  In this paper, we derive a topological pattern of urban street networks using
a large sample (the largest so far to the best of our knowledge) of 40 U.S.
cities and a few more from elsewhere of different sizes. It is found that all
the topologies of urban street networks based on street-street intersection
demonstrate a small world structure, and a scale-free property for both street
length and connectivity degree. More specifically, for any street network,
about 80% of its streets have length or degrees less than its average value,
while 20% of streets have length or degrees greater than the average. Out of
the 20%, there are less than 1% of streets which can form a backbone of the
street network. Based on the finding, we conjecture that the 20% streets
account for 80% of traffic flow, and the 1% streets constitute a cognitive map
of the urban street network. We illustrate further a peculiarity about the
scale-free property.
