Minnesota Horizontal Curve Safety Improvement Project

Project summary:

One low-cost technology that may help drivers select an appropriate speed when approaching a horizontal curve is the dynamic curve warning sign (DCWS). A DCWS generally consists of a warning sign combined with a speed-measuring device (e.g., radar) that activates a variable message (e.g., slow down) when vehicles are traveling above a set threshold. This research project evaluated the vehicle speed impacts of a DCWS at three study sites on low-volume, local rural highways. Vehicle speed data were collected one month before and one month, six months, one year, and 18 months after the installation of a DCWS at the visually identified point of curvature (PC). Data were collected at three locations within each study site. These locations were the PC, within the curve, and at an approach tangent control. Before and after differences in average speed were calculated for vehicles traveling into the horizontal curve. The overall range of difference in the unadjusted average vehicle speeds at the PC was -8.8 mph to -1.0 mph. This range changes to -8.6 mph to 1.9 mph after a naive adjustment of these differences for the average vehicle speed changes at the control locations. It was concluded that the DCWS appeared to have a greater impact at horizontal curves with lower advisory speeds and on the number of higher-speed vehicles. It is recommended that the installation of DCWSs on low-volume roadways be considered on a case-by-case basis and that a lower-cost version of the sign that was evaluated be explored.