A. Jafari, A. Al-Kaisy, S.S. Washburn
Two-lane highways constitute a large proportion of the US road network. Limited passing opportunities on these highways could lead to poor driving conditions and risky passing maneuvers. One of the alternative means to improve passing opportunities and level of service on two-lane highways is the use of the 2+1 design concept. In this design, the highway cross section consists of three lanes: a single lane in each direction of travel and a middle passing lane alternating between the two directions. Two plus one highways are very common in Europe, but have not seen broad application in the US, with the exception of reported implementations in a couple of states. This study aims at investigating the primary design parameter of 2+1 highways, i.e. the length of passing section alternating between the two directional lanes. Field data from two passing lane study sites in Oregon were used to calibrate and validate a traffic simulation program, SwashSim. The calibrated model was subsequently used in examining the passing section length under different traffic levels, percentage of heavy vehicles, and free flow speed of the highway network. The optimal length of passing lane was found using average performance over the linear study network. The results showed that free-flow speed is the major determinant of the optimal length of passing section on a 2+1 highway. As the free flow speed increases, longer passing sections are required for an optimal performance on the highway network. This research only investigated the operational performance of 2+1 highways and safety issues were not studied.
Keywords: two-lane highways; operational performance; 2+1 road; passing lane