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Transportation Models & Forecasts

This section explains the purpose, procedures, and technical details of how TPB staff use models to develop forecasts for the region's transportation network. It also contains the latest information on current model versions and how to obtain them, along with coverage of comments from the public and the current peer-review process. The article "Forecasting Future Travel" in the 2003 issue of The Region magazine provides a general overview and explanation of transportation modeling and forecasting.

About Models

A regional transportation model, also known as a regional travel demand forecasting model, is a mathematical representation of the supply and demand for travel in an urban area. The travel supply is generally represented by a highway network and a transit network. The highway network represents all major roads in the region and the transit network represents all public transportation service in the region. The demand for travel is developed using a series of mathematical models. The most common paradigm for travel demand models in the U.S. is known as the "four-step" process or "four-step" model, because of the four main steps:

• Trip generation
• Trip distribution
• Mode choice
• Trip assignment

The four-step model is a trip-based model that is used, in one form or another, by almost every MPO (metropolitan planning organization) that performs regional modeling. The first three steps are used to estimate the demand for travel. In the fourth step, trip assignment, the travel demand is equilibrated with the travel supply, as trips are loaded onto one or more networks. The geographic unit of analysis in a four-step model is the transportation analysis zone (TAZ). Usually an urban area is divided into hundreds or thousands of transportation analysis zones.

The TPB travel forecasting model has a modeled area covering 6,800 square miles, or 22 jurisdictions/counties. This area is divided into about 2,000 zones. The highway network represents over 27,000 road segments and the transit network includes over 600 routes, including such modes as Metrorail, Metrobus, other local bus, commuter bus, and commuter rail. Depending on the application, each model "run" can take as much as 11 hours of processing time on a personal computer.

The cost of maintaining and applying the models constitutes approximately 40 percent of the region's transportation planning budget (including data support). Modeling to test the air quality conformity of the proposed Transportation Improvement Program (TIP) required the expenditure of more than $500,000 for staff and other resources over a recent four-month period.