9.2.2.1 Travel Demand Model

TDM is a macroscopic analysis tool that is best for analyzing a large area with low precision. There are different types of TDMs, which include four-step/trip-based models and ABM. The traditional four-step model includes trip generation, trip distribution, mode split, and traffic assignment for a single OD from traffic analysis zone to traffic analysis zone. Conversely, ABMs examine continuing trips from one zone to another over time.
TDMs are best used for subregional/subarea-, regional-, and state-level planning and serves as a support tool for long-range areawide transportation planning. TransCAD and Cube are common software programs used to complete this analysis (for detailed guidance on inputs and outputs for these software programs, see their respective user guides). Highway and corridor networks are coded into the software program, along with socioeconomic data for each traffic analysis zone. Facility type (e.g., major and minor arterials, collectors, and locals), number of lanes, and area type impacts both the intensity and usage of links within the model.
TDMs are typically maintained by the local MPO. TxDOT’s SAM can be used in rural areas where an MPO TDM is not available. TDM outputs are typically all that is used for an analysis, however, there may be circumstances for updating and running the TDM for detailed analysis depending project scope and on project impacts to the existing network. A TDM is typically updated, calibrated, and validated for large projects or when major data is updated (i.e., census data, land-use data). When an update is necessary, the basic steps to develop/update and calibrate a TDM are:
  • TDM Development and Update; and
    • Collect data (see data and input parameters section)
    • Update Traffic Analysis Zone (TAZ) structure using base year demographic and socioeconomic data
    • Use HCM-based (mesoscopic) procedures for roadway capacities and intersection delay
    • Verify coding of urban and rural area types
    • Follow NCHRP Report 716 Travel Demand Forecasting and FHWA’s Model Validation and Reasonableness Checking Manual
  • TDM Calibration
    • Use big data resources (StreetLight, INRIX, Replica, etc.) to develop, calibrate, validate, and refine the TDM. Reinforce big data with statewide travel survey data regarding trip purpose, occupancy, and multi-modes
    • If corridor extends beyond TDM limits, calibrate external stations with historical and collected data
TDM outputs include multimodal time-dependent OD tables, link volumes, vehicle miles traveled, turning movement percentages, and network performance. These outputs can be used in alternatives analysis, traffic projections, HCM-based, mesoscopic, microsimulation, and other forms of traffic analysis for segments. Traffic and Safety Analysis Procedures Manual | 2024 9-6 When there are discrepancies between actual counts and TDM outputs, actual count data is typically used.