In the thirty years of postwar prosperity – known as the Trente Glorieuses – the construction of new towns and cities brought together, in a single metropolitan project, many different goals involving regional housing plans, mobility and competitiveness. The “mechanics of size”, which governed the development of new towns and cities, was based on the cult of performance and the cult of the “grand design”, written in stone. These logical development systems still exist today, but the operational context has nevertheless evolved.

Like metropolitan developments, planning thus has to initiate a transition: a shift from the design of a looped transport network to that of a gridded and interconnected system; encouraging a model of territorial specialization to evolve towards greater integration; seeking forms of urbanistic potential in the intensification of certain intermediary areas, getting beyond the centrality/periphery contrast and the precedence given to notions of contiguity, density, and centre. Discovering today’s “new cities” calls for regarding the metropolis as an open system, organizing territories and networks. The sphere of action of this openness lies in these intermediary situations and these metropolitan areas, where projects cannot be contained within perimeters. To do this, the actual planning method needs to be reformed.

But how? Based on what models? The Scenarisation method proposes an adaptable model which creates a link between long time (the usual planning horizon) and present time, which takes external ups and downs into account and permits a retroactive re-appraisal of the metropolitan narrative based on its developments. This study offers a scenarisation example of the future of the Ile-de-France in four seasons, anticipating the endogenous innovations, societal changes and impact of major projects.