Controlling air pollution from automobiles has proved to be one of the most difficult objectives of the Clean Air Act to achieve.¹ Motor vehicle emissions account for a large percentage of the total concentrations of hydrocarbons, carbon monoxide (CO), photochemical oxidants (ozone), and lead in urban air, and most of the 200 largest cities in the country have health-threatening levels of at least one of these pollutants in their air.² Nonetheless, attempts to control these emissions have generated intense political controversy.

The Clean Air Act establishes a dual strategy to reduce such pollution. It requires installation of emission controls on new vehicles to reduce emissions per vehicle mile³ and it provides a variety of transportation control measures to reduce total vehicle miles traveled.⁴ Central to the success of emission controls is the vehicle in-use inspection and maintenance (I/M) program.⁵ However, the Environmental Protection Agency's (EPA's) early attempts to require the states to implement transportation control plans and I/M programs were frustrated by staunch local resistance and largely successful court challenges. In the 1977 Amendments to the Clean Air Act,⁶ Congress provided EPA with powerful new authority to prod the states into action while attempting to avoid the constitutional infirmities of the existing program.