Part-time shoulder use operational impact on the safety performance of interstate 476

With every change in a roadway to improve the network operational performance, the roadway safety behavior is impacted. Part-time shoulder use (PTSU) is one strategy that significantly impacts the operational performance of a roadway for several hours each workday. This study evaluates the impact of PTSU on safety behavior and the possibility of crash prevention.

Current predictive safety measures do not account for the part-time dimensional changes of PTSU effectively. Using Vissim-based microsimulation trajectory data in the Surrogate Safety Assessment Model (SSAM) program, developed by the Federal Highway Administration, this study gauged the safety disposition of Interstate 476 (I-476) in southeast Pennsylvania (PA). The two key safety evaluation factors include the number of conflicts and the time-to-collision (TTC) of each conflict. The number of conflicts represents unsafe scenarios that could become crashes if neither possibly impacted vehicles adjusts their route. TTC is the minimum time for the conflict to become a crash if either vehicle's route was not adjusted to avoid the collision. Both factors were averaged across six 20-min periods during the 2-h commuting period simulated.

Due to the improved operational performance, the TTC values for the PTSU scenarios decreased, reducing safety; however, the number of conflicts occurring reduced significantly, counterbalancing the reduced safety from the TTC reduction.

PTSU has great operational improvement effects which create a faster-moving transportation network. The higher speeds caused by the enhanced operations increases the risk of a crash during unsafe situations, but the number of unsafe situations occurring will significantly reduce. The reduced number of unsafe situations indicates an increased probability of crash avoidance but does not indicate a definitive reduction in crashes.