Assessing Measures Which Reduce Incident Related Delays and Travel Time Variability

Assessing Measures Which Reduce Incident Related Delays and Travel Time Variability


T van Vuren and S Ahuja, Mott MacDonald;



Generally, transport planning and forecasting work concentrates on the representation of average conditions, ignoring the relevance of travel time uncertainty (due to day-to-day variability and incidents) on behaviour and travel conditions.

Traditionally, when assessing the benefits of incident reduction measures, only delay impacts have been assessed, for example using QUADRO. However, particularly in congested situations, travel time variability can have an important effect on behaviour and on benefit calculations. The former was dealt with in a previous paper presented at the 2001 ETC (Andrew Gordon et al); the latter is the subject of a new tool developed for the DTLR: INCA (INcident Cost benefit Assessment).

INCA is a spreadsheet-based application, which makes extensive use of a database of the frequency atwhich five types of incident occur, their build-up duration, and their severity, expressed as the number of lanes affected. Incident variability is represented as variations above expected journey times, and is calculated as a function of:

* the probablity of a journey encountering an incident
* the average delay per vehicle affected
* the variance per vehicle affected.

Travel time variability benefits are linked to scheduling constraints and/or reductions in the standard deviation of journey times, and in both case calculations cannot be solely link-additive: whole journey impacts must be calculated. Those calculations need to be sophisticated enough to assess the impact of alterations to the road network and an allowance for the impact of day-to-day variability must be incorporated.

The paper discusses the relative importance of the assumptions underpinning the program. Specific attention is paid to the diversion mechanism assumed when incidents occur, and the representation of variability in the severity and duration of incidents. Examples are presented of the comparative monetised impacts of reductions in delays and travel time variability on different road types and under different flow regimes, and the importance of delay versus travel time variability savings.

INCA has been used in the assessment of Active Traffic Management (ATM) of a Motorway, Motorway widening and for accident remedial measures in Yorkshire. It has proven to be a flexible toolto assess travel time variability impacts of such schemes.


Association for European Transport