Some implications of the new speed–volume curves to highway user cost and sizing of roads

Abstract

This paper investigates some implications of the speed–volume relationships for two-lane rural highways as proposed in the new edition of the U.S. Highway Capacity Manual, which is expected to be completed soon. The effect of the new speed–volume curves on road user costs is analysed by considering the case of Alberta Transportation. The analysis carried out in this paper indicates that the new speed–volume curves, when employed with the traditional values of other variables, such as vehicle running costs and value of travel time, produce cost–volume relationships that might appear unacceptable from both the highway agencies' and the road users' perspective. To overcome such a limitation it is suggested that a cost of comfort and convenience should be computed and added to the user cost.Basing their proposal on a survey of Alberta households and the actual practice of road sizing by Alberta Transportation, the authors present a generalized mathematical model to estimate the cost of comfort and convenience. The structure of the model and its independent variables include the considerations of (a) the relative importance of driving comfort as perceived by road users, (b) the unsatisfied demand (overtaking demand minus actual overtaking) curve, (c) the type of road use, (d) volume-to-capacity ratio, (e) desired highway speed, and (f) vehicle classification. The paper discusses and demonstrates how this model can be calibrated and used for economic analysis for designing and upgrading of rural highways. Key words: transportation, highway economic analysis, road user costs, cost of comfort, speed–volume curves, highway design.