To reduce congestion and improve safety, the Minnesota Department of Transportation (MnDOT) has deployed active traffic management (ATM) technology on two freeways in the Twin Cities. The ATM system incorporates intelligent lane control signals (ILCS) placed over selected lanes at half-mile increments to warn motorists of incidents or hazards ahead.
Using this existing ATM infrastructure, U of M researchers have developed and field-tested two prototypes for queue warning systems in a new MnDOT-funded project. The warning systems specifically focus on preventing rear-end collisions—the most frequent type of crash on freeways.
“Rear-end freeway crashes are a serious safety and mobility problem,” says John Hourdos, director of the Minnesota Traffic Observatory at the University of Minnesota. In 2014, these crashes accounted for 38 deaths and more than 5,000 injuries in Minnesota alone. “Research has shown that these dangerous crashes tend to occur during traffic slowdowns and at end-of-queue locations, so warning a driver to these conditions in advance allows them to be more alert and possibly avoid crashes.”
The new prototypes aim to reduce rear-end crashes by addressing stop-and-go traffic and end-of-queue situations as well as shockwaves, a crash-facilitating condition in which a sudden change in traffic movement causes a cascade of braking. The long-range goal of the project is to develop a unified queue warning system that can be deployed at other locations in Minnesota’s freeway network.
Development of the two prototype warning systems began in 2014, and they were subsequently deployed on two high-traffic freeways in the Twin Cities: I-35W and I-94.
“These two locations have significantly different traffic conditions,” Hourdos says. “On I-35W, congestion creates expanding queues, while on I-94 crashes are most likely to occur due to shockwaves that often develop quickly.”
To capture traffic data, researchers merged live video from existing camera detector stations with data from in-pavement loop detectors. With this data, researchers developed two algorithms that were used to create a rear-end collision warning system. The system can prompt the ILCS units to display warning messages for drivers, such as Prepare to Stop, Slow Traffic Ahead, and Traffic Ahead 10 MPH. In the future, the algorithms could be used to develop a rear-end collision warning system that could be installed at other freeway locations where similar queuing conditions exist.
Results of the study show that warning messages delivered by the two prototype systems can be effective at alerting drivers to queuing conditions, with the ultimate benefit of reducing rear-end collisions. At the I-94 test site, the system substantially reduced crashes and near-crashes: crashes decreased by 22 percent and near-crashes dropped by 54 percent. At the I-35W location, messages delivered by the warning system reduced speed variances by helping drivers maintain a steady speed and curbing stop-and-go traffic.
“The big lesson learned was that the detection method had to function quickly and display a message that was timely and accurate. This gains the trust and confidence of the motoring public,” says Brian Kary, MnDOT freeway operations engineer.
Going forward, the researchers would like to pursue a longer trial period of the queue warning systems. “Testing over a period of two or three years could help us ensure this cost-effective system can deliver sustainable benefits,” Hourdos says.