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, such as bus, Metrorail, and commuter rail. In addition to transportation networks, the other major input to the travel model is the land activity data for each transportation analysis zone (TAZ). The demand for travel is developed using a series of mathematical models. The most common two paradigms for travel demand models in the U.S. are known as the "four-step" model (FSM), due to its four main steps, and the activity-based model (ABM).
The TPB’s regional travel forecasting models have a modeled area covering 6,800 square miles, including 22 jurisdictions/counties. This area is divided into about 3,700 transportation analysis zones (TAZs). A typical highway network used by the travel model includes about 45,000 links (road segments) and a typical transit network includes over 1,400 transit routes, including such modes as Metrorail, Metrobus, other local bus, commuter bus, and commuter rail. Depending on the application, each model run requires about 10-15 hours of processing time on a high-end workstation. Additional processing time is sometimes necessary for modeling special scenarios.
The cost of maintaining and applying the models constitutes almost half of the region's transportation planning budget, including data support.
Different aspects of COG/TPB’s regional travel models are discussed in detail in the following subpages: