  Understanding urban growth is one with understanding how society evolves to
satisfy the needs of its individuals in sharing a common space and adapting to
the territory. We propose here a quantitative analysis of the historical
development of a large urban area by investigating the spatial distribution and
the age of commercial activities in the whole city of Rome. We find that the
age of activities of various categories presents a very interesting double
exponential trend, with a transition possibly related to the long-term
economical effects determined by the oil crisis of the Seventies. The
diversification of commercial categories, studied through various measures of
entropy, shows, among other interesting features, a saturating behaviour with
the density of activities. Moreover, different couples of commercial categories
exhibit over the years a tendency to attract in space. Our results demonstrate
that the spatio-temporal distribution of commercial activities can provide
important insights on the urbanisation processes at work, revealing specific
and not trivial socio-economical dynamics, as the presence of crisis periods
and expansion trends, and contributing to the characterisation of the maturity
of urban areas.
