Resumen
This paper provides simple explanatory models for variability of road travel times with the aim to include reliability benefits in transport projects appraisal based on mean-dispersion models. It consists in an empirical investigation of the travel time distribution on urban roads in the metropolitan region of Paris, France. The analysis is based on accidents, major road works and detection loop data collected over a year on 23 urban highways. Explanatory models for several reliability indicators based on average travel time and road characteristics have been established and should allow to conduct rough estimations of reliability benefits of transport projects.