Further modelling of the benefits from urban road improvements

Improvements to urban road network capacity can, in theory, have a number of effects on the demand for use of the road system, including peak contraction, reassignment, modal transfer, trip redistribution, trip generation, increased car ownership, and land use changes. The extent to which these effects occur in reality will govern the scale of the benefits which accrue from urban road improvements. The nature of the research described in this report was essentially that of an exploratory exercise, designed to contribute to the TRRL's wider understanding of the sources and scale of benefits from urban road improvements. To this end, a state-of-the-art four-stage transport model of Belfast, created using 1985 data, was used. An earlier part of the research had shown the need to achieve a good degree of convergence of the various iterative procedures in the model. The first part of this study was therefore concerned with establishing procedures for minimising the errors of convergence in the model. A series of model runs was then carried out to investigate the contribution made by each of the trip redistribution, modal split, and assignment components of the model to the estimates of benefit. These model tests were performed for one major package of road improvements in Belfast under two levels of travel demand. The results showed that the effects of the modal split submodel were negligible, but those of trip redistribution submodel were to reduce the benefits yielded by reassignment alone by between 7 and 12%. (A)