Impact of Variable Speed-Limit System on Driver Speeds during Low-Visibility Conditions

摘要

Low-visibility conditions can inhibit a driver's ability to perceive appropriate operating speeds, particularly during foggy conditions in which the characteristics of the fog can vary spatiotemporally. Numerous agencies have installed countermeasures such as weather advisory systems and variable speed limits (VSLs), but the results have not been analyzed quantitatively. This paper assesses how the I-77 fog VSL system in Fancy Gap, Virginia, affected driver speeds since its activation in October 2016. Prior to implementing the VSL, mean speeds were twice the safe speed based on stopping site distance (SSD) during fog. The VSL system sought to encourage drivers to travel closer to the SSD-based safe speed by posting appropriate reduced speed limits. The results indicated statistically significant reductions in mean speeds after the VSL was activated, and drivers drove closer to the safe speed based on available visibility. Compliance with the posted variable speed limit generally improved once drivers encountered reduced visibilities. Given the short period since activation, insufficient crash data existed to perform a complete safety analysis. However, the preliminary results indicated very promising safety trends for fog-related crashes. The results of this VSL implementation may serve as a reference for other agencies contemplating alternatives to improve safety in fog-prone areas and indicate that the countermeasure had a positive effect. (C) 2019 American Society of Civil Engineers.