TECHBRIEF WIM Scale Calibration:

A Vital Activity for LTPP Sites

Publication No. FHWA-RD-98-104

FHWA Contact: Monte Symons, HNR-30 (703) 285-2730

Introduction

Long-Term Pavement Performance (LTPP) data are the foundation for

new pavement designs for years to come. As such, data collected at LTPP

test sites need to be as accurate and complete as possible. For the collection of truck weight data, this requires direct calibration of weigh-inmotion (WIM) scales at LTPP test sites. The purpose of this TechBrief is

to highlight the significance of scale-calibration error on LTPP data, to

describe the drawbacks of auto-calibration techniques currently used by

some States to offset calibration errors, and to provide recommendations for implementing direct WIM scale calibration. This TechBrief is

based on preliminary work to develop a calibration/commodity study.

Effects of Scale-Calibration Error

Current design procedures compute equivalent single-axle loads (ESALs)

from measured axle weights using a mathematical formula developed

from the American Association of State Highway Officials (AASHO) Road

Test. The fourth-order relationship in this formula heavily magnifies the

effects of poor scale calibration, which can lead to significant errors in

determining the load experienced by a pavement and thus computing

the expected pavement life.

Figure 1 (on the following page) shows the general effects of scalecalibration error on the computation of ESAL values. Although the effect

of scale drift varies somewhat from site to site, the basic trend is that

every 1 percent error that a scale is under-calibrated results in slightly

more than a 3-percent under-estimation of the true ESAL value. (ESALs

computed for heavy axles are affected more by calibration drift than

ESALs computed for light axles. So the ESAL error for a site with lots of

heavy axles is greater than the error for a site with mostly light axles.)

Every 1-percent over-estimation in axle weight represents a 4.5-percent

over-estimation of ESAL values. Thus, even an over-calibration of only

10 percent would result in a 45-percent error in estimated damage.

The Long-Term Pavement Performance (LTPP) program is a 20-year

study of inservice pavements across

North America. Its goal is to extend

the life of highway pavements

through various designs of new and

rehabilitated pavement structures,

using different materials and under

different loads, environments,

subgrade soil, and maintenance practices. LTPP was established under the

Strategic Highway Research Program,

and is now managed by the Federal

Highway Administration

Research and Development

Turner-Fairbank Highway

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296
Drawbacks of Auto-Calibration

Techniques

Many States attempt to work

around the cost of scale calibration

by relying on a variety of auto-calibration techniques provided by

WIM equipment vendors. Auto-calibration is the practice by which software calculates and applies an adjustment to the scale calibration factor. It is based on a comparison of

the average of a number of measurements of some specific variable

against its expected value. Some of

these techniques adjust scale-calibration factors to known sensitivities in axle sensors for changing environmental conditions, “known”

truck conditions, and equipment

limitations. Common techniques

embedded in WIM software include:

• Using the average front-axle

weight of Federal Highway Administration (FHWA) Class 9 trucks.

• Using the average weight of

specific types of vehicles (often

loaded five-axle tractor semi-trailers).

Although these techniques have

considerable value, they are only

useful after the conditions being

monitored at the study site have

been confirmed.

For example, average front-axle

weights for Class 9 vehicles are

fairly constant (if a large enough

sample is taken) at most sites. However, these weights often vary from

site to site across the country or

even within a State. Part of this

variation is due to different weight

laws and truck characteristics, and

part is due to different truck loading conditions at each site.

Another large part of the variation is controlled by vehicle drivers. Most drivers of modern tractors can change the location of the

“kingpin” (the point at which the

semi-trailer connects to the tractor).

Setting the kingpin close to the cab

pulls in the trailer, reducing air resistance and improving fuel consumption. However, it also magnifies the roughness of the ride in the

cab and decreases driver comfort.

Setting the kingpin farther away

from the cab smoothes the ride in

the cab, but results in higher fuel

consumption. Consequently, out

on the highway, drivers tend to set

the kingpin close to the cab on

smooth roads where road quality

is no problem. However, when road

roughness begins to make a ride

uncomfortable, drivers shift the

kingpin farther back in order to

make driving more tolerable.

If no other changes are made,

simply moving the kingpin setting

can shift as much as 907 kg (2000

lb) onto or away from the front axle

Figure 1. Relationship between calibration drift and error in ESAL computations.
of a fully loaded heavy truck. This is

a change of 10 to 15 percent in axle

weight. By not accounting for these

fairly common fleet changes at a

specific WIM scale location, errors

can be auto-calibrated into the WIM

system. In fact, LTPP has confirmed

several cases in which auto-calibration settings forced scales to become uncalibrated, simply because

the auto-calibration setting was incorrect for a particular site.

Auto-calibration is not, in itself,

a bad idea. However, before it can

be used, a State must determine

the following:

• Which procedure will be used.

• Whether that procedure is

based on assumptions that are true

for a particular site.

• How that procedure complements the limitations in the axle

sensor (and sensor installation)

being used.

• Whether enough test trucks are

crossing the sensor during a given

period to allow the calibration technique to function as intended.

Recommendations for Direct

Calibration

Only direct calibration of a WIM

scale after it has been installed at a

site ensures that it is measuring

axle weights correctly. This includes a comparison of static axle

weights with axle weights that are

estimated from multiple vehicle

passes with more than one vehicle.

For short-duration counts, calibration should be performed immediately before the start of LTPP data

collection.

For longer duration counts, the

scale should be calibrated initially,

the traffic characteristics at that site

should be recorded, and the scale’s

performance should be monitored

over time. The State should also

perform additional, periodic on-site

calibration checks (at least two per

year). These steps will ensure that

the data being collected for LTPP,

as well as for State use, are accurate and reliable.

More information on WIM scale

calibration can be found in the following documents:

1. ASTM Standard E1318-94,

Highway Weigh-in-Motion (WIM)

Systems With User Requirements

and Test Method, Annual Book of

ASTM Standards.

2. Bahman Izadmehr and Clyde

Lee, “On-Site Calibration of Weighin-Motion Systems,” Transportation Research Record 1123,

Pavement Management and

Weigh-in-Motion, 1987, pp. 136-

144.

3. Long Term Pavement Performance Program, Protocol for Calibrating Traffic Data Collection

Equipment, April 1998 (available

from the LTPP team of the FHWA

Pavement Performance Division).

4. “On-Site Evaluation and Calibration Procedures for Weigh-inMotion Systems,” NCHRP Research Results Digest #214, 1996.

5. Peter Davies and Fraser

Sommerville, “Calibration and Accuracy Testing of Weigh-in-Motion

Systems,” Transportation Research

Record 1123, Pavement Management and Weigh-in-Motion, 1987,

pp. 122-126.

 JULY 1998 FHWA-RD-98-104

Researcher: The study on which this TechBrief is based was performed by Law PCS, 12104 Indian Creek Court, Suite

A, Beltsville, MD 20705 and the Washington State Transportation Center, University of Washington, University District

Building, 1107 NE 45th Street, Suite 535, Seattle, WA 98105-4631. Contract No. DTFH61-97-C-00002.

Distribution: This TechBrief is being distributed according to a standard distribution. Direct distribution is being

made to the Regions and Divisions.

Availability: This TechBrief is based in part on a publication entitled, Detection of Calibration Versus Commodity

Shifts, by Mark Hallenbeck and Cindy Cornell-Martinez. It was published in June 1996 by Chaparral Systems Corporation,

649 Harkle Road, Suite E, Sante Fe, New Mexico 87505, (505) 983-5594. It is available on Chaparral Systems Corporation’s

home page at http://www.chapsys.com//ltpp_web/ltpp7.htm.

Key Words: Weigh-in-Motion, WIM, LTPP traffic, WIM calibration.

Notice: This TechBrief is disseminated under the sponsorship of the Department of Transportation in the interest of

information exchange. The TechBrief does not establish policies or regulations, nor does it imply FHWA endorsement

of the conclusions or recommendations. The U.S. Government assumes no liability for the contents or their use.