Transportation

Currently Adopted Travel Model

TPB Travel Demand Forecasting Model

The National Capital Region Transportation Planning Board is the federally designated Metropolitan Planning Organization (MPO) for the metropolitan Washington region and is also one of several policy boards that operate at the Metropolitan Washington Council of Governments. The TPB is staffed by COG’s Department of Transportation Planning. Staff develops and maintains, with consultant assistance, a series of regional travel demand forecasting models that are used for the regional transportation planning process in the metropolitan Washington region. These regional travel demand models are developed under the guidance of the Travel Forecasting Subcommittee (TFS), a subcommittee of TPB’s Technical Committee.

The TPB Travel Demand Forecasting Model serves several functions. It provides current-year and future-year estimates of travel in the metropolitan Washington region on its road and transit system. These travel estimates are used to gauge the performance of the region’s Long-Range Transportation Plan (LRTP) and to estimate, with the aid of a mobile emissions model, the amount of air pollution caused by motor vehicles (e.g., cars, trucks, buses) using the region’s surface transportation network. Since the metropolitan Washington region has been designated by the U.S. Environmental Protection Agency (EPA) as a “non-attainment” area for one or more criteria pollutants, the TPB must conduct an air quality conformity (AQC) analysis of both the LRTP and the Transportation Improvement Program (TIP) whenever there is a major change to either of these planning documents. Additionally, the TPB Travel Model supports ongoing corridor and sub-area studies, which are typically performed by state or local departments of transportation (DOTs), often with consultant assistance. Lastly, the travel model is used to conduct scenario studies and to examine the mobility of various population segments.

At any given time, staff maintains at least two regional travel demand models: one or more production-use models and one or more developmental models. The production-use models are the ones that are used in planning studies conducted by COG/TPB and are made available to outside parties. The developmental models are the ones that are currently under development by staff and are generally not made available to outside parties since they are not yet considered a finished product.

The current adopted, production-use TPB travel demand forecasting model is the Gen2/Ver. 2.4 Travel Model. More information about the model can be found on the Data Requests webpage or the Model Documentation webpage.

Model Description

The following lists some of the key distinguishing features of the TPB's current production-use model (i.e., the Gen2/Ver. 2.4.6 family of travel demand forecasting models):

  • An aggregate, trip-based, "four-step" travel model, calibrated and validated to observed conditions.
  • Key data sets used for calibration and/or validation:
    • 2007/2008 TPB Household Travel Survey
    • Transit on-board surveys
      • 2008 Metrorail Passenger Survey
      • 2008 Regional Bus Survey
      • 2007-2008 On-Board Survey of Maryland Transit Administration (MTA) Riders, which includes MTA bus service and MARC train service (commuter rail in Maryland)
      • 2005 Virginia Railway Express (VRE) Passenger Survey (commuter rail in Virginia)
    • Recent traffic counts
  • Modeled area
    • Covers 22 jurisdictions and about 6,800 square miles
    • Includes the District of Columbia and parts of three states: Maryland, Virginia, and West Virginia
  • A transportation analysis zone (TAZ) system containing 3,722 TAZs
  • Transportation network
    • Highway network: Contains about 45,000 links, of which about 30,000 represent non-centroid links
    • Transit network
  • Trip generation for five primary trip purposes: Home-Based Work (HBW), Home-Based Shop (HBS), Home-Based Other (HBO), Non-Home-Based Work (NHW), and Non-Home-Based Other (NHO).
  • Three other trip purposes:
    • Non-freight, commercial vehicles (including autos, vans, and light duty trucks)
    • Medium trucks (two axles and 6 tires)
    • Heavy trucks (all combination vehicles)
  • Trip generation of both motorized person trips (person-trips in cars, buses, and trains) and non-motorized person trips (walk and bike). Only motorized person trips continue through the model to trip distribution, mode choice, and trip assignment.
  • Trip distribution model uses the standard gravity model formulation and makes use of a composite-time function that represents a blending of transit and highway travel times. The gravity model is doubly constrained for all five trip purposes.
  • Mode choice model:
    • One for each of the five trip purposes
    • Nested-logic mode choice model with 15 travel modes, including
      • Three auto modes (drive alone, shared ride 2-person, and shared ride 3+person)
      • Four transit modes (commuter rail, all bus, all Metrorail, and combined bus/Metrorail)
      • Three modes of access to transit (park-and-ride, kiss-and-ride, and walk)
    • Although the nesting structure does not include explicit branches for specialized transit modes - such as light-rail transit (LRT), bus rapid transit (BRT), and streetcar - the model is designed to deal with these special transit modes.
  • Time-of-day model apportions daily resident travel among four time periods:
    • AM peak period (6:00 AM to 9:00 AM),
    • Midday (9:00 AM to 3:00 PM),
    • PM peak period (3:00 PM to 7:00 PM), and
    • Nighttime/early morning hours (7:00 PM to 6:00 AM).
  • Traffic assignment
    • Static traffic assignment (STA), following user equilibrium
    • Six user classes arranged across six traffic assignments
    • Stopping criterion: A “progressive” relative gap threshold, which varies from a value of 0.01 (10^-2) in the early speed feedback iterations to a value of 0.0001 (10^-4) in the final (iteration 4) speed feedback iteration
  • Equilibrium of model results
    • Speed feedback loop recycles constrained traffic speeds from highway assignment back into earlier modeling steps
      • Peak-period transit accessibility measures are fed back to demographic sub-models
      • Peak and off-peak composite (highway/transit) travel times are fed back to trip distribution.
      • Five executions of the travel model for each alternative tested, via five iterations of the “speed feedback” process (one initial or “pump prime” execution of the model followed by four normal executions of the model).
  • Updated truck models
  • Incorporation of parallelization/distributed processing, via both native Windows techniques and Cube Cluster’s intra-step distributed processing (IDP) and multi-step distributed processing (MDP). This allows two or more parts of the model to run simultaneously, helping to minimize model run time.

Click here for information on how to request models and data.

Page updated 7/20/22.