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

Gonzales, Daniela; Fontaine, Michael D. · 2018 · ROSA P / Virginia Transportation Research Council

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Summary

This study evaluates the effectiveness of a Variable Speed Limit (VSL) system installed on a 12-mile section of Interstate 77 in southwestern Virginia, an area prone to severe, recurring fog events that historically caused high-severity, multi-vehicle crashes. The research addresses the gap in quantitative analyses of VSL countermeasures by assessing how the system, activated in October 2016, influenced driver speeds and crash characteristics during low visibility conditions. The primary objective was to determine if the VSL system successfully encouraged drivers to travel closer to safe speeds based on stopping sight distance (SSD), thereby reducing crash frequency and severity. The methodology involved a before-and-after analysis using data collected from Road Weather Information System (RWIS) stations, traffic sensors, and police crash reports. The study focused on the southbound direction, which exhibited higher speeds and greater safety risks. Researchers analyzed speed data at a single site (Milepost 4.4) where continuous data existed for both periods, as well as corridor-wide speed trends and crash data. Statistical models, including stepwise regression, were developed to examine speed as a function of visibility, weather conditions, and VSL status. Visibility conditions were categorized into bins corresponding to SSD-based safe speeds to ensure consistent comparison. The results demonstrated statistically significant reductions in both mean speeds and speed variances after the VSL system was activated. Drivers traveled closer to the safe speed dictated by available visibility, and compliance improved as drivers encountered reduced visibility zones. However, speed adjustments were less pronounced in transition areas leading into fog zones. Crash analysis revealed only two fog-related crashes in the post-activation period, indicating a reduction in crash rates during low visibility conditions. The authors note that these safety findings are preliminary due to the limited duration of the after-period data. The study concludes that the VSL system effectively mitigated unsafe driving behaviors during fog events. The findings suggest that the current VSL control algorithm can be refined to further improve driver compliance, particularly in transition zones. The authors recommend that the Virginia Department of Transportation modify the algorithm and conduct a re-evaluation after at least three years of data collection to definitively determine the system’s safety effects and return on investment. This research provides a quantitative reference for other agencies considering VSL implementations in fog-prone areas.

Key finding

The variable speed limit system produced statistically significant reductions in mean speeds and speed variances, resulting in only two fog-related crashes during the first year of operation.

Methodology

on_road

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

Topics

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• weather rain fog snow
• speed choice
• speed management
• speed distance perception

Information type

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• Empirical Findings: crash risk outcomes