The street-network efficiency of tens of British cities in relation to transport fuel consumption
and CO2 emissions are analyzed. The results show a strong linear positive correlation
between length entropy and average street length, and a negative correlation between
entropy and street density. Also, the results suggest that in a large city the street network
is used more efficiently than in a small city, as indicated by the sublinear relations between
city size (population) and the number of streets, total length of streets, and the area covered
by the street network. The sublinear relation means that these parameters grow more
slowly than the city size. By contrast, because a larger fraction of the street network is used
at close to full capacity during considerable part of the time in a large city than a small one,
the fuel consumption and the CO2 emissions show a linear relation with city size and
superlinear relation with total street length. The superlinear relation means that the CO2
emissions increase faster than the total street length, a measure of the network size. Thus,
large cities may be less energy efficient and environmentally friendly than small cities. In
every city the street network needs to interconnect all the buildings, which requires a certain
minimum size. In a small city, however, the network is used to a low capacity most of
the time so that its relations