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Executive Summary
Building new highways will
do little to alleviate traffic congestion in the long run and likely will exacerbate
already severe air pollution problems in metropolitan areas across the country.
Despite tougher limits on
tailpipe emissions and federal rules requiring that air quality play a role
in transportation decision-making, cars and trucks remain a leading source of
air pollution, particularly in urban areas, because of the dramatic rise in
vehicle-miles traveled (VMT).
• Today's vehicles
are 80 to 99 percent cleaner per mile than vehicles produced in the late 1960s.
However, since 1970, the number of vehicle-miles traveled nationwide has increased
by 159 percent, from 1.1 trillion in 1970 to 2.87 trillion in 2002, wiping
out many of these potential gains.
• VMT has increased
most rapidly in urban areas, where exposure to ozone smog and toxic air contaminants
can have the greatest impacts on health. Between 1970 and 2002, VMT on urban
roads and highways tripled from 570 billion to 1.73 trillion.
A growing body of evidence
suggests that expansion of the nation's highway network has helped fuel the
increase in driving. The expansion of highways triggers changes in driver behavior
and land use that spur additional vehicle travel—a phenomenon called "induced
travel."
To examine the link between
highways and air pollution, we analyzed data on highway capacity and vehicle
emissions for 314 metropolitan areas in the U.S. Key findings include the following:
• Per capita, cities
with more major highway capacity have higher levels of air pollution from vehicles.
The correlation holds for small (under 250,000 population), medium (250,000
to one million), and large (one million and up) metropolitan areas. In all cases,
the relationship between highway capacity and air pollution from vehicles is
highly significant. The probability that these two factors are directly correlated
is greater than 99.9 percent.
• The link between
highway capacity and air pollution from cars and trucks is strongest in America's
largest cities—those with at least one million people (see Figures ES-1
and ES-2.)
• All other things
being equal, this correlation suggests that an average large city that expands
its highway capacity by 14.6 percent—the national rate of growth in urban
areas during the 1990s—could expect a 10.9 percent increase in emissions
of nitrogen oxides (NOx) and a 10.7 percent increase in emissions of volatile
organic compounds (VOCs). Both pollutants contribute to the formation of smog,
and many VOCs are toxic to humans. Small and medium-sized cities could expect
emissions of each of these pollutants to climb by 2.1 to 5.7 percent.
To check the growth of vehicular
air pollution in metropolitan areas, state and federal officials should allocate
a greater share of transportation resources to programs to reduce growth in
the number of cars on the road and encourage alternative transportation modes
such as transit. In addition, federal and state law must ensure that new transportation
projects do not worsen air quality in metropolitan areas.
• The Transportation
Equity Act for the 21st Century (TEA-21) gives states flexibility to spend federal
transportation dollars to upgrade transportation alternatives and improve air
quality. However, many states have failed to use this flexibility and continue
to dedicate the lion's share of transportation funding to highway expansion.
Rather than investing billions in new highway capacity, states should move forward
with prudent, reasonable investments in transportation infrastructure while
working to reduce the growth in vehicle-miles traveled on our highways.
• The Clean Air Act
requires states to demonstrate that their planned transportation investments
will not cause or exacerbate violations of national health-based air quality
standards. State and federal officials must vigorously enforce this "transportation
conformity" requirement to ensure improvements in air quality in metropolitan
areas.
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