To the apparent dissatisfaction of all the parties involved, the Supreme Court on May 2, 1977, refused to resolve the issue of whether the Clean Air Act empowers the Environmental Protection Agency (EPA) to require states under pain of federal enforcement sanctions to implement EPA-drafted transportation controls as part of their air quality implementation programs. Seizing upon an admission in EPA's brief that the Agency's transportation control regulations require substantial modification, the Court, in a per curiam opinion,¹ simply vacated and remanded for consideration of mootness decisions by the Fourth,² Ninth,³ and D.C. Circuits,⁴ which held the regulations illegal for exceeding EPA's statutory authority and suggested that they violated the states' Tenth Amendment rights as well. The Court's failure to reach the merits of the issue basically continues the legal uncertainty that has stymied progress in the implementation of urban transportation controls, which are necessary for meeting the health-related primary ambient air quality standards in many major metropolitan areas.