Reliability Analysis for Roadway Curves with Horizontal Sightline Obstructions

Objects on the inside of a horizontal curve may limit the stopping sight distance (SSD) available to drivers. Crash analysis shows that SSD-related crashes on horizontal curves are limited in number and difficult to quantify. As an alternative, a reliability analysis model was developed to quantify the number of drivers per year who may encounter a stopped vehicle in a sight-restricted area, creating an opportunity where an SSD-related crash might occur on a roadway with a sight obstruction on the inside of a horizontal curve. The model considers roadway alignment in three dimensions. The model formulation is documented and an analysis of the sensitivity of crash opportunities to traffic volume and offset to a sight obstruction is performed. A sensitivity analysis of the reliability analysis model shows that the percentage of vehicles that may encounter a crash-involved vehicle or a queue of stopped vehicles over the course of a year can range from essentially zero to a value approaching 1% of the total yearly flow. The reliability analysis model is a flexible tool that can be used by planners and designers to compare, in a relative sense, the need for sight distance improvements on specific horizontal curves.