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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 2 Contribution of Motor Vehicle Transportation to Air Pollution and Energy Consumption Motor vehicles run on fossil fuels, emitting pollutants that are a major cause of poor air quality in metropolitan areas and consuming a large fraction of the nation's petroleum resources. In this chapter, the impacts of motor vehicle transportation on air quality and energy consumption are described and the models that are commonly used to analyze these impacts are introduced. TRANSPORTATION AND AIR QUALITY The four principal sources of polluting emissions from man-made sources are transportation (primarily highway vehicles), stationary fuel combustion (especially electrical utilities), industrial processes such as chemical refining, and solid waste disposal (Horowitz 1982, 21). Emissions that either directly cause or combine to form pollution (called primary and secondary pollutants, respectively) may also be classified as stationary, area, or mobile, depending on the magnitudes and geographical distributions of their emissions (DOT and EPA
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 1993, 2).1 Pollutants from motor vehicle transport, the focus of this study, are commonly referred to as mobile source emissions. To comply with the requirements of the 1970 Clean Air Act, the Environmental Protection Agency (EPA) developed national ambient air quality standards (NAAQS) that set allowable concentration and exposure limits for six pollutants considered harmful to public health. The NAAQS are expressed as average concentrations of pollutants over some period of time (see box).2 EPA tracks both the emissions or flows of harmful materials from polluting activities, such as factories and transportation, on the basis of the best available engineering and modeling estimates, and the accumulation of these emissions in the air as concentrations of pollutants, which are directly measured at selected sites throughout the country (Curran et al. 1994, 20; Horowitz 1982, 3–4). Regulatory activities are directed toward attainment of NAAQS (i.e., concentration standards), because health effects are directly related to public exposure to pollutants at specific concentrations. Transportation-Related Pollutants According to current estimates, transportation sources account for about 45 percent of nationwide emissions of EPA's six criteria pollutants. The range is considerable for each pollutant source (Table 2-1) and there is a high degree of uncertainty with respect to many of the estimates. Ground-level ozone is the most pervasive of the transportation-related pollutants; in 1993 approximately 51 million persons lived in counties that exceeded the ozone standard. Nearly 12 million persons lived in counties that did not meet the carbon monoxide (CO) standard in the same year (Curran et al. 1994, 15). Highway vehicles are the largest source of transportation-related emissions for nearly every type of pollutant (Table 2-1). In total, they contribute slightly more than one-third of nationwide emissions of the six criteria pollutants. Formation of Motor Vehicle Emissions The primary sources of motor vehicle emissions are exhaust emissions from chemical compounds that leave the engine through the tail pipe system and the crankcase and evaporative emissions from the fueling
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use National Ambient Air Quality Standards (NAAQS) in Effect in 1991 (Curran et al. 1994, 19) POLLUTANT PRIMARY (HEALTH RELATED) SECONDARY (WELFARE RELATED) TYPE OF AVERAGE STANDARD LEVEL CONCENTRATIONa TYPE OF AVERAGE STANDARD LEVEL CONCENTRATION CO 8-hrb 9 ppm (10 mg/m3) No secondary standard 1-hrb 35 ppm (40 mg/m3) No secondary standard Pb Maximum quarterly average 1.5 μg/m3 Same as primary standard NO2 Annual arithmetic mean 0.053 ppm (100 μg/m3) Same as primary standard O3 Maximumc daily 1-hr average 0.12 ppm (235 μg/m3) Same as primary standard PM-10 Annual arithmetic meand 50 μg/m3 Same as primary standard 24-hrd 150 μg/m3 Same as primary standard SO2 Annual arithmetic mean 80 μg/m3 (0.03 ppm) 3-hrb 1300 μg/m3 (0.50 ppm) 24-hrb 365 μg/m3 (0.14 ppm) NOTE: CO = carbon monoxide; Pb = lead; NO2 = nitrogen dioxide; O3 = ozone; PM-10 = particulate matter; SO2 = sulfur dioxide; ppm = parts per million; μg/m3 = micrograms per cubic meter; mg/m3 = milligrams per cubic meter. a Parenthetical value is an approximately equivalent concentration. b Not to be exceeded more than once a year. c The standard is attained when the expected number of days per calendar year with maximum hourly average concentrations above 0.12 ppm is equal to or less than 1, as determined according to Appendix H of the Ozone NAAQS. d Particulate standards use PM-10 (particles less than 10μ in diameter) as the indicator pollutant. The annual standard is attained when the expected annual arithmetic mean concentration is less than or equal to 50 μg/m3; the 24-hr standard is attained when the expected number of days per calendar year above 150 μg/m3 is equal to or less than 1; as determined according to Appendix K of the PM NAAQS.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use TABLE 2-1 Transportation Contribution to Emissions of Major Air Pollutants in the United States, 1992 (Millions of Short Tons) (Nizich et al. 1994, 3-11–3-16) SOURCE CATEGORY POLLUTANT CO NOx VOC PM-10 Pb SO2 TOTAL Transportation Total 75.3 10.4 8.3 0.6 1.6 0.7 96.9 Highway vehicle share 60.0 7.4 6.1 0.2 1.4 0.4 75.5 Fuel combustion 5.4 11.7 0.6 1.2 0.5 19.3 38.7 Industrial processes 5.2 0.9 3.1 0.6 2.3 1.9 14.0 Solid waste disposal 1.8 0.1 10.4 0.3 0.5 0 13.1 Miscellaneous 9.5 0.3 0.9 42.8 0 0 53.5 Total 97.2 23.4 23.3 45.5 4.9 21.9 216.2 Share of total (percent) All transportation 77 44 36 1 33 3 45 Highway vehicles 62 32 26 0.4 29 2 35 NOTE: CO = carbon monoxide; VOC = volatile organic compounds; NOx = oxides of nitrogen; PM-10 = particulate matter; Pb = lead; SO2 = sulfur dioxide.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use system [mainly volatile organic compounds (VOCs)] (NRC 1992, 69). For most motor vehicles (i.e., those powered by gasoline), exhaust emissions are formed in a two-stage process: emissions originate as a result of the combustion of fuel in the engine (engine-out emissions) and are then reduced by passing through a catalytic converter (tail pipe or exhaust emissions). For diesel-powered vehicles, the process of producing exhaust emissions is simpler, because there is presently no aftertreatment (i.e., catalytic converter). Carbon monoxide and VOCs are the product of incomplete combustion of motor fuels and, in the case of VOCs, of fuel vapors emitted from the engine and fuel system (NRC 1991, 257). Oxides of nitrogen (NO x) are formed differently; they are the product of high-temperature chemical processes that occur during the combustion process itself (NRC 1991, 261). Particulates, another compound mainly found in diesel exhaust, are formed primarily from incomplete combustion of diesel fuel and lubricating oil (Weaver and Klausmeier 1988, 2–7; Conte 1990, 58). The air/fuel (A/F) ratio, which is controlled by the carburetor or fuel injection system, is the most important variable affecting the efficiency of catalytic converters and thus the level of exhaust emissions (Johnson 1988, 40). Because concentrations of key emissions are not at a minimum at the same A/F ratio (CO and VOCs are highest under fuel-rich conditions and NOx is highest under fuel-lean conditions), manufacturers must optimize catalytic converter operation within a narrow A/F ratio range, known as stoichiometry, to achieve the greatest control efficiency for all three pollutants (Figure 2-1). Major Pollutants by Type Transportation is the dominant source of U.S. CO emissions, and highway vehicles contribute nearly two-thirds of the total (Table 2-1). Carbon monoxide is an odorless gas that forms from incomplete combustion of motor fuels. The higher the share of fuel in the air-fuel mixture, the more CO is produced (NRC 1992, 69). Fuel-rich operations occur under cold-start conditions, when the vehicle has been turned off for some time and the catalytic converter is cold, or under heavy engine loads (e.g., during rapid accelerations, on steep grades, or at high speeds). CO concentrations tend to be high on and near con-
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use FIGURE 2-1 Variation of CO, VOC, and NOx concentration in the exhaust of a conventional spark-ignition engine with fuel/air equivalence ratio. Adapted from J. B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill, 1988, p. 571. Reproduced with permission of McGraw-Hill, Inc.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use gested roadways and at other locations where traffic densities are high. These concentrations are often referred to as CO hot spots. However, CO can also be viewed as a regional problem, with frequent reported exceedances of the 8-hr average concentration standards. Finally, CO contributes indirectly to greenhouse gas emissions (Gordon 1991, 60).3 Motor vehicles are also a major contributor to smog, the haze that hangs over many large urban areas, which has harmful health effects, contributes to the greenhouse problem, and adversely affects crops and vegetation (MacKenzie and Walsh 1990, 7). Ground-level ozone, an important constituent of smog, is not emitted directly into the atmosphere. Rather it is formed as a secondary pollutant through a chemical reaction between the ozone precursors, VOCs and NOx, which is stimulated by heat and sunlight (Gordon 1991, 62). Highway vehicles account for about one-quarter of total VOC emissions and about one-third of total NOx emissions (Table 2-1). EPA's estimates of VOC emissions, in particular, have been challenged in a National Research Council report as understating actual emission levels by a factor of 2 to 4 (NRC 1991, 7).4 Because it is a chemically reactive pollutant, ozone behaves quite differently from CO. The relation between ozone concentrations and VOC and NOx emissions is both nonlinear and synergistic; thus, changes in VOC and NOx emissions can have impacts on ozone that are difficult to predict. For example, ozone concentrations often are lower near large sources of motor vehicle emissions, because exhaust emissions of nitrogen oxide (NO) break down the ozone molecule.5 This is referred to as ozone scavenging. Also, spatial variations in ozone concentrations tend to be much more gradual than in CO concentrations (Horowitz 1982, 63). The role of NOx in urban ozone pollution has received attention recently from the scientific and regulatory communities. NOx emissions from motor vehicles consist of a mixture of NO and nitrogen dioxide (NO2) (NO being the dominant constituent), which is formed by high-temperature chemical processes during the combustion of fossil fuels (Horowitz 1982, 17; NRC 1991, 261). High concentrations of NO2, which are responsible for the yellowish-brown color of the sky in many smoggy areas, are caused primarily by the oxidation of NO from engine exhaust and other sources to NO2 through the chemical
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use processes that produce ozone (Horowitz 1982, 77). The NRC report (1991) argued that it is the balance between ambient levels of VOCs and NOx that determines ozone levels in a particular area, and that efforts to reduce NOx may be the most effective ozone abatement strategy in many of the nation's most polluted urban areas (NRC 1991, 7).6 In its final conformity regulations following the Clean Air Act Amendments of 1990 (CAAA), EPA requires that transportation improvement programs proposed by metropolitan planning organizations (MPOs) show reductions in NOx as well as VOCs from a 1990 baseline scenario (Federal Register 1993, 62,226).7 Particulates from diesel-fueled vehicles, primarily trucks and buses, contribute to pollution from inhalable particulate matter (PM-10). 8 Overall, tail pipe emissions of highway vehicles account for less than 1 percent of total PM-10 emissions (Table 2-1). The major source of particulates is road dust, which is a function of vehicle traffic, wildfires, and agricultural activity (Curran et al. 1994, 53). Particulate emissions are raising renewed concern because of medical evidence of their contribution to lung cancer (Dockery et al. 1993, 1753). Transportation no longer accounts for a large share of pollution from lead; use of unleaded gasoline has resulted in a 99 percent reduction in total lead emission levels from highway vehicles since 1970 (Nizich et al. 1994, 3-16). Finally, transportation is not a major contributor to sulfur dioxide (SO2) (Table 2-1). Although heavy trucks and buses emit oxides of sulfur because of the high sulfur content of diesel fuel, coal-fired electric utilities are the dominant source of SO2 emissions (Curran et al. 1994, 12). Pollutants by Vehicle and Fuel Type Emissions of specific pollutants vary by vehicle and fuel type. The primary emissions of gasoline-powered, passenger vehicles—the most common vehicle on the road—are CO, followed by much smaller emissions of VOCs and NOx. Most heavy-duty diesel vehicles—combination trucks and buses—use diesel fuel.9 Their primary emissions are NOx, followed by smaller emissions of CO, PM-10, SO2, and VOCs (Figure 2-2).
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use FIGURE 2-2 Comparison of national emission estimates (measured in short tons) for gasoline-powered, light-duty automobiles and diesel-powered heavy-duty vehicles (Nizich et al. 1994, A-4, A-8, A-15, A-19, A-24). Heavy-duty diesel vehicles produce about 5 percent of total emissions from all highway vehicles, roughly proportional to their share of highway travel but small compared with the emissions of gasoline-powered passenger vehicles, which represent nearly two-thirds of total emissions from highway vehicles.10 Diesel-powered vehicles, however, contribute a disproportionate share of total highway vehicle emissions of PM-10, SO2, and NOx: 72, 47, and 27 percent, respectively (Nizich et al. 1994, A-8, A-19, A-24). High levels of NOx emissions from heavy-duty vehicles are caused by the characteristics of diesel engines. Diesel engines typically run at higher combustion chamber pressures and temperatures than gasoline engines (Lilly 1984 in Guensler et al. 1991, Appendix A). Both conditions are conducive to high NOx emission levels. PM-10 and SO2 emissions are also higher for heavy-duty diesel vehicles than for gasoline-powered automobiles. Catalytic converters
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use have not been used with diesel engines because of particulates and concentrated sulfur gases in the exhaust gas, which could clog or deactivate the catalyst (Guensler et al. 1991, Appendix A). Particulates in diesel exhaust originate mainly from unburned fuel and oil (Weaver and Klausmeier 1988, 2–7; Conte 1990, 59, 61).11 However, introducing higher combustion temperatures to burn the fuel more completely and reduce particulates leads to higher NOx emissions. The challenge facing diesel engine manufacturers is to reduce emissions of both pollutants at the same time to meet NOx and particulate standards. Emissions of SO2 are also substantially higher for diesel than for gasoline engines because of the high sulfur content of diesel fuel. However, mandatory use of low sulfur or “clean” diesel fuel, which began in October 1993, should substantially reduce SO2 emissions as well as PM-10 emissions12 from heavy-duty, diesel-powered vehicles. Factors Affecting In-Use Emission Levels Actual emission levels from transportation sources are a function of several variables that can be grouped under four main categories: travel-related factors, driver behavior, highway network characteristics, and vehicle characteristics. Highway projects that add capacity and smooth traffic flows should affect emissions related to travel, driving patterns, and physical characteristics of the highway itself. Travel-Related Factors Trips and Vehicle Use Emissions are a function of trip taking as well as distance traveled. Trips matter because emissions vary depending on the share of the trip associated with different vehicle operating modes. Exhaust emissions, one of the major sources of emissions from motor vehicle operation, include vehicle start-up emissions (start-ups are classified as cold or hot starts depending on how long the vehicle has been turned off 13) and running emissions, which occur when the vehicle is warmed up and operating in a hot stabilized mode (Sierra Research 1993, 18, 19). Evaporative emissions, the other major source, consist entirely of
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use VOCs. They include running losses, which occur when the vehicle is operating in a hot stabilized mode; hot soak emissions, which result from fuel evaporation from the still-hot engine at the end of a trip; and diurnal emissions, which result from evaporation of fuel from the gasoline tank whether the vehicle is driven or not (Sierra Research 1993, 19, 20).14 Vehicle technology improvements have been focused primarily on reducing running emissions, which are a function of vehicle miles traveled (VMT). However, vehicle emissions from a cold start when the catalytic converter is not functioning at optimal temperatures, which are a function of trip making rather than VMT, can account for more than half of total CO and VOC emissions (FHWA 1992, 6).15 The importance of trips relative to VMT is most evident for VOC emissions as illustrated by the following example of a prototypical 32-km (20-mi) trip (Figure 2-3). In this example, vehicle start-up contributes approximately one-third of total VOC emissions and trip end contributes one-sixth. Neither of these emissions is a function of VMT, but together they account for about half of the total VOCs emitted.16 Thus, the impact of highway capacity additions on trips as well as VMT is of interest in assessing the effect on emissions. Speed, Acceleration, and Load Emission levels depend not only on the number of trips taken and VMT but also on the speed and acceleration of the vehicle and the load on the engine over the distance of the trip.17 In current emission models, vehicle speed and acceleration are combined into a single average speed for various trip types (i.e., drive test cycles) so that emission levels vary with average trip speed. The severe limitations of this approach are discussed in Chapter 3. Engine loads are generally not varied to reflect different vehicle operating or highway conditions (e.g., road grade) in modeling emission estimates.18 Figure 2-4, Figure 2-5 through Figure 2-6 show current model estimates of emission factors for key pollutants expressed in grams per mile for light-duty, gasoline-powered automobiles representing the 1990 fleet mix, for a range of average trip speeds. 19 The data are based on the most recent emission models—MOBILE5a developed by EPA and EMFAC7F developed by the California Air Resources Board (CARB) and approved
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Gasoline-powered passenger vehicles—the most common vehicle on the road—are the primary source of CO highway vehicle emissions and contributors to the ozone precursor emissions from highway vehicles (VOCs and NOx). Heavy-duty diesel vehicles contribute a disproportionate share of total highway vehicle emissions of PM-10, SO2, and NOx. The transportation sector accounts for nearly two-thirds of the petroleum consumed in the United States. The highway mode accounts for about three-quarters of the transportation total. The gasoline-powered motor vehicle fleet also contributes about 20 percent of total U.S. CO2 emissions, the principal greenhouse gas. Vehicle emission levels are a function of trip taking as well as distance traveled, because emissions vary depending on whether the vehicle is warmed up. Emission levels are sensitive to average vehicle speed over the distance of the trip and vary as a nonlinear function of average trip speed. In addition, emissions are affected by smoothness and consistency of vehicle speeds, which vary by trip type. Sharp accelerations, in particular, are an important source of CO and VOC emissions, which are not well reflected in current emissions models. Thus, average trip speed alone is not a good predictor of emission levels. Fuel economy is also sensitive to average vehicle speed but somewhat less so to aggressive accelerations and braking. Thus, average trip speed is a good predictor of fuel economy for most urban trips. Highway capacity additions, which will increase average trip speeds and smooth traffic flows, should directly affect emission levels and fuel economy. A range of models are available to estimate the effects of motor vehicle transportation on emissions, air quality, and energy use. However, many were developed to predict macro-level, regional effects; they are not well suited to assessing the impacts of link-specific highway capacity enhancement projects at the level of precision that is being required of them today. Nor were the different types of models (e.g., land use models, travel demand models, emission models) designed to be easily integrated in their operations. Data requirements —both the currency of the data and the detail needed for impact analyses —are also problems. Finally, the models are based on a limited understanding of the underlying relationships. Greater knowledge of
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use travel and driving behavior, in particular, is critical to improved modeling of the travel demand and emission effects of highway capacity enhancement projects. NOTES 1. A stationary or point source is a large, geographically concentrated emitter, such as a coal-fired electrical power plant, whose emissions rates are large enough to be significant by themselves even if no other emission sources are present (Horowitz 1982, 7). An area source is a collection of small, geographically dispersed emitters that are not significant individually but that are important collectively, such as dry cleaning establishments (Horowitz 1982, 7). A mobile source, such as an automobile, is characterized as not emitting from a fixed location. 2. Short-term (24-hr or less) averaging times were designated for some pollutants, such as CO and O3, to protect against acute, or short-term, health effects; long-term averaging times (annual average) were established for other pollutants to protect against chronic health effects (Curran et al. 1994, 20). 3. CO contributes to the buildup of tropospheric (ground-level) ozone (the principal ingredient of smog) and methane, both major greenhouse gases. First, CO helps convert nitric oxide to nitrogen dioxide, a crucial step in ozone formation. Second, CO reacts with the hydroxyl radical (OH), which eventually removes CO from the atmosphere; however, OH is also the principal chemical that destroys ozone and methane. Thus, if carbon monoxide levels increase, OH concentrations will fall, and regional concentrations of ozone and methane will rise (MacKenzie and Walsh 1990, 8). 4. After the report was completed, EPA recomputed emissions from highway vehicles using the most recent version of its emission factor model, MOBILE5a (Curran et al. 1994, 22), but EPA estimates are still thought to be low by many in the scientific community. 5. Nitrogen oxide (NO), the dominant constituent of vehicle exhaust emissions of NOx, combines with ozone (O3) to form NO2 and O2. However, ozone is subsequently regenerated by further chemical reactions stimulated by the presence of sunlight (see NRC 1991, 168 for a more detailed discussion). 6. The problem is the consistent underestimate of VOC emissions, which leads to estimates of relatively low VOC to NOx ratios. The nation's ozone reduction strategy has been based largely on the premise that VOC/NOx ratios in the most polluted areas, where VOC control is more effective, are low (i.e., in the less-than-10 range). An upward correction in VOC emission inventories could indicate the need for a fundamental change
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use in ozone abatement strategies to greater use of NOx controls in many geographic areas (NRC 1991, 7). 7. This regulation, however, is an interim requirement, that is, it applies to all projects contained in new or revised transportation improvement programs until EPA approves state implementation plans. In the latter, a state could choose to accommodate NOx emissions generated by new transportation projects by reducing emissions from other sectors (AASHTO Journal 1994, 10–11.) 8. In 1987 EPA replaced earlier standards for particulate matter with the more stringent PM-10 standard, which focuses on the smaller particles likely to be responsible for adverse health effects because of their ability to reach the lower regions of the respiratory tract (Curran et al. 1994, 10). 9. The 1987 Truck Inventory and Use Survey reported that 67 percent of combination vehicles used diesel fuel, but most trucks operating only in local areas (96 percent) are fueled by gasoline (Bureau of the Census 1990, 37, 48). 10. In 1993 emissions from all highway vehicles, gasoline and diesel powered, for CO, NOx, PM-10, SO2, and VOC were 74,155 thousand short tons. Emissions from diesel-powered, heavy-duty vehicles for the same pollutants were 3,986 thousand short tons, or 5.4 percent of the total; emissions from light-duty passenger vehicles were 46,941 thousand short tons, or 63 percent of the total (Nizich et al. 1994, A-4, A-8, A-15, A-19, A-24). In the most recent year for which data are available (1992), all motor vehicles accounted for 2,239,828 million vehicle miles of travel; combination trucks and buses accounted for 104,771 million vehicle miles of travel, or nearly 5 percent of the total, and passenger vehicles accounted for 1,595,438 million vehicle miles of travel, or 71 percent of the total (FHWA 1993, 207). 11. The primary components of diesel particulates are soot formed during combustion (40 to 80 percent of the total); particulate sulfates, which depend on operating conditions and the fuel's sulfur content (5 to 10 percent of the total); and heavy hydrocarbons condensed or adsorbed on the soot from the fuel and lubricating oil and also formed during combustion (the remainder) (Weaver and Klausmeier 1988, 2-7–2-8). 12. Even if all fuel were burned in the combustion process, thereby eliminating particulates from incomplete combustion, impurities in the fuel would burn and appear in the exhaust as particulates; the primary offender is sulfur (Conte 1990, 61). With lower sulfur levels in fuel, this source of particulates should also be reduced. 13. EPA considers a cold start for a catalyst-equipped vehicle to occur after the engine has been turned off for 1 hr. For noncatalyst vehicles, a cold start occurs after the engine has been turned off for 4 hr (Sierra Research 1993, 18).
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 14. Refueling losses and crankcase emissions are also generally considered in the evaporative emissions category as is a new category, resting losses. The latter was previously included under the hot soak and diurnal categories (Sierra Research 1993, 20). 15. VOC and CO emissions are higher when a cold engine is first started, because a fuel-rich mixture must be provided to achieve adequate combustion during warm-up and the excess fuel is only partially burned. In addition, the catalytic converter does not provide full control until the vehicle is warmed up (Sierra Research 1993, 18). It takes between 1 and 3 min for modern, properly operating vehicles to warm up. Catalysts also cool off faster than engines and are completely cold in 45 to 60 min (EPA 1993, 115; Enns et al. 1993, 3). Preheated catalytic converters may ameliorate the problem. The California Air Resources Board estimates that they would decrease cold-start emissions by half or more but would not eliminate the problem (FHWA 1992, 28). 16. The number would be even larger if running loss evaporative emissions, which are included under running emissions, were separated out. 17. Loads are a function of vehicle operating conditions (e.g., number of passengers, whether a trailer is being towed, whether the air conditioning is on), highway conditions (e.g., road grade), and driver behavior (e.g., aggressive driving with sharp accelerations). The latter two conditions are described in subsequent sections. 18. The exceptions are air conditioning and towing corrections, which can be input by the user in running the emissions models. 19. Emissions were calculated under hot stabilized operating conditions. 20. CO emissions, which are a product of incomplete combustion of motor fuels, are most affected. Engine-out CO emissions increase because of incomplete fuel combustion under fuel-rich conditions and exhaust emissions increase because the catalyst is overridden (personal communication, John German, EPA, Feb. 4, 1994). VOCs are affected but to a lesser extent. They result from unburned fuel in the engine. As fuel is increased with the richer air-fuel mixture, the level of engine-out VOC emissions goes up proportionately; these emissions are not handled by the catalyst, which is overridden under fuel-rich conditions, thereby increasing exhaust emissions (personal communication, John German, EPA, Feb. 4, 1994). NOx engine-out emissions decrease under rich operation, but NOx reduction efficiencies in the catalyst also drop (EPA 1993, 19). Overall, there may be a slight increase in exhaust NOx emissions under rich operation, but the effect is relatively minor and varies from vehicle to vehicle (EPA 1993, 19). 21. From a sample of 24,000 emissions measurements made over a 4-day period, Naghavi and Stopher (1993) found that more than half of the CO was emitted by 6.9 percent of the vehicles and that about half of the VOC was emitted by 20 percent of the vehicles (p. 1).
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 22. These programs are required in areas designated “serious” or above for ozone and “high moderate” or above for CO. EPA estimates that innovative inspection and maintenance programs could yield a 28 percent reduction in emissions (DOT and EPA 1993, 33). 23. Exhaust emissions of CO and VOC also increase at temperatures above 24°C (75°F), but not as sharply as at lower temperatures. The increase is primarily the result of an increase in vapors purged from the evaporative emission control system, leading to rich operation (Sierra Research 1993, 122). 24. For example, if VOC emission formation in the engine (engine-out emissions) is reduced because less fuel is being delivered to the engine chamber per engine cycle, these gains will show up as lower tail pipe exhaust emissions only if manufacturers do not cut back on catalyst emission-control systems (tail pipe emissions) and do not take advantage of some of the savings (DeLuchi et al. 1993, 7-8). 25. The problem is not the quick acceleration, but the delay in gear shifting. Drivers with manual transmissions shift later (at higher engine speeds); with automatic transmissions, the system delays shifting up, both with the same result—high engine speeds and high fuel consumption (An et al. 1993, 4). 26. A fully loaded diesel truck realizes 3 to 3.4 kpl (7 to 8 mpg) on the highway, or approximately 108.9 to 123.4 metric ton-km per liter (280 to 320 ton-mi per gal). A car weighing 2268 kg (5,000 lb) can realize 11 to 12.7 kpl (26 to 30 mpg), or 25 to 28.8 metric ton-km per liter (60 to 75 ton-mi per gal) (Duleep 1992 in O'Rourke and Lawrence 1993, 11). 27. Emission inventories contain the relative contributions, current and projected, of emissions and pollution levels from mobile and stationary sources, drawing upon regional models and data (Harvey and Deakin 1993, 2-1–2-2). 28. Population and employment forecasts for a region may be provided by econometric models or derived from federal or state sources (Harvey and Deakin 1993, 3-11). Land use data are obtained from local land use plans. Local development policies are important to understanding potential constraints on land availability and development intensity (Shunk 1992, 107). 29. Putman reports, for example, that there are 14 MPOs in various stages of implementing DRAM-EMPAL for regional forecasting and policy evaluation efforts (Putman 1994, 1). Eleven have completed preliminary calibrations of both models using their own region's data and four are working on developing direct linkages between their transportation and land use models (Putman 1994, 2). Other land use models in use in the United States include POLIS in the San Francisco Bay Area, EMPIRIC in Atlanta, and PLUM in Washington, D.C. (Shunk 1992, 107).
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 30. In the early 1980s the entire ITLUP was distributed as a supplement to the Urban Transportation Planning System (UTPS) package, a travel demand modeling system package developed by the Federal Highway Administration (Harvey and Deakin 1993, 3-16). 31. The DRAM model, however, includes the income distribution of residential households, which is a proxy for several of these factors. 32. Many MPOs use the UTPS software package (Harvey and Deakin 1993, 3-5). 33. The MTC of the San Francisco Bay Area and the Puget Sound Council of Governments in the Seattle area have formal land use models, which are integrated into their regional travel demand models in a manner that allows for feedback between transportation and land use over time (Cambridge Systematics, Inc. 1991, 42). Cambridge Systematics, Inc., has been working with the Portland, Oregon, metropolitan area through the Land Use Transportation Air Quality Connection project to develop this capacity. Finally, the Southern California Association of Governments has recently completed a full test run of an integrated land use (DRAM-EMPAL)–transportation model (Putman 1994, 3). 34. Direct estimates of travel speed are not an output of travel demand models. Instead, link speeds are adjusted through an iterative process of assigning trips to the shortest network path to arrive at travel volume estimates (Meyer and Ross 1992, 6). Traffic volumes are often calibrated with actual traffic counts, but no attempt is made to check travel speeds or travel time against observed speeds (DeCorla-Souza 1993b, 5-6), with the result that model-derived speeds tend to overestimate actual link speeds, particularly under congested conditions (Meyer and Ross 1992, 6; Harvey and Deakin 1993, 3-63). Estimated traffic volumes on specific links may be in error by as much as 15 to 50 percent, depending on the total traffic volume on the link (DeCorla-Souza 1993b, 2). 35. Travel demand models provide no information on cold starts, because trips are not chained and travel is not tracked by time of day (Ducca 1993, 3). Travel demand models typically provide data on average weekday traffic levels by traffic zone. Hourly data on episode days by grid square, however, are needed for photochemical modeling (DeCorla-Souza 1993a, 3; Ducca 1993, 3). Information on vehicle type and age and vehicle operating mode cannot be directly obtained from travel demand model output (DeCorla-Souza 1993a, 4-5). 36. Some of the most common models for simulating traffic flows on freeways and estimating the effects of bottlenecks and ramp metering are FREQ, TRAFLO, and INTRAS. NETSIM was designed to simulate traffic changes from traffic signalization and intersection design improvements. TRANSYT and PASSER simulate traffic flows on arterials and changes in performance, such as travel times and delays that result from traffic flow improvement measures (Harvey and Deakin 1993, 3-77).
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 37. The U.S. Department of Transportation and the Environmental Protection Agency have already authorized $3 million; Department of Energy support is also being sought for a long-term program total of $25 million. 38. An analysis of instrumented vehicles driven in the Baltimore area for the FTP review found that about 18 percent of total Baltimore driving time was composed of higher speeds and sharper accelerations than those represented on the FTP [i.e., maximum speeds of 90.7 kph (56.7 mph) and maximum acceleration rates of 5.3 kph/sec (3.3 mph/sec)] (EPA 1993, 3-4). 39. An engine-based test procedure was adopted because engine manufacturers are distinct from truck manufacturers and because the same engine can be used with a wide variety of trucks with different transmissions and axles (see Appendix A). 40. The models are validated for predictive purposes on the basis of their ability to simulate adequately a base-year episode day of high concentrations of ozone (NRC 1991, 308). 41. Validation of emissions data is further complicated, as is discussed in the following chapter, by identifying what actual data (e.g., what drive cycle) to measure. REFERENCES ABBREVIATIONS DOT U.S. Department of Transportation FHWA Federal Highway Administration EPA Environmental Protection Agency NRC National Research Council TRB Transportation Research Board AASHTO Journal. 1994. Browner Responds on NOx Issue. Feb. 18, pp. 10–11. An, F., and M. Ross. 1993a. Model of Fuel Economy and Driving Patterns. Presented at 72nd Annual Meeting of the Transportation Research Board, Washington, D.C., 22 pp. An, F., and M. Ross. 1993b. A Model of Fuel Economy and Driving Patterns. No. 930328. Society of Automotive Engineers, International Congress and Exposition , Detroit, Mich., March 1–5, pp. 63–79. An, F., M. Ross, and A. Bando. 1993. How To Drive To Save Energy and Reduce Emissions in Your Daily Trip . RCG/Hagler, Bailly, Inc., Arlington, Va., and The University of Michigan, Ann Arbor, 10 pp. Bureau of the Census. 1990. Truck Inventory and Use Survey. 1987 Census of Transportation, TC87-T-52. U.S. Department of Commerce, Aug., 166 pp.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Cambridge Systematics, Inc. with Hague Consulting Group. 1991. Making the Land Use Transportation Air Quality Connection: Vol. I: Modeling Practices. Oct., 84 pp. Conte, F. 1990. Trucking in the '90s: Emissions. Owner Operator. Sept., pp. 58–65. Curran, T., T. Fitz-Simons, W. Freas, J. Hemby, D. Mintz, S. Nizich, B. Parzygnat, and M. Wayland. 1994. National Air Quality and Emissions Trends Report, 1993. 454-R-94-026. U.S. Environmental Protection Agency. Research Triangle Park, N.C., Oct., 157 pp. Davis, S.C. 1994. Transportation Energy Data Book: Edition 14. ORNL-6798. Center for Transportation Analysis, Energy Division, Oak Ridge National Laboratory, Tenn., May. Deakin, E. 1991. Scoping Study: Impact of Highway Congestion on Air Quality. University of California at Berkeley, March, 22 pp. DeCorla-Souza, P. 1993a. Travel and Emissions Model Interactions. Presented at Transportation-Air Quality Conference, Washington, D.C., Feb. 23–26. DeCorla-Souza, P. 1993b. Travel Forecasting Process. Presented at Transportation-Air Quality Conference, Washington, D.C., Feb. 23–26. DeLuchi, M., D.L. Greene, and Q. Wang. 1993. Motor-Vehicle Fuel-Economy: The Forgotten Hydrocarbon Control Strategy? UCD-ITS-RR-93-3. Institute of Transportation Studies, University of California, Davis, Jan., 25 pp. Dockery, D.W., A. Pope, X. Xu, J.D. Spengler, J.H. Ware, M.E. Fay, B.G. Ferris, and F.E. Speizer. 1993. An Association Between Air Pollution and Mortality in Six U.S. Cities . New England Journal of Medicine, Vol. 329, No. 24, Dec. 9, pp. 1754–1808. DOT and EPA. 1993. Clean Air Through Transportation: Challenges in Meeting National Air Quality Standards. Aug. Ducca, F.W. 1993. Future Directions in Travel Forecasting. Federal Highway Administration, U.S. Department of Transportation , 9 pp. Duleep, K.G. 1992.Analysis of Heavy Duty Fuel Efficiency to 2001. Presented at 71st Annual Meeting of the Transportation Research Board, Washington, D.C. Effa, R.C., and L.C. Larsen. 1993. Development of Real-World Driving Cycles for Estimating Facility-Specific Emissions from Light-Duty Vehicles. Presented at the Air and Waste Management Association Specialty Conference on The Emission Inventory: Perception and Reality, Pasadena, Calif., Oct. 18–20, 20 pp. Enns, P., J. German, and J. Markey. 1993. EPA's Survey of In-Use Driving Patterns: Implications for Mobile Source Emission Inventories. Office of Mobile Sources, U.S. Environmental Protection Agency. EPA. 1993. Federal Test Procedure Review Project: Preliminary Technical Report . Office of Mobile Sources, May, 161 pp.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Evans, L., R. Herman, and T. Lam. 1974. Multivariate Analysis of Traffic Factors Related to Fuel Consumption in Urban Driving. GMR-1710. Research Laboratories, General Motors Corporation, Warren, Mich., Oct. Federal Register. 1993. Criteria and Procedures for Determining Conformity to State or Federal Implementation Plans of Transportation Plans, Programs, and Projects Funded or Approved Under Title 23 U.S.C. or the Federal Transit Act . Vol. 58, No. 225, Nov. 24, 62, 188–62, 253. FHWA. 1992. Transportation and Air Quality: Searching for Solutions: A Policy Discussion Series. No. 5, FHWA-PL-92-029. U.S. Department of Transportation, Aug., 30 pp. FHWA. 1993. Highway Statistics 1992. FHWA-PL-93-023. U.S. Department of Transportation, 235 pp. Gordon, D. 1991. Steering a New Course: Transportation, Energy, and the Environment . Island Press, Washington, D.C., 244 pp. Greco, R. 1985. Motor Vehicle Tampering Survey—1984. Office of Air and Radiation, U.S. Environmental Protection Agency , July. Greene, D.L., D. Sperling, and B. McNutt. 1988. Transportation Energy to the Year 2020. In Special Report 220: A Look Ahead: Year 2020. Transportation Research Board, National Research Council, Washington, D.C., pp. 207–231. Guensler, R., D. Sperling, and P. Jovanis. 1991. Uncertainty in the Emission Inventory for Heavy-Duty Diesel-Powered Trucks. UCD-ITS-RR-91-02. Institute of Transportation Studies, University of California, Davis, June, 146 pp. Guensler, R. 1993. Transportation Data Needs for Evolving Emission Inventory Models. Institute of Transportation Studies, University of California, Davis, 27 pp. Guensler, R. 1994. Vehicle Emission Rates and Average Operating Speeds. Ph.D. dissertation. University of California, Davis. Harvey, G., and E. Deakin. 1991. Toward Improved Regional Transportation Modeling Practice. Deakin Harvey Skabardonis, Inc., Berkeley, Calif., Dec., 68 pp. Harvey, G., and E. Deakin. 1993. A Manual of Regional Transportation Modeling Practice for Air Quality Analysis. Deakin Harvey Skabardonis, Inc., Berkeley, Calif., with Cambridge Systematics, COMSIS, Dowling Associates, Gary Hawthorne Associates, Parsons Brinckerhoff Quade & Douglas, and Ann Stevens Associates, July. Heywood, J.B. 1988. Internal Combustion Engine Fundamentals. McGraw-Hill Book Company. Horowitz, J.L. 1982. Air Quality Analysis for Urban Transportation Planning. MIT Press, Cambridge, Mass., 387 pp. Johnson, J.H. 1988. Automotive Emissions. Air Pollution, the Automobile, and Public Health. Health Effects Institute. National Academy Press, Washington, D.C.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Lilly, L.R.C. (ed.). 1984. Diesel Reference Book. Butterworths, Boston, Mass. MacKenzie, J.J., and M.P. Walsh. 1990. Driving Forces: Motor Vehicle Trends and Their Implications for Global Warming, Energy Strategies, and Transportation Planning. World Resources Institute, Washington, D.C., Dec., 49 pp. McGill, R. 1985. Fuel Consumption and Emission Values for Traffic Models. FHWA/RD-85/053. Oak Ridge National Laboratory, Tenn., May, 90 pp. Meyer, M., and C. Ross. 1992. Transportation and Air Quality Modeling: Fitting a Square Block into a Round Hole. Georgia Institute of Technology, Atlanta, 11 pp. Meyer, M.D., M. Rodgers, C. Ross, F.M. Saunders, and C.T Ripberger. 1993. A Study of Enrichment Activities in the Atlanta Road Network, pp. 225–233. Murrell, D. 1980. Passenger Car Fuel Economy: EPA and Road. U.S. Environmental Protection Agency, Jan., 305 pp. Naghavi, B., and P. Stopher. 1993. Remote Sensing, Means, Medians, and Extreme Values: Some Implications for Reducing Automobile Emissions. Presented at 72nd Annual Meeting of the Transportation Research Board, Washington, D.C., 28 pp. Nizich, S.V., T.C. McMullen, and D.C. Misenheimer. 1994. National Air Pollutant Emissions Trends, 1900–1993. EPA-454/R-94-027. Office of Air Quality Planning and Standards, Research Triangle Park, N.C., Oct., 314 pp. NRC. 1990. Confronting Climate Change: Strategies for Energy Research and Development . National Academy Press, Washington, D.C. NRC. 1991. Rethinking the Ozone Problem in Urban and Regional Air Pollution. National Academy Press, Washington, D.C., 489 pp. NRC. 1992. Automotive Fuel Economy: How Far Should We Go? National Academy Press, Washington, D.C., 259 pp. O'Connor, K., L.L. Duvall, and R.G. Ireson. 1993. Intersection Air Quality Modeling: Review of Ambient Data and Current Modeling Practices: Vol. 2: Survey on Current Practices in Air Quality Modeling. Systems Applications International, San Rafael, Calif., 49 pp. O'Rourke, L., and M.F. Lawrence. 1993. Strategies for Goods Movement in a Sustainable Transportation System . Presented at Transportation and Energy Strategies for a Sustainable Transportation System, Asilomar Conference System, Pacific Grove, Calif., Aug. 23, 35 pp. Outwater, M.L., and W.R. Loudon. 1994. Travel Forecasting Guidelines for the Federal and California Clean Air Act. Presented at 73rd Annual Meeting of the Transportation Research Board, Washington, D.C., 24 pp. Putman, S. 1991. DRAM/EMPAL ITLUP: Integrated Transportation Land-Use Activity Allocation Models: General Description. S.H. Putman Associates, Philadelphia, Pa., Jan.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Putman, S. 1993. Sensitivity Tests with Employment and Household Location Modes. Presented at Third International Conference on Computers in Urban Planning and Urban Management, Georgia Institute of Technology, Atlanta, July 23–25. Putman, S. 1994. Integrated Transportation and Land Use Models: An Overview of Progress with DRAM and EMPAL with Suggestions for Further Research. Presented at 73rd Annual Meeting of the Transportation Research Board, Washington, D.C., 28 pp. Shunk, G.A. 1992. Urban Transportation Systems. In Transportation Planning Handbook (J.D. Edwards, Jr., ed.), Institute of Transportation Engineers, Prentice Hall, N.J., pp. 88–122. Sierra Research, Inc. 1993. Evaluation of “MOBILE” Vehicle Emission Model. Report SR93-12-02. Sacramento, Calif., Dec. 7. Stedman, D. 1991. Presentation at The Transportation-Land Use-Air Quality Connection: A Policy and Research Symposium. Public Policy Program, University of California Extension at Los Angeles, Lake Arrowhead, Calif., Nov. 6–8. Weaver, C.S., and R.F. Klausmeier. 1988. Heavy-Duty Diesel Vehicle Inspection and Maintenance Study. Final Report, Volume II: Quantifying the Problem. Radian Corporation, Sacramento, Calif., May 16. Wegener, M. 1994. Operational Urban Models: State of the Art. Journal of the American Planning Association, Vol. 60, No. 1, Winter, pp. 17–29.
Representative terms from entire chapter: