The Urban Dynamics Model

Forrester's urban dynamics model has had considerable influence on academicians and policymakers. However, the external validity of the model remains unsettled. Four large American cities which meet the initial assumptions of the model are used to test the model's predictive accuracy. Developments in these cities during a 20-year period are compared with the model's predictions. These comparisons indicate substantial deviations from predicted patterns of change. In general, the predictions of the model appear to be too pessimistic: The model fairly consistently understates developmental change and grossly overstates urban decay. The causes and implications of the model's poor performance are discussed.