Equilibrium versus Dynamics in Urban Modelling

The use of equilibrium formulations in urban modelling is increasingly challenged by models that explicitly address the dynamics of urban change. Equilibrium models assume that urban land use and transport converge to equilibrium between supply and demand and focus on comparative static analysis of these equilibria. Dynamic models consider the different speeds of processes of urban change and concentrate on their outcomes over time and the path dependence this implies. It is becoming increasingly apparent that without understanding the inherent inertia of different subsystems of cities it is impossible to assess their likely responses to land-use or transport policies. With new challenges from energy scarcity and climate change, the time horizon of urban planning is extending beyond the present generation; this makes a long-term perspective of urban models even more important. In this paper our aim is to revive the debate on whether modelling intended to inform decision making can reasonably represent cities as if they were in or near equilibrium or whether it needs to recognise explicitly that they are continuously changing and far from equilibrium. We start with a classification of urban change processes by speed of adjustment and show how equilibrium models fail to deal with them. We discuss options of modelling dynamics and argue for recursive dynamics or quasi-dynamics as a rational trade-off between theory and operationality in spatially disaggregate urban models. We illustrate this by comparing how three existing recursive or quasi-dynamic urban models address temporal dynamics and close by suggesting research needs.