University of California Transportation Center

Recent data indicate that many U.S. areas still fail to meet national ambient air quality standards (NAAQS) for one or more criteria pollutants. In 1990, ninety-six U.S. metropolitan areas violated the federal ambient ozone standard, and forty-one areas violated the standard for carbon monoxide (CO), affecting over 100 million people (U.S. EPA, 1990a). The federal Clean Air Act (CAA) Amendments of 1990 established more stringent control measures to further reduce air pollutants (U.S. EPA, 1990b) in an effort to attain air quality standards.