Abstract Reference Number - 139 1 Beyond Equilibrium: Advances in Urban Modelling David SIMMONDS David Simmonds Consultancy Ltd Suite 14, Millers Yard, Mill Lane Cambridge CB2 1 RQ, England Tel.: +44 1223 316098 david.simmonds@davidsimmonds.com http://www.davidsimmonds.com Paul WADDELL Institute of Urban and Regional Development University of California at Berkeley 316 Wurster Hall #1870 Berkeley, CA 94720-1870, USA Tel.: +1 510.642.4874 waddell@berkeley.edu http://iurd.berkeley.edu/ Michael WEGENER Spiekermann & Wegener Urban and Regional Research (S&W) Lindemannstrasse 10 D-44137 Dortmund, Germany Tel: +49 231 1899 441 mw@spiekermann-wegener.de http://www.spiekermann-wegener.de Abstract: There is a growing divide in urban modelling between equilibrium and dynamic models. Equilibrium models assume that urban land use and transport converge to equi- librium between supply and demand in each time period and focus on the comparative statics of these equilibria. Dynamic models consider different speeds of urban processes and forecast their outcomes over time and their path dependence. This paper argues 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. For instance, it takes many years between decisions on transport projects and their impacts on mobility and location decisions of households and firms. Land use policies show their effects only after a long time as building stocks change only incrementally. Relocations of households and firms respond to changes only gradually as moves require substantial transaction costs. New challenges of energy scarcity and climate change extend the time horizon of urban planning and make it even more important when policy impacts will arise. The recent global recession is perhaps the best example of the importance of understanding dis- equilibria. The paper starts with a classification of urban change processes by speed of adjustment and shows how equilibrium models fail to deal with them. It discusses options of modelling dynamics and argues for recursive or quasi-dynamics as a rational trade-off between theory and operationality in spatially disaggregate urban models. It illustrates this by comparing how three existing dynamic urban models address temporal dynamics and deal with feedback effects to produce results that capture the key features of urban dy- namics, such as price responses and their interaction with demand and supply choices, and points to applications of the three models in which the consideration of dynamics significantly added to understanding relevant policy issues. Keywords: Urban models, equilibrium, dynamics, modelling theory, modelling method, energy scarcity, climate change