Further Contributions to a General Theory of Movement.

Alonso (1973, 1978) has proposed a generalized spatial interaction model from which the four members of the traditional family of gravity models (Wilson 1971) can be derived as special cases. With the aid of AIonso's general theory, this paper discusses the relationship between the traditional gravity models in greater detail and describes the performance of a series of what are termed quasi-constrained gravity models. The systemic parameters in Alonso's general model are discussed in terms of trip generation and trip attraction modeling. These systemic parameters are shown to reflect the relative importance of "site" and "situation" variables in determining outflow and inflow totals. When the site variables are accurate surrogates for actual outflow and inflow totals, a doubly constrained gravity model is likely to be optimal in terms of replicating existing flow patterns or forecasting unknown patterns. When the site variables are inaccurate surrogates and when outflow and inflow totals are related to situation variables, then lesser-constrained gravity models are likely to be optimal. Members of the traditional family of gravity models are shown to be the extreme points on a continuous surface of gravity models. [ABSTRACT FROM AUTHOR]