Travel time variability - or journey time reliability - is an increasingly important issue. It causes late or early arrivals and requires travellers to allow extra time for their journeys. This causes inconvenience, frustration and imposes extra costs on businesses. With the development of modern production practices including 'just in time' deliveries, predictability of journey times has become ever more important. Conventionally, transport modelling and appraisal - especially for highway projects - have largely ignored travel time variability, focusing almost exclusively on average journey times. In January 2001 the authors were appointed by the UK Government to carry out research into how travel time variability (TTV) could be included in modelling and appraisal procedures. In this context TTV refers specifically to day to day variations in travel time. The research was split into two connected projects. Project 1 involved determining whether it was both feasible and desirable to include TTV in the heart of transport supply and demand modelling. Project 2 was a review of and enhancement to a spreadsheet model known as INCIBEN (INCIdent BENefits), developed by the John Fearon Consultancy for DETR. This model provides a means of carrying out economic appraisals of the variability element of schemes that affect highway-based incident related TTV. As part of these enhancements the model has now been made compatible with DTLR's Transport User Benefit Analysis procedures (TUBA) and renamed INCA (INcident Cost benefit Analysis). This paper describes the work carried out for Project 1.
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