As well as the new approaches to transport modelling in the 4S model, TransPosition has had broad and deep experience with traditional four step modelling. It has developed a number of four step models from scratch and has contributed and worked with many others. A deep understanding of the theory underlying traditional modelling gives TransPosition staff the ability to assess the strengths and limitations of various approaches, and to advise on the best ways to make use of the models.
TransPosition has developed a new modelling approach that overcomes many of the deficiencies of the traditional four step transport model, particularly with respect to modelling non-car modes and behaviour change—the Segmented Stochastic Slice Simulation Model (4S Model).
It uses Monte-Carlo simulation within a random-utility framework to produce a series of samples of travel behaviour. The model simultaneously considers when and whether travel will occur (induced/suppressed demand), destination choice, time of travel, mode choice and route choice. These choices are assessed within the context of a complete multi-modal network, with time-dependent congestion and full timetabled public transport. The route selection allows any combinations of mode, including complexities like: taking bikes onto public transport; chauffeuring; and using CBD public transport loops to travel from car parks to the destination. The behavioural sophistication is particularly important for mode choice and toll/pricing sensitivity, where variations in people’s value of time determine much of the response. High Resolution The model uses a very detailed description of the network and land-use; the network includes all roads, PT services and walk/cycle paths. It does not use traffic zones – all population and activities are located directly onto the transport network. It can use CD, zonal or lot-level data, such as the lot level population and employment forecasts being developed for Brisbane as part of the Brisbane Urban Growth Model.
Practical + Efficient
As the process consists of a series of samples, a trade-off can be made between running time and precision of results; less precise results can be generated very quickly by simply reducing the number of samples. To generate more precise results, the number of samples can be increased. Reasonable results can be obtained in under an hour, even with complete road and public transport networks for South East Queensland.
The congestion model includes time-varying delays and public transport capacity constraints. Under development are simplified trip chaining, and full journey modelling (including the characteristics of the return trip when making decisions). The model is implemented in custom-developed software with strong links to GIS and spatial databases. The early applications of the approach have been for Brisbane City Council (for a number of projects, including the Brisbane Urban Growth Model and Community Facilities Planning) and for the Toowoomba Regional Transport Study.
Key Benefits of 4S Model
- Very detailed– spatially and behaviourally
- Operates at the level necessary to consider walking, cycling and public transport
- Integrated approach for toll/pricing sensitivity
- Gives answers quickly—under an hour (not over 24h like complex 4 step models)
- Sensitive to time of day, PT congestion
Since 1991 TransPosition has worked on a number of Strategic modelling projects using both traditional modelling software (Four-Step model) and TransPositions newer 4S model.