By Center for Transportation Studies – University of Minnesota –
Between 2009 and 2011, 55 percent of Minnesota’s fatal intersection crashes occurred at rural two-way-stop intersections. Right-angle crashes, often a result of drivers’ inability to recognize a safe gap in traffic, account for most of these. When a curve or hill creates a limited sight distance at the intersection, safety issues are compounded.
warning sign and added LED flashing lights to the existing stop sign.
In research sponsored by the Minnesota Local Road Research Board, a team from the University of Minnesota Duluth (UMD) Department of Electrical and Computer Engineering, working with St. Louis County, developed a low-cost dynamic warning system to improve the safety of these intersections. The ALERT System uses LED flashing signs that are activated when a vehicle is detected approaching the intersection.
In the first phase of the project, the system was installed at a sight-restricted, two-way-stop intersection in Duluth. Drivers on the major road saw signs with the message “Cross Traffic When Flashing,” while drivers on the minor road saw “Vehicle Approaching When Flashing.” The aim was to help drivers on the minor road determine a sufficient gap to safely complete their turn and to warn drivers on the major road that a vehicle was stopped or entering the intersection from the minor road.
Study results showed that when the alert signs were flashing, vehicle speeds on the major road decreased, drivers on the minor road waited longer before crossing, and vehicle “roll-throughs” of the stop sign were eliminated.
However, a significant unintended consequence was also observed, says research assistant Husam Ismail. “We discovered that the drivers depend completely on the warning sign, so they’re ignoring the stop sign,” Ismail says. Consequently, when the alert sign was not flashing, drivers on the minor road may have assumed there was no cross traffic, and the number of roll-throughs increased.
This raises the risk of a crash if the device stops working and could indicate an overdependence on the warning system, Ismail says. Local residents voiced similar concerns, worried that some drivers would no longer obey stop signs or look for oncoming vehicles when the warning signs weren’t flashing.
The current phase of the project, ALERT-2, aims to reduce the percentage of roll-throughs on the minor road as well as improve the design for easier maintenance. Besides Ismail, the team for this phase includes UMD professor Taek Kwon and traffic engineer Victor Lund with St. Louis County Public Works.
As part of the new phase, researchers replaced the static stop sign with an LED blinking stop sign and removed the “When Flashing” label under the warning sign. Cameras and detectors captured both before- and after-installation data on roll-throughs at the intersection, which the team expects to analyze by fall 2013.
“We need the vehicle on the minor road to respect the stop sign, whether there’s a conflict [present] or not,” Ismail says. If the percentage of roll-throughs can be reduced, he adds, the system could significantly improve the safety of rural intersections with severe sight restrictions.
According to Lund, the system shows promise to improve intersection safety by helping drivers make good decisions for turning movements. “The next step is to deploy dozens of these systems so that further research can be completed on developing a crash-reduction factor,” Lund says.
- Presentation slides from the CTS Research Conference (1.3 MB PDF)
- Phase II research project page
- Phase I research report