Delay/Volume Relations for Travel Forecasting Based upon the 1985 Highway Capacity Manual

Abstract

This report discusses the 1985 Highway Capacity Manual in relation to travel forecasting models. It was found that important incompatibilities exist between the HCM and most travel forecasting models; ways of reconciling these incompatibilities are suggested. This report suggests parameters for speed/volume functions for uncontrolled road segments. For controlled facilities, the reports suggests values for link speed and link capacity to be used prior to network calibration. These speeds and capacities depend upon the type and manner of traffic control. The report also provides sample specifications for delay relationships that can make a travel forecasting model consistent with the HCM. Separate specifications are provided for signalized intersections, all-way stop controlled intersections, some-way stop controlled intersections, and two-lane roads. Introduction The 1985 Highway Capacity Manual provides delay relations for a wide variety of highway facilities. Travel forecasting models also must calculate estimates of delay. Delay is required for determining the shortest paths through networks, the spatial distribution of trips throughout the region, and the relative advantages of one travel mode over another. It has often been suggested that travel forecasting models should incorporate delay relations found in the HCM. Potentially, travel forecasts would be more accurate and forecasted volumes would be more consistent with operations-level traffic models and with accepted principles of highway design. Unfortunately, incorporating HCM delay relations into travel forecasting models is not easy. Not only are the HCM delay relations too complex for existing software packages, but they also are inconsistent with available theory and algorithms. To properly accommodate the delay relations, both software and theory would require substantial revision. The purpose of this report is to find ways to make travel forecasts more consistent with the HCM. Both preferred and alternative approaches are recommended. This report identifies properties and requirements of existing travel forecasting models; it then lists deficiencies and problems with the HCM procedures. Full specifications are developed for incorporating HCM-type delay relations into travel forecasting models. These specifications are illustrated by a complete test forecast. Simple delay/volume functions are recommended where possible. Finally, advice is given to planners who must cope with existing software, particularly during the network calibration process. Deficiencies in and Problems with the HCM from the Standpoint of Travel Forecasting The 1985 Highway Capacity Manual is seriously incompatible with traditional travel forecasting models. The principal reason for this incompatibility is the complexity of many of the delay relations, particularly those relations which compute delay as a function of more than a single link volume or more than a single turning movement.

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