This paper summarises the main findings of work carried out for the United Kingdom Department of Transport examining some of the theoretical and practical issues involved in the introduction to the assessment framework of trip re-distribution in the context of congested assignment. In particular, it describes the implementation of a robust algorithm for the simultaneous assignment and distribution of trips which overcomes potential convergence problems arising from a conventional iterative procedure. Formulated as the equivalent minimisation problem, the simultaneous dual trip distribution and assignment problem has been implemented with SATURN compatible demand and public transport suite SATCHMO. Trip numbers are assumed public transport suite SATCHMO. Trip numbers are assumed to depend on generalised travel costs with both incremental and absolute forms of distribution available. Flexibility is maintained through the choice of exponential, power or gamma deterrence functions and the option to use either singly or doubly constrained formulations. A further extension introduces a multiple user class capability, allowing competition between person types with different characteristics and opportunities, e.g. for the simultaneous treatment of work and social/educational trips. The formulation provides a mechanism by which transport planners can investigate the potential impacts of trip redistribution due to major retail developments as well as increasing levels of congestion. Tests on real life networks show that the simultaneous procedure produces more stable results than the conventional iterative alternative, an important contribution to model integrity where model convergence is now recognised as a major issue.
Abstract