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Variable speed limits can limit work zone congestion

It’s that time of year, the time when cars flock to the roads en masse and are inevitably met with lane closures, detours and delays for necessary road repairs, leading to traffic logjams and potential safety concerns for the workers. However, research conducted by the University of Missouri Civil and Environmental Engineering Department may have uncovered a way to help alleviate some of the backup and create fewer dangerous collisions.

Associate Professor Praveen Edara was the lead author on a paper titled “Evaluation of variable advisory speed limits in congested work zones” recently published in the Journal of Transportation Safety and Security.

Edara head shot.

Associate Professor Praveen Edara was the lead author on a paper titled “Evaluation of variable advisory speed limits in congested work zones” recently published in the Journal of Transportation Safety and Security.

With assistance from Professor Carlos Sun, former doctoral student Yi Hou and equipment from the Missouri Department of Transportation, Edara tested the impact the use of variable advisory speed limit (VASL) systems had in lessening congestion and reducing rear-end and lane-changing accidents on Interstate 270, a major four-lane highway in St. Louis.

When the study was conducted, I-270 already was equipped with VASL systems for a previous Missouri DOT pilot program, so it was a perfect location for Edara to collect data.

“Even without any incidents (of construction or accidents), there are several recurring congestion problems on the I-270 corridor,” Edara said. “Where there is queueing, if upstream approaching drivers are not aware of the queue downstream, there’s not enough time to hit the brakes to slow down or stop, thus, increasing the likelihood of a crash.”

“The idea was to see if you can warn drivers of slower speeds ahead. Instead of just a message, can we post the actual downstream traffic speed, so they know that if they’re doing 50 (miles per-hour), they shouldn’t be doing 50 because it’s 30 downstream.”

Edara’s data collection and simulation analysis uncovered a few key pieces of data. First, the use of VASL systems is effective in gradually slowing down drivers as they enter a work zone. VASL use resulted in a 39 to 53 percent decrease in average queue length, a 7 to 11 percent throughput reduction and just a 4 to 8 percent increase in travel time.

Maximum speed differences also decreased by as much as 10 mph, and rear-end collision conflicts dropped by 30 percent along with a 20-percent decrease in lane changing conflicts. Essentially, travel time was slightly longer, but lines were shorter and collisions were less frequent.

“You can get both safety benefits and mobility benefits by deploying variable advisory speed limit systems,” Edara said. “It helps lower the rush-hour congestion in urban areas.”