Researchers at the University of Missouri have studied systems to alleviate backups and delays that go hand-in-hand with highway construction projects. They found that using variable speed limits in construction zones may ease congestion, reduce crashes, and make work zones safer for workers and travelers.

With help from the Missouri Department of Transportation, Praveen Edara, associate professor of civil and environmental engineering in the MU College of Engineering, tested the use of variable advisory speed limit (VASL) systems and the effect they may have on lessening congestion and reducing rear-end and lane-changing accidents on a fairly dangerous stretch of I-270, a four-lane highway in St. Louis.

Variable speed limits may reduce work zone accidents.Variable speed limits may reduce work zone accidents.“The idea was to see if warning drivers of slower speeds ahead helped reduce crashes,” Edara says. Where there is queueing, if drivers are not aware of the queue downstream, they don’t have enough time to hit the brakes to slow down or stop. He says this increases the likelihood of a crash.

Instead of posting a message asking them to slow down, the VASL system posts an advisory speed limit based on the actual downstream traffic speed, so drivers know that if they’re driving 50 mph, they should slow to 30 mph downstream.

Analysis showed that the use of VASL systems is effective in gradually slowing drivers as they enter work zones. VASL use resulted in a 39-53% decrease in average queue length, and a 4-8% increase in travel time.

Additionally, using VASL meant that maximum speed differences also decreased by as much as 10 mph, and the chance of rear-end collisions dropped by 30%. Researchers also noted a 20% decrease in lane changing conflicts. Essentially, travel time was slightly longer, but lines were shorter and collisions were less frequent.

“Evaluation of variable advisory speed limits in congested work zones” was published in the Journal of Transportation Safety and Security. Carlos Sun, an MU professor of civil and environmental engineering, and Yi Hou a doctoral student, co-authored the study.