Before revision:  To help unveiling this spatial dependence of the betweenness , we introduce a simple model composed of a grid connected to a set of tree graphs. This structure , coined as the GT-model, allows us to analytically describe in terms of betweenness, how and why congestion emerges in particular geographical areas of monocentric cities .
After revision:  To help unveiling this spatial dependence of the betweenness , we introduce a simple model composed of a dense urban center connected to an arboreal periphery. These models , coined as the GT-model, allows us to analytically describe in terms of betweenness, how and why congestion emerges in particular geographical areas of monocentric cities .