Travel time unreliability on freeways: Why measures based on variance tell only half the story J.W.C. van Lint * , Henk J. van Zuylen, H. Tu Department of Transport and Planning, Faculty of Civil Engineering and Geosciences, Delft University of Technology, The Netherlands Received 9 August 2006; received in revised form 22 June 2007; accepted 23 August 2007 Abstract In recent years, travel time reliability has become one of the key performance indicators of transportation networks and corridors around the globe. Travel time reliability indicators are mostly related to properties of the day-to-day travel time distribution on for example a freeway corridor. On the basis of empirical data a number of key characteristics of this day- to-day distribution can be identified. Most importantly, this distribution is not only very wide but also heavily skewed. The (economic) consequences of this skew are substantial. For example, it is shown that in some peak periods the 5% most ‘‘unlucky drivers’’ incur almost five times as much delay as the 50% most fortunate travelers. We argue this implies first of all that (besides the variance of travel times) skew must be considered an important contributing factor to travel time unreliability. Secondly, it suggests that most of currently used unreliability measures (which are predominantly based on travel time variance), should be used and interpreted with some reservations, since they only account for a part of the costs (that is, delays) of unreliability. This is further substantiated by a comparison on the basis of empirical data from a densely used freeway in the Netherlands between a new travel time reliability measure based on both width and skew, and a number of travel time reliability measures commonly used in practice. The analysis clearly illustrates the inconsistency between all measures, both old and new. In illustration, in cases where the commonly used misery index dubs a particular departure period very unreliable, another common measure (buffer time) considers these periods relatively reliable. Although without objective and quantitative criteria (e.g. economic or societal costs) a choice for any of these measures in road network per- formance analyses will remain subject to debate, this article provides empirically underpinned arguments to prefer mea- sures incorporating the skew of the travel time distribution. Published by Elsevier Ltd. Keywords: Travel time reliability; Travel time reliability maps; Travel time reliability measures; Travel time reliability monitoring 1. Introduction Route travel times are considered key indicators on the reliability of a road network (Lo, 2002; Cassir et al., 2001). In the Dutch national transport policy for example, travel time reliability plays a central role and 0965-8564/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.tra.2007.08.008 * Corresponding author. Tel.: +31 152785061; fax: +31 152783179. E-mail address: j.w.c.vanlint@tudelft.nl (J.W.C. van Lint). Available online at www.sciencedirect.com Transportation Research Part A 42 (2008) 258–277 www.elsevier.com/locate/tra