  Urban morphology has presented significant intellectual challenges to
mathematicians and physicists ever since the eighteenth century, when Euler
first explored the famous Konigsberg bridges problem. Many important
regularities and scaling laws have been observed in urban studies, including
Zipf's law and Gibrat's law, rendering cities attractive systems for analysis
within statistical physics. Nevertheless, a broad consensus on how cities and
their boundaries are defined is still lacking. Applying an elementary
clustering technique to the street intersection space, we show that growth
curves for the maximum cluster size of the largest cities in the UK and in
California collapse to a single curve, namely the logistic. Subsequently, by
introducing the concept of the condensation threshold, we show that natural
boundaries of cities can be well defined in a universal way. This allows us to
study and discuss systematically some of the regularities that are present in
cities. We show that some scaling laws present consistent behaviour in space
and time, thus suggesting the presence of common principles at the basis of the
evolution of urban systems.
