TPB's Currently Adopted Travel Model

The TPB travel forecasting model serves a number of functions. It provides regional travel demand forecasts and air quality assessments to support both long-range planning and the development of the six-year TIP. It supports ongoing corridor and sub-area studies, which are typically performed for member state and local governments with consultant assistance. The model is also used to examine the mobility of various population segments.

The TPB travel model is periodically refined with newly collected data or with emerging forecasting techniques. At any point in time, there will always be an "official, adopted" travel model of record reflecting the last approval action by the TPB. The current official, adopted travel model is Version 2.2, which was adopted January 16, 2008, when the TPB approved the air quality determination for the 2007 CLRP and the FY 2008-2013 TIP. The Version 2.2 travel model was also used for the air quality determination for the 2008 CLRP and the FY 2009-2014 TIP.  Typically there will also be a "draft" travel model that is under development and available for public review. The next draft travel model will be called Version 2.3. Both the adopted and draft travel models are briefly described below.

Version 2.2 Model

The current production-use version of the travel model is known as Version 2.2. This is the version that was used for the January 16, 2008 air quality conformity analysis of the Transportation Improvement Program (TIP) and the Constrained Long-Range Plan (CLRP). The Version 2.2 travel model was also used for the air quality determination for the 2008 CLRP and the FY 2009-2014 TIP.  To perform the air quality conformity analysis, the model is coupled with a mobile emissions model known as MOBILE 6, which is developed by U.S. EPA.

The Version 2.2 travel model is very similar in structure and application to the TPB’s previously adopted model (Version 2.1D #50).  However, Version 2.2 includes numerous refinements and updates.  

The primary features of the Version 2.2 model include:

• An explicit commercial vehicle model;
• Revised external and through trip forecasts;
• Updated volume-delay functions (VDFs) used in the traffic assignment step;
• A newly developed queuing delay function that has been combined with the freeway VDF;   
• A refined approach for simulating planned High Occupant Toll (HOT) lanes in Virginia and planned Electronic Toll Lanes (ETLs) in Maryland; 
• A reduced number of HBW K-factors factors used in trip distribution, compared to the number used in the Version 2.1D #50 model;
• The removal of non-work K-factors used previously in trip distribution;
• The removal of bridge penalties used previously in trip distribution;
• The removal of geographic adjustment factors used in the mode choice model;
• An increased number of user-equilibrium (UE) iterations used in the traffic assignment process (was 20, now uses 60 iterations);
• Updated demographic models based on the 2000 Census Transportation Planning Package (CTPP); and
• Updated airport passenger auto driver trip forecasts based on TPB’s 2000 Air Passenger Survey;

An automated routine procedure has been established to couple Version 2.2 model outputs with the MOBILE6 emissions model developed by EPA to provide emissions estimates for the air quality conformity analyses of the CLRP and TIP.  The MOBILE6 emissions model was promulgated by EPA after extensive laboratory testing of emissions generated by large numbers of vehicles, including sport utility vehicles (SUV). MOBILE6 is a more disaggregate model than the previous MOBILE5b in several respects:

• Emissions factors for 28 vehicle classifications, up from 8 classifications in MOBILE5b;
• Emissions factors by different road facility types - freeways, arterials, local streets, freeway ramps;
• Emissions factors for each hour of the 24-hour day.

The MOBILE6 model is based on recent data reflecting an updated understanding of vehicle emissions. It also includes emission factors from new regulatory programs, such as Tier 2 and the diesel and truck program. Before the outputs of the travel model can be used as inputs to the emissions model, they must go through a mobile emissions post processor model.

Version 2.3 Model

The COG/TPB travel forecasting model is periodically updated and improved, via a “model development” program, a multi-year effort by TPB staff, member agency staff, and consultants. The Travel Forecasting Subcommittee of the TPB Technical Committee provides oversight for all models development work conducted at COG. The next, improved version of the COG/TPB travel model will be known as "Version 2.3." Enhancements include:

• A revised nested-logit mode choice model which will supplant the existing sequential multinomial logit mode choice model that is currently used in the Version 2.2 model;
• Updated truck models which will replace the existing truck models currently used in the Version 2.2 model; and
• Measures to streamline the model application and to reduce computation times, possibly including:
  • A reduction in the number of speed feedback iterations currently used in the Version 2.2 model;   
  • A decrease in the number of UE iterations used in the off-peak traffic assignment process;  
  • The implementation of distributed processing (DP) which enables one to combine multiple computing processors to support a single model execution, thus reducing model turn-around time.

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