Valuing travel time variability: Characteristics of the travel time distribution on an urban road

Fosgerau and Karlstrom [The value of reliability. Transportation Research Part B, Vol. 43 (8–9), pp. 813–820, 2010] presented a derivation of the value of travel time variability (VTTV) with a number of desirable properties. This definition of the VTTV depends on certain properties of the distribution of random travel times that require empirical verification. This paper therefore provides a detailed empirical investigation of the distribution of travel times on an urban road. Applying a range of nonparametric statistical techniques to data giving minute-by-minute travel times for a congested urban road over a period of five months, we show that the standardized travel time is roughly independent of the time of day as required by the theory. Except for the extreme right tail, a stable distribution seems to fit the data well. The travel time distributions on consecutive links seem to share a common stability parameter such that the travel time distribution for a sequence of links is also a stable distribution. The parameters of the travel time distribution for a sequence of links can then be derived analytically from the link level distributions.