WEIGH-IN-MOTION 

POCKET GUIDE

PART 2

WIM SITE SELECTION, DESIGN, 

AND INSTALLATION GUIDE

Publication No.

FHWA-PL-18-015

Office of Highway Policy Information June 2018
ii

NOTICE

The United States Government does not endorse products or

manufacturers. Trade and manufacturers’ names appear in this 

report only because they are considered essential to the 

objectives of the document.

QUALITY ASSURANCE STATEMENT

The Federal Highway Administration (FHWA) provides highquality 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.
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TABLE OF CONTENTS

INTRODUCTION TO PART 2 .......................................1

Overview ......................................................................... 1

WIM Standards and Performance Requirements........... 1

WIM SITE SELECTION .................................................4

Site Selection Considerations.......................................... 4

WIM Installation Site Selection Process Overview ......... 5

Preliminary Off-Site WIM Site Selection ......................... 5

Horizontal Curvature and Grade.......................................... 5

Pavement Structure ............................................................. 6

Pavement Smoothness......................................................... 7

Test Truck Turnarounds........................................................ 9

On-Site Evaluation of Proposed WIM Location............... 9

Longitudinal Alignment (Roadway Grade).........................10

Lateral Alignment (Transverse Slope of Roadway or Cross 

Slope)..................................................................................11

Lane Width and Markings..................................................11

Traffic Composition and Traffic Flow Characteristics.........12

Pavement Condition...........................................................13

Pavement Surface Smoothness..........................................14

Pavement-Truck Interaction...............................................15

WIM Cabinet Location and Safety Setbacks ......................15

Maintenance of Traffic.......................................................16

Power and Communication Services..................................17

A/C Power ......................................................................17

Landline Telephone ........................................................17
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Cellular Service ...............................................................18

Fiber Optic ......................................................................18

Solar Power....................................................................18

Drainage.............................................................................19

Existing Equipment for Possible Upgrade ..........................19

Calibration Test Truck Turnarounds...................................20

Proximity to Certified Truck Scale ......................................20

Coordination with Local Highway Office............................21

Site Selection Report and Location Approval .....................21

WIM SITE DESIGN .................................................... 22

Sensor Layout, Sensor Arrays, Dimensions, Depth, and 

Configuration................................................................. 22

Bending Plate .....................................................................22

Load Cell.............................................................................24

Quartz Piezo Sensor ...........................................................26

Strain Gauge Strip Sensors.................................................28

Polymer Piezo Sensor .........................................................30

Portable Polymer piezo Sensor...........................................32

Cabinet Location............................................................ 32

Directional Boring.......................................................... 33

Pull Boxes ...................................................................... 33

Data Acquisition (Communications).............................. 35

Modems .............................................................................35

Telephone Service...............................................................35

Fiber Optics ........................................................................36

Cellular Service ...................................................................36

On-Site Data Storage .........................................................37
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System Power................................................................ 37

Solar Power Devices...........................................................37

Solar Panels....................................................................38

Solar Regulator...............................................................39

Battery............................................................................39

A/C Service Devices............................................................40

Fans and Environmental Controls......................................41

Lightning Protection and Grounding............................. 41

Surge Suppression..............................................................41

Grounding ..........................................................................42

WIM Site Plan................................................................ 43

Standard and Manufacturer Specifications .................. 44

WIM SITE INSTALLATION ..........................................45

General Installation Requirements............................... 45

Installation QA Inspector............................................... 46

Pre-Construction Meeting............................................. 47

Site Preparation............................................................. 47

Protection of Utilities .........................................................48

Equipment Inspection.........................................................48

Maintenance of Traffic.................................................. 48

WIM Site Installation Quality Assurance....................... 49

Excavation and Conduit .....................................................49

Pole/Cabinet.......................................................................52

Solar Panels........................................................................53

Solar Panel Charge Circuit Devices.....................................54

Pull Boxes...........................................................................55
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Layout Marking and Cutting ..............................................55

Loop Sensor Installation and Sealing (General) .................58

Load Cell Sensor Installation ..............................................59

Bending Plate Sensor Installation ......................................60

Quartz Piezo and Strain Gauge Strip Sensor Installation ...61

Polymer Piezo Sensor Installation ......................................63

Cabling ...............................................................................65

Grounding ..........................................................................67

Cabinet Equipment.............................................................67

Initial Calibration........................................................... 68

Vehicle Classification Accuracy Validation.................... 72

WIM Site Acceptance and Reporting ............................ 74

WIM Site Acceptance Report .............................................74

WIM Site Non-Compliance Report .....................................75

REFERENCES ............................................................ 76

APPENDICES ............................................................ 78

The APPENDICES section at the end of this document describes 

the appendices for the FHWA WIM Pocket Guide, which are 

available on the FHWA website: 

https://www.fhwa.dot.gov/policyinformation/knowledgecenter

• Appendix A – Technical Report Documentation Page

Form DOT F 1700.7

• Appendix B – Acknowledgements

• Appendix C – WIM Procurement Resources

• Appendix D – WIM Site Design Resources

• Appendix E – Maintenance and Calibration Resources

• Appendix F – WIM Program Operation and

Management Checklists
vii

LIST OF TABLES

Table 1 – ASTM 1318E-09 Functional Performance 

Requirements for WIM Systems.......................................... 2 
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LIST OF FIGURES

Figure 1 – Acceptable Pavement Section for WIM ....................10

Figure 2 – Pavement Inspection.................................................14

Figure 3 – Truck Traversing WIM Scale Area .............................15

Figure 4 – Protected Cabinet Location.......................................16

Figure 5 – Bending Plate Installation .........................................22

Figure 6 – Bending Plate Installation Detail...............................23

Figure 7 – 2-Lane Unidirectional Staggered Bending Plate 

Configuration .....................................................................23

Figure 8 – 2-Lane Unidirectional In-Line Bending Plate 

Configuration .....................................................................24

Figure 9 – Load Cell Installation (Staggered Configuration) ......25

Figure 10 – 2-Lane Unidirectional Staggered Load Cell 

Configuration .....................................................................26

Figure 11 – Quartz Piezo Installation Detail...............................27

Figure 12 –2-Lane Unidirectional Staggered Quartz Piezo 

Configuration .....................................................................28

Figure 13 – Strain Gauge Strip Sensor Installation Detail ..........29

Figure 14 – 2-Lane Unidirectional Staggered Strain Gauge Strip 

Sensor Configuration .........................................................29

Figure 15 – Polymer Piezo Installation (not to scale).................30

Figure 16 – 2-Lane Unidirectional Polymer Piezo 4-Sensor per 

Lane Configuration.............................................................31

Figure 17 – 2-Lane Unidirectional Portable Polymer Piezo 2-

Sensor per Lane Configuration ..........................................32

Figure 18 – Bore Operation Conduit..........................................33

Figure 19 – Pull Box Interior.......................................................34

Figure 20 – Pull Box Exterior......................................................34
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Figure 21 – Landline Telephone Service Box .............................35

Figure 22 – Cellular Modem.......................................................36

Figure 23 – Solar Power System Components...........................37

Figure 24 – Solar Panel (80-Watt)..............................................38

Figure 25 – Solar Regulator........................................................39

Figure 26 – Battery.....................................................................40

Figure 27 – A/C Power Service Meter........................................40

Figure 28 – Thermostat..............................................................41

Figure 29 – Sensor and Telephone Surge Arrestor Devices.......42

Figure 30 – Grounding Rod with CadweldTM ..............................43

Figure 31 – Utility Location Markings........................................48

Figure 32 – Portable Changeable Message Sign Used for Traffic 

Control ...............................................................................49

Figure 33 – Sensor Conduits from Exit Hole to Pull Box ............50

Figure 34 – Pull Box Conduit Sealed with Duct Seal ..................51

Figure 35 – Solar Panel Angle Check..........................................54

Figure 36 – Drainage Gravel Depth Check ................................. 55

Figure 37 – Loop Separation Measurement ..............................56

Figure 38 – Reference Line.........................................................57

Figure 39 – Slot Depth Measurement........................................57

Figure 40 – Excavated Bending Plate Pit (In-Line Configuration)

...........................................................................................60

Figure 41 – Completed Bending Plate Installation.....................61

Figure 42 – Completed Quartz Piezo Installation ......................62

Figure 43 – Pre-Installation Sensor Testing ...............................63

Figure 44 – Sensor Slot Depth Check .........................................64

Figure 45 – Piezo Slot Clean and Dry Check...............................64

Figure 46 – Completed Polymer Piezo Sensor Installation........65
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Figure 47 – Sensor Lead Labeling and Service Loops in Pull Box66

Figure 48 – Terminal Panel Labels and Connections .................68
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LIST OF ACRONYMS

AASHTO American Association of State Highway and 

Transportation Officials 

AC Asphalt concrete 

ACF Axle Correction Factor

ALDF Axle Load Distribution Factors

APT Axles per Truck

B-WIM Bridge WIM

BP Bending plate

CDS Comparison data set

CPU Central Processing Unit

DC Direct Current

DOT Department of Transportation

DOW Day of the week

ESAL Equivalent single axle loadings

FHWA Federal Highways Administration

GFCI Ground-Fault Circuit Interrupter 

GPS Global Positioning System 

GVW Gross Vehicle Weight

HDF Hourly Distribution Factor

HOD Hour of the day

IRI International Roughness Index

LC Load cell

LPR License plate reader

LTPP Long-Term Pavement Performance

MAF Monthly Adjustment Factors

MEPDG Mechanistic-Empirical Pavement Design Guide

MOT Maintenance of traffic

MOY Month of the year 

MUTCD Manual for Uniform Traffic Control Devices

NCR Non-Compliance Report 

NEMA National Electrical Manufacturers Association

OWL Optimal WIM Locator 

PCC Portland cement concrete 

PPE Personal protection equipment

PV Photovoltaic
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PVC Polyvinyl Chloride

PVF Per Vehicle Format

QA Quality assurance

QC Quality control

RMC Rigid metal conduit 

SIM Subscriber identity module

SPS Special Pavement Studies

TMG Traffic Monitoring Guide

TMAS Travel Monitoring Analysis System

TPF Transportation Pooled Fund

U.S. United States

UL Underwriters Laboratories

VCD Vehicle Class Distribution

VWS Virtual Weigh Station

WIM Weigh-in-Motion
1 

INTRODUCTION TO PART 2 

Overview 

The Federal Highway Administration (FHWA) Weigh-In-Motion 

(WIM) Pocket Guide consists of three main text documents, 

four instructional video supplements, and six appendices. The 

video supplements and appendices are only available on the 

FHWA website: 

https://www.fhwa.dot.gov/policyinformation/knowledgecenter. 

Part 1 of the Guide includes a description of each element of 

the FHWA WIM Pocket Guide. Part 2 was developed to assist a 

WIM/traffic engineer or senior WIM specialist with site-level 

decision making, including the meticulous selection of the WIM 

site location, development of WIM system installation plans, 

and WIM system installation oversight. 

WIM Standards and Performance 

Requirements

The ASTM International E1318-09 Standard Specification for 

Highway Weigh-In-Motion (WIM) Systems with User 

Requirements and Test Methods is a primary WIM standard 

accepted in the U.S. (1). Other countries have their own 

standards (2, 3). The ASTM E1318-09 standard classifies WIM 

systems according to four distinct types, depending on the 

application and functional performance requirements.

Table 1 summarizes the ASTM E1318-09 WIM system 

performance requirements. The 95 percent compliance defined 

in ASTM E1318-09 specifies the minimum percentage of 

measurements (i.e., no less than 95 percent) that should be 

within the specified tolerances to satisfy the performance 

requirements. Since Type IV WIM systems have not yet been 
2 

approved for use in the United States, specifications for these 

systems are not provided.

Table 1 – ASTM 1318E-09 Functional

Performance Requirements for WIM Systems

Function Tolerance for 95% Compliance

Type I Type II Type III

Wheel Load* ±25% ±20%

Axle Load ±20% ±30% ±15%

Axle-Group Load ±15% ±20% ±10%

Gross Vehicle Weight ±10% ±15% ±6%

Speed ±1 mph

Axle-Spacing and 

Wheelbase ±0.5 ft

*Includes single or dual tires

ASTM E1318-09 WIM Types

• Type I and Type II systems: Suitable for

traffic data collection purposes, with Type

I systems having slightly more stringent

performance requirements. Vehicle speed

range to meet functional performance

requirements is 10 to 80 mph.

• Type III systems: Suitable for screening vehicles

suspected of weight limit or load limit violations

and have stricter functional performance

requirements than Type I and Type II systems.

Vehicle speed range to meet functional

performance requirements is 10 to 80 mph.

• Type IV systems: Not approved for use in the United

States but intended for use at weight enforcement

stations. Vehicle speed range to meet functional

performance requirements is 2 to 10 mph.

±0%
3 

Enhanced WIM performance requirements were developed 

under the FHWA Long-Term Pavement Performance (LTPP) 

Program in an effort to collect research-quality WIM data (4).

LTPP tolerances are the same as in ASTM E1318-09, but instead 

of 95 percent compliance, LTPP uses a statistically computed 95

percent confidence interval plus mean error to characterize the 

spread of measurement error and compares it with the 

tolerances listed in Table 1. The LTPP performance 

requirements include provisions for performance testing at 

three temperature and three speed ranges, requirements for 

road smoothness, and requirements for the accuracy of the 

vehicle length measurements. 
4 

WIM SITE SELECTION

Site Selection Considerations

WIM sensors should be installed within a desired road segment 

at a location that would minimize weighing errors due to 

adverse site characteristics. Certain site characteristics must be 

avoided, as their effect on measurement accuracy cannot be 

controlled through equipment calibration or maintenance. The 

most important aspects to consider when evaluating a 

candidate WIM site include: 

• Roadway geometrics that may introduce adverse truck 

dynamics, such as bouncing, side-to side rocking, 

swerving, acceleration, or deceleration. To avoid this,

special attention should be paid to horizontal curvature, 

grade, cross slope, lane width and markings, nearby 

bridges and ramps, and speed and exit signs. 

• The following traffic conditions should be avoided: stopand-go traffic, lane-changing, passing, signalization and 

intersections, lane shifts, exit lanes, and vehicle velocity 

changes. 

• Weak pavement structures and thin pavements should 

be avoided, along with rough pavement surfaces and 

pavements that show surface distresses like cracking, 

rutting, raveling, joint faulting, and potholes.

When selecting a site for new WIM installation, additional 

factors to consider include:

• Availability of power and communications (cellular, 

fiber optic, or landline service) 

• Technician safety

• Cabinet location 

• Drainage and existing equipment 
5 

• Test truck turnarounds 

• Anticipated roadway work within the next 5 years

Detailed criteria and processes for WIM site selection are 

provided in the ensuing sections. 

WIM Installation Site Selection Process 

Overview 

The WIM site selection process 

consists of two parts. Part 1 

involves a preliminary WIM site 

selection using information 

available in the office. Part 2 involves field verification and 

confirmation of the conditions at the potential WIM site 

locations and the final selection of the preferred site for the 

WIM system installation. 

The following sections describe a sequence of steps involved in 

the WIM site selection process. Action items are identified using 

the checkmark () icon. 

Preliminary Off-Site WIM Site Selection

Preliminary WIM site selection is often accomplished remotely, 

typically in the office, using all available supporting 

documentation and data. The entire roadway segment, or 

corridor, selected to meet traffic data needs must be evaluated 

to ensure that it meets the ASTM E1318-09 requirements for 

horizontal alignment and for structure. 

The goal of the preliminary site selection is to identify roadway

sections of no less than 1⁄2 mile in length that are acceptable for 

a WIM installation, using the criteria described below. 

Horizontal Curvature and Grade 

According to ASTM E1318-09, the horizontal curvature of the 

roadway lane for 200 feet prior to and 100 feet beyond the 

WIM sensors shall have a radius not less than 5,700 feet 
6 

measured along the centerline of the roadway. The vertical 

grade should be no more than 2 percent for ASTM E1318-09 

WIM Types I, II, and II and 1 percent for Type IV. 

 Review the available roadway construction plans to 

determine the horizontal curvature and the vertical 

grade of a proposed roadway segment and record 

findings. 

Pavement Structure 

Adequate pavement structure is needed to accommodate the 

WIM system sensors throughout their service life. The expected 

longevity of pavement structure and the ability of the agency to 

maintain the pavement within the WIM scale section should be 

evaluated based on current pavement condition and planned 

maintenance activities. 

Bending plate and load cell sensors should only be installed in 

portland cement concrete (PCC) pavements. Strip-type sensors 

such as a polymer piezo, coaxial piezo, quartz piezo, or strain 

gauge strip sensors could be installed in either PCC or asphalt 

concrete (AC) pavement. 

AC pavements should be relatively stiff with adequate AC

thickness to prevent sensor depths from achieving 75 percent of 

the total depth of the AC layer. The AC pavement thickness 

should be no less than 4 inches, to prevent breaking of the 

structural pavement layer (i.e., cracking through the asphalt 

layer). A top layer consisting of a high performance pavement 

mix of 1.5 to 2 inches in thickness is preferred. Roadways 

utilizing a slurry top coat would significantly reduce the life of a 

project and should not be considered acceptable under normal 

conditions. Pavements experiencing significant changes in 

pavement stiffness due to environmental effects may 

contribute to weighing errors. Softening of the AC layer during 

hot months or weakening of the subgrade during spring thaws 

could affect the performance of piezo sensors due to changes in 

the structural support that the pavement provides for the 

sensor. 
7 

A section of continuously reinforced concrete pavement of 

sufficient length, based on the formula below, is preferred for 

WIM installations in PCC pavements. For typical highway 

speeds, the slab length should be 300 to 400 feet, which 

includes 75 feet required after the sensors.

Slab length in ft =2.93*(Truck Speed in mph) + 150

When a jointed PCC pavement is constructed without dowels, 

slab curling and warping could contribute to vehicle dynamics 

and reduce the accuracy of weight measurements due to the 

excessive dynamic forces being measured by the sensors.

Pavement joints may also contribute to road roughness.

WIM sensor installations in smooth PCC pavement have longer 

periods of time between failures than installations in flexible 

pavements. PCC pavements are not susceptible to rutting 

caused by truck loading. 

 Request from the pavement design or management 

office information about pavement structure, pavement 

surface condition data, and road roughness.

 Record pavement type, pavement thickness, and road 

condition information.

Pavement Smoothness 

The pavement surface should be smooth and free of mediumand high-severity distresses to ensure the integrity of the WIM

sensor installation throughout its service life. The ideal roadway 

segment would have smooth pavement, or the proposed WIM 

site should be part of a new roadway construction or pavement 

rehabilitation plan. Future repaving should not be anticipated or 

planned for the next 5 years.

The candidate WIM sensor installation roadway section must 

meet the smoothness requirements specified in section 6.1.5 of 

ASTM E1318-09 (1). Pavement smoothness assessment based 

on the ASTM E1318-09 criteria is performed during the on-site 

evaluation, described later in this guide. 
8 

Preliminary pavement smoothness analysis may be 

accomplished in the office using American Association of State 

Highway and Transportation Officials (AASHTO) M331-13, 

Standard Specification for Smoothness of Pavement in Weighin-Motion (WIM) Systems (5). This analysis uses the profile data 

(collected with a high-speed profiler) and the Optimal WIM 

Locator (OWL) module of the Pro-Val profile data analysis

software. The OWL software uses pavement profile information 

to identify optimal WIM sensor locations. WIM site planners 

must note that OWL does not consider the type of sensor array 

or the sensor type and may be overly conservative for bending 

plate or load cell arrays. Both ProVAL and the OWL modules are 

free. Information on ProVAL and the OWL module can be 

obtained through the LTPP Customer Support Service Center.

When all WIM scale roughness indices calculated from the 

pavement profile records are less than the lower threshold

indicated in AASHTO M331-13, it is likely that the WIM site will 

produce an acceptable level of weighing error for a selected 

sensor type. The upper threshold value provided in AASHTO 

M331-13 is that above which a WIM site is likely to produce an 

unacceptable level of weighing error. A location should be 

chosen that will result in acceptable index values. 

According to AASHTO M331-13, an acceptable WIM scale 

roughness index value is required for a minimum of 131 feet 

prior to the chosen location.

 Use the OWL module of the Pro-Val software to 

compute WIM scale roughness index and compare it 

against threshold values provided in AASHTO M331-13.

 If the pavement condition is found to be unsuitable for 

WIM installation, consider grinding the pavement or 

applying a thin overlay to reduce roughness. If 

pavement improvement is not feasible, search for a 

different location that would meet the users’ needs and 

would be favorable for accurate weighing in motion.

 Record findings.
9 

Test Truck Turnarounds 

The test truck turnarounds should be close enough to enable 

testing to be completed in 1 or 2 days. A turnaround (one loop 

of the test vehicle) that takes 20 minutes or less to complete is 

recommended. Reducing test turnaround time makes more 

efficient use of time for WIM validation or calibration.

 Use online mapping tools to find the optimal locations 

for test truck turnarounds for the WIM site. 

 Use video logs or virtual drive software to identify 

roadside features and roadway characteristics, potential 

turnarounds, and relative pavement condition near the 

potential WIM site location.

On-Site Evaluation of 

Proposed WIM Location

After one or more acceptable WIM 

sensor locations have been identified 

through the off-site data assessment, 

on-site investigation must be performed

by a trained WIM specialist using this manual and the ASTM 

E1318-09 criteria to complete the assessment. Figure 1 is a 

photo of a site identified as acceptable.

 Safety personal protection equipment (PPE) should be 

worn, including hard hat and safety vest. 

 A spotter should watch traffic and warn the inspector of 

any oncoming vehicles.

 Appropriate lane closure maintenance of traffic (MOT) 

devices should be installed in accordance with the 

Manual on Uniform Traffic Control Devices (MUTCD)

(6). 
10

Figure 1 – Acceptable Pavement Section for WIM

Longitudinal Alignment (Roadway Grade) 

To avoid adverse driver behaviors, including braking and 

acceleration, the longitudinal gradient (roadway grade) of the 

road surface for 200 feet in advance and 100 feet beyond the 

WIM sensors shall not exceed 2 percent, per ASTM E1318-09.

The following procedure is recommended: 

1. Place a 16-foot straightedge in the center of the lane, 

parallel with the pavement markings. A digital level is 

then placed on top of the straightedge and the angle is 

measured and converted into gradient and then to 

slope as a percentage. 

2. Use the following mathematical procedure to convert 

the angle to percent slope:

o Calculate the tangent function of the angle with 

your calculator. For this example, the tangent of 2 

degrees equals approximately 0.03492. This is the 

slope as a gradient.

o Multiply the gradient by 100 on your calculator to 

obtain the percentage of the slope. For this 

example, multiplying 0.03492 by 100 equals 3.5

percent.
11

3. Repeat this process every 50 feet until the entire 300-

foot section has been tested. Any result over 2 percent

is a failure of the test.

4. Record the findings.

Lateral Alignment (Transverse Slope of Roadway 

or Cross Slope) 

To avoid adverse driver behaviors, including veering and lane 

wander, and to ensure balanced truck loads, the cross slope 

(lateral slope) of the road surface for 200 feet in advance and 

100 feet beyond the WIM sensors shall not exceed 3 percent for 

Type I and Type II WIM systems and 1 percent for Type III 

systems, per ASTM E1318-09. To check the roadway slope:

1. Position a 10-inch or 24-inch digital level on top of a 10-

foot straightedge, placed laterally in the travel lane near 

the proposed sensor location to measure the crossslope of the roadway.

2. Use the following mathematical procedure to convert 

the angle to percent slope:

o Calculate the tangent function of the angle with 

your calculator. For this example, the tangent of 2 

degrees equals approximately 0.03492. This is the 

slope as a gradient.

o Multiply the gradient by 100 on your calculator to 

obtain the percentage of the slope. For this 

example, multiplying 0.03492 by 100 equals 3.5

percent.

3. Repeat this process every 50 feet until the entire 300-

foot section has been tested. Any result over 2 percent

is a failure of the test.

4. Record the findings

Lane Width and Markings 

To properly install the WIM sensors, the travel lanes for the 

proposed WIM site must be wide enough to ensure proper 

layout of the weighing sensors and sufficient spacing between 

the inductive loops. 
12

The lane width for WIM sensor installations must be at least 10 

feet, and between 12 and 14 feet is optimal. The lanes must be

clearly marked with painted lines, 4 inches to 6 inches in width, 

throughout the WIM section. 

To measure lane width and markings: 

1. Use measuring tape to measure the lane and shoulder 

widths and the width of the pavement markings 

2. Record measurements and condition of the markings. 

Traffic Composition and Traffic Flow 

Characteristics 

If no site-specific traffic volume, vehicle classification, and speed 

data are available during the preliminary investigation phase, 

conduct a portable traffic volume, vehicle classification, and 

vehicle speed count to obtain detailed hourly and day of week 

truck volumes and speed information. Minimum count duration 

is 48 hours. Seven-day count studies may be required for some 

locations that have traffic flow issues on weekends (retail, 

recreational, etc.). Collected truck volume data will be used to 

develop an initial calibration plan and possible auto calibration 

plan if needed. 

The WIM systems should be in an area of free traffic flow with 

good sight distances, including:

• No stop-and-go traffic

• Minimal lane-changing

• Minimal passing

• Constant vehicle velocities

If poor traffic flow is identified from the data or video log 

review, the site may be excluded from further consideration. 

To assess traffic composition and traffic flow characteristics

while on site: 

1. Find a safe location that could be used for an extended 

period of time (minimum of 3 to 4 hours).
13

2. Evaluate the traffic conditions when the highest volume 

of traffic is anticipated, such as during rush hour.

3. Measure and record the distances to all intersections 

and signalization within the road segment. 

4. Measure and record distances to nearby roadway 

assets. 

5. Record any presence and frequency of stop-and-go 

traffic, lane-changing, and vehicles passing.

6. Depending on site characteristics, conduct 48-hour or 7-

day traffic volume, vehicle classification count, and 

speed data collection. Compute hourly and day of week 

truck volumes and speed frequency distribution. Speed 

by class information may also be helpful.

For two-lane roadways with two-way traffic, it is recommended 

that the state agency work with the local agency district office 

to create a “no-passing” zone within the 300-foot WIM scale 

area. WIM site planners should also avoid intersections and 

driveways that could cause irregular traffic dynamics in the area 

of the WIM sensor location.

Pavement Condition 

The pavement surface should be free of distresses (cracking, 

raveling, faulting, rutting, potholes, etc.) and any transitional 

joints. Information regarding the evaluation of pavement 

surface condition can be found in the Distress Identification 

Manual for the Long-Term Pavement Performance Program (7).

To assess pavement condition:

1. Visually inspect the pavement. If a lane closure is not 

practical, the evaluation can be conducted from the 

shoulder, as shown in Figure 2. 

2. Use 16-foot-long straightedge to identify any pavement 

rutting.

3. Record any distresses (cracking, faulting, rutting, 

potholes, etc.) within the WIM scale section that would

adversely affect the accuracy of the WIM system (by 

inducing truck bounce or non-uniform movement). 
14

Figure 2 – Pavement Inspection

Pavement Surface Smoothness 

The goal of the pavement smoothness analysis is to determine if 

a 300-foot section is available within the selected roadway 

segment that satisfies ASTM E1318-09 pavement profile and 

smoothness criteria for WIM sensor installation. The pavement 

smoothness analysis may be accomplished by any one of the 

following methods:

1. On-site profile data collection using a high-speed 

profiler, followed by in-house data analysis using ProVal 

software. The preferred equipment is an inertial profiler 

meeting the Class 1 standards as specified in ASTM 

E950-98. 

2. Pavement smoothness testing in accordance with 

section 6.1.5.1 of ASTM E1318-09, following the 

procedure of using the 16-ft-long straightedge, such as 

an I-beam or box beam that will not flex, and the 6-inchdiameter by 0.125-inch-height aluminum disc. For a 

distance of 300 feet before and 75 feet after the WIM 

sensor location, the roadway surface shall be 

maintained in a condition such that a 6-inch-diameter 

circular plate 0.125 inches thick cannot be passed 
15

beneath a 16-foot-long straightedge. This method 

requires a lane closure.

Pavement-Truck Interaction 

Trucks must be observed as they approach, traverse, and leave 

the proposed sensor area to determine if there is any visible 

truck bounce that may affect the performance of the WIM 

scales. Figure 3 shows a truck approaching the WIM scale area. 

Specific roadway locations that demonstrate consistent truck 

dynamics should be investigated closely to determine the cause, 

including dips or bumps in the roadway. Also, a verification that 

trucks track down the center of the lane must be performed.

 Based on site observations, record any adverse vertical or

horizontal truck movement, or side-to-side truck rocking 

motion. 

Figure 3 – Truck Traversing WIM Scale Area

WIM Cabinet Location and Safety Setbacks 

When deciding where to place the WIM cabinet, consider such 

elements as technician safety, equipment protection, 

accessibility to power and communication services, drainage, 

and proximity to test truck turnaround locations. 

Preferably, the cabinet should be in a safe location out of the 

clear zone or located behind a guardrail, so that it cannot be hit 

by a vehicle leaving the roadway, and it should have a clear line 
16

of sight to the sensors and an unobstructed view of passing 

traffic. Figure 4 shows a cabinet located in a safe location.

Figure 4 – Protected Cabinet Location

The WIM site should have a large flat area for the technician to 

park the service vehicle outside of the clear zone, also known as 

the recovery area, or behind a protective barrier such as a 

guardrail. The MUTCD and AASHTO’s Roadside Design Guide 

provide clear zone dimensions for roadways based on the speed 

of traffic (6, 8).

 Evaluate if the cabinet location provides a safe location 

for technicians to stand and not have their back turned 

to oncoming traffic. 

 Sketch the proposed location of the cabinet, conduit 

pushes, and pull boxes. Indicate safe parking areas for 

the service vehicle that are outside of the recovery area, 

not in the clear zone, and do not require technicians to 

cross the roadway to access the cabinet. 

Maintenance of Traffic 

From a safety standpoint and MOT design considerations, it is 

recommended that the proposed WIM site should provide the 
17

opportunity to use basic MOT designs for WIM installation and 

ongoing maintenance actions and not be too complex.

Power and Communication Services 

Although the standard WIM site 

design may include specifications for 

the type of power and communication 

devices to use, the on-site evaluation 

may be used to either confirm 

compliance with the specification or to 

develop a report justifying the 

exceptions to the specifications. Special circumstances 

discovered during the on-site evaluation may prohibit the use of 

certain types of communication services or alternating current 

(A/C) power service.

If A/C power or cellular, fiber optic, or landline telephone 

service is not available nearby, or the costs to bring power and 

phone supply line to the site are not affordable, solar power or 

other communications alternatives may prove to be the better 

option.

A/C Power

 If A/C power is preferred, determine if A/C power 

service is located nearby. 

 Record the distance from the proposed cabinet location 

to the closest A/C power service. 

 Identify the location of the nearest A/C power services 

on the site sketch.

Landline Telephone

 If landline telephone is preferred, determine the 

location of the closest telephone service box. 

 Record the distance from the proposed cabinet location 

to the nearest telephone service location.

 Identify the location of the nearest telephone services 

on the site sketch.
18

Cellular Service

 If cellular service is preferred, or landline service is not 

available, measure the signal strength of the available 

cellular services. This can be done using a cell phone.

 Note the cellular signal strength of the inspection 

checklist.

Fiber Optic

 If fiber optic communications are preferred, determine 

the distance to the fiber optic service. 

 Record the distance from the proposed cabinet location 

to the nearest fiber optic service.

 Identify the location of the nearest fiber optic services 

on the site sketch.

 Determine the location for the fiber vault, which will be 

required at the site to handle the waterproof case 

containing the fiber splice.

Solar Power

 If solar power is deemed to be the best option, conduct 

a solar survey to determine if sufficient sunlight is 

available. 

 Use a solar (sun) chart which incorporates azimuth and 

elevation. Azimuth shall be calculated starting true 

north as 0°, at solar noon the azimuth angle will be 

defined as 180°. A sample solar chart can be found at 

the following website:

http://solardat.uoregon.edu/SunChartProgram.php

 The chart program creates a sun path in Cartesian 

coordinates. 

 If solar power is to be used, check WIM manufacturer 

specifications to make sure that this is viable solution 

for the selected equipment and intended WIM data 

application. You may also need to consider the online 

current draw if the system may at some point be used 

in a continuous reporting mode, such as is done for 

emergency evacuations and other instances.
19

Photovoltaic (PV) solar electric modules require direct sun 

exposure in order to achieve the maximum level of 

performance. Any minor solar shading will significantly reduce 

the amount of energy produced. Sites with limited direct sun 

exposure may require an oversized PV solar module to 

compensate for the reduced electrical generation time periods.

Wind power may also be a suitable alternative power source.

Drainage 

To ensure that the equipment or sensor lead-ins will not be 

susceptible to water damage through flooding, it is important to 

select a site with adequate drainage or located on higher 

ground, especially in areas with heavy snow accumulation. It is 

best that the cabinet be located at a level that is higher than the 

pavement surface. Drainage for the pull boxes is also important. 

The pull boxes should not be installed in gullies or ditches. 

To ensure proper drainage design:

 Develop a sketch showing the optimal 

locations for the cabinet and pull box.

A “to scale” drawing is recommended.

 Identify on the sketch:

• Location of any drainage assets 

with respect to the proposed 

cabinet

• Orientation to north

• Any conduit pushes

• Ground rod location

• Anticipated power and/or communication conduits

• Pull box locations

Existing Equipment for Possible Upgrade 

If a traffic monitoring site is being evaluated for upgrade to a 

WIM site: 

 Inspect all existing equipment components and 

evaluate each item to determine if it is structurally 
20

sound and in good physical and operational condition, 

and will last as long as the newly installed equipment.

 Verifying the grounding at the site with a megger.

 Record information about existing equipment and all 

test readings.

Calibration Test Truck Turnarounds 

One of the greatest contributors 

to the cost of a WIM calibration is 

the amount of time spent on site. 

The on-site time can be reduced if 

the test truck turnarounds are 

reasonably close to the WIM site to provide the quickest, safest 

truck route possible. Another option is to use a racetrack circuit 

pattern using parallel streets instead of turnarounds. It is 

recommended that WIM systems be installed no closer than 1/2

mile, and preferably no farther than 5 miles from the nearest 

turnaround in each direction. 

To establish turnarounds:

 Drive to the turnaround locations identified through 

analysis of aerial maps to verify that they are suitable 

for the safe and repeated turnaround of semi-tractor 

trailer trucks. 

 If necessary, request permission from private 

landowners to use their land for turning the trucks 

around.

 Record the global positioning system (GPS) coordinates 

and or distances for acceptable truck turnaround 

locations.

Proximity to Certified Truck Scale 

The test trucks must be measured at a certified truck scale just 

prior to calibration. To reduce the amount of time spent on site, 

and to ensure that pre-calibration measurements are accurate, 

a truck scale that is closest to the WIM location should be used. 
21

Coordination with Local Highway Office 

Once on-site evaluations have been 

completed and the proposed site has 

been determined to be suitable for a 

WIM installation, coordinate with the

local highway office: 

 Contact district engineering staff and ask for a 

pavement rehabilitation schedule, other maintenance 

considerations, and any other activities that may impact 

installation and operation of a WIM site. 

 Record information about current or planned pavement 

rehabilitation, maintenance, or any other activities that 

may impact installation and operation of a WIM site. 

Site Selection Report and Location Approval 

 Prepare a WIM site evaluation report using findings 

from off-site and on-site investigations, including:

• Summary of findings from the off-site and on-site 

investigations.

• Rating of site applicability for WIM installation. 

• Recommended location for WIM sensors, cabinet, 

pull box, power and telecommunications, and 

drainage features (GPS coordinates). 

• WIM site sketch with distances noted. 

 Meet with the district engineer and the WIM data user

staff to get final approval for site location.
22

WIM SITE DESIGN

Sensor Layout, Sensor Arrays, Dimensions, 

Depth, and Configuration 

This section describes recommended 

sensor layout and configurations for 

different types of WIM sensors.

Bending Plate 

The bending plate is typically 69 inches 

long, 20 inches wide, and 1 inch thick. It 

is placed into a metal frame that has been permanently 

installed into the pavement. The frames are installed so that the 

edge is aligned with either the outside edge of the painted lane 

line or the center line, depending on the configuration of the 

sensors, as shown in Figure 5. 

Figure 5 – Bending Plate Installation

The excavation for the bending plate frame is 26 inches wide 

and 70 inches long. The maximum depth of the excavation is 4 

inches in the center drain area. The excavation for the loadbearing portion of the bending plate frame is a minimum of 2.25

inches. 
23

A cross section of the manufacturer’s recommended excavation 

depth measurements is shown in Figure 6. 

Figure 6 – Bending Plate Installation Detail

Each bending plate should be installed to cover one-half of the 

travel lane. Therefore, the preferred configuration consists of 

two bending plates installed in each lane. For ASTM E1318-09 

WIM Type I and Type II WIM systems, the sensors are typically 

installed in a staggered configuration to provide more accurate 

speed and axle spacing measurements that reduce vehicle 

dynamic effects, as shown in Figure 7. 

Figure 7 – 2-Lane Unidirectional Staggered Bending Plate 

Configuration

Alternatively, the in-line configuration provides a simpler 

installation, as shown in Figure 8. The speed and axle 

measurements are less accurate with this configuration than 

with the staggered configuration.
24

Figure 8 – 2-Lane Unidirectional In-Line Bending Plate Configuration

With either layout, the loops are placed in line across the 

roadway but should be separated by at least 5 feet to avoid 

interference (also referred to as “crosstalk”).

There may be constructability and cost differences between the 

in-line and staggered bending plate installations. The staggered 

array requires a separate set of drainage and electrical conduits 

and may take longer to install and therefore costs more.

For ASTM E1318-09 Type III WIM systems that require highly 

accurate weight data, the double in-line configuration is used. 

For this configuration, two in-line bending plates are installed 

prior to and after the second loop.

Load Cell 

Each load cell sensor is typically 72 inches long, 38 inches wide, 

and 5 inches thick. The load cell is installed in a metal frame that 

is permanently installed into a concrete pit. The frames are 

installed so that the edge is aligned with either the lane line or 

the center line, depending on the configuration of the sensors. 

The size of the load cell pit, which is able to house two sensors 

in a side-by-side configuration, is up to 14 feet wide and up to 3 

feet deep. The length and depth of the concrete vault differs 
25

among manufacturers. In addition, a concrete slab up to 20 feet 

in length may be required for the frame to rest on. 

The preferred load cell configuration consists of two load cells 

and at least one inductive loop and may include one axle 

sensor. The load cell threshold is placed across the traffic lane. 

It may use an in-line configuration or a staggered configuration 

with the scale almost touching at the corners as shown in Figure 

9, or separated by up to 2 feet. The WIM scales operate 

independently from one another. 

Off-scale detectors may be integrated into the scale assembly to 

sense any vehicles off the weighing surface, as shown in Figure 

10. The inductive loop is placed upstream of the load cell to 

detect vehicles and alert the system of an approaching vehicle. 

If a second inductive loop is used, it is placed downstream of 

the load cell to determine the completion of the vehicle 

detection. An axle sensor may be placed downstream of the 

load cell to determine axle spacing and vehicle speed.

Figure 9 – Load Cell Installation (Staggered Configuration)
26

Figure 10 – 2-Lane Unidirectional Staggered Load Cell Configuration

The arrays for each lane may be placed in line across the 

roadway, or they may be staggered to allow for loop homerun 

cuts for both lanes to go to the same side of the roadway. With 

either configuration, the loops should be separated by at least 5 

feet to avoid interference (crosstalk).

Quartz Piezo Sensor 

Each quartz piezo sensor is 1.5 or 2 meters in length and can be 

combined in varying lengths to provide half-lane or full-lane 

width coverage. The quartz piezo WIM sensor is approximately 

2 inches wide, 2 inches thick, and weighs 12 to 20 pounds, 

depending on the length of the sensor.

The slot for the quartz piezo sensor is 2.75 inches wide and 2 

inches deep, as shown in Figure 11. The length of the excavation 

depends on the length of the sensor to be installed but is 

typically 6 feet or 12 feet, depending on the overall lane width. 
27

Figure 11 – Quartz Piezo Installation Detail

The preferred configuration, which provides better weight 

measurement accuracy, consists of two sets of two half-lane 

sensors, as shown in Figure 12. The sensors are staggered in the 

travel lane, each wheel path set spaced approximately 12 to 16 

feet apart. The preferred configuration shown in Figure 12

provides a double threshold. 

The loops may be placed in line across the roadway, or they 

may be staggered to allow for homerun cuts for both lanes to 

go to the same side of the roadway. With either configuration, 

the loops should be separated by at least 5 feet to avoid 

interference (crosstalk).
28

Figure 12 –2-Lane Unidirectional Staggered Quartz Piezo

Configuration 

Strain Gauge Strip Sensors 

Each strain gauge strip sensor is 59, 69, or 79 inches long and is 

installed in sets of 1 to 4 pairs (2 to 8 strip sensors) that can be 

combined to cover different road widths, with one pair covering 

the width of a single road lane. Each strain gauge WIM strip 

sensor is approximately 3 inches wide, 3 inches tall, and weighs 

45 to 65 pounds, depending on the length of the sensor and 

cable length. Sensors less than 3 inches tall have recently 

become available.

As shown in Figure 13, the slot for the strain gauge strip sensor 

is 3 inches wide and 3 inches deep. The length of the excavation 

depends on the length of the sensor to be installed but is 

typically 6 feet or 12 feet, depending on the overall lane width. 
29

 

Figure 13 – Strain Gauge Strip Sensor Installation Detail

Each sensor is installed to cover one-half of the travel lane. 

Working in pairs, the two sensors are installed in a staggered 

configuration in each lane, the left side staggered 2 feet in front 

of the right sensor and each set of sensors for each wheel path 

is placed 16 feet apart. The inductive loop is installed before the 

leading set of WIM sensors, as shown in Figure 14. 

Figure 14 – 2-Lane Unidirectional Staggered Strain Gauge Strip Sensor

Configuration 

The loops may be placed in line across the roadway, or they 

may be staggered to allow for homerun cuts for both lanes to 

go to the same side of the roadway. With either configuration, 
30

the loops should be separated by at least 5 feet to avoid 

crosstalk.

Polymer Piezo Sensor 

The polymer piezo sensor is produced in lengths from 6 feet up 

to 12 feet. The sensor is permanently installed into the slot with 

epoxy grout. The sensors are typically installed so that the ends 

align with the inside of the painted lane line, the center line, or 

both, depending on the configuration of the sensors.

The slot for the polymer piezo sensor is 0.75 inches wide, and 

the sensor should be placed in the slot so that there is at least 1 

inch of grout cover, and held in place with plastic chairs, as 

illustrated in Figure 15 . The length of the slot depends on the

length of the sensor to be installed but is typically 6 feet or 12 

feet, depending on the lane width. 

 

Figure 15 – Polymer Piezo Installation (not to scale)

Each polymer piezo sensor is typically installed for half-lane 

coverage, with four sensors installed in each lane, 2 inches

deep, with at least 1 inch of grout cover. Each set of sensors in 

each wheel path is spaced 16 feet apart. A chair is used to hold 

the sensor in place during installation. This provides a dual-
31

threshold capability. Figure 16 illustrates the layout. A single 

inductive loop is installed between the two sets of staggered 

WIM sensors to provide vehicle presence detection and vehicle 

length measurement. The sensors are placed in the travel lane 

perpendicular to the direction of travel. 

The loops may be placed in line across the roadway or 

staggered to allow for homerun cuts for both lanes to go to the 

same side of the roadway. With either configuration, the loops 

should be separated by at least 5 feet to avoid interference 

(crosstalk).

Figure 16 – 2-Lane Unidirectional Polymer Piezo 4-Sensor per Lane 

Configuration

An alternative to replacing sensors when pavement overlays 

occur is to leave the sensors in place. Polymer piezo sensors can 

be paved over with good long term results. Ohio Department of 

Transportation (DOT) and Virginia DOT used this strategy to 

increase service life for the sensors from 3-5 to 5-7 years. It 

requires coordination with the construction crew and use of the 

smaller aggregate size in the AC mix for the top pavement layer 

to ensure the transfer of the normal force from the tire 

footprint to the sensor that may be 1.5 to 2.5 inches below the 

surface. Also, the lane striping crew needs to know the offset 
32

from the pavement edge to properly place the line sensor and 

the loop (center of the lane). Buried sensors produce decreased 

voltages.

Portable Polymer piezo Sensor 

For portable weighing operations, sensors each roughly onelane width in length are either taped or fastened to the 

roadway, perpendicular to traffic. Normally, two full lane-width 

sensors are placed 10 to 16 feet apart in the same lane. 

Minnesota uses a 24 feet long portable WIM sensor to cover 

two lanes at a time that works in either unidirectional or 

bidirectional mode.

Figure 17 – 2-Lane Unidirectional Portable Polymer Piezo 2-Sensor per 

Lane Configuration

Cabinet Location

A cabinet rated National Electrical Manufacturer Association 

(NEMA) 3R should be located outside of the recovery or “safe 

zone,” behind a protective barrier such as a guardrail, or 

constructed using a breakaway base. It should also be located 

close enough to the roadway and the door installed so that the 

technician can easily view the passing traffic.

The cabinet foundation should not be placed in a low area that 

may allow for water intrusion or on grade with a slope greater 

than 3:1.
33

For sites that will utilize solar power, the cabinet should be 

placed in a location that will enable sufficient exposure to the 

sun. Wind power has also been used at some sites.

The cabinet should be clearly marked as an electrical device, 

with digging warnings and agency contact information.

Directional Boring

If required, directional boring may be installed between pull 

boxes that are placed on either side of the roadway. The bore 

should be placed at least 24 inches below the road surface, and

the conduit, shown in Figure 18, should be no smaller than 2

inches in outside diameter. It is preferred that the bore not be 

run underneath the location of the in-road sensors to avoid 

electronic interference. Conduit material is typically polyvinyl 

chloride (PVC). 

Figure 18 – Bore Operation Conduit

Pull Boxes

Pull boxes should be large enough to accommodate the sensor 

lead-in cables, service loops, and splices. Pull boxes that are 12

inches wide, 16 inches long, and 16 inches deep should suffice 

for most WIM system applications. They should be installed 

leveled with grade and should be located at least 5 feet from 
34

the roadway, close to the location where the in-road sensor 

lead-ins meet the edge of the roadway. Figure 19 shows the 

interior of a pull box, and Figure 20 shows the exterior. 

Figure 19 – Pull Box Interior

The pull boxes should be constructed of steel, concrete, or a 

polymer that would withstand being run over by a semi-tractor 

trailer. The tops of the pull boxes should be bolted down. They 

should be properly marked with the word “traffic.” 

Figure 20 – Pull Box Exterior
35

Data Acquisition (Communications)

Modems 

Modems capable of reliably and continuously transmitting data 

for the time required for data transfers should be installed in 

the cabinet. Consider if the site is powered by alternating 

current or direct current (A/C or DC), and make sure the 

electronics needed for the site use similar power sources.

The technician should program the modems prior to installation

so that they will be able to communicate with the WIM 

controller. For cellular modems, a subscriber identity module 

(SIM) card will be provided by the cellular service company. 

Other site-specific or network-specific settings will then need to 

be entered into the modem.

Telephone Service 

If landline service is utilized for site communication and data 

download, the service must be of high enough quality to allow 

for data transmission. Grounding for the communication drop 

location is needed for lightning protection. Figure 21 shows a 

landline phone service box.

Figure 21 – Landline Telephone Service Box
36

Fiber Optics 

Fiber optics (optical fibers) are long, thin strands of very pure 

glass about the size of a human hair. They are arranged in 

bundles called optical cables and used to transmit signals over 

long distances.

The option of using fiber optics for data transmission has 

several pros and cons. The advantages of using fiber optic 

communication is the very fast speed (10 Gbps), which means 

that very large data files can be downloaded in split seconds or 

video imagery from a site can be streamed real-time. 

Additionally, optical fibers do not conduct electricity, preventing 

problems with ground loops and conduction of lightning.

Conversely, the installation of fiber optics can be very expensive 

for remote sites, and transmission over fiber is limited by the 

attenuation and dispersion.

Cellular Service 

If cellular service is utilized for site communication and data 

download, the service must be of high enough strength to allow 

for uninterrupted data transmission. The cellular signal must be 

measured at the site during the preliminary site selection and 

should be greater than -80 db. Signal dropout can be expected 

at strengths less than -90 db. Figure 22 shows a cellular modem.

Figure 22 – Cellular Modem
37

On-Site Data Storage 

The WIM controller should be capable of storing at least 7 days

of data between downloads. This would allow sufficient time for 

a technician to respond to a service call if communication 

between the host computer and the WIM controller is lost.

System Power

Solar Power Devices 

All solar-powered sites require a combination of solar panels, a solar 

regulator, and batteries. The power circuit is designed so that the 

solar panels charge the batteries during the day above the power 

consumption of the devices drawing power from the circuit so that 

the batteries are fully recharged before sunset. These systems are 

designed to run indefinitely until the solar panel or battery service 

life ends. The wattage of the solar panels (sometimes multiple 

panels) and the number of batteries is determined by the total 

power consumption of the devices running on the power circuit and 

the latitude of the location. Figure 23 is an illustration of the various 

solar power system components.

Figure 23 – Solar Power System Components
38

Solar Panels

Solar panels must generate enough power to the site to handle 

the power needs of the WIM controller and any peripheral 

devices, such as the modem, a cabinet light, or a fan. Most WIM 

systems require at least one 80-watt solar panel. WIM and 

other equipment vendors can provide guidance on the power 

requirements for their equipment. 

The PV solar power supply shall feature a universal design 

suitable for a wide range of site conditions. The design shall 

account for degraded solar conditions and extended periods of 

weather related to solar blockage without having any power 

interruptions. The PV solar wattage to load wattage ratio will be 

sized to account for degraded conditions. System design shall 

provide for a reserve capacity for the battery for at least 7 days

while maintaining normal operations. 

In regions prone to localized solar blockage from weatherrelated conditions such as fog and clouds, and site-specific 

locations where shading create conditions with less than 1 solar 

energy hour per day, this design is not appropriate.

Solar panels, like the one shown in Figure 24, should be installed 

on the cabinet service pole at a height that will prevent theft 

and allow for sufficient sun exposure. The solar panels should 

be installed so that they are facing south and at angle related to 

the latitude of the site.

Figure 24 – Solar Panel (80-Watt)
39

Solar Regulator

A solar regulator (see Figure 25) is part of the solar charging 

circuit. It receives its input from the solar panel and provides a 

regulated (controlled) DC output to the battery to charge it. It 

ensures that the battery is not overcharged and may provide 

surge protection for the charging circuit.

Figure 25 – Solar Regulator

To determine the size regulator required, note the amp ratings 

of the solar panels and add those ratings to determine the 

minimum size charge controller needed.

Battery

The battery (see Figure 26) supplies DC power to the WIM 

system devices, including the WIM controller, modem, fan, and 

heating element. The capacity of the battery is measured in 

amp hours. The size of the battery is determined by adding the 

amperage for all of the equipment that will be run from the 

battery. The battery should be able to provide continuous 

power, without charge, for at least 7 days.
40

Figure 26 – Battery

A/C Service Devices 

A/C devices, such as circuit breakers, switches, terminals, and 

fuses should be installed in the cabinet to provide 120 volts of 

power switching and protection from power surges such as 

lightning. They should be suited for outdoor use and be securely 

mounted to the cabinet in a separate enclosure, such as a 

circuit breaker box, to prevent electric shock. Figure 27 shows 

an outdoor A/C power service meter mount. 

Figure 27 – A/C Power Service Meter
41

Fans and Environmental Controls 

Although WIM controllers are designed to withstand very cold 

and very warm temperatures, in places of extreme 

temperatures, air conditioning devices such as fans, heaters, or 

the thermostat shown in Figure 28 may need to be installed to 

ensure that the system does not get overheated or freeze.

Figure 28 – Thermostat

Lightning Protection and Grounding

Surge Suppression 

Direct lightning strikes almost always cause costly damage. 

Nearby lightning strikes can also result in costly damage if surge 

prevention devices are not properly installed. Primary surge 

suppression devices for the loops and WIM sensors are typically 

built into the signal processing cards. However, secondary 

protection from lightning strikes is available and is

recommended. These are typically surge arrestors like the one 

shown in Figure 29, installed at the terminal strips where the inroad sensor lead-ins are connected. 

Additionally, all power service lines and telephone lines should 

be protected from lighting and other sudden power surges 

using surge suppression devices and have proper grounding to a 
42

grounding rod. Equipment should be isolated from ground using 

devices such as gas-filled diodes to prevent surges from causing 

significant damage. 

Figure 29 – Sensor and Telephone Surge Arrestor Devices

Grounding 

Grounding is used to shunt power surges to earth. The 

grounding requirement for the system is a function of the 

device on the circuit having the most conservative requirement. 

A ground resistance requirement of 10 ohms or less is 

recommended. This may be met at most sites with a single 8-

foot ground rod installed in the cabinet foundation or a nearby 

pull box, as shown in Figure 30. Other sites may require multiple 

ground rods to be installed on top of one another, or in an 

array. 

The number of ground rods required is a function of the soil 

composition/water content of the earth being grounding in. 

Additional ground rods are added until the circuit has the 

required resistance. For a grounding array, each ground rod is 

separated by 50 feet. 

The ground rod or array is connected to the ground terminal 

panel in the cabinet using copper wire. Ground connections for 

all electronic devices in the cabinet are then connected to the 

ground terminal, as shown in Figure 30. 
43

The ground resistance must be checked annually as it can 

change and more grounding may be needed.

Figure 30 – Grounding Rod with CadweldTM

WIM Site Plan

A WIM site plan should be developed to document the proper 

position and distances related to the road and WIM system

components. It should include the following system 

components: 

• In-road sensors

• Junction boxes

• Pull boxes

• Cabinet

• Power and phone sources

• Ground rod(s) 

• Solar panels

• Conduit

• Bore 

• Battery backup for power protection and temporary 

power

• Lightning protection panel or devices

• Other features such as guardrail
44

The site plan should include information such as road names, 

mile markers, GPS coordinates, speed limit, and cardinal 

direction. All special or non-typical installation notes should be 

provided on the sketch.

A typical WIM site design sketch and cabinet wiring diagram are 

provided in Appendix D. 

Standard and Manufacturer Specifications 

Examples of several standard WIM specifications are provided 

in Appendices C and D. They provide guidance on:

• Cabinet location

• Boring

• Pull boxes and junction boxes

• Solar panels

• Pedestals

• Cabinet foundations

• Conduit

• Grounding rod and bonding wire/joints

• Battery backup (larger capacity need if solar or wind 

powered)

• Loop wire

• Loop lead-in wire

• Splices

Reference manufacturer installation standards and 

specifications for guidance on:

• Sensor layout, sensor arrays, dimensions, depth and 

configuration (including drawings). 

• Data acquisition (communications). 

• Power requirements.

• Lightning protection. 

• Memory needed for 15 days (minimum) to 30 days 

(best) of storage, based on vehicle volumes at the site.
45

WIM SITE INSTALLATION 

The objective of the installation process is to provide a properly 

functioning WIM site for the design life. The proper installation 

of the WIM equipment is critical for collecting high-quality data. 

WIM sensors improperly installed in even the smoothest 

pavement will most assuredly provide unusable WIM data. 

General Installation Requirements 

ASTM E1318-09 requires that the WIM equipment be installed 

and maintained in accordance with the recommendations of the 

WIM system vendors. Therefore, this guide does not provide 

instructions for installing the vendor-specific WIM system

components. The agency or inspector should contact the 

manufacturer to acquire the most recent installation manual. 

The following generic installation requirements must be 

followed for any type of WIM system or sensor being installed:

• The installation should only be performed in good 

weather, not wet, freezing, or extremely hot (above 100

°F) conditions.

• The equipment must be protected from lightning and 

power surges.

• The equipment cabinet must protect the system 

electronics from vandalism, extreme temperature, dust, 

water, humidity, and insect or rodent infestation.

• The equipment must be installed so that routine 

maintenance can occur without disruption of data 

collection. 

• Installation start date and time to complete installation 

work should be established and any press release 

information provided to the agency in advance of the 

field work.
46

• A vendor representative should be on-site to ensure 

that vendor requirements are met.

• An agent representing the contracting agency and 

experienced with WIM installations—the quality 

assurance (QA) inspector—should be on-site to ensure 

that agency requirements are met.

• Pre-construction meetings should be held before lane 

closures are set up to discuss safety and the work 

planned for the day. 

• Complete site plans should be available on-site to 

ensure proper placement of equipment.

• All installations should be performed in accordance with 

the WIM manufacturer’s specifications and any 

additional contracting agency requirements.

• Due to warranty liabilities, the installation must be 

carried out by persons who have successfully completed 

a vendor training course and hold a valid vendor 

certificate authorizing them to install the sensors. 

• An accurate set of as-built plans should be developed 

after installation for future construction and 

maintenance work.

• The work is considered to be complete when all system 

components have been tested and accepted by the 

contracting agency and data verification testing has 

been completed.

Installation QA Inspector

The QA inspector represents or acts on 

behalf of the contracting agency and 

should have the authority of the 

agency. The QA inspector should be onsite to ensure that all elements of the 

WIM installation are performed in 

accordance with the contracting 

agency’s requirements. 
47

The QA inspector should be experienced with WIM installations

and have proper certifications or training on the particular type 

of WIM system being installed. The QA inspector is required to 

have a digital or hard copy of all applicable manufacturer 

installation standards and specifications on-hand. 

The QA inspector should document and photograph all stages of 

WIM site installation and prepare the WIM installation 

acceptance report and non-compliance report, as described in 

the following sections. The LTPP WIM System Installation and 

Calibration Audit can be found in the LTPP Field Operations 

Guide for SPS WIM Sites (4). 

Pre-Construction Meeting

Pre-construction meeting(s) of all personnel involved in the 

WIM system installation should be held prior to the beginning 

of work. Attendees may include:

• Contracting agency or their representative (QA 

inspector). 

• Contractor’s on-site superintendent. 

• MOT contractor’s on-site leader. 

• Local road maintenance supervisor. 

• WIM system manufacturer’s representative. 

Topics should include safety, lane closure time limits, work 

schedule, and a detailed discussion of the work to be 

performed. Contact information should be exchanged. The QA 

inspector should review all requirements related to the work to 

be performed. All equipment and materials should be checked 

for acceptable physical condition at that time.

Site Preparation

The following recommended procedures are not meant to 

supersede any established agency or manufacturer installation 

specifications. The QA inspector and the installation supervisor 

should review and be familiar with all contract, state, local and 
48

manufacturer installation standards and specifications for 

specific guidance and installation instructions prior to the start 

of any construction work.

Protection of Utilities 

Before the installation of any support equipment, such as 

cabinets, conduit, grounding rods/wires, pull boxes, or electrical 

service or telephone devices, the QA inspector and the 

installation supervisor must examine all existing utility markings 

and verify that the location of utilities within the work area has 

been performed within the past 2 weeks. Figure 31 shows a site 

where the utility locations are clearly marked.

Figure 31 – Utility Location Markings

Equipment Inspection 

Prior to installation, the QA inspector should inspect all 

equipment, and the installation supervisor should electronically 

test the equipment to verify that it is new, undamaged, and for 

electrical equipment, Underwriters Laboratories (UL) listed. 

Maintenance of Traffic

The contractor must develop the traffic control plan based on 

the MUTCD and any supplemental work zone guide required by 

the agency, and the plan must be approved by the agency

before work begins (6). The QA inspector should review the plan 

and bring any deviation from the specifications and standards to 
49

the attention of the contractor’s on-site supervisor for 

immediate remediation. The QA inspector should have the 

authority to halt work until proper remediation to traffic control 

is completed, or if they feel that the modifications are not 

meeting the needs of the road users. All portable changeable 

message signs (PCMS), such as the one shown in Figure 32, and 

arrow boards should be inspected to ensure that they are 

properly placed, fully illuminated and completely visible to the 

traveling public.

Figure 32 – Portable Changeable Message Sign Used for Traffic Control

WIM Site Installation 

Quality Assurance

This section describes installation QA 

activities and QA checks to ensure that 

the WIM system meets design life 

expectations. 

Excavation and Conduit 

The QA inspector must ensure that all excavation and 

installation of conduit used for the WIM installation is of the 

proper depth, type, and dimension, as stated in the design 
50

standards. The cabinet and junction boxes should be staked, 

conduit runs marked, and the outer limits of the array area 

marked on the edge of pavement. The QA inspector will use 

these to verify they will be installed in the correct locations 

prior to the start of any work. Adjustments may be needed and 

should be addressed prior to the start of work. 

In general, site work on the side of the road is performed first so 

that when the sensors are installed in the road the homeruns 

can be run to the cabinet right away and are not at risk of being 

damaged if site work is done after sensor installation.

Separate conduits should be run from each loop and WIM 

sensor exit hole to the nearest pull box, as shown in Figure 33. 

Figure 33 – Sensor Conduits from Exit Hole to Pull Box

The QA inspector must also ensure that the installation crew 

does not remove any more pavement material than is necessary 

and repairs any pavement damaged during the installation 

process. The QA inspector must perform the following:

 For the excavation and installation of conduit, observe 

the following to ensure that:
51

• Only material necessary for the installation is 

removed. 

• Excessive pavement damage is repaired with like 

material or epoxy.

• Homerun conduits are installed at the proper 

depth.

• Locator tape is installed on all conduit runs.

• Sensor exit conduits should be driven deep enough 

into the pavement (about 3 to 4 inch depth) so that 

the home run wires are not susceptible to being 

ground off when a mill and overlay is done.

• All conduit end points have smooth edges.

• All conduit used within the travel lane is galvanized 

rigid metal conduit. 

• Conduit openings are sealed with duct seal, as 

shown in Figure 34. 

• Drain conduits used for the bending plate or load 

cell sensors are lower than the electrical conduit 

openings.

• Proper size of PVC conduit is used from the pull box 

to the cabinet and base.

Figure 34 – Pull Box Conduit Sealed with Duct Seal
52

 Record findings on the QA checklist, and take 

photographs as necessary.

 If any of the above conditions are not met, inform the 

installation supervisor of the deficiency and ensure that 

the deficiency is acceptably resolved.

Pole/Cabinet 

The QA inspector shall inspect the pole and cabinet equipment 

to verify that they meet contract specifications. 

 Ensure that the pole and cabinet are placed where: 

• They are not subject to rain runoff, standing or 

moving water. 

• They will not be hit by moving vehicle, preferably 

behind a guardrail. 

• The grounding rod and/or 4- to 6-gauge grounding 

wire and bonding joint can be installed.

• They are accessible with clear line of sight to the 

sensors. 

• They are serviceable without technician 

endangerment.

• The cabinet door can be fully opened.

 Inspect the cabinet attributes to ensure that they meet 

contract specifications, which may include:

• Proper height and level.

• Conduit is securely and properly attached to the 

cabinet. 

• Hinged door with lock and two set of keys.

• Louvered vent with standard filter.

• Air exhaust through roof overhang with 

temperature activated forced air fan system.

• Wiring and equipment layout.

• Switched light.

• Ground-fault circuit interrupter (GFCI) duplex outlet 

and a circuit breaker for A/C input.

• Surge suppression.
53

 Inspect the pole attributes to ensure that it meets 

contract specifications, which may include:

• The pole is installed vertically. 

• The pole foundation is installed properly. 

• Break away pole mounting whenever possible.

 Record on the QA checklist that each of the items above 

is acceptable.

 If any of the above specifications are not met, inform 

the installation supervisor of the deficiency and ensure 

that the deficiency is acceptably resolved. Do not allow 

the installation contractor to use unapproved cabinet 

equipment without approval from the agency. 

Solar Panels 

The QA inspector must ensure that the proper solar panels and 

mounting equipment are installed.

 During the installation of the solar panels, the QA 

inspector should verify that:

• The panels are facing the proper direction (8° west 

of magnetic south).

• The panels are installed at the proper angle 

(latitude +15°). This pitch may vary by the state this 

is installed in. There are cell phone apps that allow 

measuring this angle from the ground using cell 

phone so if the panel is mounted beyond reach it is 

easily checked. Figure 35 shows an inspector 

checking the angle of the solar panel.

• All vegetation that may block exposure to the sun 

has been removed from in front of the solar panel.

• Panel height is high enough (16 to 18 feet) to thwart 

vandalism or theft. 

• Fastening hardware may include bolts and 

hardware that has torx or safety torx head to 

prevent stealing. 

 Record on the QA checklist whether each of the items 

above is acceptable.
54

 If any of the above specifications are not met, inform 

the installation supervisor of the deficiency and ensure 

that the deficiency is acceptably resolved. Do not allow 

the installation contractor to use unapproved solar 

panels or mounting hardware without approval from 

the agency. Theft-proof hardware can be utilized if theft 

could potentially be an issue, based on history or 

location.

Figure 35 – Solar Panel Angle Check

Solar Panel Charge Circuit Devices 

The QA inspector must ensure that the solar panel charge 

circuit devices have been verified and tested. 

 During the installation, verify that:

• Batteries are the correct type and have the 

specified rating for amp hours.

• Proper solar controller has been provided.

• Wiring of solar panels, solar controller and 

batteries.

• Settings on the solar controller.

 Record on the QA checklist whether each of the items 

above is acceptable.
55

Pull Boxes 

The QA inspector must ensure that properly sized and trafficrated pull boxes are installed at the locations shown on the 

WIM design plan and away from water collection areas. 

 During the installation of the pull boxes, verify that:

• The lid is flush with adjacent grade or surfaces.

• Adequate drainage was installed (see Figure 36). 

• If required from WIM vendor, grounding rod is 

installed. Earth ground test with megger should be 

performed to ensure proper grounding to the 

cabinet, power drop, and communication drop.

Figure 36 – Drainage Gravel Depth Check

 Record on the QA checklist that each of the items above 

is acceptable.

 If any of the above specifications are not met, inform 

the installation supervisor of the deficiency and ensure 

that the deficiency is acceptably resolved. Do not allow 

the installation contractor to use unapproved pull boxes 

without approval from the agency. 

Layout Marking and Cutting 

To ensure that the WIM sensors and loops are placed properly 

in the travel lane as shown on the WIM site design plans and in 

the manufacturer’s installation guide, the QA inspector must 

perform the following checks:
56

 Measure with a 100-foot tape measure and compare 

with installation documents to verify the following:

• Square lines and markings are correct and match 

construction plan and specification tolerances. 

• Proper spacing between array elements (WIM 

sensors, inductance loops, axle sensors). Figure 37 

shows measuring between loops.

Figure 37 – Loop Separation Measurement

 Observe the following to ensure that:

• The WIM sensor array layout is squared in the lane 

per the 6-8-10 triangle method or by using a premade template. Figure 38 shows a reference line 

marked on the pavement.
57

Figure 38 – Reference Line

• All saw cuts are neat and straight.

• Saw cuts are at the proper width and depth (see 

Figure 39): 

o Loop slot depths measured every 2 feet and at 

corners. 

o WIM sensor channels measured at both ends 

and in the middle of each perimeter cut. This 

may be done using a custom depth gauge.

Figure 39 – Slot Depth Measurement
58

• If preferred, the corners of the loop may be drilled 

to the depth of the slots using a 1- to 1.5-inch rock 

drill bit.

 During the saw-cutting process, ensure that the saw 

cuts have been cleaned and dried with water and/or 

clean high pressure air, and that all residue and 

moisture is removed for proper epoxy adhesion.

 If any of the above conditions are not met, inform the 

installation supervisor of the deficiency and ensure that 

the deficiency is acceptably resolved.

 Photograph entire site layout in lane with all markings 

visible.

Loop Sensor Installation and Sealing (General) 

The most critical element for WIM site installations is ensuring 

that the in-road loop sensors are installed in accordance with 

the contract specifications. This includes not only includes the 

proper placement (spacing, layout, depth, etc.) but also the 

proper mixing and application of the chemicals that are used to 

install the loop sensors. 

 To ensure that the loop sensors are installed in 

accordance with the contract specifications, check the 

following:

a. Loop homerun cuts are wide enough to allow for 

complete encapsulation by the loop sealant for a 

minimum cover of 3 inches. 

b. Loop wires are installed without the use of sharp 

implements and then secured with backer rod.

c. Loop wires are in bottom of the slot, wrapped 3 to 4 

times per foot, depending on the operational 

requirements of the WIM controller’s loop 

detection circuit. 

d. Loop wires run to the pull box without any splicing 

and are twisted around each other 3 to 4 times per 

foot. 

e. Each loop wire pair is clearly labeled prior to pulling 

through the conduit.
59

f. Loop wires are completely covered with sealant and 

the sealant surface is flush with the pavement, with 

no voids or dips.

g. An earth ground/insulation test is performed before 

pulling the lane closure to ensure the loops are not 

compromised in any way.

h. Inductance test is performed to ensure the proper 

range of henries is obtained for each loop based on 

the loop size and distance to the pull box.

 Videotape the loop sensor installation to have the visual 

evidence in case the sensors do not work properly after 

installation.

 Record on the QA checklist that each of the items above 

is acceptable.

 If any of the above conditions are not met, inform the 

installation supervisor of the deficiency and ensure that 

the deficiency is acceptably resolved. Do not allow the 

installation contractor to install the WIM sensors 

incorrectly.

Load Cell Sensor Installation 

The QA inspector should be experienced with the load cell 

installation process. The checks specific to the installation of the 

load cell WIM sensor include:

 During the installation of the load cell WIM sensors, 

verify that:

a. Load cell sensor insulation resistance is pre-checked 

prior to installation.

b. The load cell pit is properly installed.

c. Only vendor-specified installation grout is used.

d. Load cell sensors are installed to be as flush with 

the pavement surface as possible (within 1/8 inch). 

e. All voids in pavement around sensors are filled in 

with epoxy and/or caulk. All high spots in epoxy are 

ground flush.
60

f. All sensor lead-in wires exit the pavement through 

conduit and run all of the way to the pull box

without splices. 

 Record on the QA checklist that each of the items above 

is acceptable.

Bending Plate Sensor Installation 

The QA inspector should be experienced with the bending plate 

installation process. The checks specific to the installation of the 

bending plate WIM sensor include:

 During the installation of the bending plate WIM 

sensors, verify that:

a. Frame excavation, as shown in Figure 40, should be 

deep enough to allow enough clearance below the 

frame and around conduits for epoxy. 

Figure 40 – Excavated Bending Plate Pit (In-Line Configuration)

b. Bending plate sensor shield and Wheatstone bridge 

resistances are pre-checked prior to installation.

c. Only vendor-specified installation grout is used.
61

d. Bending plate sensors are installed (shimmed) to be 

as flush with pavement surface as possible (within 

1/16 inch). 

e. All voids in pavement around the frame are filled in 

with epoxy and/or caulk. All high spots in epoxy are 

ground flush as shown Figure 41.

Figure 41 – Completed Bending Plate Installation

f. All sensor lead-in wires exit pavement through 

frame conduit all the way to the pull box without 

severe bends. 

 Record on the QA checklist that each of the items above 

is acceptable.

Quartz Piezo and Strain Gauge Strip Sensor 

Installation 

The QA inspector should be experienced with the applicable 

strip sensor or quartz piezo installation process. Figure 42 shows 

a completed quartz piezo installation. The checks specific to the 

installation of the quartz piezo WIM sensor include:
62

 During the installation of the quartz piezo and strain 

gauge strip sensors, verify that:

a. Axle sensor channels are cut to the proper depth 

and width, based on the type of sensor. 

b. Sensor insulation resistance is pre-checked in the 

cabinet prior to installation.

c. Only vendor-specified installation grout is used.

d. WIM sensors are installed in accordance with 

manufacturer specifications.

e. Each sensor is properly connected to the ground 

lead.

f. Sensors are ground flush with pavement.

g. All voids in pavement around sensors are filled in 

with epoxy and/or caulk. All epoxy high spots are

ground flush to the surface and there are no bumps 

or dips in the surface greater than 1/8 inch. 

Figure 42 – Completed Quartz Piezo Installation
63

h. All sensor lead-in wires exit the pavement through 

conduit and run all of the way to the pull box

without splices.

 Record on the QA checklist that each of the items above 

is acceptable.

Polymer Piezo Sensor Installation 

The QA inspector should be experienced with the polymer piezo

installation process. The checks specific to the installation of the 

polymer piezo WIM sensor include:

 During the installation of the polymer piezo WIM 

sensors, verify that:

a. The sensor are not dented, twisted, or bent. 

b. The sensors are pretested for proper capacitance 

and resistance using a multi-meter and/or 

capacitance meter, as shown in Figure 43. 

Figure 43 – Pre-Installation Sensor Testing

c. Sensors are placed in accordance with the site 

layout plan.

d. Sensor channels are cut 3⁄4 inches wide and at least 2

inches deep. Saw cuts made in a single pass with a 

3⁄4-inch blade or two 3/8-inch blades ganged 
64

together are recommended. Check the slot depth as 

shown in Figure 44.

Figure 44 – Sensor Slot Depth Check

e. Pavement removal from the slots was done with 

care to ensure proper slot width and depth.

f. Slots were cleaned and dried before sensor 

placement (see Figure 45). 

Figure 45 – Piezo Slot Clean and Dry Check
65

g. W-chairs are properly attached to the sensor, a 

minimum of 6 inches apart. 

h. Completely clean the sensor no more than 20 

minutes from installation to allow for proper 

bonding of the sensor to the epoxy.

i. The sensor must be checked along its entire length 

to verify it is being installed at the proper depth.

The W-chairs must be adjusted within the slot to 

get the proper depth. Figure 46 shows a completed 

polymer piezo sensor installation.

Figure 46 – Completed Polymer Piezo Sensor Installation 

j. The sensor is post-tested to ensure proper 

capacitance and resistance readings. 

k. The sensor output signal is tested for proper voltage 

output. 

 Record on the QA checklist that each of the items above 

is acceptable.

Cabling 

During the installation of the sensor cabling from the side of the 

pavement, through pull boxes, and into the cabinet, the QA 
66

inspector must ensure that the cables are pulled carefully as to 

avoid damage to the external protection.

 During the cable installation, ensure that (see Figure 

47): 

a. All cabling is clearly labeled in each pull box and in 

the cabinet.

b. Service loops such that at least 5 extra feet of wire 

is left in each pull box and in the cabinet for repairs.

Figure 47 – Sensor Lead Labeling and Service Loops in Pull Box

c. Splices are industrial quality, watertight, 

hermetically sealed, and made with bifurcated 

terminals to prevent shorting.

d. Check each loop for proper inductance range is met 

before putting the splice kit on it.

e. Splices, if used, are used in the closest pull box to 

the sensor.

f. Each sensor is tested before final terminations are 

made.

 Record on the QA checklist that each of the items above 

is acceptable.

 If any of the above specifications are not met, inform 

the installation supervisor of the deficiency immediately 
67

to ensure that the WIM sensor cables are not damaged 

during installation. Do not allow the installation 

contractor to pull on cables that appear to be stuck or 

difficult to pull. Damaged lead-in cables may require an 

expensive replacement.

Grounding 

The QA inspector should observe the ground reading taken at 

the cabinet terminal strip after all the grounding components 

are installed and connected, i.e. all ground rods installed, with

grounding clamps or exothermic welds and grounding wire run 

to the cabinet. The ground should be tested using an approved 

method. A suggested method includes the following steps: 

 Attach the Earth Resistance Meter (megger) to the 

ground wire that is in the cabinet.

 Place the megger where the primary grounding rod is 

installed.

 Ensure that an earth ground test is performed (150-foot

minimum distance) to make sure the value meets 

contract specifications, which typically call for a value of 

less than 10 ohms. Sometimes driving another rod and 

bonding them together is necessary in rocky areas.

 Verify the meter reads a resistance that is within 

specification.

 If the reading is higher than acceptable, notify the onsite supervisor.

Cabinet Equipment 

Once the equipment has been installed, the technician has 

wired the sensors to the terminal strip, as shown in Figure 48, 

and the technician has set up the WIM system in accordance 

with the manufacturer’s specifications, the QA inspector must 

ensure that the system is operating properly by doing the 

following:

 Reference the manufacturer’s WIM user or operations 

guide for instructions to verify the proper operation of 

the WIM system.
68

 Ensure the WIM system is correctly classifying vehicles, 

reporting accurate axle spacings and axle or by-wheel 

path weights.

 Ensure that the system is properly collecting, storing, 

and transmitting data to the host computer.

 Record on the QA checklist that each of the items above 

is acceptable.

Figure 48 – Terminal Panel Labels and Connections

Initial Calibration

Calibration is the process of evaluating 

the measured weight and distance 

values reported by the WIM system 

against the known weights and axle 

spacings and making necessary 

adjustments to the WIM system 

operating parameters to compensate 

for those errors. Calibration is performed to ensure that the 

data accuracy remains consistent and within the performance 

specification. As a result of the calibration, the mean error in 

WIM measurements should be reduced to as close to zero as 

possible. 
69

Since the WIM vendor or installer is usually obligated to 

perform the initial calibration of the system under the 

installation contract, it is recommended that the acceptance 

test criteria in section 6.4 of ASTM E1318-09 be included in the 

installation contract. The ASTM E1318-09 performance 

requirements were provided earlier in this document, in Table 

1. In addition, it is important to have vehicle length measured 

within ±1.5 feet or within ±3 percent for vehicles greater than 

50 feet long, based on LTPP specifications (4). 

It is extremely important that the ASTM E1318-09 specification 

be followed closely for the initial calibration to ensure that the 

WIM system is providing the highest quality data possible. 

Other approved calibration methods may be used, if approved 

by the agency that initiated WIM installation. 

Typically, the initial calibration is performed after a 2-week 

“burn-in” and after all initial “best guess” system settings have 

been entered. This allows the in-road sensor installation to 

“settle,” the grout to fully cure, and the technician to determine 

if the system is operating as expected by analyzing a two-week 

data sample collected after the initial system settings were 

installed. 

Just prior to the initial calibration, the QA Inspector should 

visually inspect each WIM system component and ensure that a 

complete electrical and operational test of the WIM system 

components are conducted: 

 Inspect the in-road sensors from the shoulder.

 Audit the controller setup for proper settings.

 Check all cable connections.

 Test all sensor outputs.

 Observe passing vehicles in real time to verify 

reasonable weight, speed, and spacing measurement.

Critical items that the QA inspector should observe during the 

initial calibration include:
70

1. Measurement of Test Trucks

a. A loaded FHWA class 9 truck or other heavy truck 

type that is most frequently observed at the site 

should be used. FHWA class 5 trucks are too light to 

be used as calibration vehicles, unless the WIM 

system is installed on a load-restricted road.

b. Certified scales are used to obtain static weights, 

axle spacing, and overall length of test trucks. 

c. Truck tires and suspension are inspected for 

defects.

2. Pre-Calibration Test Runs

a. The truck does not accelerate or decelerate in the 

WIM scale area and travels down the center of the 

travel lane.

b. The test truck runs are conducted at the prevalent 

truck speeds and at the widest possible ranges of 

temperatures observed at the site. 

c. A sufficient number of pre-calibration runs are

performed to quantify WIM measurement error 

percent: at least 10 test runs per truck, per lane.

3. Test Truck Run Data Analysis to Evaluate WIM 

Performance before Calibration 

a. For each test vehicle pass, the percent difference in 

the WIM-measured value and the corresponding 

reference value (i.e. static measurement) for each 

parameter listed in Table 1 Is calculated. 

b. Using all passes of the test vehicles over the 

sensors, the number of calculated differences that 

exceeded the tolerance value shown in Table 1 for 

each data item is determined and expressed as a 

percent of the total number of observed values of 

this item (i.e. percent of all test measurements for a 

given data item (example: Gross Vehicle Weight) 

that exceed the tolerance).

c. If any specified WIM system function failed, or if 

more than 5 percent of the calculated differences 

for any applicable data item exceed the specified 
71

tolerance for that item from Table 1, declare the 

WIM system as failed.

4. System Calibration

a. Based on the WIM system measurement error 

calculated during Pre-Calibration runs, adjustments 

to the current WIM system weight and distance 

error compensation factors is calculated. 

b. New compensation factors are entered into the 

WIM system firmware for Post-Calibration testing.

5. Post-Calibration Test Runs and Evaluation

a. Actions described in Step 3 are repeated to evaluate 

if WIM system met or failed ASTM E1318-09 

tolerance requirements for 95 percent compliance

after new compensation factors are installed. 20 or 

more post-calibration test truck runs is 

recommended. The number of runs may be reduced 

to 10 if the system demonstrates the following:

• The speed dependency at each speed point has 

been eliminated or minimized (less than 2 

percent average error for the each speed point). 

• The overall system bias computed based on the 

data from the available test runs is as close to 

zero as possible, (less than 2 percent error for 

the mean error).

• None of the individual runs are outside of the 

ASTM E1318-09 tolerances for 95 percent 

compliance. 

b. Calibration process described in previous steps may 

be repeated until WIM system performance 

parameters are within tolerances specified in Table 

1, up to 3 times if needed. 

c. The documented system settings and final 

calibration results should be stored in the cabinet. 

d. Save WIM data collected 2 to 4 weeks immediately 

after calibration to develop the comparison data set 

for future data quality control (QC). 
72

Vehicle Classification Accuracy Validation 

To validate accuracy of vehicle classification, use 

the following steps: 

1. Obtain the agency’s vehicle classification

scheme or class tree to use for manual

classification of vehicles.

2. Perform the vehicle classification testing during peak

hours. Peak hours could be identified from the hourly

truck volume data sample during pre-visit data analysis.

3. Collect a manual sample of at least 100 vehicles in

FHWA vehicle classes 4 through 13 (using FHWA 13-bin

classification), or as many samples as can be collected in

3 hours. In addition, collecting video of the traffic

stream during manual study would be helpful for the

follow-up analysis of misclassified vehicles.

4. During the manual data collection, record the vehicle ID

and time stamp from the WIM records for any vehicle

that does not match the classification from the manual

observation.

5. Evaluate results using Method A or B.

• Method A:

a. Determine the total number of vehicle samples

and the total number of misclassified vehicles in

the manual sample for FHWA vehicle classes 4

through 13.

b. Calculate the percent of misclassified vehicles in

FHWA vehicle classes 4 through 13 in the

manual sample. Compare misclassification

percent for the sample with the agency’s

acceptable misclassification percent. The

percentage of misclassified vehicles should be

no greater than 2 percent.

c. Calculate the number and percentage of

vehicles in FHWA vehicle classes 4 through 13 in

the manual sample that were not classified by
73

the WIM system. The percentage of unclassified 

vehicles should be no greater than 2 percent. 

d. If fewer than 50 vehicles in classes 4 to 13 were 

observed in 3 hour period, record the number 

of vehicles that passed and failed classification 

and report that the minimum sample size to 

test classification was not met. In this case, 1 

unclassified or misclassified vehicle can be 

allowed for classification test to pass.

• Method B:

a. Calculate the percent of misclassified or 

unclassified vehicles in FHWA vehicle classes 4 

through 13 in the manual sample. 

b. To pass the test, each individual class of 

vehicles from 4 through 13 may have a 

maximum 10 percent of misclassified vehicles. 

c. If the number of vehicles collected during the 3 

hour testing period is under 30 for any vehicle 

class, use the following tolerance criteria for the 

maximum number of misclassified vehicles:

o 20-29 for one class = +/-3 vehicles

o 10-19 for one class = +/-2 vehicles

o 0-9 for one class = +/-1 vehicle

6. Draw conclusions about whether the WIM system 

passed or failed the classification test. If the WIM 

system failed the classification test, analyze each 

misclassification instance by comparing individual 

vehicle records from the WIM output with the video 

images of moving trucks collected during manual study. 

Draw conclusion if changes to the WIM system’s 

classification table are needed or if there are issues with 

the agency vehicle classification tree inputs. 
74

WIM Site Acceptance and Reporting

Once all of the installation and initial 

calibration procedures have been 

completed, the QA inspector shall 

review the QA checklists with the 

installation supervisor to address any 

remaining “punch list” deficiencies 

that may still need to be addressed. 

Example QA checklists are available in 

the LTPP Field Operations Guide for SPS WIM Sites (4). 

Once all deficiencies have been addressed, the QA inspector

should report about the completion of WIM site installation to 

the WIM program manager. The final acceptance should be 

reserved until the site has worked without any failures for 30 

days. At the end of the successful 30-day testing period, the 

WIM program manager will accept the WIM site installation and 

verification as complete. Warranty periods should not 

commence until after the site has been fully accepted.

WIM Site Acceptance Report 

After the WIM system installation has been completed, 

including calibration and acceptance testing, the QA inspector 

shall prepare a WIM system installation inspection report 

detailing all facets of the installation, including: 

• Conduit and supporting structures installation. 

• Sensor installation. 

• Sensor electronic test readings. 

• Support services electronic tests. 

• Operational verification test results. 

• Initial calibration report. 

• Photographs. 

• Non-compliance reports. 

• As-built plans for future construction and maintenance 

work reference. 

• Any approved exceptions. 
75

The documented system settings and final calibration results 

should be stored in the cabinet. 

WIM Site Non-Compliance Report 

A Non-Compliance Report (NCR) template should be developed

for routine use by QA Inspectors. The QA Inspector should use 

NCR template to document any refusals by the installation 

contractor to comply with the directions given by the QA 

inspector to resolve deficiencies. The report shall be signed by 

the contractor (installation supervisor) and immediately 

forwarded to the WIM program manager or other agency 

authority responsible for the installation project. An example of 

a non-compliance report is provided in Appendix D. 
76

REFERENCES

1. ASTM Standard E1318-09, Standard Specification for

Highway Weigh-in-Motion (WIM) Systems with User

Requirements and Test Methods. West Conshohocken,

PA: ASTM International, West Conshohocken, PA, 2009,

DOI: 10.1520/E1318-09, www.astm.org.

2. European Cooperation in Science and Technology

(COST): Action 323. 1998. Weigh-in-Motion of Road

Vehicles. Paris: 2002.

3. NMi International WIM Standard: Specification and Test

Procedures for Weigh-in-Motion Systems, NMi Certin,

Dordrecht, Netherlands, 2016. http://www.nmi.nl/nmiwim-standard/

4. Federal Highway Administration (FHWA). 2012. LTPP

Field Operations Guide for SPS WIM Sites Version 1.0.

Draft. McLean, VA: Federal Highway Administration:

2012.

5. American Association of State Highway and

Transportation Officials (AASHTO). 2016. M331-13.

Standard Specification for Smoothness of Pavement in

Weigh-in-Motion Systems. Washington, D.C. 2016.

6. Federal Highway Administration (FHWA). 2012. Manual

on Uniform Traffic Control Devices for Streets and

Highways. 2009 edition including May 2012 revisions.

Washington, D.C.: Federal Highway Administration:

2012. https://mutcd.fhwa.dot.gov/index.htm

7. Federal Highway Administration (FHWA). 2003. FHWARD-03-031. Distress Identification Manual for the LongTerm Pavement Performance Program. McLean, VA.:

Federal Highway Administration: 2003.

https://www.fhwa.dot.gov/publications/research/infras

tructure/pavements/ltpp/reports/03031/
77

8. American Association of State Highway and

Transportation Officials (AASHTO). 2011. Roadside

Design Guide. 4th Edition. Washington, D.C. 2011.
78

APPENDICES

The appendices for the FHWA WIM Pocket Guide are available 

on the FHWA website: 

https://www.fhwa.dot.gov/policyinformation/knowledgecenter

General Appendices

• Appendix A – Technical Report Documentation Page 

Form DOT F 1700.7

• Appendix B – Acknowledgements

Part 1 Appendices:

• Appendix C – WIM Procurement Resources

o Example WIM Contract Language (GDOT)

o Example of IRD Standard Warranty Statement

o Example of IRD WIM System Installation Standards

Part 2 Appendices:

• Appendix D – WIM Site Design and Installation 

Resources

o Example Pavement Surfacing Plans (SDDOT)

o Example Polymer Piezo Installation Plan

o Example SLC Layout (Mettler-Toledo)

o Example WIM Installation Standard (NJDOT)

o Example WIM Site Design (MT DOT)

o Example WIM Cabinet Wiring Diagram

o Example Non-compliance Report

Part 3 Appendices:

• Appendix E – Maintenance and Calibration Resources

o Example WIM Calibration Report (AZ DOT) 

o Example Calibration Test Truck Procurement (LTPP)
79

o Example WIM Site Maintenance Forms and Reports

(AZ DOT)

WIM Program Operation and Management Checklists

• Appendix F – WIM Program Operation and

Management Checklists

o WIM Technology Selection Table

o WIM Site Selection Checklist

o WIM Site Design Checklist

o WIM Site Installation QA Checklist

o Initial Calibration Checklist

o WIM Site Acceptance Checklist

o Test Truck Selection for WIM Calibration

o Routine WIM Calibration Checklist

o WIM Maintenance Tools, Materials, and Supporting

Documentation

o WIM Preventive Maintenance Checklist

o WIM Troubleshooting Checklist

o WIM Data QA/QC Checklist

U.S. Department of Transportation

Federal Highway Administration

Office of Highway Policy Information

1200 New Jersey Ave., SE

Washington, D.C. 20590 

https://www.fhwa.dot.gov/policyinformation

June 2018 

FHWA-PL-18-015