This Ninth Circuit decision arose in the context of a private enforcement lawsuit filed by environmental advocates under the Clean Air Act (CAA) to successfully challenge EPA’s approval of the South Coast Air Quality Management District (SCAQMD) State Implementation Plan (SIP) based in part on the failure of SCAQMD and EPA to calculate and require appropriate reductions in vehicle miles travelled (VMT). The Ninth Circuit decision threatens to disrupt federal highway funding and transportation projects because the CAA requires the Department of Transportation (DOT) to cut off such funding and halt such projects for nonattainment areas that do not have EPA-approved air quality improvement plans. The Ninth Circuit vacated EPA’s approval of the SCAQMD plan for Southern California, but nonattainment areas across the country will be at similar risk, as no nonattainment areas nor EPA itself has adopted or approved air quality plans that included the VMT calculations and reduction mandates that the Ninth Circuit has now concluded are statutorily required. The decision overturns a longstanding U.S. EPA interpretation of the Clean Air Act which held that there were no “growth in emissions” to be offset if aggregate total motor vehicle emissions were going down, due to fleet turnover, tailpipe standards and alternative fuels. Instead, the Court held that “growth in emissions” refers to the total emissions that are due solely to the increase in vehicle miles traveled, regardless of whether aggregate emissions are down or not. Consequently, the emissions reduction goal is calculated by looking at what total emissions are, minus what they would have been if there had been no growth in vehicle miles traveled. Those emissions would then have to be offset through transportation control measures, not tailpipe controls. U.S. EPA and the petitioners in this case agreed that vehicle miles traveled increased over 30 percent. CARB calculations indicate that the required emission reductions under the Court decision would be equivalent to a no-drive day each week for 1/5 of the Basin population. Alternatively, AQMD staff estimated that the required reduction could be 70 tons per day NOx, depending on the baseline. The Court also held that AQMD must submit a plan demonstrating attainment of the now revoked 1-hour ozone standard.