On-Site Calibration Evaluation Procedures for WIM Systems

The feasibility of two methods for evaluating and calibrating weigh-in-motion (WIM) systems is explored. The first method uses a combination of test trucks and vehicle simulation models. The computer model VESYM was used for the simulations. The models for the test trucks were calibrated using acceleration measurements on board the vehicles. Although, this approach does not allow calculation of the discrete value of the dynamic axle load over WIM sensors, it can be used effectively in establishing the extent of variation at a particular WIM site. This information leads to an effective WIM system calibration method. The second method for calibrating WIM systems compares static and dynamic axle loads of vehicles through automatic vehicle identification (AVI). The AVI facilities developed for the Heavy Vehicle Electronic License Plate project on the I-5 corridor was used for this purpose. The static axle load of AVI-equipped vehicles was obtained from the Oregon Department of Transportation for two sites, Woodburn southbound and Ashland northbound. The WIM load data were obtained from Lockheed IMS for all the AVI-equipped WIM systems on the I-5 corridor. The data were analyzed to match AVI numbers, dates, and times of weighing. Time limits for traveling between sites were established to ensure that trucks could not stop and load or unload cargo between sites. Errors were calculated as the percentage difference between WIM and static loads for individual axles and axle groups. Calibration factors were derived to minimize the residual sum of squares of the errors.