Chapter 9 Spatial interaction transportation and interregional commodity flow models This chapter presents an overview of spatial interaction transportation and interregional commodity flow models. Spatial interaction and transportation models are used to facilitate the explanation and prediction of patterns of human and economic interaction over geographic space. Theoretical methods for studying these phenomena have been modified considerably during the past 30 years to provide operational assistance to transportation planners and more recently regional economists. The chapter explores the developments made in the ongoing evolution of models describing spatial interaction transportation and the flow of commodities among different regions. It also provides some brief historical excerpts from the formative years and examines various modern theories and formulations that have been proposed for modeling spatial interactions assuming that transportation costs are fixed. The chapter demonstrates the way a broad range of hypotheses may be reconciled within a single unifying framework. It presents alternative formulations for analyzing route choice on a transportation network where the costs are flow dependent and deals with interregional commodity flow models in which the prices and service characteristics are respectively fixed and variable.