WEIGH-IN-MOTION 

POCKET GUIDE

PART 3

WIM CALIBRATION AND 

MAINTENANCE GUIDE

Publication No.

FHWA-PL-18-015

Office of Highway Policy Information June 2018
ii

NOTICE

The United States Government does not endorse products or

manufacturers. Trade and manufacturers’ names appear in this 

report only because they are considered essential to the 

objectives of the document.

QUALITY ASSURANCE STATEMENT

The Federal Highway Administration (FHWA) provides highquality information to serve Government, industry, and the 

public in a manner that promotes public understanding. 

Standards and policies are used to ensure and maximize the 

quality, objectivity, utility, and integrity of its information. 

FHWA periodically reviews quality issues and adjusts its

programs and processes to ensure continuous quality 

improvement.
iii

TABLE OF CONTENTS

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

WIM Standards and Performance Requirements.................... 2 

WIM CALIBRATION ....................................................4

What is WIM Calibration?........................................................ 4 

Purpose of Calibration ............................................................. 5 

Types of Calibration ................................................................. 5 

Initial Installation Calibration............................................... 5 

Routine Calibration .............................................................. 7 

Types of Calibration Methods................................................10

ASTM E1318-09 Criteria.....................................................10

LTPP Criteria.......................................................................11

Tools and Equipment.............................................................12

Calibration Procedure............................................................12

Step 1 – Perform Pre-Visit WIM Data Analysis ..................12

Step 2 – Perform Pavement Profile Data Analysis.............14

Step 3 – Select Test Trucks and Determine Test Speeds.....15

Step 4 – Assess WIM Site Condition ...................................18

Step 5 – Inspect WIM Equipment.......................................19

Step 6 – Inspect and Measure Test Trucks.........................20

Step 7 – Verify Communications with Test Truck Driver....21

Step 8 – Perform Test Truck Runs.......................................22

Step 9 – Evaluate WIM Performance Using Data 

from Test Truck Runs..........................................................24

Step 10 – Calibrate WIM System Parameters....................28

Step 11 – Perform Post-Calibration Test Runs 

and Evaluate Results..........................................................28
iv

Step 12 – Check Accuracy of Vehicle Classification............29

Step 13 – Conduct Post-Visit Data Analysis 

and Develop Calibration Report.........................................32

Step 14 – Develop and Submit Calibration 

Summary Report ................................................................34

Step 15 – Develop Comparison Data Set............................35

Step 16 – Conduct Office Data Quality Checks 

and Analysis to Aid Future Calibrations.............................36

WIM Systems with Quartz Piezo Sensors 

or Strain Gauge Strip Sensors.............................................40

WIM Systems with Polymer Piezo Sensors.........................40

WIM Systems with Bending Plates or Load Cells ...............41

WIM SYSTEM MAINTENANCE ................................. 42

Types and Frequency of Maintenance...................................42

Tools, Materials, and Supporting Documentation.................43

Materials............................................................................43

Hand Tools .........................................................................43

Test Equipment ..................................................................44

Computer Software ............................................................44

Supporting Forms and Documentation..............................44

Safety .....................................................................................45

Preventive (Scheduled) Maintenance....................................46

Pavement Inspection..........................................................46

Visual Equipment Inspection..............................................47

Equipment and Site Cleaning .............................................49

Electronic and Electrical Checks.........................................49

Firmware Upgrades............................................................50

Operational Testing and Verification................................. 51
v 

Communications Checks ....................................................52

Corrective Maintenance and Repair......................................52

Scheduled Corrective WIM Maintenance...........................52

Unscheduled Corrective WIM Maintenance ......................53

WIM Equipment Troubleshooting .........................................53

Troubleshooting Approach.................................................54

Logical Progression of Troubleshooting.............................55

Maintenance Documentation and Reporting........................60

REFERENCES.............................................................62

APPENDICES ............................................................63

The APPENDICES section at the end of this document describes 

the appendices for the FHWA WIM Pocket Guide, which are 

available on the FHWA website: 

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

• Appendix A – Technical Report Documentation Page 

Form DOT F 1700.7 

• Appendix B – Acknowledgements

• Appendix C – WIM Procurement Resources

• Appendix D – WIM Site Design Resources

• Appendix E – Maintenance and Calibration Resources

• Appendix F – WIM Program Operation and 

Management Checklists
vi

LIST OF TABLES

Table 1 – ASTM 1318E-09 Functional Performance 

Requirements for WIM Systems.......................................... 3 

Table 2 - Example of Test Truck Run Statistical Results............... 7 

Table 3 - Example of Compensation Factors..............................10

Table 4 - LTPP Performance Requirements for WIM Systems...11

Table 5 - WIM Roughness Index Thresholds*............................15

Table 6 – Example of Selected Test Truck Run Data..................24

Table 7 – ASTM 1318E-09 Functional Performance 

Requirements for WIM Systems*......................................25

Table 8 - LTPP Performance Requirements for WIM Systems...27

Table 9 – Example of Test Truck Run Results Based on LTPP 

Method ..............................................................................27

Table 10 - Example of Comparison Data Set Computation 

Results................................................................................36

Table 11 – Example of WIM Data Analysis Results for Typical 

(3S2) Class 9 Vehicles.........................................................38

 
vii

LIST OF FIGURES

Figure 1 – Calibration Test Truck Approaching WIM Scale.......... 4 

Figure 2 – Measuring Test Truck Axle Spacing............................. 6 

Figure 3 – Weighing Truck at Certified Truck Scale ..................... 8 

Figure 4 – Gross Vehicle Weight by Speed .................................. 9 

Figure 5 – Example of Data Comparison for GVW Distribution . 13

Figure 6 - Calibration Test Truck ................................................16

Figure 7 – Truck Speed Distribution Plot ...................................17

Figure 8 – Pavement Inspection.................................................18

Figure 9 – WIM Sensor Inspection.............................................19

Figure 10 – Test Truck Inspection ..............................................20

Figure 11 – Test Truck Trailer Load............................................21

Figure 12 – Collecting Test Truck Speed ....................................22

Figure 13 – Collecting Pavement Temperature .........................23

Figure 14 – Example of Post Visit Analysis of Class 9 Steering 

Axle Weight Distribution....................................................33

Figure 15 – Daily Average GVW Decreasing Over Time.............39

Figure 16 – Protected Work Vehicle Location ...........................45

Figure 17 – Field Technician with PPE........................................45

Figure 18 – Pavement Distress Identified During Inspection.....46

Figure 19 – WIM Sensor Observed During Inspection...............47

Figure 20 – Pull Box Interior.......................................................48

Figure 21 – Traffic Cabinet Location after Vegetation Control..49

Figure 22 – Sensor Testing .........................................................50
viii

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 loadings

FHWA Federal Highways Administration

GFCI Ground-Fault Circuit Interrupter 

GPS Global Positioning System 

GVW Gross Vehicle Weight

HDF Hourly Distribution Factor

HOD Hour of the day

IRI International Roughness Index

LC Load cell

LPR License plate reader 

LTPP Long-Term Pavement Performance

MAF Monthly Adjustment Factors

MEPDG Mechanistic-Empirical Pavement Design Guide

MOT Maintenance of traffic

MOY Month of the year 

MUTCD Manual for Uniform Traffic Control Devices

NCR Non-Compliance Report 

NEMA National Electrical Manufacturers Association

OWL Optimal WIM Locator 

PCC Portland cement concrete 

PPE Personal protection equipment
ix

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 PART 3 

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

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

four instructional video supplements, and six appendices. The 

video supplements and appendices are only available on the 

FHWA website: 

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

Part 1 includes a description of each element of the FHWA WIM 

Pocket Guide (referred as Guide through the rest of the

document). This Part 3 was developed to assist WIM technicians 

in the proper maintenance and calibration of WIM sites. It 

includes step-by-step instructions for calibrating a WIM system; 

suggestions for tools, forms, and reporting processes that can 

be used for calibration and system maintenance; and a 

recommended process for troubleshooting system faults. 

The WIM Calibration section of the Guide includes valuable 

information for conducting a successful WIM calibration that is 

based on best practices, including a pre-visit traffic and profile 

data analysis to develop WIM system accuracy expectations for 

the validation, on-site activities to ensure the proper operation 

and calibration of the WIM system, and a post-visit analysis to 

assess the effectiveness of the calibration. Properly conducting 

these activities will ensure high-quality WIM data collection. 

The WIM System Maintenance section of the Guide includes a 

scope of recommended maintenance activities, recommended 

schedules, description of field checks, and recommendations for 

field logs, acceptance testing, and maintenance reports. This 

section also includes a comprehensive WIM troubleshooting 

procedure.

Throughout this document, action items are identified using 

numerical sequencing or the checkmark () icon. Please also 

refer to manufacturers’ documentation for additional guidance
2 

on recommended maintenance schedules and specific 

calibration requirements. 

WIM Standards and Performance 

Requirements 

The ASTM International E1318-09 Standard Specification for 

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

Requirements and Test Methods is a primary WIM standard 

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

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

systems according to four distinct types, depending on the 

application and functional performance requirements. 

Table 1 summarizes the ASTM E1318-09 WIM system 

performance requirements. The 95 percent compliance defined 

in ASTM E1318-09 specifies the minimum percentage of 

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.
3 

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.

Table 1 – ASTM 1318E-09 Functional

Performance Requirements for WIM Systems

Function Tolerance for 95% Compliance

Type I Type II Type III

Wheel Load (single or 

dual tires) ±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

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 (i.e. percentage of measurements 

within the specified tolerances), 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%
4 

WIM CALIBRATION

What is WIM Calibration? 

WIM calibration is the process of 

evaluating the measured weight, axle 

spacing, overall vehicle length, and 

speed values reported by the WIM 

system against known static weights

and manually measured axle spacing, 

vehicle lengths, and speed and making 

necessary adjustments to the WIM 

system operating parameters to compensate for those errors. 

The initial calibration is performed to ensure that the WIM 

system accuracy meets contract specifications after the site has 

been installed. Periodic routine WIM calibrations are performed 

to ensure that the data accuracy remains consistent and within 

the selected performance specification.

Figure 1 – Calibration Test Truck Approaching WIM Scale

As a result of the calibration, the mean error in WIM 

measurements (i.e., measurement bias) should be reduced to as 

close to zero as practically possible for all measured 

parameters. The WIM system calibration is typically performed 

by a qualified WIM technician in accordance with the 
5 

manufacturer’s specifications and guidelines set forth in ASTM 

E1318-09 or in FHWA’s LTPP Field Operations Guide for SPS 

WIM Sites (1, 4). 

Purpose of Calibration

Per ASTM E1318-09, the function of calibration is to define 

factors that will be subsequently applied within WIM system 

calculations to correlate the observed vehicle speed and tireforce signals with the corresponding tire load, axle spacing, and 

wheelbase values for the static vehicle (1). Calibration reduces

the influence of speed, temperature, truck type (if multiple test 

trucks are used), and environmental changes in the supporting 

pavement structure on the WIM system’s measurement 

accuracy for each lane measured. 

The influence of temperature changes on measurement error 

beyond those observed during calibration cannot be addressed 

without vendor assistance in setting site-specific temperature 

compensation factors. Also, unless multiple test trucks are used, 

the WIM systems accuracy as a function of type of truck cannot 

be effectively determined.

Types of Calibration

Initial Installation Calibration 

Purpose

The purpose of the initial calibration is to verify that, upon initial 

installation or major repair, the WIM system is providing the 

level of weight and spacing measurement and classification 

accuracy stipulated in the WIM procurement contract.

Scope

For the initial calibration, the WIM vendor or their certified 

representative (WIM contractor) typically calibrates the site 

according to the manufacturer’s procedures and any additional 

standards listed in the WIM system procurement contract. The
6 

contracting highway agency may use the results of the 

calibration to determine the acceptability of the system.

To perform the initial calibration, a qualified WIM technician

may do the following: 

1. Statically weigh and measure each axle and record each 

axle spacing and total bumper to bumper length of each 

of the calibration test trucks (see Figure 2). 

Figure 2 – Measuring Test Truck Axle Spacing

2. Perform a sufficient number of test truck runs (at least 

10 are recommended) to develop a reasonable 

confidence in the accuracy of the WIM system, 

especially with regard to speed, temperature, and truck 

type. 

3. Calculate and install the system error compensation 

factors to compensate for the measured system bias.

4. Conduct additional test truck runs to confirm that the 

changes had the desired effect on the WIM system 

measurement bias for the desired degree of confidence 

(more runs means higher confidence). 

5. Recalibrate, if necessary to meet contract specifications.

6. Document the compensation factors installed in the 

WIM controller, the number of test truck runs resulting 

in pass or fail of the WIM performance requirements, 

and WIM performance parameters, such as mean error 
7 

and range of errors, for future reference. Table 2 shows 

an example of such documentation.

Table 2 - Example of Test Truck Run Statistical Results

Parameter Mean 2SD P/F 

Steering Axle -0.3% 5.0% Pass

Tandem Axles 1.7% 5.0% Pass

GVW 1.4% 3.1% Pass

Vehicle Length (ft) -0.2 1.4 Pass

Vehicle Speed (mph) 0.0 0.6 Pass

Axle Spacing (ft) -0.3 0.2 Pass

7. Develop a record of whether the WIM system passed or 

failed WIM performance requirements based on ASTM 

or agency-specific performance requirements. 

A representative from the contracting agency reviews the initial 

calibration results to ensure that the WIM system is providing 

data with acceptable measurement error tolerances. WIM 

installation or sensor replacement is accepted only after the 

contracting agency has approved the initial calibration.

Additional details are provided later in this chapter, in the 

section titled Calibration Procedure.

Routine Calibration 

Purpose

The purpose of the routine calibration is to maintain the WIM 

equipment at the maximum level of accuracy possible for the 

service life of the system. An annual calibration is 

recommended. 

Scope

Typically, the routine calibration is conducted by agency’s inhouse staff or a WIM maintenance contractor staff using 

procedures described in the ASTM E1318-09 standard. The 

guidelines provided in the FHWA Field Operations Guide for 
8 

LTPP SPS WIM Sites could also be used for calibration of ASTM 

E1318-09 Type I WIM systems (4). No matter which 

specification is used, the person performing the calibration 

needs to conduct a sufficient number of test truck runs (10 to 

40, depending on the site, sensor array, and sensor 

characteristics) to quantify the weight, axle spacing, overall 

vehicle length, and speed measurement error of the WIM 

system with a 95 percent level of confidence. 

To perform the routine calibration, a qualified WIM technician 

may do the following: 

1. Statically weigh each axle, axle group, and gross vehicle 

weight (GVW) and measure each axle spacing for the 

calibration test trucks. The length of vehicle and 

overhang also might be needed if this is being verified 

with the calibration runs. Figure 3 shows a truck being 

weighed at a certified truck scale.

Figure 3 – Weighing Truck at Certified Truck Scale

2. Perform a sufficient number of test truck runs per ASTM 

E1318-09 or agency specification to develop a 

reasonable confidence level in the accuracy of the WIM 

system, especially with regard to speed. Figure 4 shows 

GVW measured at low, medium, and high speed ranges. 

Sometimes, only two different speed ranges can be 

used.
9 

Figure 4 – Gross Vehicle Weight by Speed

3. Make changes to the system error compensation 

factors to compensate for the measured system bias.

4. Conduct additional test truck runs per ASTM E1318-09, 

LTPP, or agency specification requirement to confirm 

that the changes had the desired effect on the WIM 

system measurement bias (1, 4). 

5. Recalibrate, if necessary to meet ASTM, LTPP, or 

agency-specific accuracy requirements. 

6. Document the changes to compensation factors 

installed in the WIM controller, the number of test truck 

runs resulting in pass or fail of the WIM performance 

requirements, and WIM performance parameters, such 

as mean error and range of errors, for future reference.

Table 3 shows an example of such documentation.
10

Table 3 - Example of Compensation Factors

Calibration Factors

Overall Factor 103

Wheel Path Left Right

Sensor 1 3 2 4 

Speed Factor 1 3312 3300 3312 3300

Speed Factor 2 3255 3364 3255 3364

Speed Factor 3 3220 3343 3220 3343

Speed Factor 4 3120 3280 3120 3280

Speed Factor 5 3120 3280 3120 3280

7. Develop a record of whether the WIM system passed or 

failed the WIM performance requirements.

8. Store the calibration settings both in the field and in the 

office for possible later use.

Types of Calibration Methods

The recommended calibration methods are ASTM E1318-09 

method and LTPP method described in LTPP Field Operations 

Guide for SPS WIM Sites (1, 4). Both The ASTM E1318-09 and 

LTPP calibration methods use similar procedures but apply 

slightly different evaluation criteria to interpret calibration 

results. Other methods based on the reduced requirements for 

the number of test truck runs could be used, if test data show 

consistency over time. 

ASTM E1318-09 Criteria 

ASTM E1318-09 specifies acceptable WIM accuracy tolerances 

for four types of WIM systems (see Table 1 above) (1). Based on 

the ASTM E1318-09 test method, the WIM system has failed to 

satisfy performance parameters for a given WIM Type if more 

than 5 percent of the measurements for the test truck runs 
11

exceed the error tolerances specified in Table 1 for any 

measurement type. 

Additionally, if mean errors (like GVW mean or average error) 

calculated based on all test truck passes significantly deviate 

from zero (user specified value, 2% or more is recommended in 

this Guide), then the WIM system measures with a systematic 

bias and calibration has failed. 

LTPP Criteria 

Based on the LTPP Transportation Pooled Fund (TPF) Study 

5(004) test method, the WIM system has failed to satisfy 

performance parameters if the overall error exceeds the 

tolerance limits specified in Table 2 (4). The overall error is 

computed as mean error +/- 95 percent confidence interval of 

measurement errors. The allowable mean error value for GVW 

is typically limited to 2 percent. LTPP also provides a vehicle 

length measurement requirement of ±1.5 feet or 3 percent of 

the vehicle’s overall length, whichever is longer. Table 4 

summarizes the LTPP performance requirements.

Table 4 - LTPP Performance Requirements for WIM Systems

Measurement Type 95 Percent Confidence 

Limit of Error

Steering Axle ±20%

Axle-Group Load ±15%

Gross Vehicle Weight ±10%

Speed ±1 mph (2 km/h)

Axle-Spacing ±0.5 ft (0.15 m)

Vehicle Length ±1.5 ft or 3.0%*

*Whichever measurement is longer.

The basic statistic is the error expressed as a percentage of the 

known value (i.e., percentile difference between the WIMmeasured weight and the static weight of the test truck). The 

percentage of error values calculated for each run are then 

combined in one pool and used to compute the mean error, the 
12

95 percent confidence interval for the error, and the overall 

error. 

Tools and Equipment 

The calibration technician may need the following tools to 

conduct the calibration:

• WIM manufacturer’s operations manual

• Communication equipment for 

connecting to the WIM system

• Walkie-talkies or cell phones for each 

driver and one for the WIM calibration crew that works 

over a suitable long distance

• Laptop

• Calculator

• Multimeter – resistance, inductance, and maybe 

capacitance 

• Megger – ground/insulation resistance meter

• Jeweler’s screwdriver

• 100-foot measuring tape

• Flashlight

• Tire pressure gauge

• Tire depth gauge

• Cabinet key

• Non-contact temperature gauge

• Laser speed gun 

• Protective safety equipment

Calibration Procedure

Step 1 – Perform Pre-Visit WIM Data Analysis 

A pre-visit data analysis provides information on the current 

operational status of the WIM system. It is used to identify any 

deviations in weight and length measurements from the values 

recorded immediately after the most recent calibration and 

saved in the comparison data set (CDS). It also provides 
13

information needed to establish the requirements for the test 

truck vehicles, including type, weight, length, and speed. Figure 

5 shows an example CDS for GVW distribution.

Figure 5 – Example of Data Comparison for GVW Distribution

Prior to the site visit, perform the following:

1. Collect a recent data sample from the WIM system. Two 

to 4 weeks’ worth of data is recommended, depending 

on truck volumes, to obtain a sample of at least 5,000 

heavy vehicles in classes 4 to 13. Analyze the data to 

determine the current site characteristics, including:

o Most common heavy truck type for the site to be 

used as a test vehicle (typically FHWA vehicle class 

9). 

o 85th percentile truck speed and a range of speeds 

for calibration test truck runs.

o GVW distribution for the prevalent heavy truck type 

(typically FHWA vehicle class 9) and target weight of 

calibration truck.

o Axle spacing for the prevalent heavy truck type

(standard tandem versus spread tandem).

2. Compare the expected traffic flow characteristics 

developed based on the data collected immediately 

after the most recent calibration (the comparison data 
14

set – CDS) with the traffic characteristics based on the 

recently collected WIM data. Use the results to assess 

the likely need for calibration and to establish the likely 

range of compensation factors to be applied in the field.

3. Annotate the following information from the data 

analysis: 

o Changes in the GVW of the most common heavy 

vehicle type, including averages, by: 

- Hour of the day (HOD) 

- Day of the week (DOW) 

- Month of the year (MOY) 

o Changes in vehicle class distributions by HOD, DOW, 

and MOY

o Changes in average truck speeds

Step 2 – Perform Pavement Profile Data Analysis 

If pavement profile data are available, an analysis of the most 

recent data from the WIM site location is recommended. The 

pavement smoothness analysis may be accomplished using the 

profile data collected with an automated high-speed profiler 

and standard agency procedures, or using the Optimal WIM 

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

software, which can be obtained at 

http://www.roadprofile.com/proval-software. 

1. Process profile data to identify locations of highest 

International Roughness Index (IRI) values within the 

600-foot WIM segment (400 feet before and 200 after 

WIM scale location).

2. Compare the values computed using the OWL software 

in the vicinity of the WIM site with the threshold values 

provided in Table 5, taken from American Association of 

State Highway and Transportation Officials (AASHTO) 

specification M331-13 (5). 
15

Table 5 - WIM Roughness Index Thresholds* 

WIM Type Lower Threshold 

m/km [in./mi]

Upper Threshold 

m/km [in./mi]

Type I 1.34 [84.8] 2.70 [171.1]

Type II 1.86 [117.9] 3.75 [237.7]

*These values do not take into account sensor and array types and may be too 

conservative for multi-sensor arrays or high accuracy sensors

3. Create a record of locations that produced values 

exceeding the upper threshold value. Values above the 

upper threshold indicate that road roughness is likely 

contributing to the measurement error.

Step 3 – Select Test Trucks and Determine Test 

Speeds 

This step is accomplished prior to the WIM site visit. The 

following guidance helps to determine the type of test truck to 

use for WIM site validation and calibration, including vehicle 

type (class), weight, axle spacing, suspension configuration, and 

bumper-to-bumper length and speeds for calibration test truck 

runs. 

Test Truck Selection

Typically, an FHWA class 9 vehicle is used for calibration. Both 

ASTM and LTPP WIM calibration procedures recommend using 

this type of truck. Figure 6 shows a photo of a typical calibration 

test truck. The exceptions are load-restricted or commercial 

truck-restricted roads; the heaviest truck class that routinely 

uses the road is recommended for calibration, which could be a 

class 5 truck. Also, both ASTM and LTPP recommend using two 

calibration test trucks.
16

Figure 6 - Calibration Test Truck

Use the following guidance to select calibration truck(s):

1. Whenever possible, use more than one calibration truck 

to better calibrate the dynamic loading conditions 

encountered at the site. 

2. Select calibration test truck(s) that best represent the 

dominant heavy trucks (i.e., trucks in vehicle classes 6-

10) for the WIM site. Typically, this is class 9.

3. If two calibration trucks are used, one truck should be 

at or near full load capacity and be configured with 

typical tandem spacing (4.0 feet to 4.5 feet apart for the 

truck and trailer) and have air suspension on the truck 

and trailer. 

4. Select different loading for the second truck to better 

represent the most frequently observed loading 

conditions at the site. 

5. However the trucks are retained, they should each have 

the following characteristics:

 Working speedometer

 Suspension systems must be free of mechanical 

deficiencies, including cracks, punctures, air leaks, 

or loose fittings 

 Capable of meeting the test truck speed 

requirements 
17

 Loads must be non-shifting, such as crane 

counterweights, steel plates or beams, or concrete 

blocks 

 If loads that allow the collection of rainwater are 

used, the trailer and load must be completely 

covered to avoid the collection of water and the 

associated increase/decrease in load weight as 

water collects or evaporates

6. Locate the nearest certified weigh scale location to the 

WIM site.

Test Truck Speed Determination

It is important that the test truck’s speed is as close as possible 

to the dominant truck speed at the WIM site, without exceeding 

the speed limit. The range of the test truck speeds should 

represent a 10 to 20 mph range of typical truck speeds for the 

site. Figure 7 shows a typical Class 9 speed distribution plot that 

is used to determine the test truck speeds for the calibration.

Figure 7 – Truck Speed Distribution Plot

Testing at multiple speed points (at least 2) is recommended to 

minimize error dependency on speed, if the WIM system 

supports this option. 

Temperature Consideration 

The LTPP method recommends using two test trucks over the 

widest ranges of temperatures observed at the site. For WIM 

sensors sensitive to temperature changes, testing over a 30 o

F 

temperature range is recommended. 
18

Step 4 – Assess WIM Site Condition 

Perform the site assessment to determine the possible effects 

of pavement, equipment, and site conditions on the accuracy of 

WIM weight and distance measurements.

Pavement Inspection 

1. Conduct visual pavement distress survey from the 

shoulder to identify surface anomalies that may affect 

truck motions across the sensors. Figure 8 shows an 

inspector conducting a visual inspection.

Figure 8 – Pavement Inspection

2. Identify such items as potholes, patches, rutting, and 

the asphalt to concrete transition. 

3. Use the most recent pavement profile data to 

determine specific high roughness locations and

examine these locations closely during the on-site 

assessment. 

4. Annotate observations and photograph any pavement 

distresses that may affect the accuracy of the WIM scale 

measurements. 
19

Vehicle-Pavement Interaction

1. Observe several trucks while they pass over the site to 

determine if truck traffic is showing adverse 

characteristics such as bouncing, swerving, braking, or 

accelerating within 135 feet of the sensors. 

2. If feasible, determine whether the truck tires are in full 

contact with the sensors.

3. Document observed truck dynamics, noting any adverse 

movements that are within the WIM scale approach 

area and that may affect the measurement accuracy of 

the WIM system. Include the distance from the WIM 

scale area.

Step 5 – Inspect WIM Equipment 

1. Perform a visual inspection and static electrical and 

electronic tests of all WIM site support components, as 

illustrated in Figure 9. 

Figure 9 – WIM Sensor Inspection

2. Identify and record deficiencies involving the 

equipment that will require repair. 

3. Identify all items whose present or deteriorating 

condition will eventually adversely affect the operation 

and/or accuracy of the WIM equipment.
20

4. If any discrepancies or deficiencies exist that will affect 

the measurement accuracies and cannot be remedied 

on-site, postpone the calibration. 

5. For systems that use an auto-calibration routine, check 

the WIM manufacturer’s guidelines to determine 

whether to keep an auto-calibration routine on or off 

during calibration.

Step 6 – Inspect and Measure Test Trucks 

Once on site, the test trucks must be inspected to ensure that 

they meet the criteria outlined in step 3. Perform the following:

1. Inspect the type of truck body, the load, and its 

placement and suspension to ensure that they will not 

induce adverse truck dynamics into the test. The load 

must be evenly distributed along the trailer, must be 

firmly secured to prevent sliding or shifting, and must 

be covered to prevent the introduction of moisture in 

rainy weather conditions. 

2. Inspect the suspension to ensure that airbags are not 

leaking, springs are not cracked or broken. See Figure 

10.

Figure 10 – Test Truck Inspection

3. The load on the test truck should not be one that can 

move during transport, such as fluids, sand, soil, or 
21

other items that shift during acceleration or 

deceleration. Figure 11 shows an example of a suitable 

test load.

Figure 11 – Test Truck Trailer Load

4. Inspect the truck tires and suspension for defects. 

5. Use certified static scales to weigh test trucks.

6. Measure the calibration truck steering axle, single axles

(if applicable), and tandem weights at a certified truck 

scale. Measure the axle spacing and the overall length.

7. Record the static weights, axle spacing, and overall 

length of test trucks.

Step 7 – Verify Communications with Test Truck 

Driver 

1. Review the calibration procedures with the truck driver, 

including proper speeds and turnaround locations. The 

truck should not accelerate or decelerate while in the 

WIM scale area and must travel down the center of the 

travel lane.

2. Review safety procedures with the truck driver. 

3. Establish communications with the test truck driver.

4. Instruct the driver to make test runs over the WIM 

scales at predetermined speeds. 
22

Step 8 – Perform Test Truck Runs 

1. Determine the number of test truck runs. This number 

depends on the data variability observed at a site. Sites 

that have low variability of errors require fewer test 

runs than sites that have high error variability to collect 

a representative sample. Generally, this requires:

o At least 10 runs for sites that have a spread of GVW 

errors for the test truck of 5 percent or less after 10 

truck runs and show no dependency of weight 

measurements on temperature. 

Or

o At least 20 test runs for sites that have a spread of 

GVW errors for the test truck over 5 percent after 

10 truck runs or demonstrate measurement error 

dependency on pavement temperature.

2. Conduct test runs. 

3. Run each test truck over the widest range of speeds 

determined during the pre-visit analysis. Use a laser 

(best) or radar gun to measure the test vehicle’s speed 

when it is crossing the sensor for comparison with the 

WIM system’s output. See Figure 12.

Figure 12 – Collecting Test Truck Speed

4. For systems that use temperature compensation, check

the manufacturer’s guidelines regarding whether to 

keep this function turned on or off. 
23

5. Conduct at least four test truck runs at each of the three 

WIM system speed points.

6. For WIM systems that demonstrate temperature 

influence on measurement error, conduct test runs over

a 30 °F temperature range or the widest temperature 

range possible.

7. For each pass of the test truck, take and record a 

pavement surface temperature reading at a location 

near, but not on, the WIM sensor. Use a hand-held laser 

temperature device, positioned approximately 30

inches from the pavement surface, to collect pavement 

surface temperature after each test truck passage over 

the sensors. Figure 13 illustrates this step.

Figure 13 – Collecting Pavement Temperature

8. For each pass of the test truck, obtain the WIM system’s 

output for the following data. Record all data, as shown 

in Table 6.

a. The sequence number of the test run

b. The date of the test run

c. The time of the test run

d. Axle weights of the test trucks as they pass over the 

sensors 

e. Spacing between each axle on the test truck

f. Vehicle length

g. Speed of test truck
24

h. Pavement temperature for each test run

Table 6 – Example of Selected Test Truck Run Data

Time Veh. ID Class GVW Speed Length

9:42:09 28912 9 66.4 59 64

9:51:19 29037 9 76.7 49 63

9:51:28 29040 9 65.3 49 65

10:00:15 29150 9 77.4 59 63

10:09:36 29272 9 74.7 49 64

For WIM sites that utilize the polymer piezo sensor for weight 

measurement, and any other WIM sites that demonstrate 

measurement error dependency on temperature, it is 

recommended that a review of prior calibration be performed 

to assist in determining the proper calibration of the WIM 

system based on seasonal temperatures. Data collected over 

seasons can eventually be used to fine tune the WIM systems 

temperature compensation curve, typically performed by the 

WIM manufacturer.

Step 9 – Evaluate WIM Performance Using Data 

from Test Truck Runs 

Once the data have been collected in the field, statistics must 

be computed to determine whether the WIM site meets the 

required accuracy parameters stated in WIM performance 

specification, which is typically the ASTM E1318-09 specification

shown in Table 1 and repeated here in Table 7. 
25

Table 7 – 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 (2 km/h)

Axle-Spacing and 

Wheelbase ±0.5 ft (0.15 m)

The WIM system performance parameters can be evaluated 

using one of the two methods described below.

ASTM E1318-09 Method

1. For each test vehicle pass, calculate the percent

difference between the WIM-measured value and the

corresponding reference value (static weight

measurement) for each parameter listed in Table 7.

2. For each test vehicle pass, determine if any calculated

measurement difference exceeds the tolerance value

specified in Table 7.

3. Using all passes of the test vehicles over the sensors,

determine the number of calculated differences that

exceeded the tolerance values shown in Table 7 for

each data item and express this number as a percentage

of the total number of observed values of this item—

that is, the percentage of all test measurements for a

given data item (like GVW) that exceeded the tolerance.

o If more than 5 percent of the calculated differences

for any applicable data item from Table 7 exceed

the specified tolerance for that item, the WIM

system requires adjustment/calibration.

o If the mean error for any of the weight or load

functions specified in Table 7 calculated based on all

±0%
26

test passes significantly deviates from zero, then 

the WIM system measures with a systematic bias 

and requires calibration. A deviation of 2 percent is 

recommended as the measure of “significant,” but 

users may select another value that meets their 

customer needs. 

4. If calibration has been performed and failed to satisfy 

the above criteria, repeat the calibration process until it 

passes or until it fails on three successive attempts. The 

latter indicates that the system or sensors are 

malfunctioning and require corrective maintenance. 

The system could also fail due to deteriorated road 

conditions or high road roughness.

LTPP Method 

The basic statistic used for this test is the error expressed as a 

percentage of the known (i.e. certified static weight or length 

measurement) value. The percentage of error, calculated from 

the data collected for each run, is used to compute a series of 

summary statistics. These summary statistics are used to 

determine whether the scale produces acceptable quality data 

using the following procedure: 

1. For each test vehicle pass, calculate the percent 

difference between the WIM-measured value and the 

corresponding reference value (static weight 

measurement) for each parameter listed in Table 1. 

2. Use all computed percentile errors to compute the 

mean error, standard deviation (use t-distribution for 

sample sizes less than 38), and statistical 95th percentile

confidence interval for the mean error. 

3. Combine the mean error and confidence interval values 

(mean error +/- 2 standard deviations of error) to 

compute the overall range of errors. 

4. Compare the resulting overall error range with the 

tolerance levels specified in Table 8 from the Field 

Operations Guide for LTPP SPS WIM Sites (4). 
27

Table 8 - LTPP Performance Requirements for WIM Systems

Measurement Type 95 Percent Confidence 

Limit of Error

Steering Axle ±20%

Axle-Group Load ±15%

Gross Vehicle Weight ±10%

Speed ±1 mph (2 km/h)

Axle Spacing ±0.5 ft (0.15 m)

Vehicle Length ±1.5 ft or 3.0%*

* Whichever measurement is longer.

5. If the overall error range is within the tolerance levels 

specified in Table 8 and the mean error is close to zero 

(e.g., less than 2 percent) for GVW, axle group, and 

front axle weight measurements, then the system does 

not require calibration. Otherwise, calibration is 

required. See the example in Table 9. 

Table 9 – Example of Test Truck Run Results Based on LTPP Method

Parameter 95% Confidence 

Limit of Error Site Values Pass/Fail

Steering Axles +20 percent 0.1 ± 5.1% Pass

Tandem Axles +15 percent 1.3 ± 5.2% Pass

GVW +10 percent 1.1 ± 3.0% Pass

Vehicle 

Length ±3.0 percent (1.9 ft) -0.2 ± 1.1 ft Pass

Axle Length + 0.5 ft [150 mm] 0.0 ± 0.1 ft Pass

Vehicle Speed ± 1.0 mph 0.3 ± 0.5 mph Pass

6. If calibration has been performed and failed to satisfy 

the above criteria, repeat the calibration process until 

the system passes or until it fails on three successive 

attempts. The latter indicates that the system or WIM 
28

sensors are malfunctioning and require corrective 

maintenance. 

Step 10 – Calibrate WIM System Parameters 

1. Based on the WIM system measurement error 

calculated during pre-calibration runs, calculate the 

adjustments to the current WIM system weight and 

distance error compensation factors using the following 

formulas:

o For weight: 

New Factor = (Old Factor * Static Weight)/WIM 

Weight

o For spacing: 

New Factor = (Old Factor * WIM Measure)/Static 

Measure

2. Enter the new compensation factors into the WIM 

system firmware for post-calibration testing.

3. Keep a record of all changes to the WIM system 

compensation factors.

Step 11 – Perform Post-Calibration Test Runs and 

Evaluate Results 

Determine Number of Post-Calibration Test Truck Runs

The minimum of 10 post-validation test truck runs (20 runs is 

recommended) may be performed if the system does not 

demonstrate the following:

1. A temperature dependency, or the weather conditions 

on the day of testing do not provide conditions for a 

wide range of temperature changes.

2. The data from the test runs demonstrate that the speed 

dependency at each speed point has been eliminated or 

minimized (less than 2 percent average error for the 

each speed point). 

3. The overall system bias computed based on the data 

from the available test runs is as close to zero as 

possible (less than 2 percent error for the mean error).
29

4. None of the individual runs are outside of the ASTM 

E1318-09 tolerance range for 95 percent compliance (1 

run outside of the range is allowed for samples of 20 

test truck runs or larger) or the 95 percent confidence 

interval computed based on the test data is within the 

acceptable LTPP WIM performance tolerances.

If a particular site demonstrates a dependency on temperature, 

up to 20 runs per test truck spread over a longer period of time 

may be required to accurately determine WIM measurements 

and confirm that calibration worked as expected.

Test and Finalize New Compensation Factors

1. Conduct post-calibration test runs using the procedure 

described in step 8 and collect post-calibration test 

truck weight, axle spacing, and length measurement 

data. 

2. Check if WIM system performance parameters are 

within the tolerances specified in Tables 1 and 2. If all 

parameters are within the tolerances, skip #3 below. 

3. Repeat steps 8 through 11 until the WIM system 

performance parameters are within tolerances (up to 

three times if needed). 

4. Install the new/final compensation factors into the WIM 

system firmware.

5. Keep a record of the final values of each WIM system 

performance parameter from Table 1 and final changes 

to the WIM system compensation factors.

Step 12 – Check Accuracy of Vehicle 

Classification 

Vehicle classification testing involves two checks:

• Check 1 – Test whether the WIM system is classifying 

vehicles properly as compared to the agency’s 

classification scheme or class tree input. 
30

• Check 2 – Test whether the agency’s classification 

scheme or the class tree itself classifies vehicles 

effectively at the WIM site location.

For example, if a short two-axle bus is classified as class 5, verify 

the axle spacing. If the axle spacing is within the class 5 range 

based on the agency’s classification scheme or class tree input, 

it passes the first check but fails the second (and it will always 

fail the second check for that class tree input). 

To test vehicle classification accuracy for a WIM site, perform 

the following:

1. Aim to perform the vehicle classification testing during 

peak hours. Peak hours could be identified from the 

hourly truck volume data sample during pre-visit data 

analysis. 

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

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

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

3 hours. In addition, collecting video of the traffic 

stream during manual study would be helpful for the 

follow-up analysis of misclassified vehicles.

3. During the manual data collection, record the vehicle 

IDs and time stamps from the WIM records for any 

vehicle that does not match the classification from the 

manual observation.

4. Evaluate the results using either Method A or Method 

B, as described below. 

Method A: 

a. Determine the total number of vehicle samples and 

the total number of misclassified vehicles in the 

manual sample for FHWA vehicle classes 4 through 

13.

b. Calculate the percentage of misclassified vehicles in 

FHWA vehicle classes 4 through 13 in the manual 

sample. Compare misclassification percent for the 

sample with the agency’s acceptable 
31

misclassification percent. It is recommended that 

the percentage of misclassified vehicles should be 

no greater than 2 percent. 

c. Calculate the number and percentage of vehicles in 

FHWA vehicle classes 4 through 13 in the manual 

sample that were not classified by the WIM system. 

The percentage of unclassified vehicles should be 

no greater than 2 percent. 

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

observed in 3 hour period, record the number of 

vehicles that passed and failed classification and 

report that the minimum sample size to test 

classification was not met. In this case, one

unclassified or misclassified vehicle can be allowed 

for classification test to pass.

Method B: 

a. Calculate the percentage of misclassified or 

unclassified vehicles in FHWA vehicle classes 4 

through 13 in the manual sample. To pass the test, 

each individual class of vehicles from 4 through 13 

may have a maximum of 10 percent misclassified 

vehicles. If the number of vehicles collected during 

the 3-hour testing period is under 30 for any vehicle 

class, use the following tolerance criteria for the 

maximum number of misclassified vehicles:

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

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

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

5. Draw conclusions as to whether the WIM system passed 

or failed the classification test.

Once the samples have been collected, review each 

misclassification instance to determine the cause and whether

the WIM system failed the classification checks described 

above. Analyzing video images of moving trucks collected during 

manual study would be very helpful for this task. The 

information collected for the classification verification tests will 
32

provide information on possible changes that need to be made 

to the WIM system’s classification table or issues with the 

agency vehicle classification tree inputs. This is very important 

for sites that have dominant types of vehicles with atypical axle 

configurations.

Step 13 – Conduct Post-Visit Data Analysis and 

Develop Calibration Report 

The post-visit data analysis allow technicians to evaluate the 

effectiveness of the calibration. It includes a comparison 

between the data samples collected just prior to and 

immediately following the calibration. 

For this analysis, a traffic data sample from the 14 days (30 days 

for low truck volume roads with less than 100 class 9 trucks per 

WIM lane per day) immediately following the calibration is 

collected and compared with the Comparison Data Set (CDS) 

(see CDS discussion in Step 15) from the previous calibration 

event and the pre-visit data sample. To conduct post-visit 

analyses, follow these steps:

1. Download a WIM data sample for the period of 2 to 4 

weeks immediately after the calibration visit for each 

lane instrumented with WIM sensors. 

2. Compare the following parameters between the CDS 

data, pre-visit data sample, and the post-visit data 

sample for each WIM lane. See Figure 14 for an 

example.

a. Class 9 average GVW and GVW distribution by load 

bins (tabular or plot)

b. Class 9 average front axle weight and front axle 

weight distribution by load bins (tabular or plot) 

c. Class 9 tandem axle group axle load distribution by 

load bins(tabular or plot)

d. Load bins corresponding to maximum percentage of 

GVW, steering axle loads, unloaded and loaded 

tandem axle loads (i.e. peak loads in the axle load or 

GVW distribution)

e. Class 9 tractor average tandem axle spacing
33

f. Class 9 total length of vehicle

g. 85th percentile speed for heavy trucks

h. Vehicle class distribution (FHWA classes 4 through 

13)

Figure 14 – Example of Post Visit Analysis of Class 9 Steering Axle 

Weight Distribution

3. Note whether differences from the previous CDS 

observed in pre-visit data set have been resolved or 

reduced as a result of calibration. 

4. Determine if the calibration had the desired effect on 

the data values by comparing the change in the 

computed parameters with the changes made to the 

WIM system calibration factors.

5. Document findings in a WIM calibration summary 

report (see step 14 below). 
34

Step 14 – Develop and Submit Calibration 

Summary Report 

To assist in proper recordkeeping and 

future planning, it is recommended that 

the field technician develop and submit 

to the program manager a WIM 

calibration summary report that 

provides the following information:

1. WIM site inventory information 

and equipment status

2. Date of calibration and calibration protocol used (ASTM, 

LTPP, or agency-specific) 

3. Test truck inspection results, including static weights, 

axle spacing, and length measurements

4. Pavement condition assessment

5. Temperature during testing and weather conditions 

6. Equipment visual inspection

7. Results of electrical WIM sensor testing

8. Initial pre-calibration test truck runs results (records of 

individual measurements for each test truck run) 

9. Total number of test truck runs and the number of 

individual test truck runs that passed or failed any of 

the performance criteria specified in Table 1 before 

calibration

10. Computed error statistics, if LTPP method is used

11. Calibrate/do not calibrate decision and reasoning

12. Changes to system calibration factors

13. Post-calibration test truck runs results (records of 

individual measurements for each test truck run)

14. Total number of test truck runs and the number of 

individual test truck runs that passed or failed any of 

the performance criteria specified in Table 1 after 

calibration

15. Results of classification verification test (if conducted)

16. Conclusions whether WIM system met accuracy 

requirements after calibration
35

17. Any corrective actions and recommendations

18. Pre-visit and post-visit data analyses and findings 

(optional)

19. Supplemental documentation, including photographs

Step 15 – Develop Comparison Data Set 

If the post-visit data analysis confirms that the calibration was 

successful, the data used for the post-visit analysis and the 

statistics computed based on these data are considered the 

Comparison Data Set (CDS). The following parameters are 

recommended for CDS computation:

1. Class 9 average GVW and GVW distribution by load bins 

(tabular or plot)

2. Class 9 average front axle weight and front axle weight 

distribution by load bins (tabular or plot)

3. Class 9 tandem axle group axle load distribution by load 

bins (tabular or plot)

4. Load bins corresponding to the maximum percentage of 

GVW, steering axle loads, unloaded and loaded tandem 

axle loads (i.e. peak loads in the axle load and GVW 

distributions)

5. Class 9 tractor average tandem axle spacing

6. Class 9 total length of vehicle

7. 85th percentile speed for heavy trucks

8. Vehicle class distribution (FHWA classes 4 through 13)

These data and statistics could be used to compare periodically 

(biweekly or once a month) with the statistics based on the 

current data and/or data sample collected prior to scheduling 

calibration visits. See Table 10 for an example of CDS 

computation results. 
36

Table 10 - Example of Comparison Data Set Computation Results

Parameter CDS 

Average Steering Axle Weight (kips) 11.0

Average Axle 4 Weight of Split Tandems (kips) 11.9

Average Axle 5 Weight of Split Tandems (kips) 11.9

Average Tandem Axle Weight (kips) 20.5

Average Loaded Tandem Axle Weight (kips) 32.4

Average Standard (3S2) Class 9 GVW (kips) 52.8

Percent of Overweight Vehicles 9.5%

Peak Steering Axle load (kips) 12.0

Peak loaded tandem (kips) 34.0

Peak GVW empty (kips) 36.0

Peak GVW empty (kips) 80.0

Average Tractor Tandem Axle Spacing (feet) 4.3

Total Vehicle Length (feet) 48.6

Depending on the analysis results, the site calibration schedule 

may be accelerated or postponed. For example, an accelerated 

schedule may be warranted if there are changes in the average 

weight and length measurements or shifts in the weight 

distribution greater than 5 percent.

Step 16 – Conduct Office Data Quality Checks 

and Analysis to Aid Future Calibrations 

1. Sample WIM data every 1, 2, or 4 weeks, depending on 

site truck volumes. A minimum sample of 200 class 9 

trucks per lane is needed; a sample of 500 or more class 

9 trucks is recommended to identify and analyze GVW 

and axle load distribution characteristics. 

2. Use the data sample to develop the following statistics

for each lane with WIM sensors: 

a. Class 9 average GVW and GVW distribution by load 

bins (tabular or plot)

b. Class 9 average front axle weight and front axle

weight distribution by load bins (tabular or plot)
37

c. Class 9 tandem axle group axle load distribution by 

load bins(tabular or plot)

d. Load bins corresponding to the maximum 

percentage of GVW, steering axle loads, unloaded 

and loaded tandem axle loads (i.e., peak loads in 

the axle load and GVW distributions) for class 9 

vehicles

e. Class 9 tractor average tandem axle spacing

f. 85th percentile speed for heavy trucks

g. Vehicle class distribution (FHWA classes 4 through 

13)

3. Use the above statistics to compute differences with 

the same parameters based on CDS. 

4. Flag and report data samples that show differences of 

more than 5 percent (or other agency-selected value) 

between the current data set and CDS. These data 

require close monitoring and trend analysis. An example 

of a comparison is shown in Table 11. 
38

Table 11 – Example of WIM Data Analysis Results for Typical (3S2) 

Class 9 Vehicles

Parameter CDS Current %Change

Average Steering Axle 

Weight (kips) 11.0 10.6  -3.6

Average Axle 4 Weight of 

Split Tandems (kips) 11.9 11.3  -5.0

Average Axle 5 Weight of 

Split Tandems (kips) 11.9 11.2  -5.9

Average Tandem Axle 

Weight (kips) 20.5 19.3  -6.0

Average Loaded Tandem 

Axle Weight (kips) 32.4 31.5  -2.8

Average Class 9 GVW (kips) 52.8 49.7  -5.8

Peak Steering Axle load 

(kips) 12.0 11.0  -8.3

Peak loaded tandem (kips) 34.0 33.0  -2.9

Peak GVW empty (kips) 36.0 36.0  0.0

Peak GVW loaded (kips) 80.0 76.0  -5.0

Percent of Overweight 

Vehicles 9.5% 3.9%  -6.0

Average Tractor Tandem 

Axle Spacing (feet) 4.3 4.3  0.0

Total Vehicle Length (feet) 48.6 48.2  0.8

5. Subject data that deviate from CDS to monthly trend 

analysis. Compare data from the current month with 

the data for the same month from the previous year(s) 

or to the data from the previous month of the current 

or past year(s). 

6. In areas of harvest or other seasonal production, like 

timber, analysis of monthly or seasonal average for 

several values like GVW distribution, peak load values,
39

and percentage of overweight of class 9 trucks with 

standard configuration is recommended.

7. If deviation over 5 percent from CDS and seasonal 

trends persists or increases, an out-of-cycle calibration 

may be needed.

8. If calibration drift is frequently observed at the site, 

daily data quality monitoring and daily trend analysis is 

recommended. For the daily trend analysis, use time 

series plots to monitor the daily changes in the WIM 

statistics described in #2 of this list. 

An example of a daily average GVW plot is shown in Figure 15. 

As shown in the figure, from the end of July to the beginning of 

October, the average daily GVW has been steadily drifting from 

59 kips to 50 kips (-15.2 percent).

Figure 15 – Daily Average GVW Decreasing Over Time 

Calibration frequency depends on the available resources, 

individual site characteristics, sensor arrays, and sensor types. 

The following are general recommendations provided for 

different sensor types. 
40

WIM Systems with Quartz Piezo Sensors or Strain 

Gauge Strip Sensors 

Typically, WIM systems with quartz piezo or strain-gauge strip 

sensors should be calibrated every 12 to 18 months. However, 

more frequent calibration may be needed if any of the following 

is observed: 

• Recent maintenance or hardware change is performed

• Demonstrated system dependency on pavement 

smoothness

• Sensors installed in pavements subjected to seasonal 

changes in pavement stiffness

• System shows the repeatable drift from past calibration 

at a higher rate over time, whatever the reason

• Presence of commodity shipment cycles

• Changes in truck classification distribution

• Changes in average truck speed

• If key weight statistics (such as class 9 GVW) drift more 

than 5 percent from the CDS values

WIM Systems with Polymer Piezo Sensors 

Systems with polymer piezo sensors that are used for collection 

of axle weight data should be calibrated seasonally or have 

other means for consistent compensation of temperature effect 

on weigh measurement accuracy. Temperature compensation 

based on passenger vehicles or front axle weights is frequently 

not effective for measurements of heavy axle weights. In 

addition, temperature compensation that is adjusted based on 

the number of truck samples is problematic when traffic 

volumes are low and during periods in the day when pavement 

temperature changes rapidly. Site-specific adjustments to 

temperature compensation curves may be needed to address 

accuracy issue for heavy axles. 

If seasonal calibrations cannot be scheduled for whatever 

reason, users should be informed about seasonal deviations in 

weight measurement accuracy. If seasonal calibrations are not 

feasible, calibrate every 12 months during the season that has a 
41

daily temperature as close to the annual average temperature 

as possible to avoid introduction of extreme bias, as could occur 

when calibrating during seasons with extreme cold or hot 

temperatures.

Data processing and quality assurance 

personnel can provide inputs on when a 

WIM site needs to be calibrated based 

on the comparison analysis of the 

loading statistics computed based on the 

recently downloaded 2 to 4 weeks of 

WIM data and the CDS statistics. For 

sites that have other contributing factors 

that lead to changes in “typical” truck weights observed at the 

site, such as seasonal crop or other commodity movement, 

these factors must be accounted for during data checks to 

determine if the data are “typical” for the site or unexpected 

deviations have occurred.

WIM Systems with Bending Plates or Load Cells 

For bending plate and load cell WIM systems, calibration 

frequency could be greatly reduced if the systems are being 

properly maintained in a timely manner. In some cases, bending 

plate WIM did not require calibration over 5-year periods; in 

other cases, calibration every 2 years was sufficient. 

Data processing and quality assurance personnel can provide 

inputs on when a WIM site needs to be calibrated based on the 

comparison of the loading statistics based on the recently 

downloaded WIM data and the CDS statistics. For sites that 

have other contributing factors that lead to changes in “typical” 

truck weights observed at the site, such as seasonal crop or 

other commodity movement, these factors must be accounted 

for during data checks to determine if the data are “typical” or 

unexpected deviations have occurred. In addition, major WIM 

maintenance events may trigger the need for calibration.
42

WIM SYSTEM MAINTENANCE

Implementation of comprehensive 

maintenance program helps to maximize the 

life of the WIM system. This section of the 

Guide includes a scope of recommended 

maintenance activities, recommended 

schedules, description of field checks, and 

recommendations for field logs, acceptance 

testing, and maintenance reports. In addition to these 

guidelines, manufacturers’ recommendations for maintenance 

of the specific WIM equipment should be followed. 

Types and Frequency of Maintenance

Maintenance activities include regularly scheduled preventive 

maintenance and event-driven corrective maintenance. 

Preventive maintenance is performed to prevent future 

equipment failures and to extend longevity of different WIM 

parts and components. Corrective maintenance is performed 

after a problem is detected in the system or a scheduled part 

replacement or upgrade is due.

It is important to perform routine preventive maintenance 

every 6 to 12 months. For corrective maintenance, procedures 

should be developed to facilitate timely operational checks and 

repairs of system failures. 

The maintenance activities described here are not sensorspecific. For guidance on performing regular maintenance on 

specific types of sensors, reference the manufacturer’s 

maintenance manuals.
43

Tools, Materials, and Supporting 

Documentation

An effective WIM maintenance program must have the proper 

tools, materials and spare parts to perform regular preventive 

maintenance and quickly resolve system problems without 

significant down time and lost data. Manufacturers’ 

maintenance guides typically contain a list of required spare 

service parts, tools, and equipment necessary to properly 

maintain the specific vendors’ WIM systems. 

Materials 

• Clipboard

• Shop rags or clothes

• Cleaning solvent

• Pens and recordkeeping paper/forms

• Electrical tape

• Splice kits

• First aid kit

• Spray silicone

Hand Tools 

• Digital camera

• Screwdriver sets – slotted and Philips

• Jewelers’ screwdrivers – slotted and Philips

• Small socket driver set – those that fit the hardware at 

the site (solar panel, cabinet WIM controller) 

• Pliers – long nose, round nose, curved. Both smooth 

and serrated types are useful

• Adjustable wrench, small

• Cutters – diagonal and flush

• Linesman's pliers

• Wire strippers – fixed and adjustable

• Wet/dry shop vacuum

• Crimping tool

• Global positioning system (GPS) receiver
44

• Power inverter

• Tape measure

• Measuring wheel

• Small shovel

• Pry bar

Test Equipment 

• Digital multimeter with well insulated test probes

(capability for resistance, inductance and capacitance)

• Megger (loop and ground tester)

• Oscilloscope – dual trace, 10 to 20 MHz minimum 

vertical bandwidth and 10x probes (optional, to check 

the sensor output waveforms)

• Logic probe – for quick checks of digital circuitry for 

activity

• Laser speed detection gun

Computer Software 

• WIM system connectivity and data download software

• Modem setup script files and/or setup guide

Supporting Forms and Documentation 

The following forms and documentation are recommended for 

use by the WIM maintenance personnel: 

• WIM Site Maintenance and Inspection Form

• WIM System Troubleshooting Form

• WIM Site Maintenance Log (to be kept in the WIM 

cabinet)

Additionally, maintenance personnel should always have on 

hand:

• Manufacturers’ maintenance guides or service manual

• WIM site as-built plans showing locations of WIM 

components
45

Safety 

When working on-site, several safety procedures must be 

followed:

• Park any vehicles off the road in a safe area away from 

traffic (see example in Figure 16).

• If permitted, leave vehicle running and wear the 

seatbelt when in the vehicle. 

• All vehicles must be equipped with safety lights.

Figure 16 – Protected Work Vehicle Location

• Always wear personal protection equipment (PPE), 

including a reflective safety vest, hard hat, and steeltoed shoes (see Figure 17). 

Figure 17 – Field Technician with PPE
46

• Always use a safety spotter when working near the 

roadway to alert the inspector of oncoming traffic. 

• Carry adequate amounts of drinking water. 

• Wear weather-appropriate clothing and protection. 

• Carry sunscreen and bug repellent. 

Preventive (Scheduled) Maintenance

During each preventive maintenance visit, all in-road sensors 

should be checked for proper electronic values. The entire WIM 

system should be visually inspected, including the pull boxes, inroad sensors, communication and power service equipment, 

and the roadway leading up to and within the WIM scale area. 

All deficiencies should be recorded, photographed, and 

reported for correction.

Pavement Inspection 

A visual inspection of the pavement in the WIM scale approach 

will provide an indication of whether the pavement condition 

may be contributing to any inaccuracies of the WIM system.

Figure 18 shows an example of distressed pavement that could 

affect WIM system accuracy. 

Figure 18 – Pavement Distress Identified During Inspection
47

Any pavement distresses that appear to influence truck 

dynamics at or near the WIM scale area must be documented 

on the WIM Site Preventive Maintenance and Inspection Form.

1. From the shoulder, walk the distance from the WIM 

sensor installation to a distance 400 feet prior to the 

WIM sensors and 200 feet after the WIM sensors.

2. Photograph and annotate any pavement distresses that 

may influence truck dynamics.

Visual Equipment Inspection 

A visual inspection of all WIM system components is important. 

It is recommended that a lane closure be set up at least 

annually so that a close inspection of the roadway and the inroad sensors can be performed. Carefully inspect each WIM 

system component and annotate any deficiencies on the WIM 

Site Preventive Maintenance and Inspection Form, including the 

following: 

1. Check the pavement condition around the WIM sensors 

for cracks, broken pavement, or potholes. Figure 19 

shows a close-up of a WIM sensor and surrounding 

pavement.

Figure 19 – WIM Sensor Observed During Inspection
48

2. Check cabinet interior for broken components such as 

thermostats, lights, shelving, heaters, terminal panels, 

battery and modem.

3. Remove the covers to each pull box and inspect the 

interior for signs of improper drainage or infestation

(see Figure 20). Inspect the exterior for cracks and 

ensure that all securing hardware is in place. Suspend 

wiring as needed to keep it out of any water that may 

be present in the pull box.

Figure 20 – Pull Box Interior

4. Inspect the service pole, pedestal, and foundation for 

cracks and make sure that it is plumb.

5. Visually inspect power service equipment for structural 

deficiencies. DO NOT TOUCH BROKEN WIRES, AND DO 

NOT OPEN ANY LIVE SERVICE BOXES! Check earth 

ground using a megger.

6. Inspect communication equipment for damage, cut or 

damaged wires, or a broken antenna.

7. Check exposed conduit for extensive rust, breaks, or 

cracks. 

8. Annotate all readings on the WIM Site Preventive 

Maintenance and Inspection Form.
49

Equipment and Site Cleaning 

Clean the interior and exterior of all WIM components, 

including the following:

1. Remove dust or debris with a shop vacuum. 

2. Wipe down with cleaning solvents. DO NOT SPRAY 

COMPONENTS WITH SOLVENT. SPRAY THE RAG OR 

CLOTH AWAY FROM THE EQUIPMENT. 

3. Replace the cabinet filter, if necessary. 

4. Clear out clogged drainage pipes. 

5. Perform vegetation control in the area of the cabinet 

and pull boxes. Figure 21 shows the cleared area around 

a traffic cabinet.

Figure 21 – Traffic Cabinet Location after Vegetation Control

6. Look for indications of insect or rodent problems. Clean 

up and seal any openings. Consider installation of 

mothballs or other items to repel the insects or rodents.

Electronic and Electrical Checks 

1. As shown in Figure 22, perform electronic and electrical 

checks of all WIM system components to confirm that 

each component is operating within the manufacturer’s 

tolerances, including: 

o WIM sensors – list voltage, resistance, and maybe 

capacitance ranges

o Loop sensors – list values in microhenrys 
50

o Power voltages

o Telephone – list on-hook and off-hook voltages

o Grounding – list typical distance and ohm range to 

get

2. If any readings are out of tolerance, double-check the 

proper readings against the previously documented 

ranges, specification, or manufacturer’s maintenance 

manuals and begin troubleshooting the system, if 

necessary, to determine the cause. 

Figure 22 – Sensor Testing

3. Annotate all readings on the WIM Site Preventive 

Maintenance and Inspection Form.

Firmware Upgrades 

1. Once the electronic values for the WIM sensors and 

support equipment have been verified as correct, 

perform any scheduled software upgrades. Ensure that 

the instructions provided in the manufacturer’s 

operations guide are followed to protect the equipment 

from damage caused by electronic surge or improper 

installation of the firmware.

2. After installing the latest firmware, re-enter all default 

values and/or system parameters to ensure proper 
51

operation. Verify that the following system settings are 

correct:

a. Date/time

b. Site information

c. Weight compensation factors (by lane recorded)

d. Axle and loop spacings (by lane settings recorded)

e. Length of vehicle setting (loop setting)

f. Loop sensitivities

g. Lane configurations, sensor assignment and sensor 

spacing

h. Violation settings

i. Communication settings

j. Data logging information

k. Password information

l. Self-calibration settings (if used)

3. Annotate that the firmware was replaced and record 

the current version on the WIM Site Preventive 

Maintenance and Inspection Form.

Operational Testing and Verification 

After local, direct communications with the WIM system user 

interface have been established, perform the following series of 

operational checks to verify the proper operation of the WIM 

equipment.

1. Verify the proper operation of the WIM sensors and 

inductive loops by observing the individual vehicle 

characteristic values provided by the WIM system after 

a vehicle passes over the WIM scale area. Determine 

whether the speeds, distance measurements, and 

vehicle/axle weights are reasonable and that each axle 

and axle spacing is reported for each vehicle. 

2. Conduct vehicle classification study by observing several 

vehicles as they pass over the WIM scales and verify 

that they are being classified properly, according to the 

classification method that is installed in the WIM 

system. 
52

3. Conduct weight study. Record the front axle weight for 

the front axle for a sample of at least 50 class 9 trucks

(or the number counted over 3 hours if less than 50). 

Typically, the average front axle weight for the class 9 

trucks should be between 10,000 and 11,000 pounds. If 

the front axle weight average is less than 9,000 pounds 

or greater than 12,000 pounds, a calibration of the 

system may be necessary.

4. Document findings on the WIM Site Preventive 

Maintenance and Inspection Form.

Communications Checks 

As part of the preventive maintenance function, a 

communication check is important to prevent a return trip to 

the site. 

1. Prior to leaving the site, and once all other preventive 

maintenance activities have been performed, contact 

office personnel to verify that the WIM system can 

communicate remotely with the host computer.

2. Annotate findings on the WIM Site Preventive 

Maintenance and Inspection Form.

Corrective Maintenance and Repair

Scheduled Corrective WIM Maintenance 

Corrective maintenance could include changes to systems 

parameters, adjustment to sensor operating parameters, 

firmware upgrades, calibration, or replacement of parts. Many 

factors must be considered when determining the corrective 

action: cost, loss of data, availability of replacement parts, and 

degree of difficulty of the repair. The repair must be costeffective—a component should not be repaired if the cost of 

doing so would be comparable to replacing it with a new one or 

if the pavement condition would prevent long-term data 

collection. 
53

Many circuit board components are not readily available for 

replacement. The technician should have “ready-service spares” 

on hand for all components that may have to be ordered from a 

manufacturer, vendor, or other supplier. 

All replaced parts must be clearly marked, indicating that they 

are “bad,” so that they are not reused. 

After all repairs have been completed, the technician should 

perform the preventive maintenance procedures described 

earlier to ensure that the system is fully operational.

Unscheduled Corrective WIM Maintenance 

Unscheduled maintenance is performed after a problem is 

detected in the system that prompts a visit to the site. Checking 

WIM system results by lane and sensor helps with 

troubleshooting WIM data errors. 

If the source of the problem is not already known, preventive 

maintenance and WIM equipment troubleshooting routines 

should be performed to attempt to identify the source of the 

problem. 

Once a fault has been found, the problem must be investigated 

to identify and correct the deficiency. Problems that are 

detected may be corrected in several ways, including changes to 

systems/on-site parameters, adjustment to sensor operating 

parameters, firmware upgrades, calibration, or replacement of 

parts.

WIM Equipment Troubleshooting

This section provides a general guide for identifying the system 

fault, troubleshooting the fault to determine the cause, and 

repairing WIM systems and components. It is based on the 

information presented in FHWA’s WIM Field Inspectors’ 

Troubleshooting Guide and the Traffic Detector Handbook (6, 7). 
54

Troubleshooting Approach 

The first requirement in troubleshooting is developing a general 

troubleshooting approach—a logical, methodical procedure of 

narrowing down the problem. It is important to recognize the 

fault correctly, find the cause, determine the proper corrective 

measure, and repair or replace the faulty system component(s). 

Additionally, it is important to keep in mind that:

1. Intermittent problems are typically 

due to bad connections and can be 

fixed by cleaning, removing, 

inspecting, and replacing connectors 

or circuit boards. 

2. Problems that result in a completely 

non-functional system or affect a 

number of different functions or components are 

usually related to power. Sometimes, just turning the 

system off and back on may fix the problem. 

3. If a power problem is suspected, use sight and smell to 

search for evidence of burnt components.

4. Intermittent or chattering axle sensors can lead to ghost 

axles and increases in class 10 and/or class 13 vehicles.

5. Many WIM system vehicle capture problems are 

created by incorrectly set loop frequencies. Always 

begin the troubleshooting process by checking system 

settings.

6. Misclassified vehicles can mean a detuning loop as a 

vehicle is passing. Class 9 vehicles will be improperly 

classified as classes 1 and 6. Check to ensure the WIM 

system properly identifies class 9 vehicles. Did both the 

class1 and class 6 number jump the same number from 

the same DOW for a previous week?

7. Improper class data can lead to potential WIM issues, 

and vice versa.

8. Seasonal changes in traffic streams (agricultural

harvests), changes in local economics, and additional 

rezoning in the area can change WIM readings due to 
55

changes in vehicle characteristics. Knowing the local 

vehicle travel patterns can help when checking if a site 

is working properly.

Logical Progression of Troubleshooting 

Read the troubleshooting procedure in its entirety before 

beginning work. Information from one step may directly assist 

efforts in another. Refer to manufacturer guides for specific 

troubleshooting techniques and procedures related to WIM 

system components, including common problems and 

associated solutions.

The LTPP WIM System Troubleshooting Outline (TRAFFIC SHEET 

23) (4) is a valuable tool that can be used to guide the user

through a systematic troubleshooting routine. It provides a 

visual way of investigating and fixing the problem, rather than 

trying to remember what has been already found and remedies 

that have already been tried.

Use the LTPP WIM System Troubleshooting Outline or other 

similar form to take detailed notes and follow the step-by-step 

troubleshooting procedure described next.

Step 1 – Develop a Detailed Description of the Problem 

The identification of WIM system problems typically begins with 

the user’s inability to communicate with the system or 

download system data. The user may also report that the data 

received are questionable or incorrect. When the problem is 

reported, it is important to obtain very specific and detailed 

problem descriptions and symptoms. 

A lot can be learned from “by lane,” “by sensor,” and “by 

vehicle” WIM data analysis. This data troubleshooting does not 

involve a field visit to track down issues. 

1. Download data from the previous day or within the 

timeframe in question. Compare data values with the 

CDS values. This process may narrow down the problem 

to a particular function, such as weight measurement or 

distance and speed measurement. 
56

2. Determine if the travel characteristics of the site 

changed due to weather, construction, seasonal 

agriculture, or some other reason. 

3. Conduct thorough WIM data analysis. Drill down to the 

“by lane” data and even conduct “by sensor” readings if 

need be. 

4. Review “by vehicle” data. The individual by vehicle data 

will help the user see if the issue is related to heavy or 

light axles certain vehicle types or maybe some vehicle 

characteristic, such as time of day when the error 

occurs.

5. Provide this information to the technician before the 

site visit. It will help the technician to have the correct 

parts and information to narrow the issue down to 

more quickly get the site back working properly.

A WIM system malfunction may also be detected during any 

regular maintenance routine, such as periodic preventive 

maintenance, system upgrade, or equipment replacement or 

repair. Again, it is important to write down exactly what the 

problem is and the problem’s characteristics. 

Before proceeding further, if it is available in the manufacturer’s 

maintenance or operational manual, refer to the 

troubleshooting section to see if the reported problem is listed. 

If it is not, or the recommended corrective action is not 

successful, proceed with step 2 below. 

Step 2 – Evaluate System Parameters and Data 

In order to properly diagnose a system malfunction, accumulate 

all system operating information, including system data and 

current operating parameters. 

1. If remote connection with the site is possible, connect 

with the site and record all operator interface mode 

information prior to traveling to the site. If contacting 

the site remotely is not possible, or the system data 

already collected do not provide an indication of the 
57

problem, or the problem discovered through system 

data cannot be fixed remotely, visit the site. 

2. While connected to the site, open a log file and record 

all system characteristics, including:

o Weight compensation factors

o Lane configurations, sensor assignment and sensor 

spacing

o Violation settings

o Communication settings

o Data logging information

o Real time view (collect several samples of as many 

types of vehicles as possible)

o Vehicle classification and speed data from the past 

hour

o Time and date

o Password information

o Auto-calibration settings

3. Record all system sensor values available in the system 

diagnostics mode. Compare current system settings 

with historical records.

4. After reviewing the site operating parameters, site data, 

and sensor analog values, add any additional 

information to the “Problem Description” portion of the 

troubleshooting form. 

5. Double-check all measurements that appear incorrect 

or out of tolerance.

If required, follow the remaining steps on the LTPP WIM System 

Troubleshooting Outline according to the following guidelines: 

• The source of many problems may be discovered during 

the data collection and analysis portion of the 

troubleshooting process. If it is strongly believed that 

the problem has been discovered during step 1 or this 

step, skip to step 4 – Corrective Maintenance. 

• If any system parameters are found to be incorrect, you 

may typically fix the problem by entering the correct

setting. If the setting is off by a small amount, or if the 
58

setting appears to have been typed incorrectly, it is 

likely that the problem was user-related. Type the 

setting in as it should be, and restore the system 

parameters to the system firmware according to the 

manufacturer’s instructions. Correct classification is 

sensitive to 1/10th-foot increments, so accuracy of the 

WIM classification settings is paramount with good 

results. 

• If a setting is drastically incorrect, chances are the 

problem was created by a system “glitch,” and if one 

exists, many exist. The best way to fix these settings is 

to perform a hard reset of the system that would return

all system operational parameters to their default 

settings. Download all of the data files from the WIM 

system’s memory. Follow the manufacturer’s 

instructions for performing a system reset and restoring 

the default parameters.

Step 3 – Finding the Source of the Problem

Use the information gathered from the previous steps to 

determine the probable malfunction. Consider during this 

process that multiple problems are rare and that all symptoms 

should be considered to be the result of one problem. 

There are two steps that are used to determine the system’s 

faulty component. Each involves the drawing of a conclusion 

based on the information gathered in steps 1 and 2, as well as 

the added information gathered during the performance of 

each level’s procedures. 

Determine the probable functional failure. There are six primary 

functions of any WIM system: 

• Power – Most power problems are the result of loss of 

alternating current (A/C) power service, dead battery, 

blown fuse, or tripped circuit breaker. Investigate these 

possibilities first. Check the voltage for lower than 

expected power. A/C voltage should be over 105 volts.
59

Annual load testing of any system batteries should be 

performed. 

• Classification and Weight (including speed and spacing) 

– Problems related to the measurement of various 

vehicle characteristics are usually caused by improper 

system settings. Verify that all settings are correct 

before performing further troubleshooting. 

• Loop Settings – The second most common problem 

associated with misclassification of vehicles is the loop 

frequency and sensitivity settings. Verify that all loop 

frequencies and sensitivities have been set in 

accordance with the manufacturer’s setup routine. 

• Communications – Communication issues usually are 

caused by disrupted cellular or landline service, corrupt 

modem settings, or power problems associated with 

the modem.

• Data Storage – The inability of the WIM system to store 

collected data is typically associated with an improper 

system setting or corrupt system memory.

• Data Download and Processing – The failure of the WIM 

system to properly download data is typically a result of 

a communication error. Investigate deficiencies 

associated with the communication devices first.

Based on the information gathered in this step, make a 

determination of the probable faulty function. Indicate the 

faulty function on the WIM System Troubleshooting Outline

Form. 

Once the faulty function has been determined, troubleshooting 

flow charts and step-by-step guides are typically provided in the 

manufacturer’s maintenance manual that can assist in the 

process of determining the faulty component. 

If the faulty component cannot be determined, contact the 

manufacturer and provide the information gathered above. 

Many times the manufacturer can provide additional checks to 
60

perform or provide the technician with the probable fault 

component without further troubleshooting.

Maintenance Documentation and 

Reporting

Proper recordkeeping is an essential 

function of a WIM system maintenance 

program. Data from previous 

maintenance actions can be invaluable 

for determining high failure rate 

components and best practices for 

correcting common problems. Most 

importantly, past records can provide 

the WIM program manager with valuable information on 

predicting the failure of WIM system components so that they 

can plan and budget for upcoming maintenance costs and 

prepare the technician with replacement parts before the 

system fails.

Maintenance records should include preventive and 

troubleshooting efforts as well as final analysis and repair 

actions performed. Develop and use standard reporting forms 

to create historical records of all site maintenance performed, 

including:

• WIM Site Maintenance and Inspection Form – Detailed 

record of preventive or corrective maintenance 

activities and repairs/replacements performed at the 

site and the results of WIM system operational tests 

performed before and after maintenance activities.

• WIM System Troubleshooting Form – Detailed record of 

investigation activities performed at the site to identify 

problem with the equipment, including steps taken to 

identify and resolve system problems.
61

• WIM Site Maintenance Log – This form is kept in the 

WIM cabinet and provides a historical record of site 

visits and work completed.

• WIM Site Maintenance Management Tool – A 

management tool that contains a record of scheduled 

and completed WIM site maintenance visits and a brief 

summary of work performed during each visit. This is 

typically done in the form of a spreadsheet or a 

maintenance management software application.

Examples of WIM system maintenance and inspection forms 

and reports are provided in Appendix E. Further troubleshooting 

recommendations can be found in the manufacturer’s 

operations guides. 
62

REFERENCES

1. ASTM Standard E1318-09, Standard Specification for 

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

Requirements and Test Methods. West Conshohocken, 

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

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

2. European Cooperation in Science and Technology 

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

Vehicles. Paris: 2002.

3. NMi International WIM Standard: Specification and Test 

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

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

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

Field Operations Guide for SPS WIM Sites Version 1.0. 

Draft. McLean, VA: Federal Highway Administration: 

2012.

5. American Association of State Highway and 

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

Standard Specification for Smoothness of Pavement in 

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

6. Quinley, Rich. 2009. WIM Field Inspectors' 

Troubleshooting Guide. Publication No. FHWA-IF-10-

003. Washington, D.C.: Federal Highway Administration: 

2009

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

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

Washington D.C.: Federal Highway Administration 

(FHWA): 2006.
63

APPENDICES

The appendices for the FHWA WIM Pocket Guide are available 

on the FHWA website: 

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

General Appendices

• Appendix A – Technical Report Documentation Page 

Form DOT F 1700.7

• Appendix B – Acknowledgements

Part 1 Appendices:

• Appendix C – WIM Procurement Resources

o Example WIM Contract Language (GDOT)

o Example of IRD Standard Warranty Statement

o Example of IRD WIM System Installation Standards

Part 2 Appendices:

• Appendix D – WIM Site Design and Installation 

Resources

o Example Pavement Surfacing Plans (SDDOT)

o Example Polymer Piezo Installation Plan

o Example SLC Layout (Mettler-Toledo)

o Example WIM Installation Standard (NJDOT)

o Example WIM Site Design (MT DOT)

o Example WIM Cabinet Wiring Diagram

o Example Non-compliance Report

Part 3 Appendices:

• Appendix E – Maintenance and Calibration Resources

o Example WIM Calibration Report (AZ DOT) 

o Example Calibration Test Truck Procurement (LTPP)
64

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