Traffic Impacts of Bicycle Facilities is a research project funded by the Minnesota Department of Transportation (MnDOT) and the Local Road Research Board (LRRB) to inform the design of multimodal transportation facilities. The project involved a review of design guidelines for bicycle facilities, observation of bicycle-vehicle interactions at nine roadways in Minnesota with different types of bicycle facilities, analysis of results, and description of implications for design. The field observations involved review of video recordings and documenting driver behaviors while interacting with cyclists. The types of bicycle facilities that were observed included buffered bicycle lanes, 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 full passing maneuver, and queuing behind cyclists. The results show generally that drivers are less likely to alter their trajectories and deviate from their positions in the travel lanes or queue behind cyclists when facilities are clearly demarcated. Across the nine locations, drivers on roadways with bicycle lanes (buffered or striped) 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. The results also show that variation within and across types of facilities and that the outcomes of interactions on specific types of facilities cannot be presumed to be the same. Statistical modeling confirmed the descriptive results from the field studies. These results have several implications for design. Given an objective of increasing the predictability of driver behavior, buffered or striped bicycle lanes offer advantages over other types of facilities where space and resources allow. Whether sharrows are associated with more consistent driver behaviors during interactions with cyclists may depend on site-specific circumstances. Although sharrows may alert drivers to the potential presence of cyclists, 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 potential presence of cyclists, but there is no evidence from the cases in this study that interactions on roadways marked only with signs differ from roadways with no facilities. Thus, from the perspective of reducing potential traffic impacts such as queuing behind cyclists, bicycle lanes are to be preferred over sharrows or signage. (Author/publisher)
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