Assessing the Impact of Increasing Tractor-Trailer Speed Limit on the Safety and Mobility of Three-Lane Highways in Egypt

Currently, the speed limit for tractor-trailers (TTs) on Egyptian highways is 30 km/h lower than the speed limit for passenger cars (PCs). The main purpose of this article is therefore to evaluate the effects of an increase in TT speed limit on both the traffic safety and mobility of Egyptian three-lane highways, with speed limits of 90 and 100 km/h for PCs. A SUMO simulation model was used in this analysis. Average vehicle delay was used as a measure of mobility, while the simulated conflict ratio based on time to collision (TTC) and deceleration rate required to avoid crash (DRAC) surrogate safety measures were used as performance indicators of safety. All simulated vehicles were equipped with surrogate safety measurement (SSM) devices to extract traffic conflicts, with TTC ≤ 2.50 representing less sever conflicts; TTC ≤ 1.50 s and DRAC ≥ 3.35 m/s2 representing potential conflicts; and TTC ≤ 0.50 s and DRAC ≥ 6.0 m/s2 representing severe conflicts. The results show that increasing the speed limit for TTs would significantly reduce the average delay by approximately 27–76% for traffic volumes 500–2000 vehicle/hr/lane, with TTs comprising 2.5–15% of total traffic volume. Furthermore, in most cases, when comparing the situation after the speed limit increase for TTs with the base case (i.e., TTs with a speed limit of 60 km/h and 70 km/h for highways, and a speed limit of 90 km/h and 100 km/h for PCs), the results show no statistically significant change in the conflict ratio more than 1.0, suggesting that there is no safety change. Furthermore, due to the increase in speed limit for TTs, in some cases and for different thresholds, there is a statistically significant reduction in the conflict ratio, indicating improvements in terms of safety.