Report offers guidance for applying rumble strips at rural intersections
Rumble strip and warning sign in Lincoln County
Some counties in Minnesota use rumble strips at many of their rural stop-controlled intersections while other counties hardly use them at all. How effective are rumble strips, and what types of sites are good candidates for them? The third in a series of studies by Kathleen Harder and John Bloomfield of the Center for Human Factors Systems Research and Design at the University of Minnesota provides guidelines for more standardized usage.
In this project, the researchers studied the effect of rumble strips on the stopping behavior of drivers at 10 Minnesota intersections. They found that drivers slowed down earlier on approaches with rumble strips than on those without: the difference was, on average, 2.0 mph to 5.0 mph (depending on vehicle category and type of approach). In addition, speeding “outliers” (such as one driver nearing 80 mph) were more likely to slow down earlier on approaches with rumble strips.
A key factor, the researchers found, is whether an intersection is obscured by vegetation or man-made structures. Rumble strips had a bigger impact at sites where the driver on the minor road cannot see cross-traffic on one or both sides of the intersection.
Based on these results, Harder says, “we believe the addition of rumble strips to rural intersections with poor sightlines is likely a good idea.” Where the traffic is visible on both sides of the intersection, they do not recommend installing rumble strips, unless the stop-controlled intersection in question is preceded by miles of uninterrupted roadway.
For the project, the researchers visited 151 intersections in 16 Minnesota counties. The intersections, chosen with the help of county engineers, yielded 274 approaches, from which pairs of intersections with similar sightlines were selected; one of each pair had rumble strips while the other did not. Research assistant Ben Chihak then used a radar gun to collect speed data from more than 400 vehicles at 10 selected intersections.
This third study, funded by the Minnesota Local Road Research Board, built upon findings of two earlier studies (The Effects of In-Lane Rumble Strips on the Stopping Behavior of Attentive Drivers, Mn/DOT 2002-11, and The Effects of In-Lane Rumble Strips on the Stopping Behavior of Sleep-Deprived Drivers, Mn/DOT 2005-16).
In all three studies, drivers reduced speed earlier and to a greater extent at intersections with rumble strips.
Harder cautioned, however, that while rumble strips are likely to reduce crashes, they cannot eliminate them, nor will they prevent a driver from turning into or crossing a major road because of poor gap perception. She also noted that none of the drivers in the studies actually ran a stop sign with or without rumble strips.
Stopping Behavior at Real-World Stop-Controlled Intersections With and Without In-Lane Rumble Strips (Mn/DOT 2006-42) by Kathleen Harder, John Bloomfield, November 2006, www.lrrb.org.