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Federal Highway Administration > Publications > Public Roads > Vol. 62· No. 1 > Clean Air and Transportation: The Facts may Surprise You

July/August 1998
Vol. 62· No. 1

Clean Air and Transportation: The Facts may Surprise You

by Michael Koontz

Mountain wildflowers in bloom.

The week of May 24-30 was Clean Air Week. All across the nation, people were encouraged to make choices that would contribute to cleaner air. Transportation choices are an element of this decision-making process, but the balance between the roles of technology and trip-making behavior is being questioned. While many areas continue to place great emphasis on the travel choice in the quest for cleaner air, the benefits and the track record of technological solutions are coming more into focus.

Faced with the full gamut of lingering air quality challenges, Congress enacted the Clean Air Act Amendments (CAAA) of 1990. These amendments confront many of the same environmental issues that prompted the original act in 1970 and the amendments of 1977, and they continue to look toward transportation to reduce air pollution.

Along with the familiar image of billowing smokestacks and sprawling industrial agglomerations, automobiles and trucks have become symbols of air pollution. Whether this is a rightful role, and whether the onus for cleanup should be on the transportation community is now at the forefront of the clean air debate. Regardless of where future mitigation efforts are based, the transportation sector - and especially the automobile industry - has done its part in the quest for better air quality.

Background - CAAA of 1990

Despite the full force of the original act and its 1977 amendments, more than 175 areas of the United States were classified in 1990 as "air quality nonattainment areas" - parts of the country failing to meet at least one air quality standard.1 If you were a resident of any major metropolitan area, you were probably breathing air that did not meet the National Ambient Air Quality Standards (NAAQS) that the Environmental Protection Agency (EPA) establishes for six principal air pollutants. The 1990 CAAA expanded the existing regulatory procedures and set forth a matrix of deadlines attached to each of these nonattainment areas. State implementation plans (SIPs) addressing measures and milestones are mandatory under the 1990 CAAA.

Urban area with poor air quality. Not only are the many milestones and requirements challenging, but the sheer level of data management and record-keeping has been daunting. Each state is required to develop an emissions budget, an accounting mechanism creating a progressive track toward attainment of the standards. This budget can evolve into an elaborate strategy involving tradeoffs among point, area, and mobile sources of emissions. Prior to settlement on a budget for the nonattainment area, the transportation community must complete a theoretical analysis that contrasts emissions from "build" and "no-build" scenarios, as state and regional planning organizations attempt to reach "conformity" of their transportation plans and programs with SIPs. The bottom line? Transportation must contribute to lower emissions - at least until the area's budget is approved.

Air Quality Trends - A Report Card

Many of the nonattainment areas designated in 1990 have achieved their air quality goals and have been dropped from the rolls of nonattainment areas. We have seen the passing of a pair of major attainment deadlines for ozone and carbon monoxide (CO) levels, and approximately 70 areas, most of which previously had excessive ozone levels, have been redesignated as attainment areas.

EPA's National Air Quality and Emissions Trends Report, 1996 indicates that CO levels were down 37 percent from 1987 to 1996. In addition, during the same period, the average number of monitored exceedances of the CO standard fell a staggering 92 percent nationally. Ozone levels are down as well, according to the EPA's annual report published in January 1998. The average concentrations for 532 long-term air quality monitors were 15 percent lower in 1996 than in 1987. The number of times that the ozone standard of 0.12 ppm (a simple ratio of parts of ozone per million parts of generic air) was exceeded was also down, falling 73 percent over the same period.2

Gross emissions levels have also dropped for pollutants from mobile sources. These levels are based on estimates of total tonnage of the pollutants released from combined mobile sources. Based on the 1987 to 1996 time frame, both CO emissions and hydrocarbons fell 18 percent. Even the persistent problem of particulate matter - airborne pollutants that irritate the respiratory system - has been lessened; from 1988 to 1996, particulate matter was reduced by 12 percent. Interestingly, lead pollution in the air has been checked substantially, dropping 75 percent between 1987 and 1996, due to the removal of lead from gasoline.

Transportation as a Contributor

Since the passage of the Clean Air Act in 1970, aggregate emissions have declined for virtually every pollutant that EPA monitors. Within these overall reductions, most analyses point to steadily declining pollution, including hydrocarbons and nitrogen oxides, from highway mobile sources. Reducing hydrocarbons and nitrogen oxides is important because they are precursors of ozone. A precursor is a pollutant that contributes to the formation of other pollutants.

Due to the removal of lead from gasoline, lead pollution in the air has dropped 75 percent between 1987 and 1996. For hydrocarbons, 36 percent of total emissions were attributed to automobiles in 1970. That figure now rests at approximately 25 percent with a further decline expected through 2005.

Similarly, in 1970, about 20 percent of nitrogen oxides emissions were generated by automobiles. Today, only 12 percent is attributed to automobiles, and a further decline to 11 percent is projected for 2005.

These improvements are partially offset by a slowly growing share of emissions from stationary sources and by an erratic, but expanding, share from off-road sources, such as construction and farm equipment.3

The greatest boon to the decline in mobile-source emissions has been the gradual ratcheting of vehicle-emission standards - lowered targets that have been met consistently by the automobile industry. Prior to the implementation of engine-emission controls, light-duty vehicles were spewing forth levels of emissions that were manifold higher than today's improved levels. For each mile driven, a typical mid-1960s passenger vehicle emitted 11 grams of hydrocarbons, four grams of nitrogen oxides, and a staggering 80 grams of CO.

Improvements are partially offset by a slowly growing share of emissions from stationary sources and off-road sources, such as construction and farm equipment. With the advent of vehicle-emission standards, tailpipe-exhaust readings plummeted. By 1972 - two years after the Clean Air Act - hydrocarbon-emission standards were set at 3.4 grams per mile (gpm), and the CO standard was 39 gpm. After nine years and the implementation of the 1977 CAAA, the standards were 0.41 gpm for hydrocarbons, 3.4 gpm for CO, and 1 gpm for nitrogen oxides.

Passenger-vehicle standards remained constant for most of the 1980s. With the 1990 CAAA, the standard for nitrogen oxides was established for model year 1994 vehicles at 0.4 gpm, down from 1 gpm between 1987 and 1993. In addition, a more stringent standard for particulates was implemented, dropping from 0.6 gpm in 1982 down to 0.08 gpm, also effective with model year 1994 vehicles. Effective in 1996, hydrocarbons dropped even further, down to a level of 0.25 gpm.4,5

Although standards for light-duty trucks have not matched the aggressive pace set for cars, the plunge in passenger-vehicle-emission standards illustrates the progress that can be linked directly to engine technology. While the specter of ever-growing vehicle-miles traveled is disconcerting, gains in emission-control technology have more than offset this trend.

Other technologies and programs have entered the contemporary battle for cleaner air. Straddling the technological and programmatic approaches, reformulated gasoline (RFG), oxygenated gasoline, and inspection and maintenance (I&M) programs have been successful in addressing both CO and ozone. All three programs are mandated by the 1990 CAAA in specified areas. In addition, these programs have been applied further as other areas see the benefits of "opting-in" to these strategies.

None of these technology-based strategies is expected to deliver the "knockout punch" to transportation-related air pollution. However, their value as part of an integrated strategy has been well-documented. Research conducted by the Volpe National Transportation Systems Center, for example, indicated that enhanced I&M, potentially, would account for nearly 15 percent of mobile-source-emission reductions. The study also found that implementation of an RFG program reduces average mobile-source emissions by more than 10 percent.6

Disappointments and Roadblocks - More $$$ May Not Be the Answer

As advances in transportation technology have held emissions in check, other strategies associated with transportation have found little success despite very high costs. While EPA's transportation conformity process has been less than embraced by state and regional planners, it has been instrumental in isolating the minimal air quality benefits ascribed to the traditional transportation control measures (TCMs) that form most capital improvement programs. A 1993 survey of transportation plans in the San Francisco Bay area highlighted the air quality benefits of an $11 billion investment program largely devoted to transit system and high-occupancy-vehicle lane construction. Using the required build and no-build assessments, the study revealed a reduction in hydrocarbons of less than 1 percent over the analysis period with a similar result for CO.7

New performance standards for diesel trucks, effective in 2004, are expected to yield a 15-percent drop in overall truck-fleet nitrogen oxides by 2010. A 1994 assessment of the costs and effectiveness of transportation control measures projected small fiscal and emission-reduction efficiencies across the TCM spectrum. The study looked at the weak performance projected for high-capital TCMs in hydrocarbon-reduction effectiveness. This appears to be a typical track record for the country, as models in most areas attribute a decrease in pollutants of less than 2 percent to traditional TCMs - a poor return on a huge investment.8

Performance figures for the Dallas metropolitan area shed even more light on these inefficiencies. Officials in this rapidly growing Texas region report that more than 90 percent of the area's required reductions in highway-vehicle emissions were generated through technological improvements and just under 10 percent through traffic flow improvements, such as wider lanes. The remaining 0.3 percent were credited to behavioral measures focusing on reduced highway travel.9

In the midst of such capital-intensive choices are several new developments on the regulatory front that will carry substantial impacts of their own. After several years of debate, EPA published new NAAQS for both ozone and particulate matter in July 1997. Including both new and revised standards, these more stringent requirements will put states in the position of searching even further for new sources of emission reductions. Similarly, EPA has developed a regional nitrogen-oxide-reduction strategy that is also making its way through the federal rule-making process. Complete with state-by-state nitrogen-oxide-reduction budgets, the regional strategy is an effort to combat ozone transport - the downwind drift of both ozone and nitrogen oxides - that especially hinders the attainment efforts of northeastern states.

The Future - New Concepts and Technologies

States have accrued enormous air quality benefits from lowered vehicle-emission standards and more modest, but nevertheless important, gains from developments such as RFG and I&M programs. The common denominator among these successes has been technology, and it is in this direction that future efforts toward attainment will likely reap the greatest benefit.

Low-emission, zero-emission, and electric vehicles - also known as LEVs, ZEVs, and EVs, respectively - are also expected to have an effect on future regional emissions and the push toward attainment. California has a ZEV-sales mandate for model year 2003 as part of its state strategy toward clean-air attainment. Similarly, the Northeast's Ozone Transport Commission has reached an agreement with the automobile industry on a compromise plan that will introduce a "national LEV" to every state except California, Maine, New Jersey, New York, and Vermont. EPA has declared that the national LEV program is underway, and cleaner cars are slated to be available in the Northeast for model year 1999.10

Other regulatory developments may help bring transportation into an even more positive light. New performance standards have been promulgated for heavy-duty diesel trucks and locomotives. The standards for diesel trucks, effective in 2004, are expected to yield a 15-percent drop in overall truck-fleet nitrogen oxides by 2010 and a 40-percent reduction by 2020. For railroad locomotives, the reduction in nitrogen oxides as a result of the new performance standards is projected to be about 8 percent by 2020.

Looking further into the next century, many automobile industry officials are convinced that the electric drive train will be the vehicle platform of the future. Consequently, many new applications of the EV are making their way through the automotive labs, including a number of "hybrid" vehicles that use combinations of different technologies. Two of the more promising configurations are the gasoline-electric and diesel-electric hybrids that use smaller fuel-based engines running at constant - and efficient - speeds to generate electricity for the vehicle's battery pack. Even further beyond the horizon, the hydrogen fuel cell is also being projected as a viable, almost pollution-free, power platform.

Common sense also plays an important role in the reduction of air pollution. Many states and metropolitan organizations have engaged in widespread marketing campaigns to educate and warn the public of impending air quality events or episodes. Referred to as "episodic measures," these market blitzes are being used to encourage the public to refrain from participating in polluting activities, such as lawn mowing or vehicle refueling, during bad air days. Maryland and Oklahoma initiated programs to forecast the bad air days.


In many areas, especially in America's expanding metropolitan regions, improving air quality remains a difficult challenge. There have been few sources of consistent improvement in air quality over the quarter-century under the Clean Air Act. An unmistakable success story, however, has been the contribution of the automakers in their quest to keep pace with tightening engine-emission standards. American motorists are clearly driving more miles each year, and they are adding these extra miles to vehicles that they're keeping longer than ever.11 Despite these trends, pollution from on-road mobile sources has plummeted since the implementation of tailpipe standards. Indeed, the CO problem has been virtually wiped out due largely to engine technology.

More than 119 million Americans still live in air quality nonattainment areas. While the recent past has been marked with success, future reductions from on-road mobile sources will be more labored.12 Reductions have become more difficult and much more expensive to achieve as standards have been made more stringent. The dwindling benefits of complete fleet turnover, coupled with the steady growth in vehicle-miles traveled, represent clear challenges for technology.

A "silver bullet" may remain elusive, but in its place, a host of contributing technologies and measures may drive attainment efforts. Positive roles can be played by the greater use of viable alternative fuels, publicly supported I&M programs, and the development of the next generation of alternative vehicles. In the three decades of vehicle-emission standards, the purveyors of automobile technology were leaders in the push for air quality attainment through the reduction of pollution from on-road mobile sources. In the past, motor vehicles deserved their reputation as a principal cause of the country's air pollution; however, as a result of innovation and hard work, the industry deserves a new reputation as the leader in the search for solutions.


1. Our Nation's Highways: Selected Facts and Figures, Office of Highway Information Management, Federal Highway Administration, Washington, D.C., May 1995.

2. National Air Quality and Emissions Trends Report, 1996, Office of Air and Radiation, U.S. Environmental Protection Agency, January 1998.

3. Clearing the Air: A Report on Emission Trends in Selected Cities, Energy and Environmental Analysis Inc. (prepared for the American Automobile Association), September 1994.

4. Clean Air Act as Amended August 1977, 95th U.S. Congress, U.S. Government Printing Office, Washington, D.C., November 1977.

5. Clean Air Act Amendments of 1990, 101st U.S. Congress, U.S. Government Printing Office, Washington, D.C., November 1990.

6. Technical Report on Clean Air Act Programs, Effectiveness of Emission Control Measures, Volpe National Transportation Systems Center, U.S. Department of Transportation, November 1994.

7. Hank Dittmar. "Beyond Conformity: Transportation, Air Quality, and Community Values," a presentation in Rapid City, S.D., October 1992.

8. Costs and Effectiveness of Transportation Control Measures (TCMs): A Review and Analysis of the Literature, Apogee Research Inc. (prepared for the National Association of Regional Councils), January 1994.

9. Frank R. Moretti. Clean Air and Improved Mobility: Meeting the Challenge, The Road Information Program, Washington, D.C., July 1997.

10. "Air Quality Officials, Automakers Differ on 49-State LEV Costs," Inside EPA's Clean Air Report (newsletter), October 1995.

11. Alan E. Pisarski. "New Perspectives in Commuting," prepared for the Office of Highway Information Management, Federal Highway Administration, Washington, D.C., July 1992. 12. "House Republicans Question EPA's Push to Revise Ozone Standard," Inside EPA's Clean Air Report (newsletter), November 1995.

Michael Koontz is a transportation planner in FHWA's Office of Environment and Planning. He has worked in the Environmental Analysis Division since 1994, coming from the FHWA's Maryland Division Office. Koontz joined FHWA in 1992. Prior to then, he worked for the Department of Defense as a transportation systems analyst and as a regional geographer, specializing in logistics and intermodal developments in the Middle East and North Africa. Before government service, he worked as a business location planner and as a land-use consultant in the private sector. He has a bachelor's degree in geography and environmental planning, and he earned a master's degree in transportation geography from Towson State University, Baltimore.

United States Department of Transportation - Federal Highway Administration