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Evaluation of several model forms in the 1960s concluded that "the gravity model and intervening opportunity model proved of about equal reliability and utility in simulating the 1948 and 1955 trip distribution for Washington, D.C." (Heanue and Pyers 1966).
A gravity model cannot accurately predict flows, but is instead a measure against which actual observed values can be compared, highlighting where those flows are unexpectedly high or low. Social science gravity models: Gravity model of trade; Trip distribution; Gravity model of migration; Two-step floating catchment area (2SFCA) method
It is the fourth step in the conventional transportation forecasting model, following trip generation, trip distribution, and mode choice. The zonal interchange analysis of trip distribution provides origin-destination trip tables. Mode choice analysis tells which travelers will use which mode. To determine facility needs and costs and benefits ...
Trip generation determines the frequency of origins or destinations of trips in each zone by trip purpose, as a function of land uses and household demographics, and other socio-economic factors. Trip distribution matches origins with destinations, often using a gravity model function, equivalent to an entropy maximizing model .
A basic conception within it is that large numbers of people, in a city for example, actually behave as an attractive force for other people to migrate there. It has been related [4] [5] to W. J. Reilly's law of retail gravitation, [6] [7] George Kingsley Zipf's Demographic Energy, [8] and to the theory of trip distribution through gravity models.
He wrote "A General Theory of Traffic Movement" (1956), which applied the gravity model to trip distribution, which translates trips generated in an area to a matrix that identifies the number of trips from each origin to each destination, which can then be loaded onto the network.
The implication of this model is that, as opposed to other more traditional forms of random walks such as brownian motion, human trips tend to be of mostly short distances with a few long distance ones. In brownian motion, the distribution of trip distances are govern by a bell-shaped curve, which means that the next trip is of a roughly ...
The Gravity model gives bad predictions both on short and long distance commuting, while the prediction of the Radiation model is close to the census data. Further empirical testing shows [ 2 ] that the Radiation model underestimates the flow in case of big cities, but generalizing the fundamental equation the model can give at least as good ...