The continued innovation of aggregate transport demand models

A decade ago the land-use transport interaction model of Greater Manchester had only 50 zones covering the County, a highly abstract network representation making use of area speed/flow curves, and so was only appropriatefor indicative strategy testing. The latest model has 250 zones and has acomprehensive representation of transport networks. A 250 zone system is still coarse for an area with a population of 2.5 million. This coarsenessand model run times preclude incorporating representation of traffic interaction at junctions, and the resulting delays, in as sophisticated a way as some mainstream highway assignment packages. However, these challenges can be addressed using a tiered approach. The tiered model system developed for the two Transport Innovation Fund (TIF) bids has a lower tier containing spatially disaggregate highway (SATURN) and public transport (TRIPS) models that comply with UK Department for Transport (DfT) validation requirements. The upper tier consists of a bespoke aggregate 5-stage transport model known as TRAM which is consistent with DfT guidelines. For example the ratio between numbers of zones in the two tiers for Greater Manchester is approximately 3:1. TRAM takes as input turn flow-delay curves derived from SATURN together with public transport inputs from TRIPS. Demonstratingconsistency between the model tiers, through highway journey time and public transport generalised cost comparisons, is crucial to ensuring the validity of forecast and appraisal results. A vital element in this modellingapproach involves demand forecasts being passed down from the upper tier TRAM model to the spatially disaggregate SATURN and TRIPS models. Subsequent assignment in these detailed models provides more accurate highway and public transport link flows and travel times to inform a range of appraisals. Whilst significant advances have been made in the development of tiered models further progress would be facilitated by changes to software architecture. A blueprint is proposed for future development, which would integrate specialised highway and public transport assignment packages, mainstream IT databases, and compiled programmable code for implementing demand modelling. This would be a truly innovative aggregate demand modelling system. Such a system would readily take advantage of future developments in mainstream IT and transport software, being fully interoperable with standard IT data formats. For the covering abstract see ITRD E145999