The Westa (Weigh Station) simulation model was used to study truck congestion. The model evaluated weigh station throughput based on variations in entrance ramp length (1000 or 2000 feet), deployment of electronic screening transponders (5 to 50 percent), and the use of weigh-in-motion (WIM) scales (threshold limit 65,000 pounds with error rate of 5 percent). Representative weigh station traffic information and vehicle characteristics data were logged into each model design and then vehicle operations were simulated in order to evaluate impacts on station queue length, station overflow, accident risk, and the number of delays to trucks of legal weight.

The following information was used to develop the Seymour Station model:

• Scale drawings of the weigh station located in Seymour, Indiana.
• FHWA vehicle weight classification definitions.
• The average time between trucks at the scale (25 seconds).
• The average arrival rate of traffic at the station during peak periods (1 vehicle per 2.56 seconds).

The average time between trucks at the scale was derived from on-site observations in March of 1998. The vehicle peak arrival rate data was derived from information recorded at the Edinburgh Strategic Highways Research Program (SHRP) site located north of Seymour from April 5, 1998 through April 16, 1998.

RESULTS

The results of this study indicated that longer entry ramps had no bearing on the average number of minutes a station was closed due to overflow, however, longer entry ramps did reduce the number of times trucks had to brake suddenly (harder than -0.2g).

As the percentage of trucks with transponders increased the queue overflowed less often since fewer trucks had to enter the station. It was estimated that approximately 40 to 50 percent of trucks would need to be equipped with transponders to make significant reductions in station overflow.
The WIM was very effective at reducing the number of trucks required to be weighed at the static scale. The significant reduction in the number of trucks required to wait in the queue resulted in significant reductions in average queue length, time spent in the queue, and queue overflow. TheWIM scale; however, provided less opportunity for station operators to scan trucks and drivers for safety concerns.