The most basic formulation of the gravity model simply substitutes the populations at i and j for the R and A factors. In the early years of such models, this data base was developed largely through household surveys, a time-consuming and expensive undertaking. Such surveys were instrumental in developing the transportation models that have been used during the past 30 years. Since 1980, over 30 urban areas have conducted new home interview surveys to update their data base and ensure the validity of their modeling process. However, given limited resources, many planning agencies have had to rely on other means to validate their system models.
Trip distribution is an important and complex phase of the transportation planning process.
It provides the planner with a systematic procedure capable of estimating zonal trip interchanges for alternate plans of both land use and transportation facilities. These zonal interchanges constitute a basic part of the travel information necessary for transportation planning. This manual documents in detail the process of trip distribution utilizing the gravity model as it is now defined. Transportation Modeling, or Travel Demand Forecasting, estimates the amount of travel on the transportation system and provides an abstract view of travel on study facilities. The travel patterns are based on relationships developed from survey data between housing, employment sites and transportation facilities.