Analysis of alternative treatments for left turn bicycles at tandem intersections

To improve the practical capacity of the tandem intersections and eliminate the conflicts between left-turn bicycles and left-turn vehicles at the main signal, four alternative designs are proposed from the aspects of time separation and spatial separation. In terms of time separation, left-turn bicycles and left-turn vehicles are separated by the two-step crossing for left-turn bicycles design and the exclusive phase for pedestrian and bicycles design. In terms of spatial separation, exclusive passing space for left-turn bicycles is provided by using partial left-turn design in the sorting area and moving the stop line backward. A linear programming model, which combining four optimization designs into one unified optimization framework, is established with the objective of maximizing the practical capacity of vehicles. A case study and extensive numerical analysis are used to validate the effectiveness and identify the application domain of these alternative designs. The results demonstrate the promising effect of the two-step crossing for left-turn bicycles design and the moving the stop line backward design in avoiding conflicts between left-turn bicycles and vehicles while improving the vehicular capacity. The two-step crossing for left-turn bicycles design can improve 10–20% vehicular capacity under almost all analysis conditions, but it increases the delay of left-turn bicycles. The benefit of the moving stop line backward design is sensitive to cycle length and the proportion of left-turn vehicles. It performs well when the cycle time exceeds 90 s, and the proportion of left-turn vehicles exceeds 30%.