Variable Speed Limit Study Upstream of an Indiana Work Zone with Vehicle Matching

Traffic management in work zones presents significant mobility and safety challenges for agencies. The goals of a work zone traffic management plan are to safely slow vehicles ahead of the work zone, maintain speeds that provide for the safety of motorists and construction workers, and manage the growth of queues. Historically, variable speed limits (VSLs) have been presented as a form of technology that can dynamically regulate speed in response to prevailing traffic conditions. However, techniques used to evaluate the impact of VSLs typically use aggregated statistics such as mean and standard deviation to determine the typical speed reduction. This paper presents a new methodology to evaluate the impact of VSL signage on the basis of individual vehicle matching. The speeds and speed changes of these matched vehicles were used to analyze individual driver response to the VSLs. This approach allows agencies to understand the impact that VSL signage has on the distribution of vehicle speeds. It was concluded that vehicles would need to observe multiple signs before any tangible reduction in speed limit would occur. The new vehicle-matching methodology showed that, after drivers observed a 15-mph drop in the speed limit for cars (10 mph for trucks) on three consecutive VSL signs, they reduced their speed by a median of 7.5 mph (5.8 mph for trucks). Overall, 4% of cars and 10% of trucks complied with the 55-mph speed limit after the observance of three VSL signs.