Dynamic Variable Speed Limit Strategies for Real-Time Crash Risk Reduction on Freeways

Recent research at the University of Central Florida involving crashes on I-4 in Orlando, Florida, has led to the creation of new statistical models capable of determining the crash risk on the freeway in real time. The potential benefits of variable speed limit (VSL) implementation for reducing the crash risk along the freeway at different loading scenarios was studied. VSL strategies were used in a networkwide attempt to reduce rear-end and lane-change crash risks where speed differences between upstream and downstream vehicles were high. The idea of homogeneous speed zones also was introduced in this study to determine the distance over which variable speed limits should be implemented from a station of interest. This idea is unique because it is the first time a dynamic distance has been considered for VSL implementation. This study shows that VSL is an effective crash prevention strategy when the freeway is operating in uncongested conditions. Specifically, in free-flow conditions and conditions approaching congestion, VSL can be used to reduce crash risk and prevent crash occurrence. It was also confirmed that the effects of crash migration increase as the level of congestion increases, and specific implementation techniques were found to better resist those effects. VSL was not found to effectively reduce crash risk in congested situations. This finding is sustained with logic and is supported by previous research.