TxDOT and 23 MPOs collaborate in the transportation planning process and in the development of TDM outputs (TDM outputs) and their inputs. These 23 MPOs are listed on the following page. The MPO TDM outputs can be referenced to determine corridor growth rates for projects, if available. Outside these MPO boundaries, TPP’s SAM can be referenced to determine growth rates for projects. SAM involves travel demand modeling at a statewide level. It includes different passenger and freight modes and the interaction among those modes. Access to the MPO TDM and the SAM needs a data request.

There are different types of TDM, which include 4-step/trip-based models and activity-based models (ABM). The traditional 4-step model includes trip generation, trip distribution, mode choice, and traffic assignment for a single OD pair from one traffic analysis zone (TAZ) to another. Conversely, ABMs examine continuing trips from one zone to another over time.

The TDMs available through an MPO are as follows:

For non-attainment zones, only the approved TDM developed, calibrated, and validated by MPOs can be used. MPOs in the nonattainment zones are:

A TDM with project details is necessary for project level analysis for major investment type studies or a project that changes the MPO’s approved plan. Forecasting with a TDM output is also applicable in special cases, such as for a new route location; where historical data is not available; or when considerable development and land use changes are expected in the project area in the future. Travel demand modeling includes the selection of an applicable model, calibration to local conditions, validation of model results, and revisions of forecast volumes. A TDM output is calibrated for regional trip generation, trip distribution, mode choice, and assignment. TDM outputs can have inconsistencies on a link-by-link basis since these models are not calibrated at the link level. Therefore, further adjustments or post-processing of the model’s daily or peak-period outputs are typically applied prior to use in corridor or project analysis. Detailed travel demand modeling methodologies are described in National Cooperative Highway Research Program (NCHRP) Report 716 Travel Demand Forecasting: Parameters and Techniques.

It is recommended that TDM outputs and growth rates be compared with other sources, such as historical data, adjacent projects, past projects, state demographer forecasts, and US Census data population. Differences in volumes more than 10% for freeways, 15% for arterials, and 20% for ramps or low volume roads may need to be further evaluated and documented to explain the discrepancy. If no model is available for the project, then see

Section 3.3.7.

When forecasting volumes with a TDM, the existing year of a TDM coincides with the date the data was collected. For example, a TDM that uses census data for population information would have a base year that was the same as the census data year.

When forecasting traffic volumes for new locations (e.g., new routes through greenfield), it is recommended that a diversion analysis is conducted. Diversion analyses are typically conducted for a proposed highway or bypass, proposed frontage roads, ramp reversals, proposed Texas U-turns, and proposed land use development. TxDOT’s TPP Traffic Forecasting Analysis SOPs and Guide outlines how to complete a diversion analysis. Coordinate with the TxDOT project manager to gain access to the latest documents.

TDMs forecast travel demand using the following traditional four-step model: