An Empirical Analysis of Freeway Traffic Breakdown Probability Based on Bivariate Weibull Distribution

From an operation viewpoint, traffic breakdown from free-flow was defined as when the average speed of traffic drops below a certain threshold. It is known that traffic breakdown is a stochastic phenomenon which can happen even when the traffic flow is below the capacity. This study investigates the probability of breakdown at freeway bottlenecks. A bivariate Weibull distribution is adopted to model the probability of breakdown as a function of the combination of mean speed and occupancy of the incoming traffic. A case study with data collected from a section of I-80W Freeway in West Berkeley, California is carried out to test the performance of the methodology. Empirical results are derived and their implications on designing traffic control strategies are discussed. The outcome of this study is useful for developing various control strategies including Variable Speed Limit (VSL) and ramp metering.