A sensitivity based approach to network reliability assessment

This paper presents a methodology for evaluating the reliability of transpo rtation networks. While tools already exist to determine the expected benef its of travel demand management or new infrastructure, tools have yet to be developed which take into account disbenefits arising from randomly occurr ing disturbances. The paper focuses on the sensitivity of both path travel rimes and expected minimum origin-destination travel times to normal within -day demand and supply variation, where demand variation takes the form of perturbations to origin-destination flows and supply variation takes the fo rm of perturbations to link saturation flows. Two extreme cases are disting uished; one where route choices fully respond to the perturbations, corresp onding to the more major, longer-term incident and the other where route ch oice does not respond, corresponding to the more minor, shorter-term incide nt. A legit assignment model, referred to as the Path Flow Estimator (PFE), is linearised with respect to the parameters affected by within-day variat ion, using sensitivity expressions. As analytically derived sensitivities a re sometimes difficult to calculate for large networks, their approximation by finite differencing is considered. Results obtained for a large network (5000 links, 8000 OD pairs) in York are discussed, as well as results obta ined for a much smaller network (100 links, 60 OD pairs) in Leicester.