Estimating Passing Demand

This paper, from the 2004 conference proceedings of the Institution of Professional Engineers New Zealand (IPENZ) Transportation Group, addresses the issue of the assessment of passing lanes and their adequacy in various settings in New Zealand. The authors note that nearly all of New Zealand's strategic routes have been built as two-lane highways, often through mountainous and rolling country. Increasing traffic flows have increased the demand for passing lanes to increase passing opportunities for users. Currently, passing lane benefits are assessed using computer microsimulation, which can be expensive. In this paper, the authors report on the use of a simplified procedure for estimating passing demand. They found that the equations used in the simplified procedure for estimating passing demand between two streams has a critical flaw that occurs when the standard deviation of the speeds of the faster stream is low, causing the equations to estimate near zero passing demand when some demand would in fact exist. This raises doubt about the accuracy and robustness of the equations used in estimating passing demand in the economic evaluation of passing lanes. The authors briefly propose additional research that would continue the development of a single equation for determining passing demand, using a single speed distribution for all traffic.