Full-Feedback Modeling

Travel Demand Models

A transportation model, also known as a travel demand forecasting model, is a computer-based mathematical model that simulates supply and demand for travel in a given (usually urban) area. Models are used in a sequence of steps to answer a series of questions about future travel patterns. The basic questions asked in each transportation modeling step are as follows:

• 1.
  What will our community look like in the future?

  • a.
    How many people will there be? (population forecasts)
  • b.
    What will they be doing? (economic forecasts)
  • c.
    Where will activities take place? (land use)
• What will be the travel patterns in the future?
  • a.
    How many trips will be made? (trip generation)
  • b.
    Where will the trips be? (trip distribution)
  • c.
    What modes will be used? (mode split)
  • d.
    What routes will be used? (traffic assignment)
  • e.
    What will be the effects of this travel? (impact analysis)

These models are highly complex and require (as inputs) extensive current information on roadway and transit system characteristics and operations, as well as current and forecast demographic information. Creating and operating the models requires a high degree of technical training and expertise. Once a model is developed that replicates existing travel conditions, future conditions and alternatives can be evaluated in terms of their impact on the transportation system.

The most common paradigm for travel demand models in the United States is known as the “four-step model” (FSM) due to its four main steps: trip generation, trip distribution, mode choice, and trip assignment. This model is known as a trip-based model (TBM). 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, such as bus, rail, and commuter rail.

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