  Recent studies have revealed the importance of centrality measures to analyze
various spatial factors affecting human life in cities. Here we show how it is
possible to extract the backbone of a city by deriving spanning trees based on
edge betweenness and edge information. By using as sample cases the cities of
Bologna and San Francisco, we show how the obtained trees are radically
different from those based on edge lengths, and allow an extended comprehension
of the ``skeleton'' of most important routes that so much affects
pedestrian/vehicular flows, retail commerce vitality, land-use separation,
urban crime and collective dynamical behaviours.
