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Delay-Volume Relations for Travel Forecasting: Based on the 1985 Highway Capacity Manual


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.

Updated: 3/25/2014
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