Some safety countermeasures designed for motor vehicles are seen as impediments to bicycle travel. Right-side rumble strips, or shoulder rumble strips (SRS) provide a proven motor vehicle collision reduction in areas where run-off-road crashes are common. However, rumble strips pose a safety risk and decrease comfort for bicyclists, particularly for bicyclists traveling at higher speeds on steep, downhill roadway segments. Existing literature provides recommendations for placement of regular gaps in rumble strips for bicycle maneuvers, but the testing was limited to low and moderate bicycle speeds. NCDOT personnel in Division 14 have reported bicyclist concerns about high speed (25-40 mph) maneuverability when trying to exit or enter a paved shoulder using 12-foot gaps between SRS, despite using best known practices for rumble strip placement and gap length where bicycle use is legal (non-freeways). These concerns are important to study, particularly for roadways which experience a high-level of bicycle traffic such as routes that are designated as part of the NC bicycling highway system or a signed local route.
To promote and allow for the safety of bicyclists on state roadways, additional guidance is needed for when and how to apply SRS on roads with steep grades where bicycles are present or likely to use the facility. General direction on providing for safe bicycle travel is contained within NCDOT’s Bicycle Policy, but additional specificity is needed for the application of rumble strips. This research will evaluate rumble strip gap lengths, bicycle maneuverability and vehicular impacts for increased gaps between SRS. Additionally, the research will provide guidance on optimal rumble strip gap lengths for various grades, shoulder widths, and bicycle speeds as well as general considerations for bicycle and motor vehicle tradeoffs of countermeasures with differing objectives.
The findings suggest that as gap length increases, bicyclist comfort is maintained while downhill speed also increases, with subjects reporting fewer instances of discomfort as the gap size increased. The likelihood of a bicyclist hitting a rumble strip while crossing a gap decreased modestly as the gap size increased. Shoulder width did not appear to significantly influence a bicyclist’s capability of maneuvering across different gap lengths and had only a minor effect on bicyclist speeds.
