The suite of regulations that comprise California's mobile source strategy has resulted in large aggregate reductions of emissions of multiple pollutants from diverse fleets that make up the state's on-and off-road vehicles . Here, we examine how changes in on-road vehicle emissions from 2000 to 2019 have impacted exposure to PM2.5. Over this time period, on-road emissions have been shaped by several aggressive state regulations targeting specific vehicle fleets, including California Air Resources Board's (CARB) Light-Duty Vehicle Emissions Standards, Advanced Clean Cars, and the Truck and Bus Regulation . Despite statewide and fleetwide on-road vehicle miles traveled increasing ~24% -from 292 billion (2000) to 364 billion (2019) -emissions of the four species that principally drive population-weighted PM2.5 exposures from on-road vehicles have decreased. Regulatory emissions data indicate reductions of ~70% for primary PM2.5, nitrogen oxides (NOx) and volatile organic compounds (VOC), while ammonia (NH3) decreased ~15% (Fig. ) . Notably, non-exhaust primary PM2.5 emissions (e.g., brake-and tire-wear) have increased by ~20% over this time period, causing the relative non-exhaust share of primary PM2.5 to increase substantially (14% to 50% from 2000-2019) . Diverse measurement and observational datasets (see SI) corroborate overall declining emissions of PM, NOx, VOC, NH3, and other key traffic-related air pollutants (TRAPs) . Considering all species that contribute to total PM2.5, California's on-road emissions reductions outpaced the national aggregate, especially for NOx and VOC (51).