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The Congestion Mitigation and Air Quality Improvement Program

The Problem

Note: This information was archived in September 2013. For current information, see

Since the 1950s, we have known that vehicle exhaust fumes play a major role in the deterioration of air quality in urban areas. This knowledge led to widespread State and Federal regulatory activity, which eventually resulted in the passage of the modern Clean Air Act (CAA) in 1970.

The CAA gives the U.S. Environmental Protection Agency (EPA) the responsibility and legal authority to control air pollution by setting limits on pollution from stationary, area, and mobile sources of emissions. Federal standards, known as National Ambient Air Quality Standards (NAAQS), are required to be set at levels that protect human health. There are currently NAAQS for six pollutants. Those for which transportation sources are significant include carbon monoxide, particulate matter, and ozone. The most persistent pollution problem is ground level ozone, which is not emitted directly but is produced in the air during a complex photochemical reaction involving volatile organic compounds (VOCs) and oxides of nitrogen (NOx) contained in automobile exhaust emissions and other similar gasses.

Figure 1. Comparison of Growth Areas and Emission Trends
Graphical representation of information in cell to right. Years 1970 to 1999

US Gross Domestic Product Increased 147%

Vehicle Miles Traveled Increased 140%

US Population Increased 33%

Aggregate Emissions Decreased 31%
(Six Principal Polluntants)


Figure 2. Decrease in National Concentrations and National On-Road Mobile Source Emissions
  Percent Decrease in Concentrations
Percent Decrease in Emissions
CO -39 -24
O3 -4 (1 hour) -26 (VOC)
+1 (NOx)
PM10 -25 -30

The air is much cleaner than it was in 1970 even though further progress is necessary. For example, the EPA estimates that from 1989-1998 national emissions from mobile sources for carbon monoxide (CO) decreased 24 percent, 26 percent for VOCs and 30 percent for particulate matter (PM) less than 10 microns in size. These dramatic emission reductions occurred simultaneously with significant increases in economic growth and population.

Despite substantial progress in reducing emissions, the impact of mobile source air pollution continues to be large. EPA estimates that over 5,000 tons of VOCs from transportation sources were emitted in 1999 and that approximately 62 million people were living in areas that do not meet the health-based standards.

Fine particulate matter, or PM2.5, is defined as particles less than 2.5 microns in size. This pollutant causes adverse health effects by depositing in the lungs where it interferes with the respiratory process. The health risk from an inhaled dose of PM may depend on the size, composition, and concentration of the particulate. Combustion sources, including on-road vehicles, are thought to be significant to overall pollution levels of PM 2.5.

Figure 3. Number of People Living in Counties with Air Quality Concentrations Above the Level of the NAAQS in 1999
In Millions of Persons - CO: 9.1; Pb: 0.04; NOx: 0; O3: 53.8 (1 hour), 122.5 (8 hour); PM10: 20.3; PM2.5: Data not yet available; SO2: 0; Any NAAQS: 62.1 (1 hour), 125.3 (8 hour)

Large and densely populated metropolitan areas experience increased traffic congestion problems. The cost of traffic congestion to travelers is measured in hours of delay and wasted fuel. Travelers in the nation's 68 largest metropolitan areas spent over $72 billion in hours of lost time and wasted fuel in 1999. Between 1982 to 1997 the annual hours of delay per driver in the country's largest metropolitan areas increased by 125 percent, and in the small urban areas, the average increase was 400 percent.1 Figure 4 displays the congestion increases experienced in many urban areas throughout the country.2

Figure 4. Percentage Change in Urban Congestion, 1982 to 1997
Seattle +20%; Portland +33%; San Francisco +28%; Los Angeles +29%; San Diego +40%; Salt Lake City +53%; Phoenix +20%; Denver +40%; Albuquerque +52%; Dallas +35%; El Paso +30%; Houston -2%; New Orleans +11%; Minneapolis +61%; Milwaukee +33%; Chicago +36%; Detroit +20%; Cleveland +35%; St. Louis +27%; Louisville +44%; Boston +36%; New York +18%; Baltimore +35%; Washington, DC +34%; Charlotte -4%; Atlanta +45%; Miami +30%

1 & 2 Texas Transportation Institute. 1999 Annual Mobility Report, Appendix A-4

Updated: 09/23/2013
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