  Recent theoretical and empirical studies have focused on the structural
properties of complex relational networks in social, biological and
technological systems. Here we study the basic properties of twenty
1-square-mile samples of street patterns of different world cities. Samples are
represented by spatial (planar) graphs, i.e. valued graphs defined by metric
rather than topologic distance and where street intersections are turned into
nodes and streets into edges. We study the distribution of nodes in the
2-dimensional plane. We then evaluate the local properties of the graphs by
measuring the meshedness coefficient and counting short cycles (of three, four
and five edges), and the global properties by measuring global efficiency and
cost. As normalization graphs, we consider both minimal spanning trees (MST)
and greedy triangulations (GT) induced by the same spatial distribution of
nodes. The results indicate that most of the cities have evolved into networks
as efficienct as GT, although their cost is closer to the one of a tree. An
analysis based on relative efficiency and cost is able to characterize
different classes of cities.
