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CONTEXT OF THE CMAQ PROGRAM

As noted in Chapter 1, the primary policy goal of the CMAQ program is to improve air quality; congestion mitigation is another program objective to the extent that it supports this goal. In this chapter, the role of the CMAQ program in meeting both goals is discussed. The chapter begins with a brief overview of the air quality problem in the United States, its effect on human health and the environment, the contribution of transportation to the problem, the costs imposed by motor vehicle pollution, and the regulatory and planning process for pollution control. Within this broader context, the specific role of the CMAQ program in helping meet air quality standards is addressed. The discussion then turns to the role of the CMAQ program in reducing congestion. Congestion is defined, measurement of the extent and costs of congested travel on U.S. highways is reviewed, the link between congestion and air quality is examined, and the specific role of the CMAQ program in helping alleviate traffic congestion is discussed. In a final section, the changing air quality and travel context within which the CMAQ program operates and the effect of this context on the future direction of the program are considered. The chapter ends with conclusions and a review of implications for evaluation of the CMAQ program.

THE CMAQ PROGRAM AND AIR QUALITY IMPROVEMENT

Air Quality Standards

Protection of public health is the primary purpose of air quality regulation. The Clean Air Act Amendments (CAAA) of 1970 (Public Law 91-604, 84 Stat. 1676) required, and the U.S. Environmental Protection Agency (EPA) developed, National Ambient Air Quality Standards (NAAQS) for six criteria pollutants considered harmful to



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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 2 CONTEXT OF THE CMAQ PROGRAM As noted in Chapter 1, the primary policy goal of the CMAQ program is to improve air quality; congestion mitigation is another program objective to the extent that it supports this goal. In this chapter, the role of the CMAQ program in meeting both goals is discussed. The chapter begins with a brief overview of the air quality problem in the United States, its effect on human health and the environment, the contribution of transportation to the problem, the costs imposed by motor vehicle pollution, and the regulatory and planning process for pollution control. Within this broader context, the specific role of the CMAQ program in helping meet air quality standards is addressed. The discussion then turns to the role of the CMAQ program in reducing congestion. Congestion is defined, measurement of the extent and costs of congested travel on U.S. highways is reviewed, the link between congestion and air quality is examined, and the specific role of the CMAQ program in helping alleviate traffic congestion is discussed. In a final section, the changing air quality and travel context within which the CMAQ program operates and the effect of this context on the future direction of the program are considered. The chapter ends with conclusions and a review of implications for evaluation of the CMAQ program. THE CMAQ PROGRAM AND AIR QUALITY IMPROVEMENT Air Quality Standards Protection of public health is the primary purpose of air quality regulation. The Clean Air Act Amendments (CAAA) of 1970 (Public Law 91-604, 84 Stat. 1676) required, and the U.S. Environmental Protection Agency (EPA) developed, National Ambient Air Quality Standards (NAAQS) for six criteria pollutants considered harmful to

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 public health—carbon monoxide (CO), lead, nitrogen dioxide (NO2), ozone, particulate matter (PM10),1 and sulfur dioxide (SO2). Primary standards were established that set allowable concentrations of and exposure limits for these criteria pollutants to protect public health with “an adequate margin of safety” (NRC 2000, 16). Secondary standards were also established to protect the public welfare against environmental and property damage (NRC 2000, 16). EPA is required to review and update the NAAQS for major air pollutants every 5 years on the basis of the latest scientific evidence. Another category of pollutants, known as hazardous air pollutants or air toxics, is also regulated under the Clean Air Act. Air toxics are emitted from thousands of sources, such as electric utilities, automobiles, and dry cleaners. The CAAA of 1990 mandated the development of technology-based emission standards for the 188 relevant pollutants, as well as an assessment of remaining risks (EPA 2001a, 80). According to the most recent EPA inventory, highway vehicles are responsible for about 30 percent of the 4.6 million tons of air toxics released nationwide (EPA 2001a, 82). The inventory does not include diesel particulate matter, which EPA recently listed as a mobile source air toxic and is addressing in several regulatory actions discussed in the final section of this chapter. In 1997 EPA revised the NAAQS for ozone and PM on the basis of a review of the adverse health effects of exposures to ambient pollutant levels allowed by the then-current standards. The new standard for ozone extended the exceedance period from a 1-hour averaging time to an 8-hour standard to protect against longer exposure periods, and also tightened the standard for most nonattainment areas, changing from a 1-hour daily maximum of 0.12 parts per million (ppm) ozone concentration to a 0.08 ppm 8-hour standard (Federal Register 1997a, 38,856).2 Moreover, whereas prior standards focused on PM10, the new standards for PM targeted PM2.5 for the first time 1  PM10 is composed of coarse particles (i.e., between 2.5 and 10 micrometers in mean aerodynamic diameter) and fine particles (PM2.5) with mean aerodynamic diameter of less than 2.5 micrometers. 2 The new 8-hour standard is not a daily maximum, like the 1-hour standard, but instead is based on the 3-year average of the fourth-highest daily maximum 8-hour average.

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 on the basis of epidemiological studies that revealed associations between ambient PM concentrations and various adverse health effects, including mortality (Federal Register 1997b, 38,652). The 24-hour averaging standard for PM10 was also made more stringent. Challenges to the new standards in the Appellate and Supreme Courts have stalled the initial phase of implementation, but these standards were not to take full effect until 2012 and 2018 for ozone and PM2.5, respectively. As of this writing, EPA still needs to satisfy the Court that its new ozone standard can be implemented in a manner compatible with the 1990 CAAA.3 As of September 2000, the most recent period for which data are available, 101 million people, slightly more than one-third (35 percent) of the U.S. population, were living in 114 areas designated as being in nonattainment for at least one of the criteria pollutants (EPA 2001a, 76). Health and Environmental Effects of Criteria Pollutants and Air Toxics Concentrations of criteria pollutants that exceed regulated levels are believed to contribute significantly to adverse health effects, which can range from illness to premature death. The adverse health effects of CO and ozone have been known for some time. CO enters the blood stream and links to hemoglobin, reducing delivery of oxygen to the body’s organs and tissues. The health threat from lower levels of CO is most serious for those who suffer from cardiovascular disease (EPA 2001a, 11). However, impairment of cognitive skills, vision, and work capacity may occur with elevated CO levels in healthy individuals (EPA 2001a, 11). The health effects associated with exposure to levels of ozone above the 1-hour standard range from short-term effects, such as chest pain, decreased lung function, and increased susceptibility to respiratory infection, to possible long-term consequences, such as premature lung aging and chronic respiratory illnesses (EPA 2001a, 29). 3 The issue is that requirements for controls in nonattainment areas depend on the areas’ classification (e.g., moderate, serious, severe, extreme), which is keyed to the 0.12 ppm standard in the CAAA. If the standard changes, it is not clear how areas should be classified.

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 New epidemiological evidence, obtained largely during the 1990s, led to the promulgation of revised PM standards in 1997 and intense scrutiny concerning PM’s adverse health effects, including premature death (NRC 1998, ix). Both coarse and fine particulates can accumulate in the respiratory system. Coarse particles aggravate respiratory conditions such as asthma. Fine particles are also associated with exacerbation of asthma and other respiratory-tract diseases, decreased lung function, increased hospitalization for cardiopulmonary diseases, and premature death (EPA 2001a, 41).4 Air toxics are known to cause or are suspected of causing cancer and having other serious human health effects (EPA 2001a, 79). Relative to criteria pollutants, however, less information is available about the health and environmental impacts of individual hazardous air pollutants (EPA 2001b, 26). Pollutant deposition can also have adverse effects on ecosystems. SO2 is a well-known precursor to acid deposition (acid rain), as is the ozone precursor NO2 (EPA 2001a, 61). Acids are delivered to ecosystems through the deposition of dry particles and gases (such as nitric acid vapor); rain and snow; and, in coastal and high-elevation areas, clouds or fog. Although nitrogen is an essential plant nutrient, deposition of atmospheric nitrogen in some regions of the United States contributes to acidification of sensitive soils and surface waters; groundwater pollution; and eutrophication5 of downstream waters, such as estuaries and near-coastal ecosystems (Driscoll et al. 2001). 4 The issuance of new standards for PM2.5 has focused considerable attention on the need to review the science that underlies the standards. For example, Congress directed the EPA Administrator to arrange for an independent study by the National Research Council (NRC) on the most important research priorities relevant to setting PM standards, among other tasks, and added substantial funds to EPA’s budget to support the expansion of PM research. Three NRC reports addressing this issue have been published to date (NRC 1998; NRC 1999; NRC 2001a). In addition, EPA has funded five national centers to conduct PM research. The Health Effects Institute, a nonprofit independent research institute that addresses the health effects of air pollution caused by motor vehicles, has also conducted several major reviews and reanalyses of a number of key studies (HEI Perspective 2001), and the American Lung Association has published a review of recent peer-reviewed studies on the health effects of PM air pollution (ALA 2000). 5 The process by which a body of water acquires a high concentration of nutrients, especially phosphates and nitrates, that lead to excessive algae growth and depletion of oxygen in the water.

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 For example, approximately 30 percent of the nitrogen loading to the Chesapeake Bay and the New York Bay caused by human action is due to atmospheric deposition (Hinga et al. 1991; Fisher and Oppenheimer 1991). Ozone and its precursors can also affect sensitive vegetation and ecosystems. Specifically, they can lead to reduced crop and commercial forest yields and increased plant susceptibility to disease, pests, and the adverse effects of harsh weather (EPA 2001a, 29). Overall, acidic deposition can significantly affect the cycling of nutrients and the acidity of land or water ecosystems. In addition, fine particulates are a major cause of haze and poor visibility in a number of areas, including many national parks (EPA 2001a, 41). Formation of Criteria Pollutants Air pollutants either are directly emitted from sources (“primary” pollutants) or are formed in the atmosphere through physical and chemical processes (“secondary” pollutants), resulting in ambient concentrations that can adversely affect the health of exposed populations. Of the six criteria pollutants, CO, SO2, and lead are primary pollutants; NO2 has both primary and secondary origins; and ozone is a secondary pollutant formed by the action of sunlight and chemical reactions involving volatile organic compounds (VOCs) and oxides of nitrogen (NOx)6 (NRC 2000, 16–17). Airborne PM is a combination of primary and secondary pollutants (NRC 2000, 17). Carbonaceous particles from combustion sources (i.e., motor vehicles; utilities; industrial, commercial, and institutional boilers; and area source combustion) and windblown dust account for most of the primary PM. Ammonium sulfate and ammonium nitrate from the oxidation of SO2 and NOx, respectively, are important components of secondary particles, though a significant fraction of organic carbon PM can also result from the chemical reactions of VOCs. Other important constituents of airborne PM include heavy metals and polycyclic aromatic hydrocarbons (PAH). The distinction between primary and secondary pollutants is important in designing appropriate pollution control strategies. For 6 NOx emissions from motor vehicles, the primary focus of this report, consist of a mixture of NO and NO2 (TRB 1995, 44).

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 example, emissions of transportation-related PM, CO, and NO2—primary pollutants—tend to be concentrated on and near congested highways and at other locations where traffic densities are high. Thus targeted improvements, such as relieving traffic bottlenecks or otherwise reducing emissions (e.g., substituting cleaner-burning fuels) can reduce CO, NO2, and PM. In contrast, ozone and secondary fine particles typically are regional problems, not amenable to geographically targeted projects. Furthermore, ambient concentrations of secondary pollutants are not always proportional to their source emissions because the rates at which they form are not necessarily proportional to quantities of precursor gases. In the case of ozone, knowing the relative concentrations of precursor VOCs and NOx is critical to selecting appropriate abatement strategies. For example, in regions with low levels of VOCs relative to NOx, characteristic of some highly polluted urban areas, strategies that lower VOCs will reduce peak ozone concentrations; however, lowering NOx can lead to lower or higher ozone in the urban center, depending on the relative concentrations of VOC and NOx, the specific mix of VOCs present, and the proximity to NOx emissions, as well as the effects of local meteorology on transport and dispersion. These processes are complex and depend on many meteorological and chemical variables, which are described in more detail in Appendix B. Contribution of Transportation to Pollutant Formation The principal sources of polluting emissions are as follows: Transportation (on- and off-road vehicles); Stationary sources (e.g., fuel combustion by utilities and industrial, commercial, and residential sources); Industrial process sources (e.g., chemical manufacturing, petroleum refining, solvents, and waste disposal); and Other sources [e.g., biogenic emissions from natural and agricultural sources and from other combustion (NRC 2000, 17)]. In 1999, the most recent year for which data are available, emissions from transportation sources, also known as mobile source emissions, contributed to more than half (53 percent) of nationwide

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 emissions of EPA’s criteria pollutants (see Table 2-1). Nearly two-thirds (64 percent) of mobile source emissions are from highway (on-road) vehicles, although the range is considerable for each pollutant source (see Figure 2-1). For example, highway vehicles are the dominant source of U.S. CO emissions. In 1999 highway vehicles contributed 51 percent of total CO emissions nationwide (see Table 2-1 and Figure 2-1). In many urban areas, mobile sources account for more than 90 percent of total CO emissions, for example, as documented in the emission inventories for the San Francisco Bay Area and the South Coast Air Quality Management District (Los Angeles area). Nevertheless, in 1999 CO levels were the lowest recorded in the last 20 years; currently there are only six areas of the country with CO levels violating the NAAQS (EPA 2001a, 2). More specifically, CO emissions from highway vehicles have decreased by approximately 50 percent during the past 20 years despite nearly a 60 percent increase in vehicle-miles traveled (VMT) (EPA 2001a, 13). TABLE 2-1 Contribution of Transportation to Emissions of Criteria Pollutants in the United States, 1999 (millions of short tons) Source Category Pollutant Total CO NOx VOCs PM10 Lead SO2 Transportation Total 75.1 14.1 8.5 0.8 0.5 1.3 100.3 Highway vehicle share 49.9 8.6 5.3 0.3 0.02 0.4 64.5 Fuel combustion 5.3 10.0 0.9 1.0 0.5 16.1 33.8 Industrial processes 7.6 0.9 8.0 1.3 3.2 1.5 22.5 Miscellaneous 9.4 0.3 0.7 20.6a 0.0 0.01 31.0 Total 97.4 25.3 18.1 23.7 4.2 18.9 187.6 Share of total (percent) All transportation 77.0 56.0 47.0 3.0 12.0 7.0 53.0 Highway vehicles 51.0 34.0 29.0 1.3 0.5 2.1 34.0 Note: CO = carbon monoxide; VOCs = volatile organic compounds; NOx = oxides of nitrogen; PM10 = particulate matter (with mean aerodynamic diameter less than 10 micrometers); SO2 = sulfur dioxide. aIncludes windblown dust and natural sources (i.e., wind erosion). Source: EPA (2001a).

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 FIGURE 2-1 Sources of criteria air pollutants. Estimated total annual emissions of criteria pollutants from stationary sources, industrial processes, transportation (on-road and nonroad), and miscellaneous sources for 1999. Emissions are shown in thousands of short tons except for lead, which is shown in short tons. (Source: EPA 2001a, Appendix A.)

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 During the last 20 years, ozone levels (1-hour and 8-hour) have improved considerably (EPA 2001a, 29). Urban ozone levels, historically the most severe, have shown marked improvement in response to stringent control programs (EPA 2001a, 29). Mobile source emissions are a major source of VOCs and NOx—the precursors of ozone and fine particulate matter. In 1999 highway vehicles contributed 29 percent of VOCs and 34 percent of NOx emissions nationwide. VOC emissions from highway vehicles declined 18 percent during the past 10 years, but NOx emissions increased by 19 percent during the same period (EPA 2001a, 37). This poor performance of NOx emissions may reflect the lack of attention paid to the role of this important pollutant in ozone formation until relatively recently (NRC 1991). According to the national emissions inventory for 1999, tailpipe emissions from highway vehicles represented a small share (1.3 percent and 3.4 percent) of directly emitted (i.e., primary) PM10 and PM2.5, respectively, from all sources (see Figure 2-1). However, tailpipe emissions account for a substantially higher portion of PM in urban areas, where the majority of mobile source emissions occur. For example, ambient source apportionment studies show that particulate emissions in motor vehicle exhaust account for nearly 40 percent of the fine PM in Denver and Los Angeles (Watson et al. 1998; Fujita et al. 1998; Schauer et al. 1996). Including dust from paved roads and secondary ammonium nitrate from NOx emissions, motor vehicles may contribute as much as 50 to 75 percent of the fine PM in Denver and Los Angeles. In contrast, windblown dust from unpaved roads and, to a lesser extent, agriculture and forestry, wildfires, and managed burns occurs mainly in rural areas. Coarse particles are relatively short-lived in the atmosphere, tending to be removed rapidly or deposited within a short distance from the point of their release.7 Carbonaceous fine particles from combustion sources and secondary particles (i.e., nitrates, sulfates, and organic carbon formed in the atmosphere from the conversion of gaseous NOx, SO2, and VOCs), which range in size from 10 nanometers to 1 micrometer, are much longer lived and 7 Little is known, however, about the influence of exposure to road dust on the risks of mortality and disease.

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 are transported longer distances than coarse particles. Fine and ultra-fine particles also occur in far greater numbers than coarse particles. The greater numbers and longer lifetimes in the atmosphere of fine and ultrafine particles, as well as their ability to be inhaled into the deep lung, result in greater human exposure and potential health impacts than is the case for coarse particles. Transportation is a minor source of SO2 and no longer accounts for a large share of pollution from lead. Highway vehicles currently account for less than 1 percent of total lead emissions, primarily because of the use of unleaded gasoline (see Table 2-1 and Figure 2-1). Highway vehicles contribute about 2 percent of directly emitted SO2; coal-burning power plants are consistently the largest contributor (see Table 2-1). However, these percentages are somewhat misleading. Similar to emissions of NOx, SO2 emissions from motor vehicles react in the atmosphere to form sulfate aerosols and hence are an important precursor to PM2.5 (EPA 2001a, 61). The transportation sector also contributes to the formation of greenhouse gases. Approximately one-third of total U.S. anthropogenic emissions of CO2 comes from the transportation sector (NRC 2000, 20).8 About one-quarter of the total is attributable to highway vehicles (NRC 2000, 20). Emissions from highway vehicles vary by vehicle and fuel type. The primary emissions of gasoline-powered vehicles—passenger vehicles and panel trucks—are CO, VOCs, NOx, and SO2, although research is under way to characterize PM emissions from high-emitting gasoline vehicles (see Figure 2-2).9 The primary emissions of diesel vehicles—mainly heavy trucks and buses—are NOx, CO, PM, VOCs, and SO2 (see Figure 2-2). Emissions of NOx and PM from heavy trucks and buses are of greatest concern. Heavy-duty vehicles are a dominant source of directly emitted fine and ultrafine particles 8 Note, however, that there is no air quality standard for CO2—the principal green-house gas—because CO2 is not toxic and therefore has no direct negative health effects. 9 Estimates of PM emissions from light-duty vehicles are highly uncertain. They are generally based on EPA’s PART5 model, which a recent NRC study characterized as “seriously out of date” (NRC 2000, 70).

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 FIGURE 2-2 Estimated mobile source emissions by vehicle and fuel type. MOBILE5 and PART5 estimates of 1999 emissions from the on-road motor vehicle fleet. It is likely that MOBILE5 underestimates gasoline VOC and diesel NOx emissions. Emissions are shown in thousands of short tons. (Source: EPA 2001a, Appendix A.)

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 accelerations and grades. One sharp acceleration may cause as much pollution as the entire remaining trip (Carlock 1993). High-emitting vehicles are the other major contributors to on-road vehicle emissions. The distributions of emission rates among in-use vehicles are highly skewed, such that a relatively small fraction of high emitters accounts for a disproportionate fraction of total emissions (NRC 2000, 77). This fraction is likely to increase during the next two decades as the Tier 2 emission standards are implemented and absorbed into the vehicle population. TCMs that are focused on these two pollutant sources (e.g., strategies to reduce vehicle cold starts, remote sensing to detect high-emitting vehicles) are likely to have big payoffs.48 Emission standards for heavy-duty diesel engines will also be tightened. Beginning with the 2004 model year, all heavy-duty vehicles will be required to meet an NOx level approximately 80 percent below the initial standard established in 1985 (see Table 2-6).49 PM emission standards will also be significantly tightened starting in model year 2007 (see Table 2-6). A related rule, reducing sulfur in diesel fuel and thereby enabling new diesel engines to run cleaner, is slated to take effect in 2006. As previously discussed, however, much remains to be done to reduce diesel emissions, especially particulates, and this could well become a more important focus area of the CMAQ program. The impact of cleaner vehicles, however, both diesel- and gasoline-powered, may be retarded by growth in VMT. In the past, travel growth appears to have offset some of the projected gains from stricter vehicle emission standards (TRB 1995, 16).50 The question thus arises of whether metropolitan travel growth and related 48 Remote sensing refers to a method for measuring pollution levels in a vehicle’s exhaust while the vehicle is in use. If OBD systems are effective, they could also prevent vehicles from becoming high emitters. 49 The 2004 standard will be implemented in October 2002 for engine manufacturers, subject to a settlement agreement with EPA concerning the use of devices to defeat emission testing on earlier vehicles (Schimek 2001, 436). 50 For example, when the 1990 CAAA was passed, EPA estimated that gains in tailpipe emissions could be offset by 2002 for CO and VOCs and by 2004 for NOx. Thus, the act mandated measures designed to limit automobile trips in the most severely polluted areas and required strict monitoring of VMT growth in less severe nonattainment areas.

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 TABLE 2-6 Federal Exhaust Emission Standards for Heavy-Duty Diesel Engines Model Year NOx PM, Heavy Duty PM, Urban Bus 1985 10.7 NA NA 1988 10.7 0.60 0.60 1990 6.0 (44) 0.60 0.60 1991 5.0 (53) 0.25 (58) 0.25 (58) 1993 5.0 (53) 0.25 (58) 0.10 (83) 1994 5.0 (53) 0.10 (83) 0.07 (88) 1996 5.0 (53) 0.10 (83) 0.05 (92) 1998 4.0 (63) 0.10 (83) 0.05 (92) 2004 (2002) 2.0 (81) 0.10 (83) 0.05 (92) 2007–2010 0.2 (98) 0.01 (98) 0.01 (98) Note: Standards are in grams per brake-horsepower hour; NA = not applicable. Percentage decreases from precontrol levels are in parentheses. Source: Adapted from Schimek (2001, 437). congestion are likely to worsen in the future. Arguing for a slowing in the rate of VMT growth are findings that travel effects due to the entrance of women into the workforce have largely been absorbed, that the ratio of vehicles to licensed drivers is 1 to 1 (Hu and Young 1999, 9), and that a growing proportion of the population of older motorists drive less.51 FHWA, for example, forecasts an average annual urban VMT growth rate of 2 percent for 1998 through 2017, a sharp decline from the 3.2 percent average annual rate of growth in urban travel between 1987 and 1997 (FHWA and FTA 2000, 2-10, 9-3). More flexible work policies and electronic advances that enable working at home or from a nearby telecommuting center may also limit work trips and peak-period travel, although there is some evidence that telecommuting can result in an increase in non-commute-related personal vehicle trips (Koenig et al. 1996). More essential, telecommuting may change the time of day and location of travel, with important 51 However, there is evidence that older drivers are driving more than in the past. For example, in 1995 older drivers took more trips and drove more than their corresponding cohorts in 1990 (Hu and Young 1999, 49).

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 effects on emissions.52 The results of the NPTS, which show relatively constant average commuter trip times over a period of several years, suggest that in the longer run, households respond to increasing VMT and higher levels of congestion by moving farther away from metropolitan centers. As jobs follow people, commute times are kept relatively constant (TRB 1994, 114–115). On the other hand, arguing for continuing growth in congestion for many metropolitan areas are projected increases in population and income—major determinants of travel in a region (Hansen et al. 1993, 6–29). Thus a definitive judgment about growth in VMT and congestion is simply not possible on the basis of the available data (Meyer 1994, 58). Both are likely to persist in many metropolitan areas, but some regions may see a slowing in the rate of travel growth, which in turn would decrease the benefits of traffic-related CMAQ strategies. Advances in Analytic Methods for Estimating Strategy Effects Estimating the pollution reduction potential of many CMAQ-eligible strategies may become easier in the future as new measurement tools become available and more appropriate models are developed. For example, although it may never be possible to measure changes in concentrations of important regional pollutants, such as ozone and PM, due to a particular project, methods for measuring changes in vehicle emissions at the tailpipe and human exposure levels are being developed. Remote sensing of vehicle exhaust emissions is already possible, as are remote readings of exhaust measurements (NRC 2001b, 103).53 A new generation of real-time instruments and sophisticated experimental designs has also been developed for characterization of human exposure to PM2.5 and gaseous pollutants in many micro environments, including a wide range of in-vehicle and 52 For example, travel at midday or in the afternoon under noncongested conditions and in locations removed from a central city may be less polluting than travel in the morning peak-period commute. 53 CO emissions can be measured reliably using remote sensing techniques. Less-certain results are available for VOCs and NOx, and measurement of PM is an important research priority. Attention to quality assurance and quality control is essential (NRC 2001b, 116–117).

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 indoor atmospheres affected by the penetration of vehicle-related emissions (Monn 2001).54 New models are also under development that will be more appropriate for estimating the emission effects of many small-scale CMAQ projects. Future generations of mobile emission models will predict emissions as a function of vehicle operation, such as idle, steady-state cruise, and various levels of acceleration and deceleration. Two modal modeling approaches currently under development are the Comprehensive Modal Emissions Model (CMEM) (Barth et al. 2000) and the Mobile Emissions Assessment System for Urban and Regional Evaluation (MEASURE) (Guensler et al. 1998).55 USDOT, EPA, and the Department of Energy are sponsoring the development of a suite of integrated analytical and simulation models and supporting databases for transportation and air quality analysis (TMIP 1999). Known as the TRansportation ANnalysis and SIMulation System (TRANSIMS), the modeling system pairs data from a second-by-second traffic simulation model with a modal emission model (CMEM) to derive microscale-level emission estimates from changes in traffic signalization and other traffic operational changes; inputs are also provided for air quality modeling at appropriate temporal and geographic scales. The application of these new models should provide for more accurate microscale assessments of the travel-related effects (e.g., changes in traffic flows, speeds), emission effects, and possibly even air quality impacts of many CMAQ projects. CONCLUSIONS AND IMPLICATIONS FOR PROGRAM EVALUATION Transportation is one of the many sources of poor air quality in the United States. The primary goal of the CMAQ program is to reduce pollution from motor vehicles. Program funds are targeted to areas with the worst air quality (nonattainment and maintenance areas). 54 Other references on exposure assessment of air pollutants include Rodes et al. (1998), Long et al. (2000), Moosmuller et al. (2001), and Janssen et al. (1998). 55 The modal model under development at the University of California, Riverside, by Barth et al. is based on 300 vehicles tested under a variety of laboratory driving cycles. The modal approach under development at the Georgia Institute of Technology is a modal emissions model based on geographic information systems, using statistical analysis of historical laboratory and instrumented vehicle data.

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 Ozone and its precursors and CO are the primary pollutants of concern, reflecting the critical pollution problems at the time the 1990 CAAA and the 1991 ISTEA were passed. Projects aimed at reducing PM10 emissions became explicitly eligible for CMAQ funding when TEA-21 reauthorized the program, but particulates are not reflected in the funding formula. A region’s particular air quality problems and conformity requirements can influence how program funds are deployed. For example, nonattainment areas with significant air quality problems often look to CMAQ to help fund TCMs or other eligible projects for which credit can be taken toward meeting rate-of-progress requirements or SIP commitments. The type of local air quality problem may affect project choices as well. For example, areas having NOx problems may not undertake certain traffic flow improvements that would significantly increase vehicle speeds, even if such projects are CMAQ eligible, because those improvements can exacerbate ozone formation. CMAQ program regulations require that states report annually, by the relevant affected pollutants, on the potential emission reductions of funded projects. No attempt is made to determine how these projects might affect pollutant concentrations, human exposure levels, or public health. Estimating emission reductions with any degree of certainty is often difficult because the available emissions models for making such projections, or their inputs, are not well suited to the purpose. The models were developed to assess regional emission effects, not to evaluate TCMs, whose impacts are modest and often focused on particular transportation corridors or subregions. Congestion is a major problem in many large metropolitan areas. Congestion mitigation is another important goal of the CMAQ program; however, the legislation authorizing the CMAQ program prohibits spending on certain traditional congestion relief projects. For example, projects to provide new capacity for SOV travel, such as the addition of general-purpose lanes to an existing facility or a new highway at a new location, are ineligible even if those projects could help alleviate congestion. The reason for this is that such projects are viewed as not supporting the CMAQ program’s primary goal of reducing motor vehicle emissions because they encourage vehicular travel. Nor is it likely that many of these projects would meet con-

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The Congestion Mitigation and Air Quality Improvement Program: Assessing 10 Years of Experience - Special Report 264 formity requirements, another program requisite. Nevertheless, CMAQ funds can be used to support a wide range of other congestion relief strategies. The context within which the program operates has changed since the program’s inception and will continue to do so. For example, as vehicles become cleaner, some TCMs may become less effective, while other strategies (e.g., vehicle scrappage programs) that target remaining air pollution sources (e.g., high-emitting vehicles) will become more valuable. Moreover, emerging knowledge about the health effects of various pollutants may require some redirection of CMAQ funds when the program is reauthorized. For example, as knowledge about the adverse health effects of particulates and air toxics has grown, projects that address the key transportation-related sources of these pollutants (e.g., heavy trucks and buses) may warrant greater attention. Fortunately, advances in measurement tools and models should make it easier to assess the pollution reduction potential of many CMAQ strategies and may even enable the analysis to be extended to an assessment of project effects on human exposure levels. This chapter has provided information about the air quality and congestion context within which the CMAQ program operates to help the reader understand how the program has developed, provide perspective on the problems it attempts to address, and highlight some of the key changes that may affect its future direction. In the following chapter, an overview of program operations and spending trends to date is provided. REFERENCE Abbreviations ALA American Lung Association BTS Bureau of Transportation Statistics EPA U.S. Environmental Protection Agency FHWA Federal Highway Administration FTA Federal Transit Administration HEI Health Effects Institute NCHRP National Cooperative Highway Research Program NRC National Research Council

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