Traffic Impacts of Bicycle Facilities

Engineers need information about interactions between vehicles and bicyclists to design efficient, safe transportation systems. This study involved a review of design guidelines for bicycle facilities, observation of bicycle-vehicle interactions at nine roadways with different types of bicycle facilities, analysis of results, and description of design implications. Facilities observed included buffered and striped bicycle lanes, sharrows, signed shared lanes, and shoulders of various widths. Driver behaviors were categorized as no change in trajectory, deviation within lane, encroachment into adjacent lane, completion of a passing maneuver, and queuing behind cyclists. Drivers on roadways with bicycle lanes were less likely to encroach into adjacent lanes, pass, or queue when interacting with cyclists than drivers on roadways with sharrows, signs designating shared lanes, or no bicycle facilities. Queueing behind cyclists, the most significant impact on vehicular traffic flows, generally was highest on roads with no facilities or shared facilities without marked lanes. Statistical modeling confirmed the descriptive results. Given an objective of increasing predictability of driver behavior, buffered or striped bicycle lanes offer advantages over other facilities. Sharrows may alert drivers to the presence of cyclists, but traffic impacts on roadways with sharrows may not differ significantly from roadways with no facilities. Signs indicating bicyclists may occupy lanes also may alert drivers to the presence of cyclists, but this study provided no evidence that interactions on roadways marked only with signs differ from roadways with no facilities. From the perspective of reducing potential traffic impacts, bicycle lanes are to be preferred over sharrows or signage.