For nearly 60 years, California led the U.S. in reducing on-road vehicle emissions. Because California's motor vehicle emission regulation preceded the Clean Air Act of 1970, California is delegated the authority to set vehicle emissions standards more stringently than the federal equivalent . In the present analysis, we model exposure concentrations for the years 2000 through 2019, during which California's regulatory agencies pursued an aggressive and interlinked suite of multi-pollutant policies to reduce emissions across the entire on-road vehicle fleet . Examples include requiring cleaner fuels and technological advancements (e.g., hybrid drivetrain, alternative fuel and propulsion technologies, advanced emissions controls) specific to light-duty, medium-duty, and heavy-duty vehicle classes (respectively: LDV, MDV, HDV).