Reliability Analysis of Sight Distance for Stopcontrol Intersections on Horizontal Alignments

Intersection sight distance (ISO) for stop-control intersections refers to the provision of adequate sight distance between a minor-road stopped vehicle and a major-road vehicle. The AASHTO policy for ISO for intersections on straight roadways Is based on the extreme values of the component design variables, such as major-road design speed and time gap, and assumes that these variables are deterministic. This research presents a reliability method that considers the moments (mean and variance) of the probability distribution of each random variable instead of the extreme values. This reliability method also accounts for the correlations among the component random variables. A performance function in terms of a safety margin is defined as the difference between the expected available and expected required ISO. Relationships for the mean and standard deviation of the safety margin are developed using First- Order Second-Moment analysis. Design graphs for the obstruction location are established for different radii of horizontal curves, design speed, and probability of failure. The reliability method is very useful as it provides the reliability associated with I8D design values. For evaluation purposes, the method can be used to determine the probability of failure of a particular intersection for an existing obstruction and current traffic conditions. The method can also be used to design the obstruction location for a given probability of failure. It was found that the deterministic method generally provides a higher probability of failure when the obstruction is closer to the minor road.