Investigation of Influential Variables to Predict Passing Rate at Short Passing Zones on Two-Lane Rural Highways

The passing zone (PZ) is that part of two-lane highways in which drivers can safely overtake slower vehicles. Several studies have presented passing rate models in the PZ. However, there is no model to predict the passing rate in PZs shorter than 350 m. Furthermore, the effect of variables such as lane width and the proportion of motorcycles on the passing rate were not investigated in previous works. This study assessed the effects of these variables on the passing rate and presents a prediction passing model for short PZs. Data were collected from seven PZs using a drone on three different two-lane rural highway segments in Iran. The results showed that the passing rate depends on the lane width, the absolute vertical grade, the flow rate in both directions, the directional split, the proportion of heavy vehicles in the subject direction, and the proportion of motorcycles in the subject direction. Short PZ length values did not have a significant effect on the passing rate. The passing capacity occurred at a flow rate of 680vehicle/h in both directions irrespective of the directional split.