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162 � Transportation Statistics Annual Report 1996
are based on information collected within 30 days
of the incident, which affects the damage figures.
Damages that are not assessed until later are not
included, which potentially overlooks costly environmental
remediation.
Land-Use and Habitat
Effects of Transportation
Transportation has important direct and indirect
impacts on land use and environmental
habitats: 1) land is used directly for transportation
infrastructure, and 2) other development
may be stimulated by the construction or expansion
of transportation infrastructure. Both of
these transportation-related forms of development
have the potential to reduce and/or fragment
wildlife habitat and disrupt ecosystems.
� Land Requirements for
Transportation Infrastructure
The transportation infrastructure of the United
States is extensive. The highway system reaches
all but the most remote and unpopulated parts of
the country; most small cities have a rail line
passing though them, and airports of all sizes are
common throughout the United States. In addition,
ports, intermodal facilities, pipeline facilities
(where such facilities are above ground), rail
yards, and other transportation infrastructure
also require land.
According to one estimate, the land devoted to
roads in 1991 totaled approximately 20,627
square miles, and land devoted to parking ranged
from 1,910 to 3,035 square miles. (These estimates
include only the paved portion of the highway.
Other portions of highway right-of-way
such as land between divided Interstates and the
cleared areas outside the shoulders of highways
are not included.) (Delucchi 1995)
Like other forms of development, transportation
infrastructure—through direct use of the
land and through land fragmentation—reduces
the amount of land usable by wildlife and occupied
by vegetation. Currently, national-level information
about the cumulative impacts of this
infrastructure on ecological systems is very limited.
Most transportation-related impacts on
habitats are studied at the local level. The federal
government requires that Environmental Impact
Statements be prepared for federally funded
development projects that could have a significant
impact on the environment. These statements
address impacts on mammals, birds,
aquatic life forms, endangered species, air quality,
water quality, ambient noise levels, cultural
and historical sites, and other aspects of the
environment. The environmental impact statement
process does not prohibit development nor
require any specific decision, but relies on full
disclosure to identify and assess impacts. Mitigation
measures must be considered to minimize
impacts and a monitoring program adopted
where appropriate.
� Habitat Fragmentation
Transportation infrastructure can form barriers
to wildlife, especially highways and rail lines.
While highways and rail lines do not occupy
much of the area of the land they traverse, their
linear nature divides wildlife habitat into smaller,
more isolated units of land or creates barriers
between functional areas. Impacts such as traffic
noise, emissions of xenobiotic substances, artificial
lighting, and vehicle-fauna collisions also
affect species in fragmented areas. Mitigation
measures, such as lowering speed limits to
reduce noise levels, erecting warning signs and
fences to avoid collisions, and constructing ecoducts
(fauna bridges and tunnels) to allow freer
movement of fauna between isolated fragments
are sometimes used. Also, countries such as the