Skip to contentUnited States Department of Transportation - Federal Highway Administration FHWA Home
Research Home
Public Roads
Featuring developments in Federal highway policies, programs, and research and technology.
This magazine is an archived publication and may contain dated technical, contact, and link information.
Federal Highway Administration > Publications > Public Roads > Vol. 67 · No. 1 > Air Quality and Transportation

July/August 2003
Vol. 67 · No. 1

Air Quality and Transportation

by Gary Jensen

Emissions are on the decline, and efforts from the Federal to the local levels will help continue this trend.

Although the Clean Air Act contributes to cleaner air quality, the greatest success can be found in the control of emissions from on-road mobile sources. According to calculations based on data from the National Air Quality and Emissions Trends Report, 1999, published by the U.S. Environmental Protection Agency (EPA), emissions reductions from motor vehicles account for 84 percent of the total emissions reductions of the six criteria pollutants since 1970. The automotive, fuels, highway, and transit communities have managed to achieve this success in cleaning up the Nation's air with the help of tight EPA emissions standards and fuel requirements, while at the same time meeting increasing demands for improved mobility and safety.

By reducing traffic congestion on highways such as this one, States and metropolitan areas help to improve air quality.

By reducing traffic congestion on highways such as this one, States and metropolitan areas help to improve air quality.

Although the United States enacted air quality legislation during the 1950s and 1960s, the Clean Air Act of 1970 marked the first time that the country seriously addressed air pollution on a national scale. Amended in 1977, and most recently in 1990, the Clean Air Act provides the principal framework for Federal, State, and local efforts to protect air quality from all pollution sources.

In testimony before the U.S. Senate Committee on Environment and Public Works on July 30, 2002, Federal Highway Administrator Mary E. Peters described the progress made by the U.S. Department of Transportation in reducing transportation-related emissions of pollutants.

"As a Nation, we have made remarkable improvements in reducing air pollution, especially pollution that comes from transportation sources," she said. "Where transportation is a significant source of pollutants, the [U.S.] Environmental Protection Agency reports that ozone—formed by the reaction of volatile organic compounds and nitrogen oxide (NOX), carbon monoxide, and particulate matter—have all decreased substantially since 1970." Added to the atmosphere at the Earth's surface, particulate matter (PM) is fine solid or liquid particles such as dust, smoke, soot, pollen, and soil that could have a potential for significant adverse health and/or environmental impacts.

Controlling Pollution

Air pollution comes from many different sources: stationary (point) sources such as factories and power plants; smaller area sources like dry cleaners and painting operations; on-road mobile sources including cars, buses, and trucks; and non-road mobile sources such as construction equipment, airplanes, boats, and trains. Air pollution also comes from natural sources such as windblown dust and volcanic eruptions.

Under the Clean Air Act, EPA established Federal controls and standards to reduce emissions. States must develop and enforce State implementation plans (SIPs) to clean up polluted areas and protect and maintain air quality. Motor vehicle controls are only one part of the picture, but they play a significant role.

EPA established increasingly tighter national standards requiring cleaner motor vehicles and fuels. Also, where Clean Air Act goals are not met, the Act challenges State and local transportation officials to find ways to reduce vehicle emissions by reducing the number of single-occupant vehicles and making alternative modes of transportation, such as transit and bicycles, an increasingly important part of the transportation network.

Good intermodal connections, such as the one between this transit station and a major roadway, encourage transit use and reduce emissions.

Good intermodal connections, such as the one between this
transit station and a major roadway, encourage transit use and
reduce emissions.

Nonattainment Areas

To determine which areas have air pollution problems, EPA and State and local agencies established monitoring networks to measure the concentration of pollutants in outside air. Monitoring data is analyzed to determine if the standards are met. If levels of any pollutant violate the standards, then EPA, in cooperation with the State, designates the contributing area as nonattainment. Once the area again meets the standards for healthy air and has a plan in place to maintain air quality, EPA may redesignate that area back to attainment. Such areas are known as "maintenance areas." Since 1992, the number of nonattainment areas has decreased 46 percent.

In the early 1970s, the Denver, CO, metropolitan area failed to comply with air quality standards for carbon monoxide, PM10 (particles with diameters of 10 micrometers or less), and ozone. The carbon monoxide problem was so severe that the area violated the standard more than 130 times a year. At times, the Denver area would be out of compliance continually for days at a time.

Denver air quality improved from 1990 to 1999 with carbon monoxide concentrations decreasing 44 percent, PM10 concentrations decreasing 15-17 percent, and ozone concentrations decreasing 12 percent. In 2001, because of these improvements, EPA redesignated the region as attainment for both carbon monoxide and ozone. In 2002, EPA redesignated the area to attainment for PM10.

The number of nonattainment areas alone, however, does not tell the whole story. EPA has kept many areas designated as nonattainment for procedural reasons, even though actual monitoring data shows that they are meeting the standards. The most recent EPA data for 1998-2000 show that only 34 areas violated the 1-hour ozone standard (down from 98 areas in 1991), and only 3 areas violated the carbon monoxide standard in 1999-2000.

There are a number of reasons for this. An area may need additional time to resolve technical issues associated with demonstrating that the standards will be maintained. Another reason involves coordination among transportation and air agencies and the public on which projects to fund to maintain the standards or how future emissions targets should be allocated among stationary, area, and mobile sources. Also, State and local legislative bodies may need to act in order to demonstrate that control measures are enforceable.

Number of Areas Designated As Nonattainment
Pollutant 1992 2002
Carbon Monoxide 78 24
Lead 13 3
Nitrogen Dioxide 1 0
Ozone 134 74
Particulate Matter (PM10) 84 68
Sulfur Dioxide 53 26
All Pollutants 363 195
Source: EPA “Green Book” Web site, Nonattainment Status for Each County by Year, as of January 15, 2001,
This table shows that the number of areas designated as nonattainment for air quality has decreased between 1992 and 2002, demonstrating that air quality is improving in the United States.

Air Quality Standards

EPA's National Ambient Air Quality Standards (also known as "air quality standards") are Federal standards, established through extensive scientific review, that set allowable concentrations and exposure limits for certain pollutants in order to protect public health and welfare. EPA published criteria documents for six pollutants: ozone, carbon monoxide, particulate matter, nitrogen dioxide, lead, and sulfur dioxide. On-road mobile sources primarily contribute to four of these criteria pollutants: ozone, carbon monoxide, particulate matter, and nitrogen dioxide.

In 1997, EPA updated air quality standards for ozone (known as the "8-hour" standard as it is based on the measurement of average concentrations over an 8-hour period) and fine particulate matter (known as the PM2.5 standard, for particles with diameters of 2.5 micrometers or less). These standards were challenged in court, and until recently, litigation blocked their implementation. The Supreme Court now has upheld these standards, and a lower court dismissed further challenges. EPA is developing a plan for implementing the standards and expects nonattainment areas to be designated and required to develop SIPs to meet them in the upcoming years.

The Federal Highway Administration (FHWA) anticipates that these updated standards will affect a much larger number of areas than are currently in nonattainment. Identifying strategies and measures that will enable nonattainment areas to meet the standards may be substantially more difficult. In addition, the transportation contribution to PM2.5 emissions is unclear, so additional research will be necessary to determine how transportation strategies can control PM2.5 emissions.

Cleaner Air

As a Nation, and as transportation officials and citizens, we have had great success under the Clean Air Act. National levels of all criteria pollutants are down over the last 20 years. Ozone levels nationally have improved considerably. Although some areas have shown increases, ozone levels in urban areas where problems historically have been the most severe show marked improvement in response to stringent controls. Nationally, carbon monoxide levels are the lowest recorded in the last 20 years, and this air quality improvement is consistent across all regions of the country. The most recent 10-year period (1990-1999) shows that the national average of annual mean PM10 concentrations decreased 18 percent.

For example, air quality in the Los Angeles area—the only area in the country classified as extreme nonattainment for ozone—has improved significantly, thanks to the comprehensive control strategies implemented to reduce pollution from mobile and stationary sources. For instance, the total number of days the area exceeds the 1-hour ozone standard has decreased dramatically over the last two decades from more than 200 days to fewer than 50 days per year.

On July 30, 2002, Jeffrey Holmstead, assistant administrator of EPA's Office of Air and Radiation, offered testimony before the Senate Committee on Environment and Public Works. Holmstead explained that the United States has made "considerable progress" in achieving better air quality since the passage of the Clean Air Act Amendments in 1990.

"Air quality monitoring data show that in the period from 1991 to 2000, concentrations of all six criteria pollutants have declined," he said, "including the four criteria pollutants that are most affected by the transportation sector: carbon monoxide, nitrogen dioxide, ozone, and particulate matter. For example, air quality concentrations of carbon monoxide declined 41 percent and concentrations of coarse particulate matter declined 5 percent."

He concluded, "These air pollution data are good news, and are attributable to the transportation and air quality programs currently in place."

ISTEA and TEA-21

The Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) revamped the Federal highway and transit programs to provide State and local officials with additional tools to improve air quality, including flexible funding increases, a strengthened planning process, and programs specifically directed to improving air quality and transit. ISTEA required that States and metropolitan planning organizations (MPOs) carry out a comprehensive process for transportation planning and offered State and local officials flexibility in choosing among highway, transit, and other transportation alternatives that would enable them to select the best mix of projects to address air quality.

ISTEA also created the Congestion Mitigation and Air Quality Improvement Program, which directs funding to projects and programs that reduce emissions in nonattainment and maintenance areas. In 1998, the Transportation Equity Act for the 21st Century (TEA-21) continued the provisions of ISTEA and significantly increased funding for transportation programs and projects that reduce emissions from motor vehicles.

Transportation Planning And Conformity

ISTEA strengthened the planning process by requiring States and MPOs to develop better transportation plans that could help improve air quality. The requirements of ISTEA were matched with a provision in the 1990 amendments to the Clean Air Act limiting Federal transportation activities in nonattainment and maintenance areas under certain circumstances. This provision in the Act, intended to integrate the transportation and air quality planning processes, is known as "transportation conformity."

The provision is a way to ensure that Federal funding and approval goes to those transportation activities that are consistent with air quality goals. A conformity determination demonstrates that the total emissions projected for a transportation plan and program are within the emissions limits (or budgets) established by a SIP, and that transportation control measures (TCMs) are implemented in a timely fashion.

By 2001, a very high percentage (94-100 percent) of nonattainment and maintenance areas had developed transportation plans that met emissions reduction goals.

Population and Travel Growth

The United States achieved these improvements in air quality despite dramatic increases in population and personal and freight-related travel. According to the U.S. Census Bureau, between 1970 and 1999, the U.S. population increased 38 percent, and the number of people employed grew 68 percent. The Bureau of Economic Analysis in the U.S. Department of Commerce reports that the gross domestic product, adjusted for inflation, increased 147 percent during that time. And, according to FHWA statistics, the number of drivers increased 68 percent, the total vehicle miles traveled (VMT) per year grew 142 percent, and heavy-duty truck travel increased 227 percent. At the same time, however, EPA estimates that total on-road motor vehicle emissions decreased 77 percent.

Transportation planners not only face increases in personal and freight-related travel, but they also contend with other challenges in accommodating growth. For example, actual construction of new and expanded lanes on the Nation's highway system during the last 20 years increased system capacity by only 3 percent, according to FHWA's Office of Highway Policy Information.

Not surprisingly, congestion has grown steadily over the last two decades in urban areas of every size. Severe congestion lasts a longer period of time and affects more of the transportation network (as seen in 1999 statistics) than it did in 1982. In fact, in its 2001 Urban Mobility Study, the Texas Transportation Institute reported that the average annual delay per person rose from 11 hours in 1982 to 36 hours in 1999.

Other challenges include reducing the number of people who travel by single-occupant vehicles, encouraging travel by other modes, and decreasing the number of trips that motorists make. According to the U.S. Census Bureau, the majority of commuters in the United States rely on single-occupant vehicles to travel between home and work.

Emissions Trends in Transportation

Despite large increases in population, personal travel, and freight transportation; limited highway expansion; and the public's choices of transportation modes, on-road motor vehicle emissions declined 77 percent since 1970, as noted earlier. And EPA expects this downward trend to continue well into the future.

In addition to the reduction in emissions levels, the contribution of emissions from on-road motor vehicle sources has decreased as a percentage of total emissions. In fact, in 1970, motor vehicles contributed 59 percent of total emissions of carbon monoxide, nitrogen oxide, volatile organic compounds, and particulate matter, compared to stationary, area, and non-road mobile sources. By 1999, the motor vehicle portion of emissions of these pollutants dropped to 48 percent.

Stricter Standards

The majority of these emissions reductions resulted from stricter standards, improved engine technology, and cleaner fuels. Engines and fuel will become even cleaner under recent EPA-issued emissions standards and cleaner fuel requirements.

Between 2004 and 2007, more protective tailpipe emissions standards will be phased in for all passenger vehicles, including sport utility vehicles (SUVs), minivans, vans, and pickup trucks. This regulation marks the first time that larger SUVs and light-duty trucks will be subject to the same national pollution standards as cars. In addition, EPA lowered standards for sulfur in gasoline, which will ensure the effectiveness of low-emission control technologies in vehicles and reduce harmful air pollution.

Once implemented, the new tailpipe and sulfur standards will benefit Americans by offering the clean-air equivalent of removing 164 million cars from the road. The new standards require passenger vehicles to be 77 to 95 percent cleaner than those on the road today, and reduce the sulfur content of gasoline by up to 90 percent.

EPA recently issued new standards as well for heavy-duty highway engines that will take effect in model year 2007. These standards are based on the use of high-efficiency catalytic exhaust devices to control emissions or comparably effective advanced technologies. Because sulfur damages the devices, EPA also is reducing the level of sulfur in highway diesel fuel by 97 percent by mid-2006. As a result, each new truck and bus will be more than 90 percent cleaner than current models. EPA expects the clean air impact of the program to be dramatic when fully implemented. The program will provide annual emissions reductions equivalent to eliminating the pollution from more than 90 percent of today's trucks and buses—about 13 million—from America's roadways.

Even in fast-growing areas such as Atlanta, GA, motor vehicle emissions are expected to continue a downward trend. The Atlanta Regional Commission estimates that emissions of NOx and VOCs will decrease 37 percent and 25 percent respectively between 2005 and 2025.

Cleaner Air for Tomorrow

On a national level and at the local level in almost all metropolitan areas around the country, air quality is improving. From 1990-1999, only 9 percent of metropolitan areas had an upward trend in ozone concentrations, only 1 percent of metropolitan areas had an upward trend in the concentrations of particulate matter, and no metropolitan areas had an upward trend in carbon monoxide.

Reducing pollutant emissions from motor vehicles has been a major factor in this trend toward cleaner air. Technological innovations, cleaner fuels, and highway and transit programs have reduced emissions significantly over the past 30 years and will continue this trend well into the future.

People descending escalator into a subway station
Urbanites descend an escalator into a subway station that received Federal funding.

Gary Jensen is a member of the Transportation Conformity Team in FHWA's Office of the Natural and Human Environment. He is involved in developing policies and guidance associated with transportation conformity and other air quality issues. He has been with FHWA for 8 years. Prior to his current assignment, he spent 4 years in FHWA's Tennessee Division Office managing transportation planning and environmental programs. He holds a degree in civil engineering from the University of Idaho.

For more information about transportation and air quality, visit or contact Gary Jensen at 202-366-2048 or

United States Department of Transportation - Federal Highway Administration