202 Urbanization models and the regions
around the city (e.g., many North American cities). But dispersed development
can also be combined with the bulges, greenbelt, transportation corridors, and
satellite cities patterns. Irrespective, the dispersed development pattern is associ-
ated with a massive fine-scale road net, which in turn is connected to the major
radial (and ring road, if present) transportation network.
Various factors affect whether development is dispersed or compact (Yaro
et al.1990,Troy1995,Theobold and Hobbs1998). For instance, the use of septic
systems for human wastewater facilitates dispersed residential development. In
contrast, in Australia, Germany and certain other nations, homes in a devel-
opment near a city cannot be built until after the sewer and other utilities
have been installed, an effective way to create compact growth and neighbor-
hoods. With a percieved financial gain, typically any town or municipality can
expand, whereas in Britain, to protect especially valuable land, many communi-
ties receive a government subsidy not to expand.
The much-discussedcauses of urbanizationbear mention since some strongly
affect the location and degree of development (Theobold and Hobbs 1998,
Hansen2002,Hall2002,Burgi et al.2004). Consider a large flood or cyclone
(hurricane) or earthquake that causes extensive building destruction (e.g., San
Francisco 1906, New Orleans 2005). The area destroyed is usually rebuilt, though
perhaps in a form better able to deal with a repeat disturbance. Some of
theresidents affected, however, may relocate to a more secure location out-
side the metropolitan area. Or consider rising sea level associated with climate
change. Low-lying urban areas near coasts may become largely inundated, caus-
ing some residents and businesses to relocate to the outskirts of the city (e.g.,
Bangkok, San Diego/Tijuana). Immigration may stimulate urbanization around
certain communities where arrivals from particular nations or cultures wish to
aggregate. Immigrants arriving without financial resources often live in squat-
tersettlements within a city or near its perimeter (Perlman 1976 , Main and
Williams1994), and may later move outward as land prices rise. Transportation
corridors, both radial and ring-road, are especially associated with urbanization.
Forinstance, adding a radial highway, or even new traffic lanes to an existing
one, is apt to produce an urbanized bulge in that direction. Constructing a ring
highway around a metro area facilitates concentric-zone or dispersed develop-
ment over extensive areas.Four urbanization models
Since the causes of urbanization vary widely and numerous recognizable
repeated patterns are produced by the process, modeling has been frequently
used to analyze such a complex system. A sampling of several types of models to
understand urban expansion is outlined below. Much of the literature focuses on