IN NOVEMBER 1990, President George Bush signed into law the Clean Air Act Amendments of 1990, far and away the most sweeping and expensive clean air legislation in the nation’s history. In many respects, the Amendments followed the well-traveled path of “command and control” regulation laid out by the original 1970 Clean Air Act and its 1977 Amendments. Once again, Congress tightened emissions standards for industrial plants and automobiles (“stationary” and “mobile” sources in the terms of the trade). Once again, Congress established purportedly ironclad statutory deadlines by which cities must attain the stringent ambient-air-quality standards mandated by the Act.