The 1985 Highway Capacity Manual and Road User Costs for Two - Lane Roads

R oad user cost is an important consideration in highway economic analyses. The two main components of this cost are (a) motorvehicle running cost and (b) travel time. Highway engineers and economists know that, for road user costs (running costs plus travel time), lower traffic volume usually provides the least unit cost; as volume increases, the cost to the user increases because of congestion. Because the road user costs are a function of travel speed, speed-volume relationships are used to determine the user costs for given roadway and traffic conditions. Over the past two decades, highway engineers have used speedvolume curves contained in the 1965 Highway Capacity Manual as an input to computation of the user costs.’ But, in 1985 the Transportation Research Board published a new version of the Highway Capacity Manua/ (HCM).2 Among the updated material in the 1985 HCM are new speed-volume curves for two-lane rural highways. These curves, along with the “percent time delay” (PTD) concept as introduced in the new HCM, are expected to have significant implications for computation of the road user costs. Note that the term percent time delay is defined in the 1985 I-ICM as the average percent of time that all vehicles are delayed while traveling in platoons because of the inability to pass. The HCM points out that the percent of vehicles traveling at headways less than 5 seconds can be used as a surrogate measure for PTD in field studies. In the new HCM, speed has been found to be much less sensitive to volume of traffic than it was shown to be in the 1965 I+CM. One of the implications of the utilization of the new speed-volume curves will be a much slower decrease in travel speed, and as a result, a reduction in the cost of travel time that can be associated with a given increase in traffic volume. The main objective of this article is to investigate the effect of speed-volume curves and the PTD parameter on user costs computed for twoIane roads. Another objective of this investigation is to provide a brief description of a proposed model for cost of comfort and convenience that may be helpful to traffic engineers when they use the 1985 HCM’S speed-volume curves for estimating user costs for two-lane roads.