Which are the figures that can show how good a given urban structure is ? it is often heard that urban density is a relevant figure, and it is a reasonable assessment; the floor-area ratio (how many sq m or sq ft you get to stack over a given area) gives an illustrative figure, that can be easily compared between different contexts. But to agree on the thresholds between high, medium and low density is more difficult, as this depends on culture; as etiologists show that the distance a person accepts as a reasonable intimacy area when surrounded by a set of persons varies with culture (in many sparsely populated regions this value is somewhat high, while in overpopulated cities it is rather small, otherwise the underground would not be used…), the threshold for high density can vary even inside a single country. For instance, in Spain some regional planning laws limit the FAR to 1 (ratio of built floor surface to the development zone area), but some as the Basque Country (in which land uses compete for scarce valley plains) or Galicia (with a tradition of dense urban cores) allow much higher values. Even from an environmental point of view, density must be defined taking into account the carrying capacity of the site.

A second figure is also important: the occupation level of the already urbanized and serviced land. An airline whose occupation ratio were to be 50% of the potential tickets would strive to get profits, and the same happens to a city: maintaining the streets has a cost, usually covered by fiscal income, so no residents means a complex situation, as Detroit painfully reminds.

So density is about how many snails you have in a shell (well, usually one…), and occupation level tells how easily the bug can move the shell…