Investigation of driver speed choice and crash characteristics during low visibility events : final report.

McCann, Katie; Fontaine, Michael D. · 2016 · ROSA P / Virginia. Dept. of Transportation

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Summary

This study investigates driver speed choices and crash characteristics during low visibility events on Interstate 77 (I-77) in Fancy Gap and Interstate 64 (I-64) in Afton, Virginia. The research was motivated by recurring severe fog events in these mountainous regions, which have historically caused multi-vehicle chain-reaction crashes. Specifically, a 95-vehicle crash on I-77 in 2013 highlighted the danger of drivers traveling too fast for visibility conditions. To address this, the Virginia Department of Transportation (VDOT) installed an Active Traffic & Safety Management System (ATSMS) with a variable speed limit (VSL) component on I-77. The study aimed to characterize existing driver behavior to develop a VSL control algorithm that balances safety with driver compliance, avoiding potential increases in speed variance if posted limits are ignored. The researchers analyzed crash, speed, and visibility data collected from 2010 to 2015. Visibility was measured using Road Weather Information System (RWIS) sensors, while speed data were gathered via Wavetronix radar detectors. The study categorized visibility into bins based on stopping sight distance (SSD) safe speeds, defining "low visibility" as conditions below 645 feet. Statistical methods, including Z-tests for mean speed differences and F-tests for speed variance, were employed to compare low visibility conditions against clear conditions. Additionally, stepwise linear regression and generalized linear models were developed to quantify the relationship between observed mean speeds and visibility levels, accounting for variables such as traffic volume, truck percentage, and site location. The results indicated that crashes during low visibility on I-77 were more likely to be severe and involve multiple vehicles compared to clear conditions. Mean speed analysis revealed that observed speeds exceeded SSD-based safe speeds across all low visibility conditions and sites. In the worst visibility conditions, drivers often exceeded safe speeds by more than 20 mph. While speed variance did not significantly increase as visibility decreased on I-77, it did differ from clear conditions at several I-64 locations. Modeling showed that although drivers reduce speeds in fog, a significant gap remains between observed and safe speeds. Notably, speeds on I-64 were less sensitive to visibility changes than on I-77, potentially due to illuminated in-pavement markers improving delineation or higher commuter familiarity with the route. The findings directly informed the development of the VSL control algorithm for the I-77 system, aiming to bridge the gap between current driver behavior and safe speeds. The study concludes that future VSL deployments should incorporate existing driver behavior models into their initial algorithms to ensure compliance and minimize speed variance. The authors recommend analyzing post-activation speed and crash data on I-77 to evaluate the system’s operational and safety effects. If the I-77 system successfully reduces crash frequency and severity while improving compliance, a similar system should be implemented on I-64 using the behavioral models derived from this study.

Key finding

Observed mean speeds exceeded safe speeds based on stopping sight distance in all low visibility conditions, with drivers exceeding safe speeds by more than 20 mph in the worst visibility, while speeds on I-64 were significantly less sensitive to visibility changes than on I-77.

Methodology

dataset

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 distance perception
• speed choice
• visibility analysis litigation
• rail grade crossings
• roadway lighting effects

Information type

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• Empirical Findings: crash risk outcomes