Assessing the Risk in the Design of Passing Sight Distances

Abstract

Mostly, design methods in transportation engineering do not address the level of risk associated with the design and use of various highway elements. Passing sight distance (PSD) is an example of a highway operational element analyzed with no risk measures. The aim of this paper is to attach risk indexes to various PSD lengths. The writers devised a unique microscopic simulation using the ARENA software for replicating passing maneuvers on two-lane two-way roads. The simulation monitors the movement of three vehicles: the passing, the impeding, and the opposing vehicles. Two thousand passing attempts are simulated for each PSD length, and a weighed risk index is obtained. The risk index is mainly based on the value of the clearance time between the passing and the opposing vehicles at the end of the passing maneuver. Six levels of risk are identified. Using the simulation results, the writers are able to assess the risk of various PSD lengths for a specific design speed. In addition, the risks associated with the current PSD standards are identified. The American Association of State Highway and Transportation Officials “Green Book” values are shown to be conservative relative to the other design values. The Manual on Uniform Traffic Control Devices values fall within the acceptable risk levels. With risk measures attached to the PSD lengths, a trade-off analysis between the level of service and risk is feasible to accomplish.