Impact of a Variable Speed Limit System on Driver Speed Choice and Crash Characteristics During Low Visibility Conditions

Low visibility conditions can inhibit a driver’s ability to perceive appropriate operating speeds, particularly during foggy conditions where the characteristics of the fog can vary spatiotemporally. By reducing visibility and contrast in the visual field, fog obscures crucial driving cues essential for perceiving depth and speed. Studies have shown that fog-related crashes tend to involve more vehicles and more severe injuries. Numerous agencies have installed countermeasures like weather advisory systems and variable speed limits (VSLs) to mitigate these conditions, but not many studies have quantitatively analyzed the results of these projects. In October 2016, the Virginia Department of Transportation (VDOT) activated a VSL system on a 12-mile section of Interstate 77 that runs through mountainous terrain in southwestern Virginia known to experience severe, recurring fog events with the objective of to reducing the quantity and severity of crashes in the corridor. This thesis assesses how the I-77 fog VSL system in Fancy Gap, Virginia affected driver speed choice and crash characteristics since its activation in October 2016. Prior to the installation of the VSL, drivers frequently drove much faster than the safe speed based on the stopping sight distance during fog. The VSL system sought to get drivers to travel closer to the safe speed based on available visibility by posting appropriate reduced speed limits.
The analysis examines the effect of the VSL system on driver speeds both before and after at a single site and across the corridor in the after period. Effects on crashes for the entire corridor are also examined. The results showed statistically significant reductions in mean speeds and variances after the VSL was activated, and drivers drove closer to the safe speed based on available visibility. Models developed to understand how the VSL system affected speed as a function of visibility showed that speeds are reduced by a statistically significant amount when VSLs are active. Trends in speed by posted speed limit were examined across the corridor, and it was found that compliance generally improved once drivers encountered reduced visibilities. Speeds did not change as much in transition areas leading into the area where the fog was present, however. Crash analysis revealed only two fog-related crashes in the after period, yielding reduced crash rates during low visibility conditions and indicating improved safety. The results of this VSL implementation may be used to further refine current VSL control algorithm to improve compliance even further and could serve as a reference for other agencies contemplating alternatives to improve safety at fog-prone areas given the indications that the countermeasure did have a positive effect.