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FHWA Home / Policy & Governmental Affairs / 2002 Conditions and Performance

Conditions and Performance

Status of the Nation’s Highways, Bridges, and Transit:
2002 Conditions and Performance Report

Chapter 19: Air Quality

Chapter 19 Table of Contents

  • Introduction
  • The Clean Air Act and Air Quality
    • National Ambient Air Quality Standards
    • Hazardous Air Pollutants
    • Cleaner Air
  • Emissions Trends in Transporation
  • ISTEA and TEA-21
    • Congestion Mitigation Air Quality Improvement Program
    • Transportation Control Measures
    • Inspection and Maintenance Programs and Other Control Measures
    • Public Education
    • Transporation Planning and Conformity
  • Transit and Clean Air
  • Conclusion



Air quality effects need to be considered when evaluating the impacts of future transportation investments. That is why two of the investment models in this report–HERS and TERM–include emissions costs. This chapter describes the general relationship between air quality and transportation.

While the Clean Air Act (CAA) has controlled pollutant emissions from all air pollution sources, the greatest success can be found in the control of on-road mobile sources. Emissions reductions from motor vehicles have accounted 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 Environmental Protection Agency (EPA) emissions standards and fuel requirements, while at the same time meeting the increasing demands of improved mobility and safety.

This chapter begins by discussing the history of air quality legislation and the sources and types of air pollution that are primarily affected by transportation. It then discusses the past and expected trends of pollutant emissions, followed by a summary of highway and transit programs that are being used to reduce motor vehicle emissions.

The Clean Air Act and Air Quality

Air pollution has been a problem for a long time. However, until the 1950s there were few laws that addressed this issue. One event that first captivated public attention occurred in October of 1948, when 20 people were killed and over 7,000 became ill because of severe air pollution over Donora, Pennsylvania. The Donora incident resulted from factory emissions and meteorological conditions that trapped those emissions, and it led to State and Federal air quality controls. Air pollution has been identified as a cause of several health and environmental problems, including respiratory illnesses and other diseases, crop damage, decreased visibility, and structural deterioration.

Although air quality legislation was enacted during the 1950s and 1960s, the 1970 Clean Air Act (CAA) marked the first time that air pollution was seriously addressed on a national scale. The Clean Air Act was amended in 1977 and most recently in 1990. The Clean Air Act, as amended, provides the principal framework for Federal, State, and local efforts to protect air quality from all pollution sources. Air pollution comes from many different sources: stationary (point) sources such as factories and power plants; smaller area sources such as dry cleaners and painting operations; on-road mobile sources such as cars, buses, and trucks; non-road mobile sources such as construction equipment, airplanes, boats, and trains; and naturally occurring sources such as windblown dust and volcanic eruptions.

Under the CAA, Federal controls and emissions standards have been established to reduce emissions. States must also develop State implementation plans (SIPs) that they enforce to clean up polluted areas and protect and maintain air quality. Motor vehicle controls are only one part of the picture, but they have played a significant role.

EPA has established increasingly tight national standards requiring cleaner motor vehicles and fuels. Also, where CAA goals were not being met, State and local transportation officials have been challenged 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.

National Ambient Air Quality Standards

The National Ambient Air Quality Standards (NAAQS, also referred to as “air quality standards”) are Federal standards, established through extensive scientific review, that set allowable concentrations and exposure limits for certain pollutants. The standards are intended to protect public health and welfare. Air quality standards have been established for six pollutants for which EPA has published criteria documents: ozone (or smog), 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 developed updated air quality standards for ozone (known as the “8-hour” standard) and for fine particulate matter (known as the “PM2.5” standard). However, these standards were challenged in court, and litigation has persisted until recently, blocking their implementation. The Supreme Court has now upheld the standards, and a lower court has dismissed further challenges. EPA is in the process of developing a plan for implementing these standards, and it is expected that nonattainment areas will be designated and be required to develop SIPs in the upcoming years to meet them.

It is anticipated that these updated standards will affect a much larger number of areas than are currently in nonattainment. It may be substantially more difficult for areas to identify strategies and measures that will allow them to meet the standards. In addition, the contribution of transportation to PM2.5 emissions is unclear, and additional research will be necessary to determine how transportation strategies can be utilized to control PM2.5 emissions.

Hazardous Air Pollutants

An emerging issue in air quality and transportation is hazardous air pollutants, also known as air toxics. These pollutants are known or are suspected to cause cancer or other serious health or environmental effects. They include pollutants like benzene, perchloroethylene, methylene chloride, heavy metals like mercury and lead, polychlorinated biphenyls (PCBs), and dioxins. Not all air toxics are emitted from transportation sources. While the harmful effects of air toxics are of particular concern in areas closest to where they are emitted, they can also be transported and affect the health and welfare of populations in other geographic areas. Some can persist for considerable time in the environment and/or accumulate in the food chain.

To address concerns about the potentially serious impacts of hazardous air pollutants on public health and the environment, the CAA includes a number of provisions that have required EPA to characterize, prioritize, and control these emissions as appropriate. On the mobile source side, many of the emission control programs put in place to control criteria pollutants reduce air toxic emissions as well. These programs have reduced and will continue to reduce on-highway emissions of air toxics significantly.

In March of 2001, EPA designated 21 compounds as mobile source air toxics (MSATs), recognizing that motor vehicles are significant emitters of these compounds. Although EPA has established this list of MSATs, it has not established that all emissions of these compounds are health risks, nor has it established any standard or measure of what concentration of these compounds might be harmful. EPA's final rule specifically states “that inclusion on the list” of MSATs “is not itself a determination by EPA that emissions of the compound in fact present a risk to public health or welfare, or that it is appropriate to adopt controls to limit the emissions of such a compound from motor vehicles or their fuels.” Further evaluation is necessary to determine the need for and appropriateness of additional mobile source air toxics controls for on-highway and non-road sources and their fuels.

Cleaner Air

The Nation has experienced considerable 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, and although some areas have shown increases, ozone levels in urban areas where problems have historically been the most severe have shown 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. This is described by Exhibit 19-1.

Exhibit 19-1

Percent Decrease in Concentration of Criteria Pollutants, 1992-2002[1]

POLLUTANT 1980-1999 1990-1999
Carbon Monoxide (CO)
Lead (Pb)
Nitrogen Dioxide (NO2)
Ozone (O3)[2]
Particulate Matter (PM10)
Sulfur Dioxide (SO2)
[1]This ozone concentration is based on the 1-hour ozone NAAQS. In 1997, EPA promulgated a new 8-hour ozone NAAQS. However, due to legal challenges, this 8-hour standard has not yet been implemented.
[2]Concentration measurements of PM10 for 1980 are not available.

Source: National Air Quality and Emissions Trends Report, 1999, EPA OAQPS, Research Triangle Park, NC, March 2001.

To determine which areas have air pollution problems, monitoring networks have been established to measure the concentration of the pollutants in the outside air. Monitors are inspected regularly and their data analyzed to determine if areas meet the standards. If monitored levels of any pollutant violate the NAAQS, then EPA in cooperation with the State designates the contributing area as “nonattainment.” Once the area has again met the standards and has healthy air, and the area has a plan in place to maintain the standards, EPA may redesignate that area back to “attainment.” Such areas are also known as “maintenance” areas. Since 1992, the number of nonattainment areas has decreased by 46 percent. This is described by Exhibit 19-2.

         Exhibit 19-2

Number of Areas Designated Nonattainment, 1992-2002 [1]

POLLUTANT 1992 2002
Carbon Monoxide
Nitrogen Dioxide
Particulate Matter (PM10)
Sulfur Dioxide
All Pollutants
[1] EPA Green Book website, Nonattainment Status for Each County by Year, as of January 15, 2001, http://www.epa.gov/oaqps001/greenbk/anayo_ak.cfml.

However, just looking at the number of nonattainment areas does not necessarily tell the whole story. Many areas are still considered nonattainment for procedural reasons, when actual monitoring data shows that their air quality is meeting the standards. For example, the most recently available data for 1998-2000 showed that only 34 areas violated the 1- hour ozone standard (down from 98 areas that were originally designated and classified in 1991), and that for 1999- 2000 only 3 areas violated the carbon monoxide standard.

There are a number of reasons why an area may still be designated nonattainment, even if the area is not violating the standards. An area may need additional time needed to resolve technical issues associated with demonstrating that the standards will be maintained. There are often coordination issues among transportation and air agencies, and the public over which projects should be given funding priority in maintaining the standards or how future emissions should be allocated among stationary, area, and mobile sources. Also, actions may be required by State and local legislative bodies to demonstrate that control measures have adequate commitments and are enforceable.

The above referenced improvements in air quality have been achieved even with dramatic increases in population, and personal and freight travel. Since 1970, population has increased 38 percent; the number of people employed has increased 68 percent; the Gross Domestic Product, adjusted for inflation, has increased 147 percent; the number of drivers has increased 68 percent; total vehicle miles traveled (VMT) per year have increased 142 percent; and heavy-duty truck travel has increased 227 percent. At the same time, total on-road motor vehicle emissions have decreased 77 percent. Exhibit 19-3 describes these trends.

As seen below, transportation planners have been faced with huge increases in personal and freight travel. They have also faced other challenges toward accommodating this growth. For example, actual construction of new and expanded lanes on the Nation's highway system over the last 20 years has only increased the system by 3 percent. Exhibit 19-4 describes this phenomenon in terms of the difference between expansion and traffic volume. Not surprisingly, congestion—a major source of air pollution—has grown steadily over the last two decades in urban areas of every size. Severe congestion, which greatly impacts air quality, lasts a longer period of time and affects more of the transportation network in 1999 than in 1982. The average annual delay per person climbed from 11 hours in 1982 to 36 hours in 1999 percent.

Another challenge has been trying to decrease the amount of people who travel by single-occupant vehicles, and encourage travel by other modes, as well as decreasing the number of trips people take. As can be seen in Exhibit 19-5, the majority of people in the United States rely on single-occupant vehicles to travel between home and work.

Emissions Trends in Transportation

In spite of the challenges, national emissions trends of on-road mobile sources have declined over the last 30 years. As shown in Exhibit 19-6, despite large increases in population, personal travel and freight transportation, and in spite of very limited highway expansion and public mode choice, total on-road motor vehicle emissions of carbon monoxide (CO), volatile organic compounds (VOCs), oxides of nitrogen (NOx), particulate matter (PM-10), lead (Pb), and sulfur dioxide (SO2) have declined 77 percent since 1970. The Environmental Protection Agency (EPA) expects this downward trend to continue well into the future.

In addition to the reduction in emission levels, on-road motor vehicle emissions have become a smaller percentage of total emissions. In fact, in 1970 motor vehicles contributed 59 percent of total emissions of carbon monoxide, oxides of nitrogen (NOx), volatile organic compounds (VOCs), and particulate matter (PM-10) when compared to stationary, area, and non-road mobile sources. However, by 1999, the motor vehicle portion of emissions of these pollutants dropped to 48 percent. This is described in Exhibit 19-6.

The majority of the emissions reductions have resulted from stricter emissions standards, improved engine technology, and cleaner fuels, and engines and fuel are to 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 pick-up trucks. This regulation marks the first time that larger SUVs and other light-duty trucks are 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. When the new tailpipe and sulfur standards are implemented, Americans will benefit from the cleanair equivalent of removing 164 million cars from the road. These new standards require passenger vehicles to be 77 to 95 percent cleaner than those on the road today and the reduction of the sulfur content of gasoline by up to 90 percent.

EPA has also recently issued new emission standards that will begin to take effect in model year 2007 applying to heavy-duty highway engines and vehicles. These standards are based on the use of high-efficiency catalytic exhaust emission control devices or comparably effective advanced technologies. Because these devices are damaged by sulfur, EPA is also 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. The clean air impact of this program is expected to be dramatic when fully implemented. This program will provide annual emission reductions equivalent to removing the pollution from more than 90 percent of today's trucks and buses, or about 13 million trucks and buses. Exhibits 19-7 and 19-8 describe this graphically, where NMHC refers to non-methane hydro-carbons, a chemical compound emitted in vehicle exhaust and evaporative emissions.

Page last modified on November 7, 2014
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