208 Urbanization models and the regions
by50 %, 100 %, and 200 %, respectively). Urbanization of course produces quite
different patterns around different cities, and tends to occur in pulses rather
than at a constant rate. The three stages or levels of urbanization chosen are
based, in part, on several US and Canadian cities that showed a 1.5 to 2 times
increase in built area in a recent two-decade period (Formanet al.2003). Another
consideration was the growing interest in raising the time horizon for urban
plans from the characteristic 3, 5, 10, or 20 yr to the decades-to-generations
timescale of sustainability.
Additional model assumptions are important. All land, including mountains,
parks, military bases, and large wetlands, can be urbanized; only major water
bodies cannot be urbanized. Only two types of land cover exist in the models,
built space and greenspace. No infill building on greenspace patches or corridors
enclosed in a metropolitan area occurs. A final assumption applies to cities with
green wedges, here defined as greenspaces that project into a metropolitan area
at least one-third of the radial distance to the city center. Since a strong reason
normally exists for the persistence of a wedge, in the models existing greenspace
wedge areas remain and cannot be urbanized. However, wedges are not extended
outward as urbanization expands, so in certain models development beyond
green wedges leaves them as elongated greenspace patches within an expanded
metro area.
The extreme complexity of existing greenspace boundaries and patchily dis-
tributed development around the 38 worldwide urban regions precluded the
feasibility of precise computer mapping of mosaic sequences on the region maps
(Color Figures2--39). Consequently, precise areas were calculated for each model,
and approximate areas and boundaries were hand-drawn on maps derived from
the Color Figures with sites (pictograms) and most land covers removed. This
provided an efficient and effective way to directly visualize and compare the
four broad urbanization models.
The mosaic sequences of the concentric, satellite, transportation, and dis-
persed urbanization models (Figure8.2)tobeused below are outlined as follows.Concentric-zones model mosaic sequence
All urbanization, except as noted above for green wedges, is adjacent to
and evenly distributed around the metropolitan-area border. In the first stage of
theconcentric-zones model (reaching 1.5 times the initial size of metro area),
urbanization ‘‘parallels” the broad outline of major built lobes and intervening
greenspaces (which are wide, irrespective of how far outward or inward they
project) of the metropolitan area. Finer-scale lobes and coves of the metro-
area border are ignored. In the second and third stages (2 times and 3 times,