Passing Sight Distance on Rural Two-Lane Highways: A Review of US Practice

The ability to pass slower vehicles on rural two-lane highways is important for breaking up platoons and improving the quality of service. However, as passing takes place in the lane of opposing traffic, there is an increased level of risk associated with passing maneuvers on two-lane highways. In practice, passing sight distance (PSD) is used in design and lane marking purposes to allow or prohibit passing maneuvers on different sections of two-lane highways. This study performed a thorough review of the PSD practice in the United States (US), considering the well-established PSD concepts and principles. The review examined the historical evolution of PSD criteria in the US leading to the current PSD guidelines used in practice while highlighting the theories, rationale, and underlying assumptions where applicable. The review results highlighted a few limitations in the current practice. Specifically, the exclusive use of the critical passing sight distance (CPSD) concept for design and pavement marking purposes and the overly optimistic assumptions used in estimating the PSD are two major concerns that may well affect the adequacy of PSD for safe passing maneuvers. Therefore, the study stresses the need for adopting a safer approach in developing PSD criteria. Meanwhile, the study strongly recommends exceeding the current minimum PSD values using site-specific information and traffic conditions. This review is important in that the engineering standards in the US are adopted in many developing countries around the world, especially when working with international engineering firms or receiving funding from international organizations.