Assessing performance reliability of transportation networks under demand and supply uncertainties

This paper proposes a new approach to access the network performance considering both travel time and its reliability in transportation networks with demand and supply uncertainties. Due to such uncertainties, travel times are no longer deterministic and so they can be viewed as random variables. The corresponding mean, variance and covariance between link travel times are deduced by an analytical method using a modified travel time function. Confronted with travel time variations travelers are assumed to allow an extra travel time, known in the literature, a safety margin, for their journeys to improve the likelihood of punctual arrival. Under this condition, the effective travel time, which is defined as the summation of mean travel time and safety margin, is adopted as the traveler´s path choice criteria. Then, the corresponding reliability-based traffic assignment problem is formulated as a variational inequality (VI) problem. And the resultant total effective travel time is proposed as the measure for evaluating the network performance on both travel time and its reliability. Such VI problem can be solved by an employed heuristic solution algorithm. The numerical results indicate that travel time variations increase as the network environment becomes more uncertain in demand and supply. It is also evidenced by the numerical results that the newly proposed performance measure is more applicable than conventional measure, particularly for networks with demand and supply uncertainties.