Reliability Analysis of Left-Turn Sight Distance at Signalized Intersections

The current method for the analysis of sight distance for left-turn vehicles at signalized intersections assumes that all component variables are deterministic. This paper presents a probabilistic approach based on such random variables as major-road speed, time gap required for left-turn vehicle, vehicle width, lateral position of left-turn vehicle, distance between driver eye of left-turn vehicle and its front, and lateral and longitudinal positions of positioned opposing left-turn vehicle. A safety margin is defined as the difference between available and required sight distances. Relationships for the mean and standard deviation of the safety margin were developed using the first-order second-moment (FOSM) method. This method identifies the offset (distance between left edge of the left-turn lane and right edge of the opposing left-turn lane) at signalized intersections for a desired probability of noncompliance based on intersection and traffic characteristics. The proposed reliability method was validated using Monte Carlo simulation, and design graphs were established. The results show that the deterministic method provides very conservative offsets and that the offset is most sensitive to vehicle width and lateral distance-related variables and less sensitive to longitudinal distance-related variables.