Section 176(c) of the federal Clean Air Act establishes transportation conformity requirements that are intended to ensure that transportation activities do not interfere with air quality progress. Transportation conformity links and coordinates transportation and air quality planning processes through motor vehicle emissions budgets (budgets) that are established in State Implementation Plans. Once budgets are established in a State Implementation Plan and approved or determined adequate by U.S. EPA, subsequent transportation plans and programs produced by transportation planning agencies must demonstrate that the emissions from the plan or project do not exceed those budgets and thus “conform” to the State Implementation Plan.
For San Diego County, CARB submitted a State Implementation Plan to U.S. EPA in 2021 for the 75 and 70 parts per billion 8-hour ozone standards that included budgets developed using the California on-road mobile source emissions model, EMFAC2017. For San Joaquin Valley, CARB submitted a State Implementation Plan to U.S. EPA in 2017 for the 75 parts per billion 8-hour ozone standard that included budgets developed using the EMFAC2014 model. In 2021, California released a new version of the on-road mobile source emissions model, EMFAC2021 that is now required for transportation conformity determinations. To align the budgets with EMFAC2021, CARB is proposing to use EMFAC2021 to update the budgets for the San Diego County and San Joaquin Valley nonattainment areas. This update to the budgets will allow future conformity determinations to be made using EMFAC2021.
On March 27, 2025, the Board will consider the adoption of the 2025 Updates to Motor Vehicle Emissions Budgets for California Ozone State Implementation Plans. If adopted, CARB will submit the 2025 Updates to U.S. EPA as a revision to the California SIP.
