The air pollution problem in many U.S. metropolitan areas is quite serious. An increasing number of them are out of compliance with the Federal clean air standards mandated under the Clean Air Acts of 1970, 1977 and 1990. A significant proportion of air pollution (averaging close to 60 percent, and as high as 88 percent for carbon monoxides) is accounted for by automobile emissions (and the indirect repercussions of automobile use such as PM10 [particulate matter of less than 10 micrometers in diameter emissions associated with highway construction, paved road dust, tire friction, etc.]).¹ Yet in Los Angeles, the long-term national leader in air pollution especially with respect to ozone², there have been major declines in automobile emissions since the mid-1970s when California's emission control mandates first began. In some other metropolitan areas, air quality has deteriorated because the effects of rapid population and employment growth and soaring VMT have not been dampened by California-level emission controls.

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