In the broadest sense, the TPB's regional travel demand model consist of three elements:
- Input data
- A series of models (mathematical procedures and representations)
- Output data (“results")
The two basic inputs to TPB's regional travel demand model are:
- Forecasts of future population, households, and employment throughout the region; and
- Information about future transportation networks -- changes that are planned, or potential changes to be tested -- that would improve today's transportation system.
Updated population, household, and employment estimates are prepared through COG’s Cooperative Forecasting Program every few years, reflecting the best judgments of local officials regarding the location of future housing, commercial and industrial development within the region. The forecasts are developed by the Cooperative Forecasting and Data Subcommittee (CFDS), reviewed by the Planning Directors Technical Advisory Committee (PDTAC), and approved by the COG Board of Directors.
- Vehicular volumes on roads (flows on links)
- Speeds on links
- Origin/destination patterns - These are represented by zone-to-zone trip tables, which are usually segmented by travel mode.
- Mode splits
- Emissions from cars and trucks (requires post processor and emissions models)