Interactions between cities intensified and developed at larger scales during the second half of the twentieth century, under the impetus of lower transport costs, the growth of global economy and technological progress. The diffusion of the automobile has been spectacular, from the 1930's onwards. In this context, relations between cities are changing: in the 1950s, insists Gottmann emphasized the existence of strong economic interactions between decision centers and production centers within an integrated region from Boston to Washington called “megalopolis” (Gottmann, 1957). The second half of the twentieth century was marked by the increase in volume and diversity of flows of goods and people between cities. The transition described here, «.. large, sudden and long-lasting changes in structure and function” according to Scheffer et al (2012), is the emergence of polycentric cities "Mega-Cities" (Hall & Pain, 2006). It corresponds to a change of scale in the operation of metropolitan areas. Networks and flows that can be called long distance in the first regime transformed in terms of operation and exchanges between places, new entities, called "megacity regions", "mega-regions", "megalopolitan regions", "Regional galaxies, "or" regional states "(Soja, 2011). The long distance becomes intra-urban in the second regime. As Soja points out (2011), "The urban, the metropolitan, and the subnational-regional scales seem to be blending together". The approach presented here combines empirical observation and theoretical modeling situations, to study several explanatory factors candidates to this transition (exogenous factors related to demographics, changing economic context and endogenous factors such as interactions social and governance modes). The MetropolSim model (Le Néchet, 2012), which is integrated model LUTI (Land Use Transport Interaction) and network dynamics, agent-based, is extended. This model allows us to test the relative influence of possible factors explaining the evolution "polycentric mega-cities" transition.