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Angeles recorded 208 unhealthful days in 1985
but only 136 in 1994. Similarly, over the same
period the number of unhealthful days in New
York dropped from 65 to 8, in Pittsburgh from 9
to 2, and in Phoenix from 88 to 7. (Unhealthful
days are those in which EPA’s pollution standards
index exceeded 100.)
This progress notwithstanding, the most
recent evidence shows that emissions reductions
are slowing and, barring any major
changes, will soon be overwhelmed by travel
growth. Some have suggested, therefore, that
policies are needed to reduce travel (such as
employee trip reduction) or to shift people from
automobiles to other modes (by improving transit
or bicycle/ pedestrian facilities). An examination
of these policies, collectively known as
transportation control measures, shows that
they tend to have a small positive impact on
reducing emissions, but are often expensive in
comparison with technology-based solutions.
The most effective TCMs include parking fees
and congestion pricing.
Longer term measures—highway construction
and land-use planning—are also considered
TCMs. Highway construction, or the decision
not to build, affects the volume and distribution
of traffic, and therefore affects emissions.
Elements of land-use planning may also affect
travel patterns and emissions, because overall
development may affect trip lengths, mode split,
and congestion. And at a more localized level,
transit-oriented development may be a means to
reduce the dominance of single-occupant vehicles.
The impact of these longer term TCMs on
emissions and air quality, however, is uncertain.
Few places have attempted to implement landuse
planning as a TCM, thereby limiting direct
empirical analyses. Moreover, analytical modeling
of the long-term impacts—of highway construction,
for instance—is generally
inconclusive because it can only address the
direct, short-term impacts on vehicle-miles traveled,
and even then imperfectly.
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While the evidence suggests that individual
TCMs have relatively modest impacts, complementary
TCMs, taken together, may have higher
impacts than current research indicates. Hence,
TCMs could play an important role in the longterm
strategy for emissions reduction. Also, it
should not be assumed that the current preferences
of Americans, such as the preference of
many for large suburban lots and single-occupancy
vehicle transportation, will continue
indefinitely. Demographic changes and changes
in tastes and attitudes may increase the popularity
of options like transit-oriented development
at some future date.
International Comparison of
Transportation and Air Pollution
The transportation community faces the
major challenge of curbing air pollution and
greenhouse gas emissions, especially carbon
dioxide (CO 2) stemming from worldwide
growth in motor vehicle use. The world’s motor
vehicle fleet has more than doubled since 1970,
exceeding 615 million vehicles in 1993. Over
three-quarters of these vehicles are owned by
citizens of the most affluent countries of the
Organization for Economic Cooperation and
Development (OECD).
The United States and several other countries,
mostly in the OECD, have reduced vehicular
emissions rates with catalytic converters
and cleaner fuels. As a result, OECD’s share of
several kinds of air pollution from transportation
is less than its share of all vehicle-miles
traveled (more than 70 percent). In 1990,
OECD countries accounted for 48 percent of
CO, 59 percent of VOCs, and 64 percent of
NOx emitted by the global motor vehicle fleet.
There is growing uncertainty, however, about
whether technological solutions to pollution,
by themselves, will be sufficient to keep up
with future growth in transportation demand in
OECD countries.