Travel through time: the story of research on travel time reliability

This paper provides a review of research on the concepts and application of travel time reliability and on the underpinning research on travel time variability. It charts the development of the topic from beginnings concerned with exploring the possible extent of variability in travel times through to the current interests in the specification of meaningful metrics for reliability, the inclusion of travel time reliability factors in traffic assignment and on ascribing economic valuations to travel time reliability for use in the economic appraisal of transport policies and projects. Travel time reliability is now recognised as a major determinant of travel behaviour and as an important performance measure for transport systems operation. Much has been achieved in understanding the symptoms and causes of variability in travel times. Yet the definitions are still not completely clear, and as a consequence the measurement and assessment of travel time reliability is still open to interpretation. Relationships between traffic congestion and reliability are inferred, and in more recent times the influence of extreme events', including weather events, on reliability has become an important research topic. The paper reviews the current situation, and provides ideas for future research.