Variable Speed Limits (VSL) cut crash rates by more than half during low visibility on I-77 in Virginia.

The VDOT pilot was found to significantly improve the safety of hazardous mountainous routes by introducing VSL to areas of persistently low visibility.


Summary Information

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.


Data were available at a limited number of locations along the corridor before VSL activation from both permanent and temporary data collection stations. After VSL installation, some stations were relocated and additional permanent data collection sites were installed. Data before VSL activation were obtained primarily through querying existing databases to update data that were previously collected by other studies of I-77. For the after period, VDOT’s Southwest Region Operations provided logs of VSLs, speeds, and visibility during fog events. Although data were available for both directions of travel, the after analysis of this study focused on the SB (downhill) direction given that previous work indicated this direction was responsible for the vast majority of safety concerns.

Police crash reports were retrieved from VDOT’s Roadway Network System in order to understand crash characteristics and frequencies before and after system activation.


    1. Crash rates during low visibility were less than one-half of pre-VSL activation crash rates. Although based on limited data, it is a positive early indication that the system is having a positive impact on safety.
    2. Average vehicle speeds and standard deviations were reduced after VSL activation. Statistically significant reductions in average observed speeds of 2 to 5 mi/h were observed when visibilities were 250-645 ft. No visibilities less than 250 ft were available
    3. At low visibilities, speeds were still above the Posted Speed Limit (PSL), but were closer to the Safe Stopping Distance (SSD) speeds than before VSL activation.

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

Author: Gonzales, Daniela E. and Michael D. Fontaine

Published By: Virginia DOT

Source Date: 09/01/2018

Other Reference Number: VTRC 19-R6



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Benefit ID: 2018-01331