BACKGROUND

Shoulder and centerline rumble strips and stripes on roadways, whether conventional or sinusoidal, provide many safety benefits to motorists. But for bicyclists, coming in contact with them can be a very jarring experience, and riding safely upon them nearly impossible. State departments of transportation (DOTs) are increasingly installing rumble strips on roads also traveled by bicyclists, particularly rural two-lane roads with speed limits over 50 miles per hour (mph). These roads often have less than four feet of clear useable shoulder space. Bicyclists, riding on such roads with rumble strips and limited rideable shoulder space, feel increasingly challenged and may find no option but to ride in the lane with high-speed mixed traffic, increasing the likelihood of a crash with a motor vehicle.

Another safety issue of much concern to the bicyclists is the motorists’ behavior when passing them on roadways with centerline rumble strips. However, there is very little information available on the motorist-bicyclist interaction on rural roads where rumble strips decrease or eliminate the rideable shoulder space. A Michigan DOT study on motorists’ behavior on rural roads with centerline rumble strips concluded that motorists were less likely to cross centerline rumble strips when passing bicyclists to avoid vibration and noise (https://www.michigan.gov/documents/mdot/RC1627_489159_7.pdf). This would most likely make them pass too closely to the bicyclist, greatly increasing the risk of the bicyclist losing control and crashing.

Federal legislation allows federal Highway Safety Improvement Program (HSIP) projects to install rumble strips if “the rumble strips or other warning devices do not adversely affect the safety or mobility of the bicyclists and pedestrians….” However, there is little research on quantifying the effects of shoulder and centerline rumble strips on bicyclists’ safety. With growing use of rumble strips nationwide to reduce run-off-road and head-on crashes of the motorists, it is imperative that the unintended safety impacts on the bicyclists also be determined to better inform state DOTs’ design policies.

OBJECTIVES

The objectives of this research are to:

1. Determine and quantify safety impacts on bicyclists on rural high-speed two-lane roadways with centerline and shoulder rumble strips.

2. Characterize motorists’ behavior when encountering bicyclists on rural high-speed two-lane roadways with centerline and shoulder rumble strips. Aspects of motorists’ behavior to be assessed could include, among other things, lateral and longitudinal controls, reaction time, and eye movement, when passing bicyclists traveling in the same direction on such roads.

3. Develop a guide on various rumble strip applications, with a focus on their impact on bicyclists’ safety.

STATUS

Work has been initiated. The final report is expected in June 2025.