Abstract:The current travel forecasting process, often referred to as the four-step process, has been in place for over thirty years. Designed during the 1960s, it used the existing understanding of travel behavior and available computing capability to create four models, developed sequentially: trip generation, trip distribution, modal split, and network assignment. The model structure is aggregate in nature, with the planning area divided into a number of homogeneous traffic zones.

In the four steps, (1) trip generation models estimate the number of trips, trip origins, and trip destinations for each zone; (2) trip distribution models assign origin and destination zones to each trip; (3) modal split models calculate the proportion of trips carried by each component of the transportation system-highway, transit, carpooling, etc.; and (4) network assignment models distribute the trips to individual routes between origins and destinations.

This four-step process was highly successful in supporting transportation planning analysis during the 1970s and 1980s. However, two major pieces of legislation, the Clean Air Act Amendments (CAAA) of 1990 and the Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991, have significantly changed the questions asked of the travel forecasting process. In addition to the legislation, changes in behavior have also begun to invalidate some of the underlying assumptions of the process. The overall effect of these changes is to require a fresh look at how travel is forecast and a redesign of forecasting procedures.

Note:Published in Transportation and Energy: Strategies for a Sustainable Transportation System, ed. Daniel Sperling and Susan Shaheen, American Council for an Energy-Efficient Economy, Washington DC and Berkeley CA