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

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.

Typical Limitations of Travel Forecasting Models

There are many travel forecasting packages; their capabilities vary greatly. The most popular packages have the following characteristics, which greatly limit users' ability to determine realistic estimates of delay.

Relative to other parts of travel forecasting models, the calculation of delay is not particularly time consuming. If turn penalties can be avoided, additional complexity in delay relationships should not cause unreasonable increases in computation time.

Data Limitations

Networks can have thousands of links and intersections, so there are severe limits to the amounts of data that can be economically provided for each. A typical model now requires only two pieces of information about each link for the purposes of delay calculations: capacity and free travel time. It is important not to burden the user with additional data requirements, unless the need has been firmly established through appropriate sensitivity tests of realistic delay relationships.

By their nature forecasts are done for future years; planners do not have very precise information about many of the important traffic characteristics affecting delay. For example, a planner doing a long-range forecast would have little knowledge about .the type of traffic control at any given intersection. The signal timing for signalized intersections would be essentially unknown, and there would be only vague information about the presence of pedestrians, bus operations, and parking maneuvers. Clearly, it would be inappropriate to construct delay relationships requiring data that cannot be obtained.

How the HCM Violates Model Limitations

The following list of violations does not include assessments of the accuracy of the estimates of delay. It is likely that more realistic and more transferable models of delay can be devised, given sufficient time and resources.

Basic Freeway Sections and Multilane Highways

Two-Lane Roads

Weaving Sections

All-Way Stop Controlled Intersections

Some-Way Stop Controlled Intersections

Signalized Intersections

General Issues

A more general problem concerns the definition of LOS C, often taken as the definition of "design capacity" in forecasting models. LOS C is largely subjective and is determined by different methods, depending upon the type of facility or type of traffic control. Thus, there no longer exists a simple method of relating LOS C to LOS E (ultimate capacity) that works across the full range of facilities or traffic controls.

For example, LOS C on freeways is determined by traffic density, while LOS on two lane roads is determined by percent time delay. The volume-to-capacity ratio for LOS C varies between 0.77 (freeway basic segment, 70 mph design speed) to 0.16 (two-lane road, mountainous terrain, 1 00% no passing).

Minimum Requirements of Forecasting Models to Reasonably Approximate HCM Delay Procedures

As indicated in the preceding paragraphs serious incompatibilities exist between the HCM and existing travel forecasting models. The incompatibilities can be fully resolved only by extensive revisions to the forecasting models. The amount of effort necessary to make these revisions depends upon the structure of the existing computer code.

Updated: 03/28/2014
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