Active traffic management dynamically manages congestion based on current and near-term traffic conditions. One ATM strategy is variable speed limit/variable advisory speed, which adjusts speed limits on roadways based on weather conditions, traffic volume, and construction. The idea being that traffic performance will improve if the posted speed limit addresses real-time traffic scenarios. But how effective are VSL/VAS systems on overall road safety? What is the crash frequency? Researchers Tarek Hasan, Mohamed Abdel-Aty, and Nada Mahmoud wanted to explore VSL/VAS and safety performance functions related to crashes.

The authors identified a research gap related to crash frequency, where average annual daily traffic-based safety performance functions did not capture crash potentiality and VSL operation data offered limited insight on crashes. In their paper, “Freeway Crash Prediction Models with Variable Speed Limit/Variable Advisory Speed” in the Journal of Transportation Engineering, Part A: Systems, the authors studied this gap by focusing on the evaluation of VSL/VAS strategy to reduce crash potential by developing short-term SPFs with detailed VSL/VAS operational data. Policymakers and practitioners will find this research helpful when evaluating VSL/VAS strategies. Learn more about this research to improve highway safety at The abstract is below.


Variable speed limit (VSL) and variable advisory speed (VAS) signs are efficient, cost-effective and among the state-of-the-art active traffic management (ATM) strategies. They adopt the idea of dynamically changing posted speed limits to improve highway safety performance and operation by harmonizing traffic speed. VSL/VAS system involves changing speed limits according to real-time traffic events and weather conditions. Hence, traditional average annual daily traffic (AADT) based crash prediction models may not capture the temporal effect of traffic characteristics due to the high level of aggregation. To address this issue, short-term safety performance functions (SPFs) with aggregation levels of average annual weekday hourly traffic (AAWDHT) and average annual weekday peak traffic (AAWDPT) along with AADT-based SPFs were developed using high-resolution traffic detector and VSL/VAS operational data. In this study, the Poisson log-normal model performed well at each level of aggregation and thus is recommended for developing short-term SPFs. In line with previous studies, traffic volume and standard deviation of speed were found to be positively associated with crash frequency in all the estimated models. In addition, it was found that implementation of VSL/VAS significantly reduced crash frequency by 15.97% and 26.14% for the AAWDHT and AAWDPT models, respectively. The safety improvement was captured in the short-term models in a more distinguished way than in the highly aggregated AADT-based model. The findings of this study pave the way for practitioners and policymakers to evaluate and select important parameters for VSL/VAS strategy implementation on freeways.

Learn more about the safety advantages of variable speed limits in the ASCE Library: