In September 2019, the National Highway Traffic Safety Administration (NHTSA) and Environmental Protection Agency (EPA) finalized the the Safer Affordable Fuel-Efficient (SAFE) Vehicles Rule Part One: One National Program. In the SAFE Rule Part One, EPA revoked California's authority to set its own greenhouse gas emission standards and zero-emission vehicle mandate under the Clean Air Act. One basis for EPA's action was a new rule by NHTSA interpreting its authority to set fuel efficiency standards as preempting states' authority to regulate greenhouse gas emissions from cars.
On April 22, 2021, NHTSA proposed repealing that rule. NHTSA's action sets the stage for EPA to restore California's authority to set its own more climate-protective auto emission standards.