ABSTRACT
When assessing infrastructure investment, it is crucial to forecast the flows on the links after the suggested improvements. In freight flow analysis, the impact on the flows in the network is conditional on the size of the investment and the complexity of the network. If the investment has a critical impact on the flows, the change may be substantial and hard to predict by linear models. In economic geography and regional economics, the gravity model has gained wide acceptance as a reasonable (although simple) model of spatial interaction between nodes in a network (Haynes and Fotheringham, 1984 or Sen and Smith, 1995).
