Operational Performance Models for Freeway Truck-Lane Restrictions

A common approach to reducing the impacts of truck traffic on freeways has been to restrict trucks to certain lanes to minimize the interaction between trucks and other vehicles and to compensate for their differences in operational characteristics. Many possible design alternatives for truck-lane restrictions exist. The aim of this study was to develop operational performance models that can be applied to help identify the most operationally efficient truck-lane restriction alternative on a freeway under prevailing conditions. The operational performance measures examined in this study included average speed, throughput, speed differentials, and lane changes. Prevailing conditions included number of lanes, interchange density, free-flow speeds, volumes, truck percentages, and ramp volumes. The VISSIM model was adopted as the simulator for this study. A program was developed to automate the process of running multiple VISSIM runs and extracting the corresponding output for various input scenarios. Nonlinear regression models were then developed to relate the average speed, throughput, number of lane changes, and speed differentials under prevailing conditions. Based on the performance models developed, a simple decision procedure was recommended to select the desired truck-lane restriction alternative for prevailing conditions. Findings, based on the analysis of the simulated data, were as follows: (1) In general, truck restriction alternatives increase the average speed under low interchange density, low truck volume, and low ramp volume condition. When a freeway corridor is congested, truck-lane restrictions reduce the average speed. However, the speed reduction is negligibly small, except when a large number of restricted lanes is used. This suggests that restricting an appropriate number of lanes to truck traffic is generally beneficial since it may improve traffic safety without worsening the efficiency of moving traffic. (2) A large number of the restricted lanes resulted in a higher rate of throughput under low truck percentages with sparsely spaced interchanges. A relatively low number of restricted lanes generally provides a higher capacity than the non-restriction alternative for truck percentages up to 25%. (3) Statistical analysis shows that the speed differentials between restricted and non-restricted lane groups are significant, and that the magnitude increases as the number of interchanges, ramp volumes, truck percentages, and free-flow speed increase. (4) Truck-lane restrictions significantly reduce the number of lane changes by separating slower vehicles from faster vehicles, potentially improving traffic safety. (5) One-lane truck restriction is suitable for 3-, 4-, and 5-lane freeways, while two-lane truck restriction is more suitable for 4- and 5-lane freeway corridors, except when the interchange density is high and truck percentage is larger than average. The report concludes with recommendations for further studies.