Growth of cities
In the absence of geographical constraints, such as terrain or oceans, cities typically have patchy, irregular, shapes. At first an aggregation system (see page 331) might seem to be an obvious model for their growth: each new development gets added to the exterior of the city at a random position. But actual cities look much more irregular. Most likely the reason is that embedded within the cities is a network of transportation routes, and these tend to have a tree- or vein-like structure (though not necessarily with a single center)—with major freeways etc. as trunks. The result of following this structure is to produce a much more irregular boundary.