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trip. That pollution could be eliminated by adding
heaters to autos' catalytic converters.38
· Operating speed: Cars emit far more of most pollut-
ants at slow speeds--0 to 20 miles per hour--than at 20
to 55 mph. The effect is most pronounced for CO and
VOCs. NOx (which contribute to smog) decline from 0 to
20 mph but then increase as speeds rise above 25 mph,
although the increase is not significant until speeds
reach 40 mph.39
· Density: Some pollutants, particularly CO, dissipate
quickly in the atmosphere and present health problems
only where automobile traffic is extremely dense or
congested.
One way to reduce automotive air pollution is to get
people to reduce their driving. New Urbanist planners hope
to do that by attracting people, especially commuters, to
other modes of travel such as walking, cycling, bus, and
rail. Their plans include several elements aimed at reach-
ing that goal:
· Higher population densities so mass transit will be
more feasible;
· Mixing housing and commercial uses so people will be
better able to walk or bicycle to work or shopping;
· Pedestrian-friendly designs aimed at making commer-
cial areas more amenable to people on foot and bicy-
cles;
· Transit-oriented designs, particularly higher density
residential developments around rail stations and other
transit centers and along transit corridors; and
· Construction of rail transit facilities to connect
high-density job and residential centers.
As attractive as those ideas may sound on paper, in
practice they fail to fulfill most of their promises. At
best, New Urban plans will slightly reduce the share of
trips people make by auto and even more slightly reduce the
distance they drive. Those reductions come about not be-
cause New Urbanism makes walking or transit more attractive
but because it increases congestion and parking costs so
much that cars are less attractive. Congestion, in turn,
increases many forms of pollution, particularly those depen-
dent on speed and density.