Impact of Short Turn Lanes on the Approach Capacity at Signalized Intersections — A Revised Model

At signalized intersections, turn lanes are often designed as turn bays with limited lengths and thus share one or more lanes with the through traffic upstream of the branching point. If the length of the turn lane is not sufficiently long, the turning and through traffic mutually interfere and the total approach capacity is reduced. The German Highway Capacity Manual (HBS) features a model to calculate this effect. However, the model has been derived only for approaches with one through lane based on a simulation study with a limited dataset. In this paper, the effect of short left-turn lanes was analyzed by extensive microscopic simulations supported by empirically observed parameters of four approaches with short left-turn lanes in Germany. Based on the simulation results, the influence of a broad range of geometric, flow and control parameters on the resulting approach capacity was analyzed. As a result, the general suitability of the current model in the HBS for one through lane could be confirmed. The model was recalibrated with simulated data which improved the goodness of fit and extended to signalized approaches with more than one through lane. Owing to the methodology of calibration, the model can also be applied to calculate the impact of short right-turn lanes and adopted to variable local or regional boundary conditions of the traffic flow. Thus, the model can also be incorporated into other quality-of-service assessment procedures like those in the U.S. Highway Capacity Manual (HCM).