Operational effects of weigh-in-motion systems in weight enforcement

The effects of weigh-in-motion (WIM) systems on traffic operations and weight enforcement were compared and evaluated. The systems studied included high speed WIM, medium speed WIM and conventional static scales alone. The major measurements of effectiveness were traffic delay, queue, and the avoidance rate for overweight vehicles. Four weighing facilities from both the eastern and western United States were chosen as the real life bases for the study.

Queuing theories, probability and statistics were the major methodologies employed in the study. The characteristics of queuing systems, such as traffic arrival patterns, weight enforcement processing time distribution, and capacity of the static scales at each weigh station were determined through field data collection at weigh stations. The proportion of the vehicle population directed to the static scale by WIM screening was analyzed, based on WIM accuracy and truck weight distributions at or near each weigh facility.

By considering delay, queue, and the avoidance rate of overweight vehicles comprehensively, optimal weighing systems are proposed for different V/C ratios, i.e., the ratio of traffic volume at a specific site to the actual capacity of the static scale. When the V/C ratio is less than 1, a weighing facility using a static scale alone is most cost effective; when the V/C ratio is between 1 and 1.5, a medium speed WIM is suggested. High speed WIM is recommended only when the V/C ratio is larger than 1.5.