Variable Speed Limit Sign: Effects on Speed and Speed Variation in Work Zones

Variable Speed Limit (VSL) signs are used across the country in conjunction with Intelligent Transportation Systems to lower posted speed limits in areas affected by conditions such as congestion, construction, accidents, fog, snow, and ice. VSL signs let operators adjust the posted speed limit without changing the physical sign. As technology advances, the ease of using VSL signs is also increasing since speeds can now be changed at preset times via email or telephone, or manually at the site. The Utah Department of Transportation desired to evaluate the effectiveness of this technology on driver behavior before using it widely in work zones on state highways. The focus of this research is driver response to VSL signs. A six-mile test site with a long-distance work zone on I-80 north of Wanship, Utah, was used to test the response of drivers to VSL signs. Five speed detectors and two VSL signs were placed and vehicle speeds were monitored for about 3 months. The speed data analyses showed that driver response was positive. Though the average speeds between static speed limit signs and VLS signs were not statistically different at a 95% confidence level, variation in speeds was reduced in general, especially at the first speed detector location downstream of the first VSL sign. Providing drivers with speed restrictions that reflect actual conditions builds “trust” in the posted speed. Long-term use of VSL signs is recommended for evaluating their long-term effect on driver compliance to reduced work zone speed limits.