19 2Built systems, built areas, and whole regions
protecting soils and locations close to the metropolitan area that are especially
valuable for market-gardening (Chapter3)isanimportant exception.[R4]In nearly 40 % of the urban regions, agriculture (cropland or mixed crops/woods) is
more prominent close to the metropolitan area than across the urban-region ring as a
whole, where natural land (desert, grassland, or forest/woodland) usually predominates
(Figure7. 15).In these regions with somewhat limited agricultural land, urbanization
spread of the metropolitan area mainly covers this valuable land. Experience
from some urban regions indicates that, since the original community began by
prime agricultural soil, urbanization over time may cause a significant loss of
theregion’s best soils.
Urbanization spread from satellite cities and towns in the urban-region ring,
on the other hand, is more likely to degrade natural land. As described above,
this may be quite significant depending on the amount and location of natural
land.Unique features near metro-area borders
[R5]Almost all metropolitan area border lengths are about 35 to 350 km (22--
220 mi) long, though three exceptions have much longer convoluted borders(Figure7. 16).
Theborder lengthof a metropolitan area is a rough overall index of how much
surrounding greenspace is degraded by the metro area. Cities with populations
from 260 000 to over 10 million almost all have metro-area borders within a
single order of magnitude. Three outliers, Chicago, Atlanta, and Philadelphia,
have much longer boundaries, in part because of their convolutions of major
built lobes and green wedges, and in part because of sprawl. The lobes generally,
but not entirely, follow major transportation routes. The greenspace wedges
generally,but not entirely, follow major stream or river corridors. Extensive low-
density ‘‘unsatisfactory” outward urbanization spread, or sprawl, has pushed the
overall metro-area border on flat or gently rolling terrain far outward from the
city center. A greenbelt (e.g., London) or urban growth boundary (Portland) has
been used to arrest further sprawl and its reverberating impacts.[R6]Almost all metropolitan areas have at least one unique natural-system-related feature
close to its border, while a quarter of the metro-areas have three or more such features
nearby(Figure7. 16).These diverse unique natural-system-related features (e.g., a scenic viewpoint,
historical site, geologic feature) are threatened by existing human activities,
as well as by potential urbanization spread near almost all metro areas. This is
particularly evident by large cities. Identifying and protecting areas around these