In the design of intersections, the method of adding lanes at approaches is a common way to improve the capacity of intersections. Limited by the length, the extended short-lane may cause the vehicle queue exceeds the length and blockage at the entrance of the short-lane, preventing the following vehicles, which is called short-lane effect. This phenomenon wastes the space of the left-turn lane and the green time, and reduces the capacity of the approach. A left-turn short-lane based probabilistic model is proposed in this paper to calculate the capacity. The capacity in this model is split into two cases. The probability and capacity of each case are calculated by the ratios of turning vehicles, length of the short-lane and signal timing. The impact extent of these factors are also be explored in case studies. Finally, data of 3 approaches at different signalized intersections is used to test and verify the probabilistic model and the HCM2000 model. The result shows that the HCM2000 model seriously overvalues the capacity of approaches with left-turn short-lanes, while the error of the probabilistic model is much smaller, within 200pch/h.
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