Uncertainty and Predictability of Urban Link Travel Time

Travel times that vehicles experience in urban road networks are intrinsically uncertain because of the stochastic character of delays at signalized intersections. The ability to capture delay characteristics at signalized intersections is critical for estimating and predicting travel times on urban links. Much research has been done to predict travel time on urban links on the basis of traffic state information (e.g., volumes or speeds). However, the results have not been promising. One important reason is that delays experienced by vehicles on urban links are uncertain because of both traffic conditions and traffic control at intersections. This paper addresses the causes of travel time uncertainty. A probabilistic delay distribution model with stochastic arrivals and departures is proposed to investigate delay uncertainty in both undersaturated and oversaturated conditions. The delay distributions with the Poisson arrival process and with the binomial arrival process are compared. Results show that different arrival patterns have little influence on the delay distribution in undersaturated conditions, although they have significant influence on the delay distribution in oversaturated conditions. The delay distributions under different degrees of saturation are overlapping, which indicates that it is difficult to determine the traffic state for a given single-valued delay and vice versa. The “width” of the delay distribution based on percentiles is used to quantify the delay uncertainty at signalized intersections. The dynamics of delay uncertainty and of delay uncertainty under different degrees of saturation are investigated.