Impacts of the I-77 Variable Speed Limit System on Speed and Crash Characteristics During Low Visibility Conditions

Report No: 19-R6

Published in 2018

About the report:

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. 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 are more likely to involve multiple vehicles and severe injuries than crashes during clear conditions. Numerous agencies have installed countermeasures such as weather advisory systems and variable speed limits (VSLs) to mitigate these conditions, but not many studies have quantitatively analyzed the results of these countermeasures.

In October 2016, the Virginia Department of Transportation (VDOT) activated a VSL system on a 12-mile section of I-77 that runs through mountainous terrain in southwestern Virginia. The area is known to have severe, recurring fog events, so the VSL system was installed to reduce the quantity and severity of crashes in the corridor. This study assessed how the I-77 VSL system has affected speeds and crash characteristics since its activation. Before the installation of the VSL, drivers frequently drove much faster than the safe speed based on the stopping sight distance during fog. The purpose of the VSL system was to influence drivers to travel closer to the safe speed during reduced visibility events by posting appropriate reduced speed limits.

The analysis examined the effect of the VSL system on driver speeds before and after activation at a single site and after activation across the corridor. Effects on crashes for the entire corridor were 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 were reduced by a statistically significant amount when VSLs were 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. These safety results are considered preliminary, however, because of limited after data. The results of this VSL implementation may be used to refine the current VSL control algorithm to improve compliance even further and could also serve as a reference for other agencies contemplating alternatives to improve safety at fog-prone areas.

Given the results, it is recommended that VDOT’s Southwest Region Operations convene a group to modify the VSL control algorithm. It is further recommended that the Virginia Transportation Research Council re-evaluate the safety effects of the system after at least 3 years of after data are available to make a more definitive determination of the safety effects of the system and to determine its return on investment. Implementation of these recommendations could further improve the efficacy of the system and result in a better quantification of the full benefits of the system.

Disclaimer Statement:The contents of this report reflect the views of the author(s), who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Virginia Department of Transportation, the Commonwealth Transportation Board, or the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks is for identification purposes only and is not to be considered an endorsement.


Other Authors

Daniela E. Gonzales 

Last updated: November 10, 2023

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