MnDOT researchers studied three intersection safety measures—lane constrictors, J-turns, and retroreflective signal backplate borders—to determine their effectiveness. Final reports are now available.
Lane constrictors
This project examined lane constrictors, which use painted median lines and rumble strips to force a gap between lanes of opposing traffic as cars approach an intersection. The overarching goal is to separate lanes of traffic and draw driver attention to the intersection.
Between 2018 and 2019, MnDOT installed 66 of these lane constrictors at stop-controlled intersections across Minnesota. Before-and-after and cross-sectional analysis showed that the constrictors do not have a significant impact on overall crash rates, but they do correlate to a 22 percent decrease in crashes that result in fatality or injury. This suggests that lane constrictors help reduce the severity of crashes—swapping injury crashes for property-damage-only crashes.
J-turns
This project explored the effectiveness of J-turns. The goal of a J-turn is to reduce severe angle crashes (T-bones) at an intersection by preventing cars from making left turns into oncoming lanes of traffic. A J-turn will typically force a vehicle to overshoot its intended left turn, then give the driver a protected space where they can perform a U-turn and head back toward their destination.
MnDOT installed 83 J-turns across the state between 2010 and 2022. Before-and-after and cross-sectional analysis showed that they correlate to an increase in rear-end crashes, but also reduce angle crashes as well as crashes that result in fatality or injury.
Retroreflective signal backplate borders
This project reviewed retroreflective signal backplate borders. This approach makes traffic lights more visible to drivers by applying a strip of retroreflective material—usually tape—to backplates.
MnDOT installed 116 of these borders across Minnesota (mostly in 2019). The Federal Highway Administration had found that such borders correlate to a 15 percent reduction in total crashes, but the MnDOT report contradicts these results—it found that backplate borders do not have a significant impact on crash rates.
—Sophie Koch, contributing writer
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