NECTAR Series on Transportation and Communications Networks Research
Edited by Ana Condeço-Melhorado, Aura Reggiani and Javier Gutiérrez
Chapter 1: Accessibility and spatial interaction: an introduction
Spatial interaction refers to economic or demographic flows between different locations, generally through a transport infrastructure. It implies a complementarity between two places engaged in a supply–demand relationship which is subject to certain costs, such as cost of transport or transaction costs. Spatial interaction models (SIMs), stemming from their analogy with Newton’s law of gravity, are especially suited to study the spatial interaction between pairs of locations, where space is measured in the form of cost–utility function (at the aggregate or disaggregate level). SIMs have a long history and have been used in a wide variety of contexts. Wilson (1970) gave SIMs theoretical strength by deriving them using the entropy maximization approach. Nijkamp (1975) offered an economic framework by deriving SIMs from a cost-minimization approach and, finally, Anas (1983) demonstrated the formal equivalence between SIMs and logit models and microeconomic theory. All in all, SIMs in all their forms (unconstrained, singly constrained and doubly constrained) can be interpreted in macroeconomic and microeconomic terms, where a key behavioural element is provided by the cost function parameter (for a review, see Reggiani, 2014). The accessibility function can be directly derived from SIMs and it therefore contains SIM behavioural cost components. Its analytical formulation is consistent with the accessibility concept and equation provided by Hansen (1959), the first author to apply a gravity-type model to the study of accessibility, defining it as ‘potential of opportunity for interaction’ (p. 73).