Theme development : “Toward integrated modelling of urban systems” 

The French Scientific Network “Urban Modelling”, on the initiative of this symposium, includes stakeholders of the sustainable city, from upstream research to practitioners, with an objective of removing barriers between disciplines and between stakeholders. 

The majority of the world population live now in urban areas which are quickly extending. This phenomenon raises questions in several dimensions : the economic one (efficiency of the city and costs for its stakeholders), the social one (cohesion, segregation and disparities of access to the amenities) and the environmental one (energy consumption, greenhouse gas emissions, loss of biodiversity, land artificialisation). The simultaneity of problems calls for an integrated approach into urban policies, their evaluation and consequently the modelling of urban processes. 

These processes involve spatial and temporal dynamics on various scales, whether short-term (less than an hour in transport networks) or long-term ones (several decades), unpredictable breaks or smooth urban transitions during one or two decades, the transformations of single districts or whole urban areas. A first issue thus relates to the characterization of the relevant spatial and temporal scales to handle urban phenomena. 

A second issue relates to the linking of various spatial and temporal scales between models (aggregate / disaggregate, micro / macro, static / dynamic). It is now a commonplace to say that the city is a complex system and the sciences of the complexity contribute today to the modelling of parts of this system by a bottom-up approach. However, how to reconcile the bottom-up models (which make emerge collective behaviour as in the case of multi-agents models) with the top-down models which are relevant to represent certain macroscopic processes ? Besides, the architectures of models become more and more complex by combining an important number of components. How to ensure a consistent calibration of the whole ? 

The third issue follows from the emergence of “big data”, as for example GPS data allowing to locate mobile items (vehicles or human beings equipped with smartphones), and more generally various data of consumption susceptible to be made available in the near future (for instance the intelligent meters of electricity, e-ticketing and e-money). How does this phenomenon contribute to renew the problem of the urban data collection ? How can these data complete the richer data traditionally collected during heavy cross-section surveys ? 

Finally, these new data may be available real time, modifying the mode of management of the city and challenging the traditional models of planning. Besides, with this injunction to real time, the models also have to lose their black box aspect and improve their legibility in order to present the results in a way which can be communicated to the decision-makers, even to the citizens, and, why not, turning them out into tools of interactive exploration placed at the heart of the public debate.