In McCarthy v. Thomas, the Ninth Circuit held that the Clean Air Act (hereinafter "CAA") authorizes the Environmental Protection Agency (hereinafter "EPA") to conditionally approve a State Implementation Plan (hereinafter "SIP"), thereby binding the states even if later EPA actions do not specifically reference the earlier conditionally approved provisions. In McCarthy, the SIP which included transportation measures for Pima and Maricopa Counties in Arizona, had never been fully approved by the EPA, but had received a conditional approval. Since Arizona had difficulty complying with the CAA, the SIP was revised several times before the EPA granted full approval. The final plan, approved years later by the EPA, did not specifically include the transportation provisions originally provided by the state. The McCarthy decision puts the states on notice that commitments made in SIPs are legally binding notwithstanding the SIP is only conditionally approved. When a state proposes changes to its SIP, it does not get a new "bite of the apple," but must adhere to measures already accepted by the EPA.
Edward P. Murphy,
Environmental Law - McCarthy v. Thomas: Are States Bound When Approval Of An SIP Is Merely Conditional?, 25 Golden Gate U. L. Rev.