Findings at the national level reveal that MDV and HDV traffic have a disproportionately large impact on emissions exposure relative to their traffic volume. For instance, despite MDVs and HDVs accounting for only 4.7% and 7.4% of total average daily VKT in the U.S., respectively, they contribute 19.6% and 26.7% of average NOX exposure, and 24.8% and 25.6% of average PM2.5 exposure (Table ). This indicates that emissions from MDV and HDV traffic collectively contribute to approximately half of the total vehicle emissions exposure in the U.S.