An Efficient Design for Very Large Transport Models on PCs
I Williams and C Lindsay, ME&P WSP, UK
As part of the development of the highway assignment module for use in the pilot version of the UK DTLR?s National Passenger Transport Model, ME&P created a 10,000 zone assignment model whichwas run on PC (though not without tedious complications). Subsequently, there was interest to upgrade the model to be iterated to a converged equilibrium solution at the 10,000 zone level while still using a detailed road network for all of GB. This paper outlines the theoretical foundations and the application in practice of the sampling approach that was designed for this purpose. Through appropriate sampling procedures the computational power and detail is focused on zone pairs in proportion to the level of traffic that they generate, while avoiding bias in the results. The approach involves:
* designing a suitable algorithmic structure and order for the main operations in the transport model software
* designing a suitable theoretical sampling and permutation structure to avoid bias * implementing the approach in practice for a large model and reviewing the performance.
It is demonstrated that major gains in computational efficiency can be achieved with minimal reductions in the accuracy of the local mode split and assignment of flows to the network. The net result is that the computational requirement for large models increases more in proportion to the number of zones, ratherthan its square.
Association for European Transport