Traffic-generated compounds impact air quality through three major pathways: vehicle running emissions of gaseous and particulate compounds, secondary formation during plume transport of gases and particles, and mechanical processes that abrade particles from brakes, tires, and the road surface. Carbon monoxide (CO), nitrogen oxides (NOx), and volatile organic compounds (VOCs), as well as particulate matter (PM) constituents such as polycyclic aromatic hydrocarbons and black carbon, are among the numerous compounds that have been identified at elevated concentrations near large roads.
Emission reduction programs implemented by government agencies throughout the world have significantly reduced emission rates of air pollutants from motor vehicles. Since 1970, average per vehicle emissions in the United States have been reduced by more than 90% for VOCs and 80% for PM10 and NOx.9,10 In spite of these reductions, motor vehicles still significantly contribute to pollution in urban areas, often due to large increases in vehicle use offsetting per vehicle emission reductions. Furthermore, emissions from some vehicle associated sources (e.g., brake and tire wear) are not regulated, and pollutants generated from these sources may also increase in the future with increased vehicle use.
Populations near roads are exposed to this mixture of primary emissions and secondarily formed pollutants. Approximately 30–45% of urban populations in the United States are likely exposed to elevated pollution levels near roads.4 In many countries with densely populated urban areas, this figure is likely higher.