In the broadest sense, the TPB's current, trip-based Gen2 regional travel demand forecasting model consists of three elements:
- Input data
- A series of models (mathematical procedures and representations)
- Output data (“results")
Input Data
The two basic inputs to TPB's current 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.
Output Data
The main outputs from TPB's current four-step regional travel demand model are:
- From the mode choice model
- Estimated zone-to-zone trips by travel mode (though not validated at this level)
- Estimated mode splits
- Output data is calibrated by geographic segment and is summarized to the jurisdictions and the region
- From traffic assignment
- Estimated vehicle trips and volumes on road segments/“road links” (though not validated at this level)
- Estimated traffic speeds on road segments (though the speed data is not validated)
- Output data is validated and summarized to screenlines, jurisdictions, and the region
- From transit assignment
- Estimated person trips/volumes on transit links (though not validated at this level)
- All transit trips are assigned to the transit network, but we currently validate only total boardings by transit sub-mode and Metrorail boardings by station group
- From trip distribution or mode choice
- Estimated origin-destination (O-D) movements, represented by zone-to-zone person trip tables.
- Although the trip distribution model is calibrated to the observed trip-length frequency distributions at the regional level and the mode choice model is calibrated by geographic segment, neither zone-level nor jurisdiction-level O-D information is validated. Thus, staff do not recommend its use.
- From the MOVES mobile emissions model and the travel-speed post-processor model
- Estimated air pollution (mobile emissions) from cars and trucks for both criteria air pollutants (e.g., NOx and VOC) and greenhouse gas (GHG) emissions.
- Estimated information summarized at the regional and jurisdiction levels.
Note: The current regional travel demand forecasting model produces many fine-grained outputs, such as link-level outputs (e.g., the number of vehicles traveling on each link in the AM peak period) and zone-interchange-level outputs (e.g., the number of bus person trips traveling from TAZ X to TAZ Y). However, the model has not been validated to these fine-grained levels, so it is not recommended that one use these fine-grained outputs from the travel model. A general rule is that, before using or reporting any model outputs, they should be summarized or aggregated to the same, or a higher, level as was used in model validation. For example, although the model produces estimated link-level traffic volumes, this information should be aggregated to the screenline level, jurisdiction level, or regional level, before it is used or reported. For more information, please see the latest user’s guide for the travel model, which can be found on the Model Documentation webpage.
Page updated 6/29/2022