TPB's Four-Step Travel Model

The TPB, like virtually all U.S. metropolitan areas, makes use of a trip-based travel demand model, which is often called a “four-step model,” due to the four major steps (shown below). 

Four-Stem Travel Forecasting Model

An alternate approach to the trip-based model (TBM) is the activity-based model (ABM). Although academics and researchers have been studying ABMs for about 40 years, it is only recently that some MPOs have begun to develop or adopt ABMs. Whereas TBMs use the trip as the basic unit of analysis, ABMs assume that travel is a derived demand (derived from the need to participate in various activities through the day) and use a combination of trips and tours as the unit of analysis (a tour is a collection of linked trips). Whereas TBMs estimate travel at the aggregate zone level, ABMs estimate travel at the disaggregate person and household level and then aggregate the results to the zone level for trip assignment. While some MPOs have begun or completed the implementation of an ABM, many others, such as TPB, have decided to defer such a move, due, in part to the complexity and perceived costs of the new models. An excellent overview of the relationships between TBMs and ABMs can be found in NCHRP Synthesis 406: Advanced Practices in Travel Forecasting (2010).

TPB staff has worked with the Association of Metropolitan Planning Organizations (AMPO) and the U.S. DOT, via a pooled funding initiative with 11 other MPOs, to collect further information about the effectiveness of ABMs in practice-based settings.