VDOT is introducing variable speed limits, but will congestion care?

August 1, 2022 12:05 am

New variable speed limits signs posted on Interstate 95 south of Fredericksburg, near Thornburg. (Virginia Department of Transportation)

“There is such thing as Hell on Earth and it’s I-95N between Richmond and DC,” quipped CBS 6 news anchor Elizabeth Holmes in a recent tweet. The post went viral, echoing a sentiment felt by the countless car users who regularly get caught in congestion between the two capitals.

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. The new electronic signs are meant to lower speeds when traffic gets thick, but in our era of increasingly dangerous driving, can such signs really improve safety?

Need for (variable) speed

Currently, 20 states deploy variable speed limits on at least one stretch of their road networks. In Virginia, the 15 miles of northbound Interstate 95 that received variable speed limit signs last month actually aren’t the first of their kind: For the last five years along I-77 around Fancy Gap, VDOT has used similar signs to warn drivers of dense fog, resulting in what the agency says is a 73% reduction in crashes. 

States where variable speed limits have been deployed. (Virginia Department of Transportation)

The variable speed limit corridor south of Fredericksburg, however, won’t rely on weather reports for its updates, but rather on an algorithm assessing traffic volumes.

The aim of the 48 new electronic signs along I-95 is to “monitor in real-time what is going on in the corridor and reduce the speed limit before things get congested,” explained Mena Lockwood, an assistant state traffic engineer with VDOT. “That way we can create an even speed throughout the corridor so folks travel at a free flow that may be slower than normal, but they won’t hit full-stop traffic.”

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. “This technology can help solve invisible traffic jams, backups that result from a chain effect of braking that ripples downstream rather than an actual crash,” said Eric Dumbaugh, associate director of the Collaborative Sciences Center for Road Safety at Florida Atlantic University.

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 adaptability of the signs and the algorithm that drives them means one “could have a scenario where there are two different slowdowns along the corridor and normal speed traffic between them,” said Lockwood. No matter how bad congestion gets, the signs will never go lower than 35 mph.

From 2015 to 2019, VDOT estimates this stretch of I-95 witnessed over 70,000 person-hours of delay due to congestion; however, the agency uses a higher estimate of the average number of persons per vehicle than the current industry standard, meaning the actual number of hours delayed may be lower. Of the 30,000 crashes that occurred along the corridor in that time frame, 55% were rear-end crashes.

In other states, variable speed limits have delivered on average a 5% reduction in travel times and an 8% decrease in crashes. Effectiveness varies based on the compliance of drivers, roadway geometry and how the system is set up, but VDOT is hoping its algorithm delivers better results than have been seen so far in other states.

Effective absent enforcement?

To alert drivers to the new variable speed limits ahead of their introduction, VDOT deployed billboards along the corridor, ads on the way-finding app Waze and a multitude of static signs at interstate on-ramps in the area. Even the speed signs themselves flash whenever the limit is lower than the standard 70 mph. Still, some road safety experts like Dumbaugh worry drivers won’t follow the variable speed limits and will undermine the system’s effectiveness.

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,” he warned. “As soon as you have someone ignoring the signs and cutting across lanes at a fast speed, you will once again trigger that ripple effect of downstream congestion. I have a hard time thinking this will be effective without concurrent enforcement.”

Lockwood said state police have voiced hope that the new variable speed limits will have an impact, but she noted that throughout the public feedback process communities along the corridor expressed skepticism that drivers will comply, as well as concerns about increased enforcement.

Although the Governors Highway Safety Association recommends the use of automated speed cameras as “powerful tools to reduce crashes,” current Virginia code only allows such technology in school and work zones. This means the new variable speed limits “will be enforced like any other: drivers need to be pulled over by a state trooper,” said Lockwood.

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. VDOT will also be conducting analyses at the six-month and full-year marks. But regardless of the project’s performance, it has no plans to decommission the new variable speed limit system.

“It’s a pilot in the sense that we will evaluate it, but we don’t have a plan to take the project out at a certain point,” Lockwood said. “We don’t have specific targets set for what we would consider successful, but we will certainly be looking at the return on investment and how long it takes to recoup what we put into this project.”

Should the pilot prove its worth, then similarly supersaturated corridors around the commonwealth could one day get their own variable speed limits.

For now, VDOT is asking for drivers’ assistance in improving the paved purgatory that is I-95 north of Richmond: “Drivers don’t like to be slowed down when they don’t need to be or else they won’t comply, so we want people to trust the system and know that we are only reducing the speed when it’s needed,” said Lockwood. 

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Wyatt Gordon
Wyatt Gordon

Wyatt Gordon covers transportation, housing, and land use for the Mercury through a grant from the Piedmont Environmental Council and the Coalition for Smarter Growth. The Mercury retains full editorial control. Previously he’s written for the Times of India, Nairobi News, Honolulu Civil Beat, Style Weekly and RVA Magazine. He also works as a policy manager for land use and transportation at the Virginia Conservation Network.