Sprawl reduces traffic only along the leaves of the commuting graph: few people live there, and no one commutes through the leaves to reach inner hubs. When a n

Sprawl reduces traffic only along the leaves of the commuting graph: few people live there, and no one commutes through the leaves to reach inner hubs. When a node stops being a leaf, more and more vehicles travel through it, which means increased congestion. It also means that people living on leaf nodes but who work in the hubs have to travel farther and farther, along more invreasingly congested nodes, than they do in their own remote neighborhoods. This is certainly the case in areas like New York (whose suburbs grew largely because Robert Moses liked cars more than trains, although he himself did not even drive) and San Francisco (which has a unique bi-directional commute traffic jam problem, where both the city and far-flung suburban sprawl nodes are job hubs). I don't have a link handy, but I believe the growth of Atlanta followed this exact pattern and became a classic case study of awful urban planning.