusses 

these factors and their impact on the ultimate design. 

SITE SELECTION 

Magnetometers will detect a vehicle whenever a sufficient portion of its 

magnetic shadow (i.e., changes in the horizontal and vertical components of 

the Earth's magnetic field caused by the passage or presence of a vehicle) 

falls on a sensor probe. The dimensions of the magnetic shadow generally 

approximate the geometric dimension of the vehicle. In some cases, the 

magnetic shadow may be offset from the probe by a few feet in any direction. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-54 Federal Highway Administration 

SENSITIVITY 

Electronics units for magnetometers provide two or more independent 

detection channels. Each channel supports 6 to 12 sensor probes, depending 

on the model. If only one probe per channel is installed in the street, the 

entire channel sensitivity is available to that single probe. However, as the 

number of probes per channel increases, the sensitivity is divided among the 

probes, thus decreasing the sensitivity of each individual probe. For example, 

when four probes are connected to a single channel, the total channel 

sensitivity remains the same, but the sensitivity at each probe will be 

reduced to one-fourth of the total channel sensitivity. 

Therefore, it is necessary to determine what types of vehicles are to be 

detected in order to select the proper spacing between probes and to define 

the number of probes connected together per channel. The number of probes 

required per lane and their optimum cross-lane position are determined by 

the lane width and the size of the vehicles to be detected. Some portion of the 

vehicle must pass over a sensor probe to be detected. Some general guidelines 

for the number of probes for a given type of vehicle and the number of probes 

per channel are given below: 

• Auto, Trucks, and Buses: Install probes at 5-ft (1.5-m) intervals. 

Six probes per channel, maximum. 

• Motorcycles (300 cubic centimeters (cc) and larger) (300 cc = 

18.31 cubic inches)): Install probes at 4-ft (1.2-m) intervals. Four 

probes per channel, maximum. 

• Motor Bikes (70–300 cc) (4.27–18.31 cubic inches): Install probes at 

3-ft (0.9-m) intervals. Three probes per channel, maximum. 

• Bicycles: Install probes at 3-ft (0.9-m) intervals. Two or three probes 

per channel. 

Installation sites should be chosen to avoid conditions that adversely affect 

operation such as those adjacent to manholes or large pipes; near very high 

current transmission lines, trolley lines, or underground power lines; or 

within tunnels or other iron-structure enclosures. 

Magnetometers are frequently the sensor of choice on bridge decks. Figure 

4-34 illustrates a typical bridge deck installation. The presence of the steel 

deck over or under the probe has little effect on system performance. 

However, vertical structural steel members may detract from performance by 

reducing the intensity of the adjacent ambient magnetic field. As with loops, 

the most appropriate location for the probes is at the maximum distance from 

the steel supports or columns as shown in the illustration. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-55 Federal Highway Administration

Figure 4-34. Typical installation of magnetometers on a bridge deck. 

A magnetic field analyzer, such as the model shown in Figure 4-35, should be 

used to measure the intensity of the magnetic environment at the selected 

location. This instrument measures geomagnetic field intensity, magnetic 

noise, and ac magnetic field strength. It is especially useful at locations 

where the use of magnetometers is questionable (e.g., within tunnels, near 

large electrical devices, etc). 

Situations exist where manmade magnetic noise is of sufficient intensity to 

impair magnetometer performance. These occur at sites where nonvehicular 

induced magnetic field perturbations exceed 5 milliOersteds such as near 

streetcar lines, some trolley bus lines, subway trains, or even elevators. Few 

sources are of sufficient intensity to affect a sensor probe located more than 

30 ft (9 m) from the source. However, magnetometers that transmit data via 

a radio frequency link to the electronics unit in the controller cabinet are 

subject to interference from communications devices operating at the same 

frequency. Therefore, a frequency survey should be performed to determine a 

clear channel on which to transmit these magnetometer data. 

PROBE PLACEMENT 

The optimum lateral placement of probes in a lane is determined by such 

factors as the width of the narrowest vehicle to be detected, the lane width, 

and the detection quality required. Some part of the magnetic shadow of the 

vehicle must fall on one or more of the probes for detection to occur. 

Consequently, the maximum probe spacing is equal to the width of the 

narrowest vehicle to be detected. Since most autos, trucks, and buses are 

wider than 5 ft (1.5 m), a single probe centered in a 10-ft (3-m) lane provides 

adequate performance. In a 12-ft (3.7-m) lane, a single probe may fail to 

detect some small vehicles traveling on the edge of the lane. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-56 Federal Highway Administration 

Figure 4-35. Magnetic field analyzer. 

By increasing the number of probes to two per lane, virtually all fourwheeled vehicles within a 12-ft (3.7-m) lane would be detected. In this type of 

configuration, probes should be placed no further than 5 ft (1.5 m) from each 

lane boundary. If bicycles are to be detected, more probes per lane may be 

needed. 

In general, the quality of detection improves as the number of probes per 

lane is increased because of the spatial averaging, which results when the 

magnetic shadow of a vehicle falls on several probes judiciously spaced. 

Reduction in field intensity at one lateral position may be compensated for by 

increases at other positions. 

PROBE DEPTH 

Vertical placement of probes is an important determinant of system 

performance properties. Deep placement such as 18–24 inches (45–60 cm) 

provides good single-count vehicle-presence detection, but results in a lower 

signal level. Conversely, shallow placement (e.g., 6 inches (15 cm)), provides 

higher signal levels, but with an increased incidence of multiple counts per 

vehicle. Multiple counts with shallow placement result primarily from the 

passage of major components of the vehicle such as engine, transmission, and 

differential, each of which may produce a separate magnetic perturbation. 

By increasing the depth to 18–24 inches (45–60 cm), most vehicles yield a 

single perturbation of the magnetic field because the deeply buried probes 

sense the overall magnetic bulk of the vehicle rather than details of the 

understructure. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-57 Federal Highway Administration

In summary, for a detection application where only passage information is 

required and where multiple counts are not detrimental, probes should be 

located near the surface. Similarly, in constant speed applications where the 

time-extension feature of the extendable-presence mode can be used 

effectively, probes may be located near the surface. Small, two-wheeled 

vehicles such as bicycles or motorbikes develop narrow, low-intensity 

shadows. Their detection requires shallow placement of probes, as well. 

SUGGESTED CONFIGURATIONS 

Different probe placements and connections to the electronics unit are used 

to support various traffic management functions and data acquisition needs 

as depicted in Figure 4-36. Several typical probe configurations are presented 

in Appendix L. These illustrate the tradeoffs made in probe depth and lateral 

distance between probes. Configurations are included for single-lane, twolane, and three-lane detection; wide-lane detection; and two-wheeled vehicle 

detection. Also shown are several suggested configurations for left-turn lanes 

with detection zones ranging from 30 to 70 ft (9 to 21 m). 

WIRE SIZE AND CABLE SELECTION 

Cable length and number of probes per channel determine minimum wire 

size. The probe excitation circuit provides current at a constant 125 mA peakto-peak, but is limited to a 15-volt peak-to-peak swing. Maximum allowable 

probe-cable assembly resistance is therefore 125 ohms. At resistance levels 

below this limit, system operation is normal. At higher resistance levels, 

instability and performance degradation will occur. 

Effective resistance of each probe is approximately 4 ohms. Probes on a 

channel are connected in series electronically, so their resistance is additive. 

Allowable cable resistance is 120 ohms, less the sum of the probe resistances. 

AWG #22 copper wire pairs have a loop resistance (going and returning total) 

of 35 ohms per l,000 ft (300 m), AWG #20 pairs have 20 ohms, and AWG #18 

pairs have 13 ohms. Because of the changes in wire resistance with 

temperature, it is recommended that #18 AWG wire be used on cable runs 

exceeding 2,000 ft (600 m) in climates where extreme temperature ranges are 

typical. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-58 Federal Highway Administration 

Figure 4-36. Magnetometer-probe placement in support of several data-gathering functions. 

Connection of the magnetometer electronics unit and probes requires two 

pair of conductors per channel. One pair supplies the probe excitation current 

and the other carries the return signal. Properties of the cable become 

especially significant in long transmission distances of over 2,000 ft (600 m). 

The cables should combine distance capability with high noise immunity and 

environmental tolerance. 

Although up to 12 probes per channel can be installed, the allowable lead-in 

length decreases as the number of probes increases. Generally, the lead-in 

length should not exceed 4,000 ft (1,200 m) for a 12-probe-per-channel 

arrangement. If 6 probes per channel are used, the lead-in length should not 

exceed 5,000 ft (1,500 m). 

MAGNETIC-DETECTOR CONFIGURATION 

Magnetic detectors first developed in the 1930s, i.e., sensors whose operation 

is based on an induction magnetometer (also referred to as a search coil 

magnetometer), are still in use today, particularly where deteriorated 

pavement or frost activity contributes to the failure of inductive-loop detector 

wires. Magnetic detectors are also applied where vehicle detection is required 

without placing sawcuts in the pavement. While magnetic detectors are 

inexpensive, reliable, and simple, most generate only pulse outputs, which 

are suitable in support of traffic actuated signal control and traffic volume 

counting. The exception is the Model 702 microloop probe from 3M, which 

can be connected in rows of three to detect stopped vehicles with applicationspecific software purchased from the microloop manufacturer. 
 Traffic Detector Handbook—2006 

October 2006 Page 4-59 Federal Highway Administration

The magnetic detector contains a coil of wire wound around a highly 

permeable core, which is placed below the surface of a roadway. When a 

ferrous metal vehicle comes near or passes over the coil, the lines of flux 

produced by the Earth’s magnetic field that pass through the coil are 

perturbed by the vehicle, thus inducing a voltage in the coil. A high-gain 

amplifier boosts this voltage to operate a relay, which sends a signal to the 

controller that a vehicle has been detected.(22) 

The following discussion is limited to the induction magnetometer or 

magnetic detector. The model shown in Figure 2-42 is cylindrical in shape, 

2.25 inches (57 mm) in diameter and 21 inches (533 mm) long. 

MAGNETIC-DETECTOR PROBE PLACEMENT 

Probe placement is critical for the proper operation of this sensor system. 

For the probe to detect a change in the Earth’s magnetic field, a vehicle must 

be moving at a speed greater than approximately 5 mi/h (8 km/h). This 

implies that the probe must be placed far enough back from the stopline 

where vehicles are normally in motion (generally at least 50 ft (15 m)). It 

must also be placed in the most appropriate location across the lane to 

generate a sufficiently strong signal to register a vehicle detection. 

Distance from the Stopline 

The setback distance of the probe from the stopline is based on the desirable 

allowable gap. Assuming an allowable gap (or equivalently, a passage time) 

of 3 seconds, the probe would be located 132 ft (40 m) from the stopline on a 

street with vehicles traveling at 30 mi/h (48 km/h). The graphs shown in 

Figure 4-37 identify the proper location of the probe relative to the stopline 

based on allowable gaps of 2, 3, 4, or 5 seconds. 

Lateral Placement of Probe 

In a single-lane approach to a stopline, the optimal location of the probe is 

under the path normally followed by the right wheels of the vehicle, as shown 

in Figure 4-38. On a two-lane approach where a single probe is used, the 

probe is placed between the lanes to provide satisfactory coverage. If the 

right lane of a multilane approach is designated for right-turning traffic, the 

probe is located in the middle of the through lane to minimize the effect of 

vehicles in the right-turn lane. 

A common practice for multilane approaches is to place a probe in each lane. 

In such designs, the probe is placed under the right wheel track in each lane. 

Up to three probes can be placed in a conduit. When magnetic probes are 

placed in a new roadway, multiple conduits should be used, one probe per 

conduit. In an existing road, conduit runs should be kept to a minimum; thus, 

more than one probe may be placed in a conduit. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-60 Federal Highway Administration 

1 mi/h = 1.6 km/h 

Figure 4-37. Setback of magnetic probe from stopline as a function 

of vehicle speed and allowable gap. 

Two lane

Single lane

Figure 4-38. Magnetic probe placement for a single-lane 

and two-lane approach to a stopline. 

SENSITIVITY 

Two or more probes used on different approaches of the same traffic phase 

should be placed in similar locations (if possible) so that the area monitored 

by each probe is the same. This permits the sensitivity of all sensors to be 

adjusted by the single sensitivity-adjusting knob. 

400 

350 

300 

250 

200 

150 

100 

50 

10 20 30 40 50 60 

Observed Speed (mi/h)

Probe Setback Distance (feet)

2 Seconds 

3 Seconds 

4 Seconds 

5 seconds

Allowable Gap

400 

350 

300 

250 

200 

150 

100 

50 

10 20 30 40 50 60 

Observed Speed (mi/h)

Probe Setback Distance (feet)

2 Seconds 

3 Seconds 

4 Seconds 

5 seconds

Allowable Gap
 Traffic Detector Handbook—2006 

October 2006 Page 4-61 Federal Highway Administration

A sensor probe generates an impulse in response to any change in the 

magnetic conditions surrounding it. The impulse strength is proportional to 

the change in the magnetic field. Changes in the magnetic field can be caused 

by movement of iron or steel objects in the vicinity of the probe or by changes 

in the current flowing through power lines. With the probe placed as close as 

possible to the path of the vehicles, the strength of the impulses caused by 

these vehicles will be maximized. Impulses caused by traffic moving in other 

lanes or by current changes in nearby power lines will be minimized. The 

sensitivity of the sensor can be decreased until the unwanted impulses are 

reduced in strength so that they do not actuate the relay, while the impulses 

from vehicles in the proper lanes remain strong enough to cause actuation. 

REFERENCES 

1. Kell, J.H., and I.J. Fullerton. Manual of Traffic Signal Design, 

Institute of Transportation Engineers. Prentice-Hall, Englewood 

Cliffs, NJ. 1982. 

2. NEMA Standards Publication No. TS-1: Traffic Control Systems.

National Electrical Manufacturers Association, Washington, DC. 

1989. 

3. Zegeer, C.V. Effectiveness of Green Extension Systems at High Speed 

Intersections, Research Report 472. Kentucky Department of 

Transportation, Bureau of Highways, Division of Research, 

Lexington, KY. 1977. 

4. “Large Area Detection at Intersection Approaches,” Report of 

Technical Committee No. 17, Traffic Engineering. Southern Section 

ITE. 1976. 

5. Courage, K.G., et al. Inductive Loop Detector Configuration Study—

Final Report, Transportation Research Center, prepared by 

University of Florida. Florida Department of Transportation, 

Tallahassee, FL. 1985. 

6. Traffic Signal Detector Loop Location Design Installation. Illinois 

Department of Transportation, District 6, Springfield, IL. 1988. 

7. Woods, D.L. Texas Traffic Signal Detector Manual, Report No. 

FHWA/TX-90/1163-3F. Texas Transportation Institute, College 

Station, TX. 1992. 

8. Lin, F.-B. “Optimal Timing Settings and Detector Lengths of 

Presence Mode Full-Actuated Control,” Traffic Control Devices and 

Rail-Highway Crossings, Transportation Research Record 1010. 

Transportation Research Board, National Research Council, 

Washington, DC. 1985. 

9. Sackman, H., et al. Vehicle Detector Placement for High-Speed, 

Isolated Traffic-Actuated Control, Volume 2: Manual of Theory and 

Practice, FHWA –RD-77-32. U.S. Department of Transportation, 

Federal Highway Administration, Washington, DC. 1977. 

10. Wu, C.-S., et al. Detector Configuration and Location at Signalized 

Intersections, Center for Transportation Research, Research Report 

259-1F, prepared by University of Texas. Texas State Department of 

Highways and Public Transportation, Austin, TX. 1983. 
Chapter 4—In-Roadway Sensor Design 

October 2006 Page 4-62 Federal Highway Administration 

11. Beierle, H. “A Method of Detector Placement,” IMSA Signal 

Magazine. Texas Highway Department District No. 15. June 1974. 

12. “Large-Area Detection at Intersection Approaches,” a section 

technical report, Technical Committee 17, Traffic Engineering. 

Southern Section ITE. 1976. 

13. “Small Area Detection at Intersection Approaches,” a section 

technical report, Technical Committee 18, Traffic Engineering.

Southern Section ITE. 1974. 

14. Traffic Signal Design Handbook, Pub. 149. Commonwealth of 

Pennsylvania, Department of Transportation, Bureau of 

Maintenance and Operations, Traffic Engineering and Operations 

Division, Harrisburg, PA. 1983. 

15. Applications Note 3217, Traffic Control Devices Department. 3M, 

Canoga Controls, Traffic Control Products, St. Paul, MN. No date. 

16. Standard Plans. California Department of Transportation, 

Sacramento, CA. 1988. 

17. Blikken, W.A. The Adjustable Diamond Loop for Vehicle Detection.

Michigan Department of Transportation, Detroit Freeway 

Operations Unit, Detroit, MI. 1979. 

18. Capelle, D.G. Freeway Traffic Management. Transportation 

Research Board, National Research Council, Washington, DC. 1979. 

19. Computerized Signal Systems, Student Workbook. U.S. Department 

of Transportation, Federal Highway Administration, Washington, 

DC. 1979. 

20. Kay, J.L., et al. Locating Detectors for Advanced Traffic Control 

Strategies Handbook. U.S. Department of Transportation, Federal 

Highway Administration, Washington, DC. 1975. 

21. Traffic Control Systems Handbook, Federal Highway Administration 

Implementation Package FHWA-IP-85-11. U.S. Department of 

Transportation, Federal Highway Administration, Washington, DC. 

1985. Also published as ITE Publication No. LP-123. Institute of 

Transportation Engineers, Washington, DC. 1985. 

22. Klein, L.A. Sensor Technologies and Data Requirements for ITS.

Artech House, Norwood, MA. 2001. 
HRDO-04/10-06(1M)E

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