Recent events have shown that our agglomerations are
		vulnerable in front of emergency situations. There are a lot of factors
		to consider and one of them is the structure of towns generated by road
		networks. One way to study these structures is to modelize the networks
		by graphs and to use the theory developed on them. Recently, some works
		have studied the de- tection of communities in large graphs. A
		community in a graph is a set of connected nodes strongly connected to
		each other and less connected to nodes from other communities. The aim
		of detection is to identify commu- nities according to a predefined
		criteria but without specifying the number of communities and the size
		of communites. The known algorithms find com- munities but without
		taking into account the time. We propose a dynamic algorithm of
		communities detection and we explain how it can be applied on Le Havre
		agglomeration, for example, to estimate the vulnerability related to
		the road network use by vehicles.