Evaluating the safety effectiveness of a weather-based variable speed limit for a rural mountainous freeway in Wyoming

This article investigates the safety effectiveness of weather-based variable speed limit (VSL) systems on a rural mountainous interstate in Wyoming using a before-after study with Empirical Bayes (EB). VSL is considered one of the important countermeasures that help in reducing crashes by regulating and minimizing the variability for the motorists' operating speed. Although the multiple benefits result from the VSL, studies evaluating its safety effectiveness using Highway Safety Manual (HSM) methodologies are scarce in literature. Interstate 80 (I-80) in Wyoming, which has a total length of 402 miles, is known by its challenging roadway geometry, adverse weather, and high truck traffic. It comprises four weather-based VSL corridors totaling 147 miles. Full SPFs were developed to account for the variability in weather, traffic, roadway geometry, and implemented countermeasures. Negative binomial (NB) and multivariate adaptive regression splines (MARS) were used to develop the SPFs. Six years in the before period and 5 years in the after period were utilized to develop the crash modification factors (CMFs). Results showed that the MARS model outperformed the NB model. Moreover, the VSL showed a significant reduction in all the investigated crash types. The safety effectiveness of VSL for the examined severity levels ranged between 15% to 29% and 12% to 34% using NB and MARS, respectively.