Assessing Driver Speed Choice in Fog with the Use of Visibility Data from Road Weather Information Systems

Abstract

Fog can create a significant safety hazard for motorists. If motorists drive faster than visibility permits, severe multiple-vehicle crashes could occur. In Virginia, sections of I-64 and I-77 in mountainous parts of the state experience significant recurring fog events. These locations have also experienced several chain-reaction crashes during fog, involving more than 50 vehicles. Those crashes were typically caused by drivers traveling too fast for the conditions. To improve safety on the I-77 corridor, the Virginia Department of Transportation is constructing a variable speed limit system that will provide dynamic speed limits based on the available visibility. Before the system is activated, it is important to understand existing driver speed choice behavior during low-visibility conditions. It is possible that posting a variable speed limit based only on stopping sight distance could create significant speed variance and decrease safety if drivers are driving much faster than conditions warrant. In this study, speed and visibility data were examined at several locations on I-64 and I-77, where there were recurring fog events. The models developed showed that while motorists are slowing down in low visibility, there is still a significant differential between observed speeds and the safe speed calculated from the stopping sight distance. The results indicate that some additional smoothing will need to occur to ensure a safe transition from current speed choices to desired safe speeds.