A Novel Design and Implementation of an Aggregate UK-based Transport Model
Y Xiang, I Wright, T Meehan, Atkins, UK; the West of England Partnership, UK
This paper presents an aggregate six-stage transport modelling framework for a large conurbation with state-of-the-art design and implementation. The integrated model complies fully to UK WebTAG and sensitive to policy changes for decision makers.
This paper presents a flexible, aggregate transport modelling framework with state-of-the-art model design and implementation that takes advantage of the rapid advances in computer power. Aimed to assess a range of potential transport interventions in the West of England sub-region and provide supporting evidence for Major Scheme Bids, the Great Bristol Modelling Framework (GBMF) has been developed as a six-stage aggregate transport framework for the West of England Partnership. The GBMF has been successfully applied to assess a range of transport interventions (including demand management) and has supported a number of a Major Scheme Bids.
Aggregate transport models continuously evolve with innovation in design and implementation for policy makers. The GBMF study has developed a very detailed network representation combined with a fine level of demand segmentation in a spatially disaggregated system to ensure model?s responsiveness to changes. The stages inside the GBMF include: frequency modelling, main mode choice, time period choice, destination choice, sub-mode choice, and highway and public transport (PT) assignments. The Bristol model adopts an integrated zoning system with 600 zones that has been used across all six stages rather than adopting a hierarchic modelling system. The demand is detailed segmented with five journey purposes, three household income bands, four time periods, and two person types with car availability, and the homogeneity within disaggregated groups has been sought ultimately for the development of GBMF. The demand model is dynamically linked to a very detailed SATURN highway network whilst the emme3-based Public Transport models similarly represent travel conditions for non-highway based modes.
An innovative Production-Attraction (PA) formulation has been constructed for the GBMF with pseudo tours for average week days to enable the latest WebTAG requirements to be satisfied. Pseudo tours have been created based on trip data for home-based trips by using return proportions obtained from UK Department for Transport (DfT). This PA formulation is consistent to the GBMF demand modelling structure with pseudo tour zonal costs properly weighted between outward and return legs.
The model has also adopted variable Values of Time (VOT) with distance has also been incorporated into the GBMF by segmentation to help achieving generally accepted elasticities. Before-and-after study results will be presented to show the impact of VOT variation to mode scales and then the impact on elasticities.
The resulting complexity of the GBMF system has placed heavy demands on the computing resources available especially in order to achieve high levels of demand/supply convergence within acceptable model runtimes and memory requirements. With the latest multi-threaded desktop PCs, GBMF runtimes have been substantially reduced by a number of innovative convergence strategies including undertaking SATURN assignment and skimming in parallel as well as adopting variable convergence strategies within the assignment. Arrays and network scenarios within the GBMF have been carefully planned and maintained to enable the model to remain within a single emme3 databank for rapid data access and retrieving.
The development of GBMF has significantly advanced aggregate-based modelling framework to meet the latest WebTAG requirements. Modelling results will be presented with real-life applications to demonstrate its responsiveness and effectiveness to policy changes, for example the promotion of Rapid Transit schemes in the Greater Bristol area. The paper demonstrates that aggregate transport models may still play significant roles in current transport planning with affordable runtimes and manageable data and resources.
Association for European Transport