Virginia Using Variable Speed Limits To Reduce Congestion

“Hoping to decrease driving delays, last month the Virginia Department of Transportation introduced variable speed limits between mile markers 115 and 130 south of Fredericksburg,” reports Wyatt Gordon in the Virginia Mercury. “Currently, 20 states deploy variable speed limits on at least one stretch of their road networks,” where research shows an average of 5 percent reduction in travel times and 8 percent fewer crashes. “Spaced six-tenths of a mile from one another and at each of the three highway on-ramps along the corridor, the 48 signs each have congestion detectors designed to predict potential traffic problems and drop the speed limit by up to 10 mph per minute.”

The problem variable speed limits seek to solve is called speed flow inversion — the phenomenon whereby even minor interruptions in vehicles’ movement cause extensive congestion due to the density of traffic.

According to Eric Dumbaugh, associate director of the Collaborative Sciences Center for Road Safety at Florida Atlantic University, “This type of technology only works if you have total compliance and a majority, if not all, road users see the sign and adhere to that speed.” The agency is using billboards, signs, and alerts on Waze to alert drivers to the variable speed limits. “The first indications of whether VDOT’s $10 million investment in the new corridor is paying off should come in September, when the agency completes its first round of data on crash rates and types, the frequency of speed reduction, compliance and person-hours of delay.”