Highway vehicles are responsible for a declining share of pollutants in the US, according to data gathered by the US Department of Energy (DOE).
More than half of all carbon monoxide (CO) emissions in 2002 were from highway vehicles; by 2016 that fell to 30%.
The share of nitrogen oxides (NOx) emissions from highway vehicles declined from 43% of all NOx emissions in 2002 to 34% in 2016.
The highway share of volatile organic compound (VOC) emissions declined by 9% during this same period.
Highway vehicles contributed less than 3% of all particulate matter (PM) emissions.