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WIM data collection are a good first step toward this type 5.8 CALIBRATE AND MAINTAIN
of program.3 CALIBRATION OF EQUIPMENT
Installation of equipment should not be considered com-
5.7 MANAGE EQUIPMENT INSTALLATION plete until that equipment has been calibrated and acceptance
testing of the device in that location has been completed.
Proper installation of sensors is key to both performance Both WIM and vehicle classification devices require calibra-
and life span, regardless of the technology involved. tion, although WIM calibration is far more complex and dif-
To ensure the quality of any given installation, it is good ficult than vehicle classification.
practice to have at least one agency representative and one
vendor representative oversee the sensor installation process
at permanent sites. This ensures that both the state's and the 5.8.1 Initial Calibration
vendor's requirements are met during the installation process.
This is particularly important when warranties are used to A number of procedures for calibrating WIM scales exist.
ensure system performance, in that it ensures that both par- Appendix 5-A in the 2001 FHWA Traffic Monitoring Guide4
ties are satisfied with the initial site conditions and installa- provides a reasonably complete description of the current
tion. (For WIM performance warranties, site conditions must state-of-the-art in WIM system calibration. Some material
usually match ASTM E 1318 site condition specifications. from this appendix is reprinted below. In addition, ASTM5
These site conditions should be verified by both parties when and the FHWA's Long Term Pavement Performance Project6
the site is first selected, well prior to the beginning of the have recommended the use of two test trucks of known
installation process.) weight but different vehicle characteristics (different classi-
fications and/or suspension types) for performing WIM scale
Installation of sensors does not just involve placement.
calibration.
For permanently mounted sensors, installation also involves
The two-test-truck calibration technique consists of obtain-
(among other items) placement of conduit for lead wires,
ing static weights for two distinctly different vehicles and then
placement and design of junction boxes, design and place-
repeatedly driving those vehicles over the WIM scale. Scale
ment of cabinets that hold data collection electronics, and
calibration factors are then adjusted to minimize the mean
provision of environmental protection (lighting and electri-
error obtained when comparing static and dynamic weights.
cal surge protection, moisture protection, temperature con-
(Both the ASTM and LTPP documents provide step-by-step
trols, defenses against insect and rodent infestation) for the
directions for calibrating scales using this technique.) Ideally,
entire system.
during the calibration effort, the two test trucks should be
Poor installation of any features can lead to early system
driven over the WIM scale at a variety of speeds and under
failure and significant increases in both sensor downtime and
varied pavement temperature conditions in order to ensure
maintenance and repair costs.
that the scale operates correctly under all expected operat-
Good practice for equipment installation includes choos-
ing conditions.
ing good equipment and sensor locations in the first place. The use of two calibration vehicles is specifically designed
For intrusive sensors, this means placing them in or on pave- to limit calibration biases that can be caused by the use of a
ment that is in good condition and likely to last well past the single test vehicle. Biased calibration when using a single test
design life of the sensors being installed. For both intrusive truck comes from the fact that every truck has its own unique
and non-intrusive sensors, it means understanding the envi- dynamic interaction with a given road profile given a specific
ronmental conditions that occur at a site and designing sen- load. Calibration of a scale to a single vehicle's dynamic per-
sor installations so that sensors are protected as much as pos- formance (motion) is acceptable when the motion of that
sible from environmental effects on system performance. (For vehicle is representative of the traffic stream. Unfortunately,
example, video cameras need to be placed so that glare and it is extremely difficult to determine if a given test truck is
other lighting problems are minimized and so that the cam- representative of the traffic stream, and consequently use of
eras are protected from rain, snow, and spray generated by a single vehicle can cause a calibration bias that forces the
vehicles. Similarly, intrusive sensors need to be protected from scale to weigh most vehicles inaccurately.
moisture intrusion, with particular attention paid to in-pave- The source of this calibration bias can be explained with
ment wiring when freeze-thaw conditions exist.) two figures. Figure 5.2 illustrates how the force applied by a
A variety of techniques exist for protection of sensors from
environmental conditions. Good management practice is to
document those practices that are successful (for future use 4
http://www.fhwa.dot.gov/ohim/tmguide/index.htm (active as of June 20, 2003).
5
by new staff within the agency) and to share those successes American Society of Testing Materials, Annual Book of ASTM Standards 2000, Sec-
tion 4, Construction, Volume 04.03, Designation: E 1318-02--Standard Specification
with other agencies. for Highway Weigh-In-Motion (WIM) Systems with User Requirements and Test
Method, ASTM.
6
Long Term Pavement Performance Program Protocol for Calibrating Traffic Data
Collection Equipment, April 1998 (with 5/10/98 revisions). http://www.tfhrc.gov/pave-
3 ment/ltpp/pdf/trfcal.pdf (active as of June 20, 2003).
http://www.tfhrc.gov/pavement/ltpp/spstraffic/index.htm (active as of June 20, 2003).
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54
G r e at er f or c e t ha n
s ta ti c w ei g h t
Axle force as the
truck moves down
Bias caused the road
by measuring
this axle at
Point A
Actual force
(axle weight)
at Point A
Static
Weight
Le s se r fo r ce th an
s ta ti c w ei g h t
Scale
Location
Distance
(Direction of Travel ---->)
Figure 5.2. Variation of axle forces with distance and the resulting effect on WIM scale calibration.
truck (or any given truck axle) varies as it moves down the The problem with the single test truck technique occurs
road. This sinusoidal oscillation (bouncing) results from the because each truck has a different dynamic motion. When the
interaction between the vehicle's suspension system(s) and test truck has a different set of dynamics than other trucks
the road's roughness. The vehicle's dynamic motion causes using that road, the scale is calibrated to the wrong portion of
the weight felt by the road (or the scale sensor) to change the dynamic curve. In the example illustrated in Figure 5.3,
from one pavement location to the next as the vehicle moves if the scale is calibrated to the dynamic motion of the test
down the road. The goal of the WIM calibration effort is to truck, it will cause the scale to overestimate the weights asso-
measure this varying force at a specific location (Point A in ciated with the majority of trucks on that road (Point B).
Figure 5.2) and relate it to the truck's actual static weight. To A change in a given vehicle's speed affects the force applied
do this, the scale sensor must be able to measure the weight by that vehicle's axles at any given point in the road. This is
actually being applied at Point A and also to correct for the because the oscillation of the suspension and load are pri-
bias resulting from the fact that, at Point A, the test truck is marily based on time, not distance. Thus, the load always
actually producing more force than it does when the truck is lands at the same time after a bump in the road is crossed, but
at rest (because it is in the process of landing as it bounces if the truck is going slowly, that landing is located closer to
down the road). the bump than if the truck is moving quickly. Thus, on roads
By using a test truck, it is possible to directly relate the where truck dynamics are high (and the trucks are bouncing
actual weight sensor measurement to the actual static weight a lot), a change in average vehicle speed (e.g., caused by con-
in one simple calculation. If the test truck is driven over the gestion or some other factor) can result in a shift in the appro-
WIM scale several times and the weights estimated by the priate calibration factor for a scale.
scale are compared with the known static values, it is possi- To solve the calibration problem caused by dynamic
ble to determine whether the scale is operating consistently vehicles, five basic approaches have been proposed in the
and, if it is, to calculate a statistically valid measure of the literature:
scale's ability to estimate that truck's axle and gross vehicle
weights. The scale's sensitivity is adjusted (the "calibration · A scale sensor can be used that physically measures
factor") until the weights estimated by the scale equal the the truck weight for a long enough period to be able to
known static weights of the truck and its axles. account for the truck's dynamic motion (this is true of
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55
Average vehicle motion for all
G r e at er f or c e t ha n trucks on this roadway
s ta ti c w ei g h t
Axle force on single
Bias caused by calibration truck
measuring this
axle at Point A
A
Static
Weight
B
Average bias for all
trucks is negative
Le s se r fo r ce th an
s ta ti c w ei g h t
Scale
Location
Distance
(Direction of Travel ---->)
Figure 5.3. Variation of axle forces with distance and the resulting effect on WIM scale calibration.
the bridge WIM system approach where the truck is on 5.8.2 Maintaining Calibration
the scale the entire time it is on the bridge deck).
· Multiple sensors can be used to weigh the truck at dif- Once a scale is initially calibrated, best practices maintain
ferent points in its dynamic motion either to average out calibration over time by a combination of periodic on-site
the dynamic motion or to provide enough data to predict calibration verification field tests and an ongoing review of
the dynamic motion (so that the true mean can be esti- the scale output against known quantities (e.g., have the loca-
mated accurately). tions of the loaded and unloaded peaks for Class 9 trucks
· The relationship of the test truck to all other trucks can be moved since the scale was calibrated?). When changes are
determined. This is often done by mathematically mod- observed in the reported values for these known quantities,
eling the dynamic motion of the truck being weighed in scale performance is investigated (i.e., the measured changes
order to predict where in the dynamic cycle the truck is trigger one of the periodic field calibration tests) to determine
when it reaches the scale. if a change in vehicle characteristics is occurring or if changes
· More than one type of test truck can be used in the cal- in pavement profile or sensor sensitivity have affected the
ibration effort (where each test truck has a different type scale's calibration.
of dynamic response) in order to get a sample of the
vehicle dynamic effects at that point in the roadway.
· Independent measurements can be used to ensure that 5.8.3 Autocalibration
the data being collected are not biased as a result of the
test truck being used. While many WIM systems feature autocalibration func-
tions, these are not an acceptable substitute for the initial site
As noted earlier, the current best practice relies on the use calibration, and, even when used for maintaining the calibra-
of multiple test vehicles (a minimum of two) for initial cali- tion of a previously calibrated WIM, they should only be used
bration of WIM scales. This technique was chosen over the with caution.
other methods because of its simplicity and its relatively low Many autocalibration techniques were originally designed
costs compared with the other alternatives, though there is to adjust scale calibration factors to account for known sen-
appreciable interest in the multi-sensor approach in Europe. sitivities in sensor performance to changing environmental
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conditions. Others were software adjustments developed to Autocalibration is not a bad idea. However, before it can
take into account equipment limitations. Common techniques be used even to maintain a scale's calibration, several factors
include must be understood:
· Using the average front-axle weight of FHWA Class 9 · What autocalibration procedure the scale is using,
trucks and · Whether that procedure is based on assumptions that are
· Using the average weight of specific types of vehicles true for a particular site,
(often loaded five-axle tractor semi-trailers or passen- · How that procedure complements the limitations in the
ger cars). axle sensor (and sensor installation) being used, and
· Whether enough vehicles being monitored as part of the
Although these techniques can have considerable value, autocalibration function are crossing the sensor during
they are only useful after the conditions they are monitoring a given period to allow the calibration technique to
at the study site have been confirmed. In fact, tests performed function as intended.
by LTPP7 showed that autocalibration functions were not
always successful at maintaining calibration of environ- The highway agency should also thoroughly test the actual
mentally sensitive sensors when environmental conditions performance of an autocalibration system before assuming
were changing rapidly. Autocalibration functions cannot be that a vendor's claims about its accuracy are valid. Only after
expected to calibrate a scale accurately if key autocalibration testing has been satisfactorily completed should a state rou-
values have not been independently confirmed at that site. tinely use autocalibration. Even then, autocalibration does not
Site-specific confirmation of autocalibration variables is eliminate the need for a state to monitor scale output or peri-
important because research has shown that those key vari- odically perform calibration verification tests in the field.
ables are not as constant as thought when autocalibration for
WIM was first developed. For example, while the average
5.8.4 Calibration of Vehicle
front axle weight for Class 9 vehicles at most sites stays fairly
Classification Equipment
constant (and can be measured accurately if a large enough
sample is taken), the average front axle weight often varies In theory, calibration of vehicle classification equipment
significantly from site to site across the country or even within is not as difficult as WIM system calibration. In reality, some
a state. Part of this variation is due to different weight laws specific installation problems can cause problems with clas-
and truck characteristics, part is due to different truck load- sifier output. Compared with WIM equipment, axle-based
ing conditions at each site, and part is due to vehicle charac- vehicle classification equipment is generally less sensitive to
teristics that are controlled by vehicle drivers. minor variations in signal strength. However, some non-
Most drivers of modern tractors can change the location of intrusive sensors can be very sensitive to input parameters
the "king pin" (i.e., the point at which the semi-trailer con- and may require careful tuning of sensor performance to work
nects to the tractor). Setting the king pin close to the cab pulls correctly.
in the trailer, reducing air resistance and improving fuel effi- There are basically three issues related to the performance
ciency. However, this setting also magnifies the roughness of of classifiers that need to be reviewed as part of the installa-
the ride in the cab and increases driver discomfort. Setting tion calibration:
the king pin farther away from the cab smoothes the ride in
the cab but results in higher fuel consumption. When operat- · Sensor configuration and layout information,
ing on rough roads, drivers tend to set the king pin farther · Conversion of the outputs into estimates of each passing
back than when they operate on smooth roads. vehicle's characteristics (vehicle speed, vehicle length,
If no other changes are made, simply moving the king pin and distance between axles), and
setting from its foremost position to its rearmost position can · The conversion of the vehicle characteristic information
shift as much as 2,000 pounds onto or away from the front axle into estimates of that vehicle's classification.
of a fully loaded heavy truck. This is a change of 10 to 15 per-
cent. By not accounting for these fairly common fleet changes Automated vehicle classifiers generally require input of
at a specific WIM scale location, similar errors can be auto- information related to the specific layout of the sensors used.
calibrated into the WIM system. In fact, LTPP has confirmed For axle classifiers, this generally means the distance between
several cases in which autocalibration settings forced scales to the two axle sensors (or two loops used for vehicle speed
adopt biased calibration factors simply because the autocali- computation). For non-intrusive detectors, it may include
bration setting was incorrect for a particular site. measurements such as the height of the camera and angle of
view or the distance of a sensor from the roadway.
These measurements are used as input to the sensor sys-
7
SPS Traffic Site Evaluation--Pilots Summary and Lessons Learned, May 2, 2002,
http://www.tfhrc.gov/pavement/ltpp/reports/lessons/Lessons.pdf (active as of June 20,
tems to convert the sensor outputs into the estimates of vehi-
2003). cle speed, length, and axle spacing, which are in turn used to
