Evaluating Traffic Capacity and Improvements to Road Geometry

This is an investigation of methods for evaluating traffic capacity and improvements to road geometry in order to determine economically justified levels of investment in road geometry. A macroscopic speed-flow model was derived for use with HDM-III (World Bank's Highway Design and Maintenance Standards Model for predicting speeds and operating costs for isolated vehicles as a function of road geometry), which makes use of the detailed free speed geometry relationships already available. This paper also reviews 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 considers the estimation of marginal construction costs with changes in geometric standards, and the sensitivity of predicted geometric standards to uncertainities in the input parameters.