Urban dynamics was studied during the eighties in terms of
the so-called ‘urban life cycle theory’ which was launched by the
Vienna group within the CURB (costs of urban growth) project.
The main elements of this theory, as well as its limits, are already
well known. It provides an elegant way of describing urban trends
and dynamics by interpreting them as the result of a natural process
of physical diffusion of urban population from core areas to the
periphery. The opposite movement, from periphery to core, is explained
on the other hand by the existence of scale diseconomies.
However, some aspects remain unresolved: for example, is there a
city size threshold at which the process of decentralisation starts?
At what city size do scale economies become scale diseconomies?
At what point in time does this happen? Should we expect cities
like Milan and Rome to grow to the size of London and Paris before
they start to decline?
Many empirical analyses have proved the existence of an optimal
city size through the measurement of economies or diseconomies
of scale, generally applied either to the costs of urban services
or to elegant econometric estimates of urban and sectoral production
functions. But, unfortunately these studies have never produced
a common result, and have often been subject to criticism
for their restrictive hypotheses.