Procedures for evaluating traffic capacity and improvements to road geometry

The geometric standards of a road, such as width, minimum curve radius and maximum grades, can have a major effect on the costs of road construction and maintenance, and the speed, safety and vehicle operating costs experienced by those who travel on the road. This paper investigates the effects of geometric improvements on the overall costs and benefits of a road project. The main objective of the paper is to develop a framework for evaluating changes in geometric characteristics, so that appropriate standards can be established for particular cases. A further objective of this paper is to review and assess methods for predicting vehicle operating costs, and the extent to which these methods take account of road geometry characteristics. While various models are available for predicting these costs for free-flowing traffic on relatively wide roads, the modelling of traffic interaction and narrow road widths is currently inadequate. Hence, much of this paper is devoted to developing approximate procedures for taking account of these effects. The paper also considers the estimation of marginal construction costs with changes in geometric standards, and the sensitivity of predicted geometric standards to uncertainties in the input parameters.

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