Built areas 177present in North America and elsewhere (Trocmeet al.2003,Iuellet al.2003,
Formanet al.2003,van Bohemen 2004). The best designs facilitate crossing by
targeted species as well as a large portion of the fauna. Local residents and
walkers on regional trail systems often also use the crossings. Wildlife-crossing
structures mitigate the barrier and habitat-fragmentation effects of highways,
and enhance regional connectivity for movement among natural areas. Thus
major wildlife-crossing structures are apriorityin the outer urban-region ring
to facilitate regional wildlife movement, walking on regional trail systems, and
connectivity for local residents.
Built areas
For built areas the following groups of patterns and results are
highlighted: (1) green patches and corridors within metropolitan areas; (2) metro-
politan-area form; (3) evidence of regional planning; (4) satellite cities; (5) towns
in the urban-region ring (or urb-region ring); and (6) ‘‘natural” disasters.
Green patches and corridors within metropolitan areas
[A1]All combinations of high, medium, and low densities of green patches and
greencorridors are present within the metropolitan areas. Most common is a high density
of patches and low density of corridors; a sixth of the metro areas has a low density of
both greenspace types(Figure7. 7).
Small and medium greenspaces dispersed over a metro area provide nearby
access to nature and recreation for residents. A high density provides stepping
stones for many species to move across built areas, and even tiny patches can
be effective for such movements. Linear green patches are corridors that fur-
ther enhance species movement, especially in the direction a corridor is ori-
ented. Green corridors also line and protect streams, though in metro areas
most streamwater is in underground pipes so the green corridors are over for-
mer streams. In the urban regions greenspace patches are considerably more
abundant than corridors.
Increasing the number and area of parks in metropolitan areas with little
greenspace per person is a valuable goal. Mapping the average distance between
housing units and parks (e.g., as in Chicago and London) pinpoints priority areas
fornewparks. These internal greenspaces facilitate accessibility of people to
nearby parks, but do not provide connected greenspace accessibility for walking
and bicycling to greenspace outside the metro area, a much-valued weekend
activity (e.g., around Dutch cities).
[A2]About 30 % of the metro areas lack a greenspace≥1km^2 (250 acres), whereas about
afifthofthe metro areas has a≥10 km^2 (4 mi^2 )greenspace present(Figure7. 7).