Minnesota DOT (MnDOT) Evaluated An Active Traffic Management System in Twin Cities, Minnesota.
Twin Cities, Minnesota, United States
Development of a Queue Warning System Utilizing ATM Infrastructure System Development and Field Testing
Summary Information
This project developed two separate systems that can identify lane-specific shockwave or queuing conditions on the freeway network along two separate interstate highways in Twin Cities, Minnesota. The system used an existing Active Traffic Management (ATM) system that included Intelligent Lane Control Signals (ILCS) spaced every half-mile over every lane to warn motorists upstream for rear-end collision prevention. The first system was based on the premise that freeway rear-end collisions tend to occur in extended stop-and-go traffic or at end-of-queue locations. The second queue warning system was based on the hypothesis that not all congestion events are dangerous but there are certain traffic conditions that are crash-prone regardless of whether they result in standing queues or not. The two systems were field tested on two interstate highways with significantly different traffic-flow conditions prone to rear-end collisions.
METHODOLOGY
The first system was tested in the field for three weeks in 2016, showing the queue warning message “SLOW TRAFFIC AHEAD” to travelers when a queue was estimated at downstream locations. The loop detector data, the queue warning system log files, and traffic camera videos were used to evaluate the queue warning system. Based on the before and after data collected, the mean value and standard deviation of speed was calculated for each detector and for each message, and then a hypothesis testing was carried out to see whether there was a significant change before and after the deployment of the system. Event observations for the second system were collected during a three-month evaluation period (from June 2016 to August 2016) based on manual reduction of video from multiple cameras recording crashes and near-crashes, as well as the exact times the sign changed states. The number of events were compared to that from a 2013 study in the same region to calculate the difference in number of crashes and near crashes.
FINDINGS
- In the first three months of one of the second systems’ deployment, crash event frequency was reduced to 9.34 crashes per million vehicle miles of travel (MVMT) and 51.8 near crashes per MVMT, a 22 percent decrease in crashes and a 54 percent decrease in near crashes.
- For most of the lane segments involved in the study, queue warning messages tested in the first system smoothed traffic by reducing the speed variance at downstream locations and speed difference between upstream and downstream locations, and therefore reduced the risk of rear-end crashes.
