WEIGH-IN-MOTION 

POCKET GUIDE

PART 1

WIM TECHNOLOGY, DATA 

ACQUISITION, AND 

PROCUREMENT GUIDE

Office of Highway Policy Information

Publication No. 

FHWA-PL-18-015 

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.
iii

TABLE OF CONTENTS

INTRODUCTION TO WIM POCKET GUIDE ................. 1

Purpose of WIM Data Collection..................................... 1 

Purpose of the WIM Pocket Guide.................................. 1 

Guide Content and Organization .................................... 2 

INTRODUCTION TO PART 1 ....................................... 3

Purpose............................................................................ 3 

Principles of Weighing in Motion.................................... 3 

WIM Standards and Performance Requirements........... 5 

WIM TECHNOLOGY REVIEW ..................................... 7

WIM System Components............................................... 7 

Inductive Loops ............................................................. 10

WIM Sensors ................................................................. 11

Sensor Types.......................................................................11

Sensor Selection Considerations.........................................11

Bending Plate .....................................................................15

Load Cell.............................................................................19

Polymer Piezo Sensor.........................................................23

Quartz Piezo Sensor ...........................................................28

Strain Gauge Strip Sensor...................................................31

Portable WIM Technologies.......................................... 34

Bridge WIM (B-WIM) .........................................................34

Portable WIM.....................................................................37

WIM Sensor Technology and Performance 

Comparison Table.......................................................... 41

WIM Controllers............................................................ 44
iv

Purpose ..............................................................................44

Main Components and Functions ......................................44

Additional Recommended Functionality ............................48

DATA REPORTING REQUIREMENTS ......................... 49

WIM Vendor Software and Firmware Requirements... 49

FHWA WIM Data Format............................................... 50

Mechanistic-Empirical Pavement Design Guide 

(MEPDG) Format ........................................................... 51

Download Method ........................................................ 51

PROCUREMENT CONSIDERATIONS ........................ 53

User Requirements and Vested Involvement............... 53

Factors Affecting WIM Technology Selection ............... 54

WIM Sensor Longevity Considerations......................... 55

Availability of Funding and Resources .......................... 55

Life Cycle Costs.............................................................. 56

Breakdown of WIM Life Cycle Costs...................................56

Pavement Condition and WIM Life Cycle Costs..................57

Cost Comparison ........................................................... 58

Sensor Costs .......................................................................59

Initial Installation Costs......................................................60

Continuous Operation Costs...............................................63

Additional Costs Considerations.........................................64

Total WIM Costs................................................................. 66

Benefits of Upgrading Existing Traffic Volume or 

Classification Monitoring Sites to WIM......................... 66

WIM Pilot Study Implementation ................................. 67
v 

Procurement Recommendations.................................. 67

Standards and Specifications.............................................67

WIM Performance Characteristics.....................................68

Technology Selection..........................................................68

Site Locations.....................................................................68

Bidding/Award ...................................................................68

WIM Data Purchase versus WIM System Purchase ...........70

Warranty............................................................................70

Installation .........................................................................71

Site Acceptance ..................................................................72

Calibration..........................................................................73

Maintenance ......................................................................74

Repairs ...............................................................................75

REFERENCES............................................................. 77

APPENDICES ............................................................ 79

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 and Installation 

Resources

• Appendix E – Maintenance and Calibration Resources

• Appendix F – WIM Program Operation and 

Management Checklists
vi

LIST OF TABLES

Table 1 – ASTM 1318E-09 Functional Performance 

Requirements for WIM Systems.......................................... 6 

Table 2 – Summary of WIM Sensor Technology Assessment....43

Table 3 – WIM Sensor Costs.......................................................59

Table 4 – Initial Installation Costs (Excludes Sensor Installation 

Costs Shown in Table 3).....................................................63

Table 5 – Continuous Operation Annual Costs..........................64
vii

LIST OF FIGURES

Figure 1 – Typical 2-Lane Bidirectional WIM System Design....... 8 

Figure 2 – Junction Box (lid cover removed)................................ 8 

Figure 3 – Cabinet Interior Components...................................... 9 

Figure 4 – Cellular Modem........................................................... 9 

Figure 5 – Slab Length for Minimum WIM Error........................13

Figure 6 – Bending Plate Sensor ................................................15

Figure 7 – Bending Plate Installation .........................................15

Figure 8 – 2-Lane Unidirectional Bending Plate – 

Staggered Configuration....................................................16

Figure 9 – 2-Lane Unidirectional Bending Plate 

In-Line Sensor Configuration .............................................17

Figure 10 – Single Lane Type III Sensor Configuration...............17

Figure 11 – Load Cell Sensor......................................................20

Figure 12 – Load Cell Installation...............................................20

Figure 13 – 2-Lane Unidirectional In-Line Load 

Cell Configuration ..............................................................21

Figure 14 – 2-Lane Unidirectional Staggered Load 

Cell Configuration ..............................................................22

Figure 15 – Polymer Piezo Sensor..............................................24

Figure 16 – Polymer Piezo Installation Depiction ......................24

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

per Lane Configuration ......................................................25

Figure 18 – Quartz Piezo Sensor................................................28

Figure 19 – Quartz Piezo Installation Depiction.........................28

Figure 20 – 2-Lane Quartz Piezo Sensors Staggered 

Double Threshold Configuration........................................29

Figure 21 – Strain Gauge Strip Sensor........................................32
viii

Figure 22 – 2-Lane Unidirectional Strain Gauge Strip 

Sensor Staggered Double Threshold Configuration...........32

Figure 23 – B-WIM Installation ..................................................35

Figure 24 – Single Lane Portable WIM Sensor...........................37

Figure 25 – 2-Lane Unidirectional Portable Piezo Full 

Lane Width Sensors ...........................................................38

Figure 26 – WIM Controller Internal Components 

Functional Diagram............................................................44

Figure 27 – Smooth Pavement Section......................................58

Figure 28 – 2-Lane Infrastructure ..............................................61

Figure 29 – 4-Lane Infrastructure ..............................................62
ix 

LIST OF ACRONYMS

A/C Alternating current

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 loading

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
x 

PV Photovoltaic

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 WIM 

POCKET GUIDE

Purpose of WIM Data Collection

Weigh-in-motion (WIM) is a primary technology used for 

monitoring vehicle weights and axle loads on roadways. Traffic 

loading data collected by WIM include wheel (single or dual 

tires) loads, axle loads, and gross vehicle weights (GVW). These 

data can be reported for each vehicle that passes over a WIM 

sensor. In addition, WIM devices collect traffic volume, axle 

spacings, vehicle classification, and speed data. 

State and other highway agencies collect WIM data for many 

reasons, including highway planning, pavement and bridge 

design, freight movement studies, motor vehicle enforcement, 

and legislative and regulatory studies (e.g., truck weight and size 

studies). Motor vehicle enforcement officers use heavy truck 

axle load data to plan enforcement activities, as well as to 

screen and identify specific vehicles that violate weight limits

during real time on-site monitoring. Highway planners and 

designers use WIM data to develop summary statistics by time 

of day, day of week, month of year, and annually. These 

summary statistics are used to develop inputs for pavement and 

bridge design, including equivalent single axle loadings (ESAL)

and axle load distributions. 

Purpose of the WIM Pocket Guide

This guide serves as a quick reference source for common WIM 

topics. The guide provides sufficient technical details that

enable traffic monitoring professionals to make sound decisions 

on WIM-related issues. It is built on states’ successful practices 

in using WIM systems and references the available standards 
2 

and guides related to WIM data collection. It contains practical 

advice and easy-to-follow instructions and illustrations. In 

addition, the guide has four instructional video supplements 

that show installation for different types of WIM sensors. The 

guide also contains references to web-accessible appendices 

that provide guidance on the various elements of procuring, 

installing, and maintaining WIM systems. 

Guide Content and Organization

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. 

The WIM Pocket Guide is divided into three 

parts intended for use by different personnel 

involved in WIM operations. Each part is as a 

self-contained document available in paper and 

electronic formats. 

• Part 1 – WIM Technology Selection, Data

Acquisition Requirements, and Procurement Guide

 Target audience: WIM program manager, WIM

program-level decision maker.

• Part 2 – WIM Site Selection, Design, and

Installation Guide

 Target audience: Traffic engineer or senior

specialist, WIM site-level decision maker.

• Part 3 – WIM Maintenance and Calibration Guide

 Target audience: Technician involved in routine

WIM maintenance and calibration.
3 

INTRODUCTION TO PART 1

Purpose

This document is designed to assist WIM program managers 

with program-level decision making, including review of WIM 

measurement principles and applicable standards, selection of 

WIM sensor and controller technologies, development of 

contract language for data acquisition and data format 

requirements, and analysis of WIM costs. 

Principles of Weighing in Motion

The process of weighing 

vehicles in motion is designed 

to capture and record axle 

spacings and weights by wheel

(single or dual tires), axle, and/or total vehicle (GVW) as vehicles 

drive over sensors installed in a roadway or under a bridge. In 

In ASTM E1318 – 09 Standard Specification for Highway WeighIn-Motion (WIM) Systems with User Requirements and Test 

Methods, WIM is described as “the process of measuring the 

dynamic tire forces of a moving vehicle and estimating the 

corresponding tire loads of the static vehicle.” (1)

Weighing vehicles at normal operational speed makes the 

weighing process more efficient and less disruptive than pullout permanent or portable static weigh stations that require 

vehicles to be stopped. These systems measure the transient 

forces applied by the tires as vehicles pass over the sensor. 

Dynamically measured forces are used to estimate the weight of

the vehicle when it is at rest. Several WIM technologies exist to 

capture the applied forces and predict static weight; these 

technologies are described in the next chapter. 
4 
5 

WIM Standards and Performance 

Requirements

ASTM E1318-09 is the 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 

approved for use in the United States, specifications for these 

systems are not provided.

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.
6 

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

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. 

±0%
7 

WIM TECHNOLOGY REVIEW

WIM System Components

The major components of permanent WIM systems include:

• WIM sensors embedded in the roadway surface or 

placed on the surface or on/under a bridge deck to 

detect, weigh, and classify vehicles. The combination of 

WIM sensors and loop detectors within a weighing lane 

is called the sensor array.

• Electronics to control system functions, measure and 

process sensor outputs, and provide vehicle records for 

display and storage.

• Infrastructure, including conduit, directional bore, 

cabinet, poles, and junction boxes.

• Support devices, such as power (A/C, wind or solar) 

devices to power the WIM electronics, lightning 

protection, grounding rods, and communication devices 

to transmit the collected data to a remote server.

• Software and/or firmware installed in the WIM 

electronics to process sensor measurements, analyze, 

format, and temporarily store collected data.

An example of a typical WIM system design is shown in Figure 1. 

In this example, two full lane-width WIM sensors are installed in 

each lane, providing a double-threshold configuration. For 

quartz piezo sensor layouts, two half-lane width sensors are 

installed adjacent to one another to create a full-lane width 

sensor. 
8 

Figure 1 – Typical 2-Lane Bidirectional WIM System Design

Most of the WIM system infrastructure is buried, either beneath 

the pavement or in the soil. Junction boxes, such as the one 

shown in Figure 2, are installed to route the sensor wires to the 

cabinet and provide access for splicing the sensor wires, if used. 

Figure 2 – Junction Box (lid cover removed)
9 

The WIM controller, supporting electronics, and 

communications devices are usually located in a roadside 

cabinet (see Figure 3). 

Figure 3 – Cabinet Interior Components

Communication devices may include telephone jacks or a 

cellular modem, which is shown in Figure 4. 

Figure 4 – Cellular Modem

The following sections provide more details about the WIM 

system components, including the in-road WIM sensors and the 

WIM controller. For information about installation of the 
10

sensors and infrastructure components, refer to Part 2 of the 

guide. 

Inductive Loops

Inductive loops are frequently installed in the roadway as part 

of the WIM array. They consist of four parts: a wire loop of one 

or more turns of wire embedded in the roadway pavement, a 

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

cable connecting the lead-in wire at the pull box to the 

controller, and an electronics unit housed in the controller 

cabinet. The electronics unit contains an oscillator and 

amplifiers that excite the embedded wire loop (5). Not all WIM 

arrays require the use of a loop sensor, such as the portable 

WIM array.

If inductive loops are used as part of the WIM array, several 

considerations need to be addressed: 

• The number of inductive loops, and their locations 

within the array, differs according to WIM controller 

manufacturer and application. 

• Loop sizes vary based on the lane width. Loops sizes of 8 

feet by 6 feet are gaining popularity to accommodate 

motorcycle counting.

• Loops within an array, and loops installed in adjacent 

lanes, must be installed far enough away from one 

another to avoid interference, or “crosstalk.” 

• The loop lead-ins must be twisted, and the twisted loop 

lead-ins for each loop must be run in separate slots to 

avoid crosstalk.

• A second loop may be installed to provide a back-up 

mechanism for capturing timing information should one 

of the WIM sensors fail to detect the wheel as it passes. 
11

WIM Sensors

Sensor Types 

Several types of in-road WIM sensors are available in the United 

States. The most frequently used sensors include bending plate, 

load cell, quartz piezo, polymer piezo, and the strain gauge strip 

sensor. Sensors like bending plate and load cell are wide and 

allow the full tire footprint to rest on the sensor. Other sensors 

are narrow and come in contact only with a part of the tire 

footprint at any given time, as a vehicle moves over the sensor. 

The narrow sensors, also called strip sensors, include polymer

piezo, coax piezo, quartz piezo, and strain gauge strip sensor. 

The WIM sensors vary in accuracy and cost, and each sensor 

type has advantages and disadvantages that must be 

considered during the selection process. Details about each 

sensor type are provided in the ensuing sections.

Sensor Selection Considerations 

The required level of data quality (WIM measurement accuracy) 

combined with the available funding often drives the type of 

installation/sensor an agency will use. A higher level of data 

quality requires more precise WIM sensors and system. 
12

If users require high accuracy weight data collection over long 

periods of time, and the budget allows for high accuracy 

sensors/arrays and maintenance of pavement smoothness and 

pavement integrity at the WIM site locations, then either a load 

cell or bending plate sensor is the preferred solution. The 

accuracy of WIM systems utilizing these technologies meets or 

exceeds all the requirements for highway truck weight data

monitoring. 

Over long periods of data collection (say, more than 7 years), 

the difference in life cycle costs of bending plate and load cell, 

Six Key Questions for Sensor Selection

When considering the type of sensor and array 

to use, five key questions need to be addressed: 

1. How many WIM sites are needed, and what 

funding is available? The requirement for a greater 

number of sites (for more comprehensive road 

network coverage) typically requires selection of a 

less accurate sensor to stay within budget 

constraints.

2. What level of weight data accuracy is acceptable 

for users? Higher levels of accuracy would require 

more expensive sensors or a different sensor array, 

which may lead to a decision to have fewer sites, to 

stay within budget constraints.

3. Do you need WIM in all lanes and directions?

4. What are the personnel, material, and equipment 

costs associated with maintaining each WIM site?

5. What is the pavement type and its current 

condition? 

6. Is it likely that pavement condition would 

deteriorate over the intended WIM service life? If 

so, what is the cost of maintaining the pavement 

over the intended WIM data collection period?
13

when compared to piezo sensors, decreases while data quality 

stays consistently high, if properly maintained/calibrated. These 

sensors have to be installed in portland cement concrete (PCC) 

pavements. If the pavement is asphalt concrete (AC), the 

installation of a continuously reinforced concrete pavement 

segment (preferably with no joints) is recommended to improve 

the structural integrity of the WIM site installation. This 

recommendation should be weighed against the agency’s 

requirements for minimum/maximum PCC slab length and 

pavement type. 

The formula below (recommended by TRB/FHWA LTPP Traffic 

Expert Task Group, author David Cebon) may be used to 

determine the appropriate length of the concrete slab based on 

the speed of truck traffic at the site. For typical highway speed, 

total slab length is 300 to 400 feet. (For a 400-foot slab, use 325 

feet before the array and 75 feet after the array.) 

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

Figure 5 – Slab Length for Minimum WIM Error 
14

If pavement conditions that initially meet ASTM E1318-09 

specifications cannot be maintained over a long time period 

(over 3 years), then polymer piezo sensors may be the only 

viable choice based on functionality and cost. The flexibility of 

the polymer piezo enables it to tolerate some pavement rutting, 

and the cost is less than that of the other WIM sensors. Also, 

when the sensor’s ability to collect accurate weight degrades, 

the WIM site could be converted to a vehicle classification site

so the agency’s investment can continue to be productive. 

Polymer piezo sensors are affected by temperature fluctuations 

and by seasonal changes in pavement stiffness. Consequently, 

polymer piezo sensors must be calibrated every 6 to 12 months 

to maintain accuracy in weight measurements. Also, due to 

precision limitations, the polymer piezo sensor is less capable of 

accurate prediction of heavy loads and loads over the legal limit. 

However, when installed on high truck volume roads and 

properly calibrated, they adequately predict the average axle 

loads and GVW, typically within ASTM WIM Type II performance 

requirements. 

Sensors like the quartz piezo and strain gauge strip sensor fall in 

between the two classes of sensors described above in terms of 

cost, accuracy, and longevity and calibration. As a result, these 

sensors frequently represent a desirable combination of cost 

and accuracy. Quartz piezo and strain gauge strip sensors can be 

installed in both AC and PCC pavements. This versatility, along 

with accuracy and cost considerations, makes these sensors a 

popular choice for new WIM installations. 

In summary, it is the required level of data quality combined 

with the ability to maintain a smooth pavement condition that 

determines how long a WIM site can be operated. Load cell or 

bending plate sensors would be the choice for projects with a 

typical life expectancy of 8 to 10 years, provided the PCC 

pavement requirement can be satisfied. Quartz piezo or strain 

gauge strip sensors are the best options for 3 to 5 years. 

Additional site longevity for these sensors could be gained by 

using a double threshold configuration. Piezo polymer sensors 
15

are best suited for short-term studies of 1 to 3 years, or where a 

smooth pavement condition cannot be achieved or maintained, 

provided that lower accuracy and average weight vs. high 

precision heavy and overweight measurements are acceptable. 

The function, accuracy, and typical application for each type of 

WIM sensor, along with the strengths and weaknesses of each, 

are discussed in greater detail in the following sections. 

Bending Plate 

Sensor Description

Bending plates utilize strain gauges bonded to the underside of 

the plate to collect loading data. The bending plate WIM sensor, 

illustrated in Figure 6, is typically 6 feet long, 20 inches wide, 

and 1 inch thick. It weighs approximately 250 pounds. It may be 

black or grey in color. Figure 7 shows a bending plate installed in 

the pavement.

Figure 6 – Bending Plate Sensor

Figure 7 – Bending Plate Installation
16

How it Works 

As axles pass over the bending plate, the system measures the 

strain on the plate approximately 2,000 times per second at 

highway speeds and calculates the load required to induce that 

level of strain.

Sensor Configuration and Layout

Each bending plate is installed to cover one-half of the travel 

lane. Therefore, there are typically two bending plates installed 

in each lane. For Type I and Type II WIM systems, the bending

plates are typically installed in a staggered configuration, as 

shown in Figure 8. In this configuration, the first loop provides 

vehicle detection and signals the presence of a vehicle. The 

bending plates provide the weights and the timing information 

to determine speed and axle spacing. A second loop may be 

installed, which is used for timing if full contact is not made with 

either of the bending plates.

Figure 8 – 2-Lane Unidirectional Bending Plate – Staggered 

Configuration

Alternatively, the in-line configuration shown in Figure 9

provides single threshold detection. In this configuration, the 

bending plates provide weight information and the loops 

provide vehicle presence and the timing information for speed 

and axle spacing. The speed and axle measurements are less 
17

accurate with this configuration than with the staggered 

configuration since they are using an inductive field rather than 

axle detection for timing. 

Figure 9 – 2-Lane Unidirectional Bending Plate In-Line Sensor 

Configuration

The configuration for Type III WIM systems, as shown in Figure 

10, utilizes four bending plates installed in two lines—before 

and after the trailing loop inductor. These systems are typically 

low-speed and are installed in a single lane either before or in a 

weigh station for pre-screening and sorting. 

Figure 10 – Single Lane Type III Sensor Configuration

Strengths

The bending plate has proven to be one of the most accurate 

WIM technologies available in the U.S., capable of producing 

consistently accurate data over time when regularly maintained. 

When installed and maintained properly in good pavement, the

bending plate sensor and mounting frame are very durable and 
18

may last 8 to 12 years when installed in new concrete 

pavements. The bending plate sensor demonstrates very little 

speed dependency, especially in smooth pavements, and almost 

no temperature dependency. These sensors are capable of

achieving a calibration accuracy range of +/- 3.0 percent. The 

initial cost of the bending plate sensor is slightly more than 

quartz piezo sensors or strain gauge strip sensors and less than 

load cell sensors. 

Weaknesses

The bending plate sensor is typically only recommended for use 

in PCC pavements. AC pavement designs with the proper depth 

and material composition have shown some success. If installed 

in AC pavement, the pavement around the frame begins to 

break up after time and allows the frame to become loose, 

creating a hazard for the traveling public. To avoid this problem, 

a concrete vault may be installed to embed the frames or a 

concrete road segment may be installed, but this increases the 

cost significantly and may lead to pavement distress over time, 

causing adverse truck dynamics and decreasing WIM system 

accuracy. 

Bending plate maintenance should be performed on an annual 

basis. Service intervals in warm climates can be increased to 2 

years. Bending plate maintenance requires short-term lane 

closure and may pose some safety risk to WIM technicians. 

Proper traffic control is essential. 

Accuracy 

This system can be classified as an ASTM Type I, II, or III system, 

depending on the application of the device and the number of 

scales placed in the lane. The bending plate system, when 

properly installed in smooth pavement, well maintained, and 

calibrated may consistently provide accuracies well within 

ASTM 1318-09 Type III performance specifications. When 

properly installed and calibrated, bending plate WIM systems 

using two 6-foot-long bending plates in each lane may be 

expected to provide GVW with an error of 10 percent or less of 
19

the actual vehicle weight for 95 percent of the trucks measured.

For example, the bending plate sensor typically provides GVW

measurement accuracy within +/- 4.0 percent over time, 

according to a study conducted on the LTPP bending plate 

installations (6). 

Recommended Use

The bending plate sensor is well suited for a variety of 

applications, including weight enforcement, pre-screening for 

static truck scales, research requiring high-quality traffic loading 

data, highway and bridge design, planning, and warranty. This 

sensor is the most cost-effective method for long-term data 

collection that requires a high degree of accuracy. Bending 

plates are frequently used to collect loading data when the user 

specifies ASTM E1318-09 Type I WIM performance 

requirements.

Load Cell 

Sensor Description

The load cell scale system consists of two weighing platforms, 

each with a surface size of 6 feet by 3 feet, 2 inches, placed 

adjacent to each other to fully cover a 12-foot traffic lane in line 

or in a staggered configuration. Hydraulic or mechanical 

transducers measure the force applied to the scale. The 

installation of a load cell scale requires the use of a concrete 

vault. The size of vault can be as large as 12 feet long, 5 feet 

wide, and nearly 3 feet deep. The load measurements are 

recorded and analyzed by the system electronics to determine 

tire and axle loads. Figure 11 shows an illustration of a load cell 

sensor. Figure 12 shows a load cell installed in the pavement.
20

Figure 11 – Load Cell Sensor

Figure 12 – Load Cell Installation

How it Works 

Load cell WIM systems utilize two scales to detect an axle and 

weigh both the right and left side of the axle approximately 

2,000 times per second at highway speeds per axle passage. The 

scale mechanism transfers loading on the weighing surface to 

the load cell sensor. The load cell sensor utilizes a transducer 

which creates an electrical signal whose amplitude is directly 

proportional to the force being measured. The system records 

the weights measured by each scale and sums them to obtain 
21

the axle weight. Storing data from the individual load cells by 

wheel weights is the best method, and this can be presented

using the data format described in the 2016 FHWA Traffic 

Monitoring Guide (TMG) per vehicle format (PVF) “Z” data

format (7). 

Sensor Configuration and Layout

Typical load cell system configurations consist of two load cell 

platforms and at least one inductive loop. It may also include an

axle detector and/or an independent off-scale detector. There 

are two standard layouts for the load cell WIM system.

For the in-line configuration (shown in Figure 13), a load cell 

threshold is placed across the traffic lane. It uses two in-line 

scales. The inductive loops are placed upstream and 

downstream of the load cells to detect vehicles and alert the 

system of an approaching vehicle. An axle sensor may be placed 

upstream of the load cell to determine axle spacing and vehicle 

speed.

Figure 13 – 2-Lane Unidirectional In-Line Load Cell Configuration

Load cells may also be installed in a staggered configuration, as 

shown in Figure 14. Off-scale detectors may be installed 
22

independently or integrated into the scale assembly to sense 

any vehicles off the weighing surface. 

Figure 14 – 2-Lane Unidirectional Staggered Load Cell Configuration

Strengths

This sensor is the most accurate among the commercially 

available WIM sensors. Based on feedback from state agencies, 

the load cell sensor could consistently measure GVW with an 

accuracy of +/- 6 percent error or better at highway speeds. The 

load cell sensor, when properly installed and maintained, can be 

expected to last up to 12 years before major refurbishing work

is required, which may add another 12 years of service life. The 

scale frame and vault have an expected service life of 25 years. 

Based on the high accuracy expectation of the load cell WIM 

system, the load cell system should be calibrated every 12 to 24

months. Sites requiring highly accurate data, such as weight 

enforcement and research-related sites, should be calibrated 

annually. Sites used for planning or freight movement may be 

calibrated up to every 24 months. 

Weaknesses

The load cell scale is the most expensive and time-consuming

WIM sensor to install. The installation of a load cell scale 

requires the use of a crane and the installation of a concrete 

vault. This installation often makes the load cell cost-prohibitive 
23

for highway applications, unless data collection is for a longer 

term (10 years or more). It is also difficult to implement in 

asphalt pavements (due to the concrete vault requirement). 

Accuracy 

Based on manufacturer information, when properly installed 

and calibrated, load cell WIM systems should be expected to 

provide GVW that are within 6 percent of the actual vehicle 

weight for 95 percent of the trucks measured (ASTM Type III). 

This system can also be classified as ASTM Type I or II, 

depending on the application. These WIM sensors have a 

history of being the most accurate WIM sensor, providing better 

than ASTM Type III accuracy over the 12-year life of the sensor.

Recommended Use

Load cell WIM scales can be used in low, medium, or high speed 

weigh-in-motion applications. Since the load cell provides high 

accuracy, it is best suited for data collection and enforcement 

pre-screening applications (both on mainline and ramp 

applications). 

Polymer Piezo Sensor 

Sensor Description 

The basic construction of the polymer piezo sensor consists of a 

copper strand, surrounded by a piezoelectric polymer material, 

which is covered by a brass sheath (see Figure 15). The polymer 

piezo sensor can be ordered in several different sensor and 

lead-in cable lengths. A WIM system utilizing the polymer piezo

WIM sensor consists of at least two full-lane polymer piezo

WIM sensors and one inductive loop (see Figure 16). Polymer 

piezo sensors are classified as class 1 and class 2 accuracy by 

manufacturers. A class 1 sensor is needed for WIM installations. 

Class 2 sensors are suitable for vehicle classification monitoring. 

To improve accuracy, four full-length sensors should be used in 

each weighing lane, as shown in Figure 17. 
24

Figure 15 – Polymer Piezo Sensor

Figure 16 – Polymer Piezo Installation Depiction

Sensor Configuration and Layout

Each polymer piezo sensor is typically installed for half-lane 

coverage, with four sensors installed in each lane, 2-inch deep 

slot, 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. Using four sensors in this 
25

way provides a dual-threshold capability. 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.

How it Works 

Polymer piezo WIM systems utilize piezo sensors to detect a 

change in voltage caused by pressure exerted on the sensor by 

the tires on an axle. When pressure is applied to the 

piezoelectric material, an electrical charge is produced. The 

larger the charge, the greater the output signal amplitude, the 

more the weight. By measuring and analyzing the charge 

produced, the weight due to a passing tire or axle group can be 

estimated. As a vehicle passes over the piezo sensor, the system 

records the electrical charge created by the sensor and 

calculates the dynamic load. 

Figure 17 – 2-Lane Unidirectional Polymer Piezo 2-Sensors per Lane 

Configuration

Strengths

Polymer piezo WIM systems are the least expensive among 

WIM systems. The sensors are the easiest to be installed and 

replaced. In addition to measuring traffic loads, polymer piezo
26

sensors are widely used for vehicle classification. The sensor 

installation is very durable, so the sensor rarely needs to be 

replaced due to the sensor becoming loose in the roadway. 

With proper calibration and temperature compensation, these 

sensors could be used for monitoring of average truck loads on 

high truck volume roads.

Weaknesses

Polymer piezo sensors are much less accurate than bending 

plates, load cells, linear strain gauges, or quartz piezo sensors 

due to temperature sensitivity and sensitivity to changes in 

pavement stiffness. The need for auto-calibration also limits 

applicability of these sensors on roads with low truck volumes. 

Some WIM vendors’ controllers utilize a temperature 

compensation function that uses a temperature probe 

embedded in the roadway as a way of reducing the effect of 

temperature on the sensor operation. To be successful, this 

function needs to be set based on the sensor behavior observed 

over time at the WIM site. Polymer piezo sensors are not 

suitable for accurate measurement of overloads or a narrow 

range of heavy loads; they are better suited for monitoring 

average truck loads. Use of piezo polymer sensors is more 

successful in temperate climates (less temperature effect) and 

on high truck volume roads (more successful auto-calibration).

These sensors require frequent calibration due to high 

sensitivity to temperature changes—a 30 to 50 percent voltage 

change in sensor output may be expected when passing 

through 32 °F. Auto-calibration used to compensate for 

temperature sensitivity may not be successful in providing

accurate measurements of heavy loads for multi-axle groups on 

low-volume roadways, since the effectiveness of the autocalibration feature is highly dependent on a high enough 

number of front-axle measurements to compute reasonably 

accurate compensation (auto-calibration) factors. Autocalibration effectiveness is also diminished if a stable 

relationship between the front axle weight and the weight of 

heavy axle groups cannot be successfully defined at the site. 
27

The service life of these WIM sensors is the shortest among all 

WIM sensors, typically 1 to 3 years.

Narrow-width sensors may have higher error when compared 

to wider bending plates or load cells due to additional 

approximations resulting from the fact that the sensor 

measures only part of the tire footprint at any point in time.

Roadway roughness resulting in adverse truck dynamics also 

plays an important role in any strip-type sensors. 

Accuracy 

When properly installed and calibrated on roads with truck 

volumes that provide conditions for an effective autocalibration and temperature compensation, a polymer piezo

WIM system using four half-lane width sensors could provide 

GVW measurement accuracy that is within 15 percent of the 

actual vehicle weight for 95 percent of the trucks measured on 

most days. This is consistent with the requirements for ASTM 

E1318-09 Type II WIM systems. This accuracy is likely to 

deteriorate with time or temperature changes or with 

pavement deterioration. Semi-annual or a data quality driven 

calibration schedule may be needed to maintain the desired 

accuracy level, especially in regions with wide changes in 

seasonal temperatures. The auto-calibration feature is 

ineffective for low truck volume roadways. 

Recommended Use

The polymer piezo sensor is well suited for determining 

whether roadways are loaded or unloaded, monitoring average 

truck weight on the roads with heavy truck volumes, and largescale WIM data coverage studies where accuracy is less 

important than the number of measurement locations. This 

sensor is most cost-effective for short- to mid-term data 

collection. Polymer piezo sensors can be installed in any types 

of AC or PCC pavements. Polymer piezo sensors are frequently 

used to collect loading data when the user specifies ASTM 

E1318-09 Type II WIM performance requirements. With proper 

installation and frequent calibration, it is possible to collect 
28

loading data that meet ASTM E1318-09 Type I WIM 

performance requirements for roads with high volumes of class 

9 trucks, especially in temperate climatic regions. 

Quartz Piezo Sensor 

Sensor Description

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

can be combined in varying lengths to provide half-lane or fulllane 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. Figure 18 

illustrates a quartz piezo sensor. Figure 19 illustrates a quartz 

piezo sensor embedded in a pavement cross section.

Figure 18 – Quartz Piezo Sensor

Figure 19 – Quartz Piezo Installation Depiction
29

Sensor Configuration and Layout

To maximize measurement accuracies by improving the 

sampling of each side of the vehicle, the sensors are typically 

installed to cover one-half of the travel lane, two for the left 

wheel path and two for the right wheel path, with each set of 

staggered sensors spaced 16 feet apart. Figure 20 illustrates this 

sensor configuration. 

Figure 20 – 2-Lane Quartz Piezo Sensors Staggered Double Threshold 

Configuration

How it Works 

The quartz piezo WIM sensor is a force sensor based on quartz 

crystal technology. A wheel rolling over the sensor applies 

vertical forces which are distributed through the quartz crystals 

in the sensor. The quartz elements produce an electrical charge 

that is proportional to the vertical forces applied. 

Strengths

Quartz piezo WIM sensors can be installed in AC (over 4 inches

thick) or PCC pavements, but the installation is much more 

durable in PCC pavements. The sensors themselves are 

ordinarily maintenance free. In case of pavement rutting, the
30

top of the sensor may be ground to conform to the pavement

surface, which helps maintain system accuracy. The quartz 

sensor material has the distinct advantage of being much less 

sensitive to changes in temperature as compared to polymer 

sensors, but it is more expensive, per sensor, than the other 

piezo style sensors. Unlike load cell or bending plate systems, 

the quartz piezoelectric sensor requires almost no maintenance 

but needs to be calibrated every 12 to 18 months to meet Type I 

WIM system accuracy requirements due to calibration drift. 

There is more than one type of quartz piezo sensor available. 

Weaknesses

Because the sensor relies on structural support from the 

pavement, if the pavement strength is significantly affected by 

environmental conditions (material softening due to high 

temperatures, high soil moisture content), the sensor output 

may be also affected by these changes in pavement support, 

even though the sensor material itself is not affected by the 

temperature. These sensors continue to be popular due to 

easier and less costly installation. 

Narrow-width sensors may have a higher range of error (less 

precision) when compared to wider bending plates or load cells 

due to additional approximations resulting from the fact that 

only a partial tire footprint is being measured at any point in 

time. However, one manufacturer of quartz piezo WIM sensors 

has been certified by the International Organization of Legal 

Metrology for vehicle weighing with quartz sensors at highway 

speeds. This certification paves the way for the use of quartz 

piezo sensors in weight enforcement applications.

Roadway roughness, which results in adverse vehicle dynamics,

plays an important factor in the quality of the data, as with any 

in-the-road WIM system. 

Accuracy 

This system can be classified as an ASTM E1318-09 Type I or 

Type II system depending on the use of the device and the 

number of scales placed in the lane. To meet ASTM E1318-09 
31

Type III requirements, the full-width, double-threshold 

configuration must be used. The quartz piezo system, when 

properly installed in smooth pavement, well-maintained, and 

calibrated may consistently provide accuracies well within 

ASTM E1318-09 Type I specifications. For example, quartz piezo

sensors typically provide GWV measurement accuracy within 

+/- 6 percent, based on the long-term monitoring of the 

performance for the FHWA LTPP WIM systems that use the 

quartz piezo sensor. 

Recommended Use

WIM systems utilizing the quartz piezo sensor are well suited 

for a variety of applications, including pre-screening for static 

truck scales, research requiring high accuracy WIM data, 

pavement and bridge design, planning, and warranty. This is 

frequently the sensor of choice for collecting axle loading data 

for pavement design. Quartz piezo sensors are frequently used 

to collect loading data when the user specifies ASTM E1318-09 

Type I WIM performance requirements. This sensor is most 

cost-effective for mid-term data collection (3 to 5 years) that 

requires a high degree of accuracy.

Strain Gauge Strip Sensor 

Sensor Description

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. Figure 21 illustrates a typical strain gauge 

strip sensor.
32

Figure 21 – Strain Gauge Strip Sensor

Sensor Configuration and Layout

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 22. 

Figure 22 – 2-Lane Unidirectional Strain Gauge Strip Sensor Staggered

Double Threshold Configuration

How it Works 

The strain gauge WIM sensor is based on strain gauge load cell 

technology. As a vehicle passes over the WIM sensor, the 

system measures the vertical strain placed on the sensor by the 

weight of the wheel. The resultant change in the electronic 

properties of the strain gauge load cells are translated into the 

dynamic load and the wheel, axle, and GVW are produced by 

the WIM controller or software. 
33

Strengths

Strain gauge strip sensors can be installed in AC or PCC 

pavement. If installed in AC, the pavement must be thick 

enough to accommodate a 3-inch-deep sensor in the top 1/3 of 

the asphalt layer (to prevent cracking through asphalt layer 

thickness). The sensors themselves are ordinarily maintenance 

free and, in case of pavement rutting, the top of the sensor may 

be ground flush up to 0.25 inch to conform to the pavement, 

which helps maintain system accuracy. Strain gauge strip 

sensors are comparable to or slightly less expensive per sensor 

than quartz piezo sensors, and they are less sensitive to changes 

in temperature than polymer piezo sensors. Reliable lifetime 

expectancy for these sensors has not been field tested beyond 5 

years.

The sensor is less reliant upon structural support of the 

pavement than quartz piezo sensors. This decreased reliance in 

structural support is due to the design of each strain gauge strip 

sensor, which is slightly larger than quartz piezo sensors. 

Weaknesses

The strain gauge strip sensor installation currently recommends 

3-inch-deep channels cut in pavement for installation. This 

installation requirement limits applicability of strain gauge strip 

sensor for thin pavements. To prevent pavement cracking at the 

sensor location, it is recommended that the sensor depth does 

not exceed 1/3 of the pavement structural layer thickness.

Narrow-width sensors may have higher error when compared 

to wider bending plates or load cells due to additional 

approximations resulting from the fact that only part of the tire 

footprint is being measured by the sensor at any point in time.

Improved accuracy can be obtained by additional rows of 

sensors and/or careful WIM site selection and installation.

Due to the new product/technology, there is limited long-term 

(beyond 5 years) sensor performance information available. 

More information is expected to be available in a few years (by 

2020), since several state agencies are currently using this type 
34

of sensor at multiple data collection locations (FL, IN, MN, OR, 

TX, and WA). 

Roadway roughness, which results in adverse vehicle dynamics, 

plays an important factor in the quality of the data, as with any 

in-the-road WIM system. 

Accuracy 

This system can be classified as an ASTM E1318-09 Type I, II, or 

III system depending on the installation, use, and the number of 

sensors placed in the traffic lane. The strain gauge strip sensor 

system consisting of a single pair of sensors, when properly 

installed in smooth pavement, well maintained, and calibrated 

is expected to provide accuracies well within ASTM E1318-09 

Type I performance specifications. Configurations with two pairs 

of strain gauge strip sensors provide GVW measurement 

accuracy within +/-5 percent. Typical WIM calibration intervals 

of once per year are recommended for proper WIM site 

performance.

Recommended Use

WIM systems utilizing the strain gauge strip sensor are well 

suited for truck and axle loading data collection and research, 

screening for enforcement, pre-selection for static weighing, 

and bridge and infrastructure protection. Strain gauge strip 

sensors are frequently used to collect data for enforcement 

screening at high speeds and when the user specifies ASTM 

E1318-09 Type I WIM performance requirements. 

Portable WIM Technologies

Bridge WIM (B-WIM) 

Sensor Description

Bridge WIM system sensors are made up of strain transducers 

that are mounted to the underside of the bridge deck or 

structural members, either temporarily with C-clamps or 

permanently with fasteners. The transducers are connected to

portable roadside electronics that collect, process, and store the 
35

data. Bridge sensors installed under the bridge deck never come 

in direct contact with the tires. There are limitations for the 

type of bridges and traffic conditions these WIM systems will 

support. Bridge selection is key to quality data collection as not 

every bridge is a good candidate for B-WIM.

Sensor Configuration and Layout

Two main approaches to B-WIM have been used. The first 

approach uses strain sensors mounted on the bridge and a 

separate axle detection sensor installed on the road. The 

second approach uses only strain sensors installed on the bridge 

for both axle detection and weight measurements. The second 

approach provides a simpler design, installation, operation, and 

maintenance of the B-WIM system. Figure 23 shows the 

installation of B-WIM sensors.

Figure 23 – B-WIM Installation 

How it Works 

Bridge WIM systems monitor the bending deformations of a 

bridge caused by vehicles crossing over the structure. These 

deformations are measured using strain sensors attached to the 

structural members. These deformations are analyzed to 

estimate the GVW and axle loads of passing traffic. Axle spacing, 

total vehicle length, and speed can also be detected. 
36

Strengths

One advantage of B-WIM systems is their portability. The 

sensors can be removed and reinstalled on a different structure, 

provided that easy and safe access is available under the bridge 

span. Another advantage of B-WIM systems is that the same 

sensors could be used for evaluating the performance of the 

structure itself. Because B-WIM sensors are installed under the 

bridge, they do not pose any risk of becoming a road hazard. BWIM is installed beneath a bridge with a simple surface 

installation as opposed to a more costly and time consuming in 

pavement installation. 

Weaknesses

Each time the B-WIM system is moved to a new location, it 

requires recalibration specific to a bridge’s response to moving 

loads, making these systems unique to each location. Various 

factors degrade the signal from the strain gauges and limit the 

accuracy of data obtained from these systems. The most 

significant of these factors are the presence of other traffic on 

the bridge at the same time a truck is being weighed (which 

significantly increases the noise in the weight signal) and the 

lack of documented information at the highway agencies about

the expected structural bridge responses to various loading 

conditions, which limits the accuracy of load estimation based 

on B-WIM sensor inputs. 

B-WIM application is limited by the type of structures and traffic 

conditions. They work more efficiently on small single-span 

bridges or large culverts with well understood and documented 

structural responses. They are good for bridges that are not 

subjected to high traffic volume (ideally, a single truck crosses 

the bridge at the time of measurement). These systems are best 

suited for estimating GVW and not with individual axle or wheel 

loads. 

Installation of B-WIM sensors may require special safety 

precautions, depending on the terrain beneath the bridge and 

the bridge’s height.
37

Accuracy 

The accuracy of B-WIM systems can be affected by the presence 

of multiple trucks, whether in parallel or serial configurations, 

and by other traffic on the bridge. Accurate axle spacing 

detection also is a concern with B-WIM. Since the accuracy of BWIM load estimation is highly dependent on each bridge 

structure and the skills of the engineer setting up the system, no 

reliable long-term information on the average B-WIM system 

performance can be provided at this time.

Recommended Use

B-WIM could be suited for a variety of applications, including 

bridge protection and safety, planning, bridge management and 

design, and warranty. These systems require availability of 

skilled personnel to make a selection of the acceptable bridge 

structures, to evaluate bridge structural responses to moving 

truck loads, and to set up and calibrate the system. It is 

expected that the cost of B-WIM sensors is far less than the cost 

of system set-up and calibration. Several states (AL, CT, NJ, and 

MS) have experience with this technology, albeit limited. 

Portable Piezo Sensors 

Sensor Description 

There are two basic styles of portable piezo sensor installations: 

a bare polymer piezo (brass linguini) sensor taped directly to the 

pavement and a piezo that is encased in a sleeve and then 

fastened to the pavement, as shown in Figure 24. 

Figure 24 – Single Lane Portable WIM Sensor
38

Sensor Configuration and Layout

For portable weighing operations, sensors (each sensor is 

roughly one lane width in length) are either taped or fastened 

to the roadway, perpendicular to traffic, as shown in Figure 25. 

Normally, two sensors are placed 3 to 6 feet apart. The 

University of Minnesota developed a sensor that is long enough 

to cover two lanes (8).

Figure 25 – 2-Lane Unidirectional Portable Piezo Full Lane Width 

Sensors

How it Works 

When a mechanical force is applied to a piezoelectric sensor, it 

generates a voltage proportional to the weight by causing 

electrical charges of opposite polarity to appear at the parallel 

faces of the piezoelectric material. When placed on a rigid 

surface, the measured voltage is proportional to the force or 

weight of the wheel or axle (if a dynamic loading component is 

present due to truck bounce, or pavement stiffness changes 

over time, it may degrade the proportionality relationship). 

The time difference between the wheel contacts on the two 

sensors is used to determine vehicle speed, which is then used 

to determine the wheel and axle spacing needed for vehicle 

classification.

Limitations

The sensor installation itself creates bumps in the road, which is 

counterintuitive to the road smoothness requirements for 

collecting high-quality WIM data.
39

There is often a burn-in time for the sensor to embed itself in 

the material used to hold the sensor in a line. Road roughness

must be considered, and rough pavement should be avoided

when locating where the portable WIM data collection should 

be undertaken. Lastly, on-site calibration of the WIM sensors 

must be performed using heavy trucks after the burn-in is 

completed. This is very important to know the accuracy with the 

variables of pavement stiffness, vehicle dynamics, and sensor 

response for different loadings. 

The installation and removal of portable WIM sensors likely 

requires temporary traffic control to ensure the safety of the 

installers.

Strengths

These systems are relatively easy to set up, although they are 

routinely placed only in the outside lane of traffic in order to 

allow the lead wires to be placed on the roadway shoulder. 

These sensors are the least expensive among WIM sensors. 

Weaknesses

Polymer piezo sensors used in portable operations suffer from 

significant limitations in accuracy for the following reasons:

• The sensor installation itself creates bumps in the road, 

which is counterintuitive to the road smoothness 

requirements for collecting high-quality WIM data.

• Because the sensor is mounted on the surface, there is 

a horizontal force acting on the sensor with each wheel 

strike, causing it to deform in the wheel path. This often 

degrades the quality of the WIM measurements and can 

cause the sensor to fail.

• Polymer piezo sensors are temperature sensitive, 

making it difficult to keep them in calibration when 

temperature changes during the day.

• The sensor narrowness allows tires being weighed to 

“fold” over them, meaning that at no time during the 

axle weighing process is an entire tire positioned on the 

sensor. Therefore, changes in tire pressure or tire tread 
40

patterns may affect the force measured by this type of 

sensor more significantly than by many other WIM 

technologies.

• Calibration of the sensors to site-specific vehicle 

dynamics and environmental conditions using heavy

trucks of known weight is necessary to obtain the level 

of accuracy needed for direct inclusion of these data 

into the pavement design process. The cost of these 

efforts often far exceeds the cost of placing and 

retrieving the data collection sensors and greatly 

increases the cost of collecting accurate weight data 

with these systems. The lack of site-specific calibration 

significantly affects the portable piezoelectric WIM 

system ability to accurately estimate the static axle 

weights.

• Because pavement response to traffic loads has a nonlinear effect on WIM sensor measurements, autocalibration based on light or moderate loads leads to

poor prediction of heavy loads.

• Depending on system set-up, weight measurements of 

lighter trucks can be significantly overestimated when 

compared with heavy truck weight estimations. 

• Sensors have a relatively high signal-to-noise ratio. 

• Narrow-width sensors may have higher error when 

compared to wider bending plates or load cells due to 

additional approximations resulting from the fact that 

the sensor measures only part of the tire footprint at 

any point in time.

Accuracy 

The accuracy of a piezoelectric portable WIM system is highly 

dependent on the set-up and calibration. When properly 

calibrated using heavy test trucks, typical accuracy for this 

system is +/– 25 percent of the actual vehicle weight for 95

percent of the trucks measured. This accuracy is likely to 

deteriorate with time or temperature changes. 
41

Recommended Use

Portable piezo data collection is best suited for a short-term

data collection application when temperatures are above 

freezing and low-accuracy WIM data are acceptable to the user. 

These systems are suitable for identifying general trends in 

traffic loading and comparing loading patterns among different 

sites. The data from these systems are not recommended as 

direct input for pavement design or motor vehicle law 

enforcement operations. 

WIM Sensor Technology and Performance 

Comparison Table

The performance, accuracy, and life expectancy of WIM sensors 

and the systems that utilize them are dependent upon several

factors. 

Although each of these factors plays an important role in the 

quality of data that the WIM system produces, it is widely 

accepted that the type of sensor and the roadway array are the 

most important considerations. The capabilities of the WIM 

controller can also be an important consideration when 

determining which technology will be used to collect data that 

will meet the customer’s needs.

Major Factors Affecting WIM Performance

1. Sensor type 

2. Number and spacing of sensors in the array

3. Pavement support and surface smoothness

4. Roadway geometry and traffic flow

5. Proper installation, including oversight

6. Type of installation material (grout)

7. Routine maintenance

8. Calibration using heavy test trucks

9. Weekly data quality monitoring for weights

and spacing
42

Table 2 summarizes information presented in the previous 

sections and could be used to compare the main characteristics 

of different sensors, and to identify sensors appropriate for 

different applications/uses.
43

Table 2 – Summary of WIM Sensor Technology Assessment

Sensor 

Type Strengths Weaknesses ASTM Type

Recommended 

Use/Applications

Load Cell High 

accuracy

Use in concrete 

only

High cost

High maintenance

I, II, III

Highway 

Monitoring, 

Highway Design, 

Planning, Prescreening

Bending 

Plate

High 

accuracy

Use in concrete 

only

High to Moderate 

cost

I, II, III

Highway 

Monitoring, 

Highway Design, 

Transp. Planning, 

Pre-screening

Quartz 

piezo

High 

accuracy,

low 

maintenance

Moderate cost I, II, III

Highway 

Monitoring, 

Highway Design, 

Transp. Planning, 

Pre-screening

Strain 

Gauge 

Strip 

Sensor

High 

accuracy,

low 

maintenance

Limited long-term 

performance 

record

Implemented with

limited number of 

controllers

I, II, III

Highway 

Monitoring, 

Highway Design, 

Transp. Planning, 

Pre-screening

Permanent

Polymer 

Piezo

Low cost,

low 

maintenance

Temperature 

sensitive

Needs frequent 

local calibration

II

Highway 

Monitoring, 

Transp. Planning

Portable 

Polymer 

Piezo

Low initial 

cost, easy 

to set up, 

portable

Low accuracy

Temperature 

sensitivity, needs 

local calibration

II Planning

Bridge 

WIM

(strain 

gauge)

Quick to 

set up, 

portable

Depends on local 

bridge structural 

response, needs 

local calibration

Safety risk & 

special equipment 

for tall bridges

II

Add-on operation 

to bridge 

monitoring
44

WIM Controllers

Purpose 

The WIM controller collects and processes the input signals 

from each of the in-road WIM sensors, computes timing 

information to determine axle spacing and speed of a vehicle, 

processes raw WIM sensor signals and computes static weight 

based on system weight compensation factor settings, provides 

vehicle record data, and stores the data for later transmission to 

the host computer.

Main Components and Functions 

The type of WIM controller depends on the WIM data collection 

application and the necessary functions required of the WIM 

system. For example, the WIM controller used for a virtual 

weigh station (VWS), which is used for enforcement prescreening, must be able to perform video data collection and 

real-time data transmission to a remote location. It may also 

require an automatic license plate reader and/or an over-height 

detector integration. A functional diagram of a typical WIM 

controller is provided in Figure 26. 

Figure 26 – WIM Controller Internal Components Functional Diagram
45

The six basic functions that are contained in most WIM 

controllers include the following.

1. Power Distribution 

The power distribution function feeds

power from the external power devices 

such as batteries, charge controllers, 

environmental power, or alternating 

current (A/C) converters to the WIM 

controller components. In the case of a power surge, power 

is diverted to grounding devices through power surge 

devices installed in line with sensors, telephone, power, and 

ground.

2. Sensor Signal Collection and Processing

The signal processing function of the WIM 

controller collects the analog signals from 

the loop inductors, WIM sensors, and 

temperature sensors, then converts the 

analog signals to digital signals and sends 

them to the central processing unit (CPU). 

3. Central Processing Unit

The WIM controller CPU contains all of 

the routines that the WIM system uses to 

process the various digital input signals 

and produce individual vehicle records 

that include date, time, speed, weight, axle spacing, and 

vehicle classification data. 

It also contains the WIM vendor’s software and/or firmware

that is used to format the vehicle records into the data files 

containing information about vehicle weight, speed, and 

vehicle classification. This information may include:

• Site identification (including global positioning 

system, GPS)

• Direction and lane number
46

• Time and date of each vehicle passage

• Vehicle sequence number

• Vehicle speed and classification

• Vehicle length

• Sequence and spacing of axles

• Weight of all wheel paths, axles or axle groups

• Code for invalid measurement

• Inductive signature 

The CPU data files could be in binary or ASCII format. These 

files are eventually downloaded to the off-site host 

computer and processed using WIM data processing 

software to produce various reports and other usable file 

formats.

4. User Interface

The user interface provides a 

communication link between the CPU

and the user. The user interface is a 

system menu that may be accessed and 

viewed remotely using an off-site computer through a wired 

or wireless communication device, or it could be accessed 

through a display mounted on the front or top of the WIM 

controller’s chassis, or both. The WIM controller’s menu is 

used to set up or change system settings, check the 

operational status of the WIM sensors, or download data. 

The individual vehicle records can be seen in real-time, as 

they are being collected, on a display provided in the 

controller cabinet or on an external computer screen.

The user interface provides access to various operating 

parameters and calibration factors that can be adjusted by 

the user. Some of the user menu functions include:

• Real-time vehicle viewing selectable by lane

• Resetting of the system clock
47

• Monitor system memory in terms of storage 

remaining

• Setup and initiate the generation of summary 

reports on data previously collected by the system

• Transfer selected raw data files 

• Classification parameters

• View and adjust calibration/system settings

• View system internal troubleshooting parameters 

such as voltages, sensor output signals, and data 

signatures

5. Data storage

The data storage function of the WIM 

controller consists of on-board memory 

devices that store the data as collected until 

the data are transmitted to the host 

computer through an external communication device. The 

WIM controller data storage function should provide data 

storage capability for a minimum of 7 days to prevent 

overlap or loss of data.

6. Communications

The WIM controller’s data output

devices, such as a serial T10 or Ethernet 

port, can be connected directly to a 

personal computer at the field site using 

a communications cable or remotely 

accessed through a wired or wireless modem connection

that transmits the WIM data off-site. The individual vehicle 

records can also be seen in real-time, as they are being 

collected, on a display screen installed on the top of the 

controller, or on the external computer screen. 
48

Additional Recommended Functionality 

The recommended additional features for a WIM controller are 

based on the data application and may include:

1. Calibration Features:

a. Front-axle weight compensation 

factor independent from GVW 

compensation factors

b. Temperature 

compensation/temperature measurement. An inpavement sensor is best. 

c. Auto-calibration (for polymer, coaxial and ceramic 

piezo sensors only). Caution: Auto-calibration works 

best on high truck volume roads and may not be 

applicable on low truck volume roads. It also should 

be set in a way to assure accurate GVW, not just 

accurate front axle measurement.

2. Self-diagnostics Features

a. Sensor waveform

b. Timing information

c. Sensor presence analysis

d. System power monitoring

3. Enforcement Features (screening 

for enforcement only): 

a. Video processing

b. Real-time data streaming

c. Over-height vehicle detection 

d. Automatic license plate reader data processing

e. Overweight limit setting for single, tandem, tridem, 

and GVW
49

DATA REPORTING 

REQUIREMENTS

WIM Vendor Software and Firmware 

Requirements

It is a good practice to provide a set of specific 

requirements for WIM data format(s) or a WIM 

data specification as part of WIM procurement 

documentation. WIM vendor software and 

firmware requirements should be based on the 

user’s needs for WIM data. In addition, WIM 

software should have data quality control (QC) functions to 

monitor and report weight data accuracy. 

FHWA recommends that users ask WIM vendors provide vehicle 

weight data as individual vehicle records, preferably in the 2016 

TMG PVF data format. Individual vehicle records provide: 

• A detailed analysis of traffic patterns and vehicle 

characteristics. 

• Development of axle correction factors and statistical 

procedures for data summarization and estimation. 

• Detailed data QC checks. 

• Troubleshooting WIM equipment performance issues, 

vehicle misclassification, or data collection and sampling 

issues. 

• The ability to store individual records for all vehicle 

types, not just classes 4 through 13.

In addition, the WIM data processing software should be 

capable of generating daily, weekly, monthly, and annual 

summary reports, including vehicle speed, classification, ESAL, 

and axle loading and GVW distribution summaries by vehicle 
50

class, by lane, and by direction, as well as data summaries in 

hourly increments. WIM systems could also be used to collect 

vehicle classification data and should be capable of reporting 

vehicle classification data sets. The software should be able to 

generate reports on errors, auto-calibration, site history, 

calibration history, and overweight vehicles. 

Axle correction factors (ACF) are an important output from 

WIM, and they are critical to assure proper animalization of 

single road tube counts. Annual updates of ACFs are necessary.

This is detailed in TMG 2016 chapter 1.2.6 (7).

Many WIM vendors have very effective controllers for data 

collection but their software has very limited data QC and 

quality assurance (QA) features. Several agencies use WIM 

vendor firmware and software to generate data records that 

could be transferred to a different software for data processing 

and QA/QC of the data. 

FHWA WIM Data Format 

If a WIM system is procured, at least in part, for 

collecting and reporting data to FHWA, the WIM 

vendor software should be capable of data 

processing and reporting in the FHWA TMG 

format (currently, TMG version October 2016). 

Data files generated by the WIM vendor software should be 

formatted for direct upload to national databases maintained 

by the FHWA, including the Travel Monitoring Analysis System 

(TMAS) and any of its successors. See chapter 7 in the 2016

TMG for the FHWA WIM data formatting requirements.

The recommended format for reporting WIM data to FHWA is 

PVF. The WIM vendor software should be capable of producing 

data in the “W” or “Z” variants of the FHWA PVF described in 

sections 7.7.4 and 7.7.5 of the 2016 TMG, respectively (7). The 

advantages include the availability of the detailed information 

that could be used to monitor WIM data quality and to 

investigate its dependency on external factors such as speed 
51

and temperature and the ability to aggregate data in different 

ways that suit the needs of different users.

Mechanistic-Empirical Pavement Design 

Guide (MEPDG) Format

If a WIM system is procured for collecting data to support 

pavement design based on the MEPDG method, then the WIM 

data processing software or other agency available software 

should be capable of generating traffic data in the MEPDG 

format, including: 

• Axle load distribution factors (ALDF) 

• Number of axles per truck (APT) 

• Vehicle class distribution (VCD) 

• Monthly adjustment factors (MAF) 

• Hourly distribution factors (HDF) 

• Wheelbase of the power unit for tractor-semitrailers

• Axle spacing, and axle spacing frequency distribution. 

WIM sensor ability to measure truck lateral wander (outer 

wheel position from the pavement edge for each truck, in 

inches) is an advantage for MEPDG use. See the AASHTOWare 

Pavement ME Design user’s guide for MEPDG traffic data 

formats (9). 

Download Method

WIM data are typically downloaded and stored by 

an off-site host computer using telemetry and 

vendor software. The sites are typically polled 

automatically every night. For sites without 

reliable telephone or cellular service, the data are

downloaded manually at the site. Manual download also 

requires vendor software to be installed on the field 

technician’s laptop computer. The field person connects to the 

WIM controller directly using a connected cable, and then 

utilizes the controller’s user menu to select the data files for 
52

transfer to the laptop. Downloaded data may be stored in the 

agency’s central traffic database or at an on-site or off-site 

database maintained by the WIM data service provider. Data 

may also be stored as flat files in a TMG or user-specified 

format.

Automated polling is the most desirable method of WIM data 

download. It is good practice to include auto polling software 

functionality capable of generating flat files in the current TMG 

format in the WIM procurement documentation. Automated 

polling of data should be accomplished every day to reduce the 

risk of missing or overlapping data.
53

PROCUREMENT 

CONSIDERATIONS

User Requirements and Vested 

Involvement

The intended use of the WIM data will help 

to determine the type of WIM equipment, 

the specific locations, and physical and 

traffic characteristics of the road segments 

selected for WIM sites installation. Therefore, the first step in 

choosing a WIM system is to clearly define the user 

requirements, including:

• Data types 

• Data accuracy

• Data storage and size of data

• Data collection frequency and length (days of collected 

data)

• Lane array and sensor type

• WIM sampling requirements (number of sites for types 

of routes) 

• WIM site locations

It is important to get the users of WIM data to advocate for 

continuous WIM program support within the agency. 

Integrating WIM data into vital Department of Transportation 

(DOT) functions such as pavement design, performance 

measurement, and safety and freight studies is a key to a WIM 

program’s longevity and continuous agency support from DOTs. 

Otherwise, it may be difficult to get a commitment to fund a 

given WIM operation over the long-term. Although decision

makers may recognize the need to collect the WIM data, a lack 

of a mandate or low utilization of WIM data in an agency’s vital 
54

operations may lead to reduction or elimination of the program 

funding when budgets are limited. 

Factors Affecting WIM Technology 

Selection

Factors influencing selection of a specific 

WIM technology include: 

• Reliability of a WIM system and 

its components (i.e., the ability of 

the system to consistently collect 

weight data that meet the 

performance requirements over long periods of time)

• Locations where a given technology can be successfully 

installed

• Array of sensors to be installed

• Sensitivity of WIM sensors to various factors 

(temperature, vehicle dynamics, traffic volume, and 

speed)

• Traffic conditions observed at the proposed site

(number of lanes and directions)

• Frequency and ease of the WIM system set-up, 

maintenance, and calibration to assure consistent 

performance

• Resilience of sensor installation (e.g., the ability to 

continue to collect data if one or more sensors fails) 

• Life span of sensors and other WIM components in 

comparison with the data collection period

• Number of WIM sites needed

• Local weather conditions

• Cost of the WIM system components and their 

installation

• Cost, time, and effort to order and replace failed parts, 

vendor’s customer support
55

WIM Sensor Longevity Considerations

When selecting a WIM sensor, in addition to accuracy 

requirements and operation costs, the period of data collection 

and sensor life cycle costs are an important consideration. Cost 

estimations for WIM sites that are intended to operate for more 

than 5 years should consider replacement of piezo sensors and 

possible pavement remediation to improve smoothness. The 

use of bending plates or load cells would be cost-prohibitive for 

sites that are installed to operate less than 5 years.

Availability of Funding and Resources

The cost of the WIM system is directly related to 

the overall WIM system performance. The 

need for resources increases as the required 

accuracy level increases to provide for 

additional data and equipment QA to 

maintain higher levels of accuracy. 

To address these financial considerations, a balance must be 

struck between the number of WIM data collection locations, 

the duration of data collection, and the accuracy of WIM data. 

User requirements and anticipated costs to procure and 

maintain WIM systems should be evaluated against the 

available or projected funding. If funding is not sufficient to 

address all user needs, traffic monitoring personnel in 

cooperation with the data users must decide what is more 

important: data sampling (number of sites/locations covered) or 

data accuracy. 

If sampling is more important, a decision may be made to 

procure WIM systems that are less expensive to install and/or

maintain. The data users must decide if lower quality data still 

would be suitable for use in their applications. 

On the other hand, if accuracy is more important, a decision 

may be made to procure WIM systems that are more accurate 
56

and inherently more expensive. Strategic selection of 

prospective WIM site locations based on functional class of the 

roadway, truck traffic distribution, and regional road usage may 

partially overcome sampling/coverage problems. 

Another cost-saving option is to approximate vehicle weights at 

vehicle classification sites, using WIM data from sites that have 

similar vehicle classification and road use characteristics. To do 

so, road usage by different truck types should be well 

understood.

Life Cycle Costs

When making a decision about purchasing a 

WIM system, costs beyond the initial WIM 

system purchase and installation need to be 

considered. Availability of the staff and other 

resources necessary for continuous WIM 

program operation also plays an important role in procurement 

decisions. Continuous funding support and resources are 

needed for routine WIM performance verification and 

calibration, site maintenance and repairs, data processing, and 

QA. The costs associated with software upgrades and pavement 

rehabilitation at the WIM site may also need to be considered.

Breakdown of WIM Life Cycle Costs 

The following is a comprehensive list of the life cycle costs 

associated with procuring and maintaining WIM systems: 

1. WIM Site Preparation and Installation Costs

a. Site selection, design, and field investigation (site 

selection report and design/construction plans)

b. Pavement preparation for WIM site installation to 

improve smoothness (if any)

c. Other site improvements

i. Vehicle pull off area

ii. Power service drop by power supply company

(if not solar)
57

iii. Telephone service drop by telephone service 

company (if not cellular)

iv. Equipment protection (guardrail or barrier)

d. WIM system installation turn-key (WIM equipment 

and supporting infrastructure purchase, installation, 

initial calibration and acceptance testing)

2. Central Computer Processing System

a. Dedicated host computer for data downloading and 

storage

b. Software for auto polling, quality control and 

quality assurance (QA/QC), processing, and 

reporting

3. Annual Operating Costs

a. System operating expenses

i. Power costs per site

ii. Telecommunication costs per site

b. WIM data processing, reporting, and QA/QC per site

c. WIM site maintenance and calibration expenses per 

site per year (cost of calibration truck is an 

additional expense)

d. Sensor replacement cost, averaged over 5 years, 

per site

e. Pavement smoothness improvement costs, when 

applicable (for calculations, spread over a 5-year 

period) 

Pavement Condition and WIM Life Cycle Costs 

To optimize the available resources, it is important to match the 

design life of the WIM equipment with the life and condition of 

the pavement. The lifespan of the in-road piezoelectric WIM 

sensor should be equal or less than the service life of the 

pavement in which it is placed. For bending plates, the sensor 

can be removed, but the frames into which the bending plates 

are set are not removable. If installing bending plates or load 

cell sensors on a road with AC pavement, the feasibility and cost 

of a concrete slab installation should be considered. If needed, 
58

the WIM controller and WIM cabinet may be successfully 

moved between different locations. 

The costs to maintain pavement smoothness near the WIM site 

over time (to minimize effects of road roughness on dynamic 

tire forces that affect WIM measurement accuracy) should be 

considered. Major pavement rehabilitation is not typically 

included in the WIM life cycle cost analysis, since the service life 

of the WIM system is usually dictated by the condition of the 

pavement. Pavement rehabilitation activities such as chip 

sealing, thin mills and overlays, thin overlays, and ultrathin 

overlays may reduce the output of polymer piezo sensor and 

consequently prevent the WIM system’s ability to provide viable 

data. However, pavement maintenance activities, such as crack 

sealing and seal coating, may be necessary to extend the life of 

the WIM system, and so these costs must be considered. 

Figure 27 – Smooth Pavement Section 

Cost Comparison 

When budgeting for a WIM site, or comparing different WIM 

site options, the following cost categories need to be 

considered:

1. Initial WIM sensor and site installation costs

2. Annual operation costs

a. Routine maintenance costs 

b. Routine calibration costs 
59

c. Annualized sensor replacement costs

3. Additional infrastructure, maintenance of traffic (MOT),

and mobilization costs

Sensor Costs 

The most significant cost difference between WIM system 

installations is the type of WIM sensor that is used. Table 3

summarizes the average costs for the different types of sensors. 

Installation costs are discussed in the next section. The costs in 

dollars shown in Table 3 reflect the following sensor layout that 

is required to produce ASTM Type I or Type II performance: 

• Polymer piezo – 4 half-lane width sensors

• Quartz piezo and strain gauge strip sensor – 2 half lane

width sensors

• Bending plate and load cell – 2 half-lane width sensors

Table 3 – WIM Sensor Costs

Sensor Type

Number of

half-lane 

sensors

Range of costs for 1

lane of sensors

Life 

(years)

Sensor 

Installation

low high 

Polymer Piezo 4 2-3 4,000 6,400

Quartz Piezo 2 3-5 16,000 24,000

Strain Gauge 

Strip Sensor 2 3-5 16,000 24,000

Bending Plate 2 6-8 18,000 28,000

Load Cell* 2 10-12 44,000 53,000

*Includes cost of the pit.

In addition to the costs shown in Table 3, installation of a 

concrete slab is required for bending plates and load cells 

installed in AC pavements. This additional cost must be added 

to the costs listed in Table 3. Due to high added costs, bending 

plates and load cells are rarely used for WIM installations in AC 

pavements.
60

Initial Installation Costs 

Table 4 provides costs associated with the WIM site installation 

that are in addition to the WIM sensor costs provided in Table 3. 

The values in the table represent the costs for installing a 2-lane 

and 4-lane WIM system for sensor layouts described in the 

previous section. The 2-lane WIM system shown in Table 4

shows necessary system elements other than WIM sensors. The 

assumptions used to develop the cost table for the 2-lane WIM 

system include:

1. The pavement condition is acceptable for a WIM 

installation

2. The National Electrical Manufacturers Association 

(NEMA)-rated roadside cabinet is mounted on a 

breakaway-design and located outside of the clear zone, 

per AASHTO Roadside Design Guide, 4th Edition (10)

3. Directional boring (recommended) 

4. Two traffic-rated junction boxes are required

5. Polyvinyl chloride (PVC) conduit is used to route the 

sensor leads to the junction boxes and to the cabinet

6. Host computer vendor software for data processing and 

download is included

7. Cellular or landline modem

8. Solar panel (typically one 80-watt panel) 
61

Figure 28 – 2-Lane Infrastructure

The 4-lane WIM system shown in Figure 28 shows necessary 

system elements other than WIM sensors. The assumptions 

used to develop the cost table for the 4-lane WIM system 

include:

1. The pavement condition is acceptable for a WIM 

installation

2. The NEMA-rated roadside cabinet is mounted on a 

breakaway-design and located outside of the clear zone, 

per AASHTO Roadside Design Guide, 4th Edition (10). 

3. Directional boring (required)

4. Three traffic-rated junction boxes are required.

5. PVC conduit is used to route the sensor leads to the 

junction boxes and to the cabinet (approximately 250 

feet)

6. Host computer vendor software for data processing and 

download is included. 

7. Cellular or landline modem

8. Solar panel (typically one 80-watt panel) 
62

Figure 29 – 4-Lane Infrastructure
63

Table 4 – Initial Installation Costs

(Excludes Sensor Installation Costs Shown in Table 3)

*The initial calibration cost may be included in the cost of the sensor 

purchase. 

**Considers only WIM vendor software for rudimentary data 

processing and reporting. 

Continuous Operation Costs 

Table 5 provides costs associated with 

maintaining and calibrating the WIM system 

throughout its expected lifetime. The 

assumptions used to develop the operations 

cost table include:

• Annual calibration per ASTM E1318-09, including cost of 

using one 5-axle semi-trailer truck 

• Routine maintenance is 2 visits per year

• Software updates

Element 

Range of costs

2 lanes 4 lanes

low high low high 

1. Installation 

(a+b+c) 20,500 56,500 32,000 78,000

a. Controller 7,500 24,000 9,000 26,000

b. Infrastructure 13,000 28,000 23,000 45,000

c. Initial 

calibration 0* 2,500 0* 5,000

2. Vendor software 

installed on host 

computer **

0* 2,000 0* 2,000
64

Table 5 – Continuous Operation Annual Costs

*Using one Class 9 truck (typically $90 to $135 per hour), maximum 20 

runs per lane. Bending plates and load cells may be calibrated every 2 

years.

** Includes parts, MOT and labor, but does not include mobilization. 

*** Based on the number of sensors in each lane and the expected life 

of the sensor shown in Table 3. 

It is important to consider that software upgrades may also be 

needed. However, these are program costs not associated with 

a particular type of sensor and are difficult to estimate.

Additional Costs Considerations 

There are several additional considerations that may affect the 

cost of the WIM installation, including the need to provide 

power service or landline telephone services at the cabinet, as 

well as traffic control requirements. The use of agency 

personnel to perform the construction or initial calibration, as 

opposed to contractors, may also affect the cost. Costs 

associated with data processing and reporting are not included 

in the estimates described in the previous sections. 

Element

Range of Annual Costs

2 lanes 4 lanes

low high low high

1. Routine Maintenance 500 1,200 750 1,500

2. Routine Calibration* 2,500 6,000 4,000 7,500

3. Sensor Replacement** Average Annual 

Cost per Lane***

a. Polymer Piezo 4,300

b. Quartz Piezo 10,600

c. Strip Sensor 10,600

d. Bending Plate 7,000

e. Load Cell 9,000
65

In addition to the costs provided above, other costs that may be 

incurred, which could affect the life cycle costs for WIM 

systems, include:

1. Mobilization costs – Costs to mobilize equipment and 

crews to the WIM site (est. $1,000 to $2,500).

2. Maintenance of traffic – The location of the WIM 

installation will dictate the level of MOT that is 

required. WIM installations on high-volume highways, 

with multiple interchanges, may increase the 

installation costs substantially. Typical lane closure costs

range from $800 to $2,500 per day. 

3. Power – Solar power requires an initial investment of 

$800 to $1,600, depending on the system’s power 

requirements, but little ongoing maintenance cost. A/C

power service requires an initial investment to get the 

power to the cabinet (est. $5,000 to $10,000) and 

ongoing minimal monthly delivery and usage fees 

(under $20 per month). 

4. Cellular service requires a small initial investment of 

$350 to $650 for the modem and ongoing $25 to $35 

monthly service fees. Traditional landline requires an 

initial investment to get the telephone service to the 

cabinet, which may range from hundreds to thousands 

of dollars (est. $5,000). Traditional landline modems 

cost from $250 to $750, depending on features and 

data transfer rates. Ongoing monthly service fees may 

range from $15 to $35. 

5. Pavement improvement costs – For asphalt pavements, 

at least minimum pavement maintenance such as crack 

sealing is typically required every 5 years (est. $1,000 

per 400-foot section). Concrete slab installation 

required for bending plate and load cell sites installed in 

AC pavements.

6. WIM installations that are incorporated into larger 

pavement design and construction projects may 

increase the cost of the WIM system dramatically. One 
66

state agency reported that it sometimes doubles their 

installation cost.

7. Additional calibration costs – Turnaround times, length 

of trip per run, number of test truck runs, truck hourly 

rates, fuel surcharges and accuracy requirements can 

double calibration costs. 

8. Staff – Additional resources may be needed for

dedicated or shared in-house staff and/or contractor 

staff. The cost depends on the size of the WIM program 

and data quality requirements; estimate 1⁄2 to 2 full-time 

equivalent personnel at the transportation specialist 

level, including: 

a. Data processing and analysis staff

b. Field technicians

c. WIM operation manager

Total WIM Costs 

To compute the total cost for installing and maintaining a WIM 

system using a specific type of sensor, first multiply the per lane 

costs in Table 3 by the number of WIM lanes and then add to

the costs calculated based on Table 4 and the costs provided in 

Table 5. Then, add estimates derived from the additional cost 

items listed above to compute the total cost. These numbers 

could be used for WIM program/operation budgetary planning. 

Benefits of Upgrading Existing Traffic 

Volume or Classification Monitoring Sites

to WIM

To reduce the cost of a permanent WIM installation, upgrading 

an existing traffic count or volume site to WIM could be an 

attractive option. The primary benefit to upgrading an existing 

site is the reduction in cost, since the majority of the required 

infrastructure is already in place. Secondary to that, the existing 

site may have in-road sensors already installed for counting or 

classifying that may be used for WIM, such as inductive loops.

However, the existing site must meet the requirements for a 
67

WIM installation with regard to pavement condition, 

geometrics, traffic flow, and safety (see ASTM E1318-09 for the

requirements). In addition, the location should meet data user 

needs.

WIM Pilot Study Implementation

First-time users or users looking into new 

technology implementation should consider 

a pilot program prior to any large-scale 

projects. A pilot study allows the opportunity 

to evaluate commercially available WIM sensor and 

controller technologies, power options, and 

communication devices before developing standard WIM design 

and procurement specifications. 

Installing the first unit in a lane directly in advance of an 

enforcement fixed scale near the agency’s office is one 

recommended approach to reduce cost and provide a way of 

tracking yearly performance. A pilot study also gives the buyer 

extra leverage in getting small issues resolved quickly by a 

contractor or vendor promoting their product. The minimum 

pilot test time should be one year.

Procurement Recommendations 

Standards and Specifications 

To ensure a durable WIM installation, 

standards for each element of the installation 

should be explicitly stated in the contract, 

leaving no room for interpretation. On-site 

observers (WIM installation QA inspectors), 

acting as an agent for the agency, should strictly hold the 

contractors to each of the installation standards and 

specifications.
68

Appendices C and D provide examples of WIM procurement 

contract, manufacturer’s installation standard, and warranty 

statement. 

WIM Performance Characteristics 

For every WIM system procurement contract, the minimum 

WIM performance characteristics (measurement accuracy and 

means for evaluating this accuracy) should be clearly stated 

with references to the applicable standard (in the U.S., ASTM 

E1318-09). 

Technology Selection

If budget permits, it is recommended to select one type of WIM 

sensor, controller, and data processing software. Use of 

multiple types of controllers and software packages could pose 

significant challenges with keeping up with hardware and 

software improvements and cross compatibility. Agencies that 

experience a high rate of personnel turnover would have the 

added financial burden of training new technicians and data 

managers on several different types of equipment and software 

packages.

Site Locations 

The site locations should be evaluated against the requirements

specified in ASTM E1318-09 and vetted by the stakeholders 

before procurement documentation is assembled. Once WIM 

site locations are finalized, GPS coordinates, road name, and 

milepost information should be included in the procurement 

documentation.

Bidding/Award 

In addition to the practices established by the 

highway agency for installation of traffic 

monitoring sites, the following recommendations 

specific to WIM installation, calibration, and/or 

maintenance contracts should be considered.
69

Contractor’s Previous WIM Experience and Certification

The installation and calibration of WIM systems is extremely 

technical in nature. An incorrect layout or the failure of an 

installation contractor to follow manufacturer specifications can 

be detrimental to the lifespan and quality of the data collected 

by the system.

It is extremely important that the installation contractor’s onsite supervisor have several years’ experience in installing and 

calibrating WIM systems and certified by the WIM manufacturer 

in installation of sensors and controller set-up.

Basis for Award

Technical capability, experience, and cost should all be 

evaluated when awarding WIM installation contracts. Agencies 

should incorporate a scoring system that evaluates the 

experience, past performance and technical aptitude of 

prospective bidders, specific to the type of WIM system being 

installed. Low cost should not be used as a single determining 

factor due to highly technical nature of the job. Minimum 

qualifications related to the extent of WIM installation and 

calibration experience should be established, and no awards 

should be made to contractors not meeting these minimum 

qualifications.

Sole-Source Justification

Although the process can be quite difficult, and the justification 

for doing so can be challenging to WIM program managers, 

sole-source justification can significantly reduce the costs 

associated with trying to maintain a consistently high level of 

expertise and can provide a means for stretching funding by 

reducing the investment on different vendor software packages 

and possibly eliminate the need to create customized software 

to handle the different data outputs from different WIM 

controllers. Sole-source can typically be justified if these initial 

and annual cost savings can be demonstrated.
70

Turn-Key System Delivery

Use of multiple contractors to install WIM 

system components (sensors, controller, 

infrastructure, software) may lead to troubles 

down the road if issues with data quality arise. 

It may be difficult and time-consuming to determine 

fault if something goes wrong. Therefore, it is recommended to 

use a single contractor responsible for turn-key fully functional 

WIM system delivery.

WIM Data Purchase versus WIM System Purchase 

Paying for WIM data as a commodity is another option. Under 

this type of contract, the contractor owns and maintains all of 

the WIM data collection equipment, either including or 

excluding the in-road sensors, and the agency pays for the 

acceptable data that they receive. Data acceptability is 

determined using a qualifying and quantifying data acceptability 

system. This arrangement incentivizes the contractor to provide 

good quality WIM data for the maximum number of days and 

takes the burden off the contracting agency in maintaining and 

servicing the WIM sites in a timely manner, thus solving the 

problem with qualified or limited human resources.

Warranty 

The WIM equipment procurement contract 

should have a warranty period that is 

reasonable in regards to the equipment 

and its intended use. For example, a 5-year 

warranty on parts may be deemed 

reasonable. To keep the warranty, a regular 

preventive and corrective maintenance program should be 

established for the site to help to ensure that the expected “site 

design life” is attained. 

Appendix C provides an example of contract warranty language. 
71

Installation 

Supervision

The solicitation shall state that the WIM installation contractor’s 

on-site supervisor must be experienced in WIM installation. 

Additionally, it is recommended to require a manufacturer’s 

representative to be on-site during WIM installation.

Quality Assurance Inspection

Even the most experienced WIM 

installation contractor can make mistakes. 

Without proper independent QA and 

oversight, it is possible that a contract 

specification will be overlooked. Therefore, 

it is in the best interests of the agency to

stipulate in the procurement solicitation that they will assign a 

QA inspector who is knowledgeable in WIM installations to act 

as an agent of the state to oversee all installation efforts.

The solicitation shall state that the on-site QA inspector shall be 

responsible for enforcing all contract standards and 

specifications, ensuring that all work proceeds safely, and that 

all processes are properly documented using checklists and 

photography, and possibly video. The solicitation shall give 

power to the QA inspector to stop work if specifications are not 

being followed or if unsafe practices are occurring and shall not 

continue until the work deficiencies are corrected.

Electrical Testing

The procurement solicitation should include language that 

requires the manufacturer’s representative to test all WIM 

components to ensure that all in-road sensors are operating 

within manufacturer tolerances, power service is sufficient, the 

equipment is properly grounded, there is lightning protection, 

and all communication devices are working properly. 

Final Inspection

As part of the payment terms, the solicitation should state that 

the QA inspector, the manufacturer’s representative, and the 

contractor’s on-site supervisor will make a final inspection of all 
72

elements of the WIM system, documenting any substandard 

aspects and ensuring that each deficiency is corrected prior to 

the contractor leaving the site or final payment will be withheld 

until proper corrections of deficiencies are made.

Site Acceptance 

The acceptance of the site includes a review of 

the installation and a determination if the 

installation has met the contract specifications 

and is installed in accordance with the 

manufacturer’s specifications. Site acceptance 

should be based on the following criteria:

1. All equipment is installed as per manufacturer’s 

specifications

2. All equipment is installed as per contract specifications

3. All sensors have been successfully tested

4. All support equipment (solar, A/C, telephone) has been 

successfully tested

5. System has been calibrated per manufacturer 

procedures and validated to be performing per contract 

specifications

6. System has successfully passed vehicle classification test

7. System has operated for 30 consecutive days without 

failure and produces data of consistent acceptable 

quality

8. All site documentation and as-built drawings have been 

submitted and accepted

Site acceptance and acceptance testing should be done by the 

contracting agency or contracting agency agent that is 

independent of the installation contractor. Similarly, WIM 

system performance verification should be done independent of 

the WIM maintenance contractor.
73

Calibration 

Accuracy Standards

Initial WIM calibration requirements 

should be included in the WIM site 

installation procurement package. If the 

routine calibration function is being 

outsourced, use the following 

recommendation for the solicitation. 

The initial calibration should be done in accordance with ASTM 

E1318-09, and references to existing standards used by state 

and federal agencies for WIM system calibration should be 

provided in the procurement documentation. Examples of 

calibration procedures are provided in Appendix E. 

Reporting

The solicitation shall state that once a calibration of the system 

has been performed, the results of the calibration should be 

reported. The report should include:

1. Site ID

2. Date of calibration

3. Air and pavement temperature and weather conditions

4. Type and weight of test trucks. At least one loaded 

FHWA class 9 vehicle is recommended. Supply a weigh 

scale ticket documenting weights. 

5. Number of calibration test truck runs (minimum 10 

before and after calibration) for each lane

6. The measurement accuracy of each WIM lane when 

compared with the static weights of the test trucks, 

including: 

a. Mean percent error (measurement bias) for axle 

weights and GVW

b. 95 percent confidence interval for the percent error 

(measurement precision expressed as statistical +/- 

percent error range) for axle weights and GVW

c. Percent of axle weights and GVW measurements 

meeting ASTM E1318-09 performance parameters 
74

for a given type of WIM system (ASTM Type I, II, III, 

IV). 

7. The classification accuracy of the WIM system for each 

lane (percent classification errors in different vehicle 

classes, and percent of unclassified vehicles, using 

agency’s vehicle classification rules or classification

method). 

a. 2 percent or fewer misclassified or unclassified 

vehicles in the manual sample in FHWA vehicle 

classes 4 through 13 combined. 

b. 10 percent or less misclassification in each 

individual class of vehicles from 4 through 13. If the 

number of vehicles collected during the 3-hour 

testing period is under 30 for any vehicle class, the 

maximum number of misclassified vehicles:

• 20-29 for one class = +/-3 vehicles

• 10-19 for one class = +/-2 vehicles

• 0-9 for one class = +/-1 vehicle

8. The axle spacing and overall length measurement 

accuracy for each lane

9. Statement if each lane of the WIM system met or failed 

ASTM E1318-09 performance parameters for given 

ASTM type of WIM system

10. Final calibration settings for each lane

Maintenance 

A WIM installation is a significant 

investment that must be protected. A 

routine maintenance program is essential 

to maintain the WIM system’s ability to 

produce high-quality data. Routine 

maintenance, such as annual or semiannual site visits to inspect, clean, and 

calibrate WIM systems can be done using in-house personnel, 

incorporated into the installation contract, or can be awarded 

separately either to the manufacturer or to an independent 

agency. If the routine WIM maintenance function is being 
75

outsourced, use the following recommendations for solicitation

development. 

Routine Maintenance

The solicitation shall state that the technician assigned to 

maintain the WIM system should have a proven acceptable 

level of expertise with maintaining the specific type of WIM 

system that is installed. The timeliness of filed maintenance 

visits should be clearly stated. Include a sample maintenance 

report in the solicitation.

Spare Parts

Lost data can be costly to a WIM program. Without replacement 

parts readily available, parts can take months to arrive and be 

installed, resulting in extensive system down times. Therefore, 

include in the solicitation a requirement for the contractor to 

maintain a stock of ready service spare parts. Pricing for 

additional parts should be requested in the solicitation. 

Repairs 

As part of the procurement process, a warranty should be 

worded so that the conditions warranting the repairs that need 

to be made are explicit. The warranty language should indicate 

the reaction time by the installation contractor and/or the 

manufacturer to correct system failures and the penalties that 

will be incurred if the reaction time is not met.
76
77

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. Klein, Lawrence A. 2006. Traffic Detector Handbook: 

Third Edition – Volume I. Publication FHWA-HRT-09-108. 

Washington D.C.: Federal Highway Administration 

(FHWA):2006. 

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

tions/its/06108/

6. D. Walker, O. Selezneva, and D. Wolf, Findings from 

LTPP SPS WIM Systems Validation Study, Proceedings of 

the 6th International Conference on Weight-In-Motion, 

Dallas, Texas, June 2012

7. Federal Highway Administration (FHWA). 2016. Traffic 

Monitoring Guide. Washington, DC: Federal Highway 

Administration: 2016. 

https://www.fhwa.dot.gov/policyinformation/tmguide/
78

8. T. M. Kwon. 2016. Weigh-Pad-Based Portable Weigh-inMotion System User Manual. Report No. MN/RC 2016-

07. Duluth, MN: University of Minnesota Duluth: 2016. 

9. AASHTOWare Pavement ME Design software. User 

Manual. 

http://www.aashtoware.org/Pavement/Pages/default.a

spx

10. American Association of State Highway and 

Transportation Officials (AASHTO). 2011. Roadside 

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

https://bookstore.transportation.org/collection_detail.a

spx?ID=105
79

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) 
80

o Example Calibration Test Truck Procurement (LTPP)

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