Weigh-in-Motion (WIM) systems are effective in capturing weight and axle configurations of vehicles and can be served as essential inputs for evaluating transportation infrastructure performance. Auto-calibration of sensors, which is an algorithmic procedure by which weights measured by the WIM sensor are adjusted by calibration factors that are periodically calculated based on presumed traffic characteristics, is often performed to ensure weights are measured accurately. This project developed a new form of auto-calibration to measure the weights of the same truck, tracked by Automatic Vehicle Identification (AVI) across multiple WIM sites, to generate a reference weight and calibration factor. Two data collection efforts were undertaken in March of 2018 and 2019 to gather WIM, static scale, and video data for algorithm validation resulting in a sample of approximately 500 trucks to use for algorithm performance evaluation.

Estimated calibration costs for 50 WIM stations by calibration method:

• Annual calibration costs using field data costs $219,826, assuming 10 percent of the WIM sites (5 sites) are near static scale and 90 percent of the WIM sites (45 sites) are calibrated using test trucks. This estimation does not include increases in staffing costs related to labor and travel associated with on-site calibration.
• Annual cost of auto-calibration using AVI-based data collection is $5,000 in this study. However, it is important to note that commercial AVI type of datasets from various vendors can range significantly.
• Annual cost of auto-calibration using a combination of field and AVI-based data collection was estimated as $153,750, assuming that 30 percent of the WIM sites (15 sites) use AVI-based calibration and 70 percent (35 sites) use field data collection to update auto-calibration parameters.

WIM sensor costs:

WIM sensor costs and life span for a two-lane site are estimated based on FHWA’s Weigh-In-Motion Pocket Guide and are shown in Table 1. Sensor costs do not include installation costs. For a typical two-lane site with initial calibration, installation costs range from $20,500 to $56,500. Operation costs include annual calibration costs, routing maintenance (two site visits per year), and software updates. Routine calibration cost is based on using one test truck ($90 to $135 per hour) for a maximum of 20 runs per lane. Bending plate and load cells may be calibrated every two years.

Table 1. WIM sensor costs and life span for a two-lane site.