Driver Response to a Slippery Curve Warning System during Winter Weather Conditions

This study assessed the effectiveness of a slippery curve warning system (SCWS) as a speed reduction countermeasure during winter weather conditions. A field evaluation was conducted during the winter of 2024 at two horizontal curves located on a rural highway in northern Michigan. The SCWS that existed at each curve included a pair of slippery when wet warning signs and a curve warning sign. Each SCWS sign included a series of LEDs along the border that flash during potentially slippery pavement surface conditions and are activated based on current weather and road surface data collected from onsite sensors. The evaluation was conducted during consistent daytime weather conditions that included temperatures in the upper 20°F to low 30°F and wet pavement surfaces. The flashing LED sign borders were manually activated and deactivated every 30 min during data collection periods. Light detection and ranging was utilized to track the speeds of free-flowing vehicles beginning upstream of the SCWS and continuing into the curve. The results showed that the SCWS had a significant speed reduction effect for drivers approaching the curve during winter weather conditions. With the LED sign borders flashing, on entering the curve, drivers were 65% less likely to exceed the curve advisory speed and average speeds were 0.9 to 1.5 mph lower compared with when the LED borders were not flashing. In addition, the speed reduction effect of the flashing LED borders was greatest for the fastest group of drivers, who are the most vulnerable from a crash severity risk standpoint.