6

Findings and Conclusions

Despite more than two decades of clean air legislation and considerable progress in reducing pollution from transportation sources, many major metropolitan areas continue to be out of compliance with national air quality standards. The Clean Air Act Amendments of 1990 (CAAA) introduced more stringent controls to help ensure further reductions in motor vehicle emissions coupled with new milestones and deadlines for attaining compliance, strict monitoring procedures, and automatic sanctions for noncompliance that include loss of federal highway funds.

Legislation similar to the CAAA has not been passed recently in the energy area. However, transportation's share of petroleum consumption in the United States has steadily increased during the past decade, and dependence on foreign oil sources has reached levels exceeded only in 1979. Thus energy officials are seeking ways to improve fuel efficiency and reduce vehicle travel to cut back energy consumption.



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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 6 Findings and Conclusions Despite more than two decades of clean air legislation and considerable progress in reducing pollution from transportation sources, many major metropolitan areas continue to be out of compliance with national air quality standards. The Clean Air Act Amendments of 1990 (CAAA) introduced more stringent controls to help ensure further reductions in motor vehicle emissions coupled with new milestones and deadlines for attaining compliance, strict monitoring procedures, and automatic sanctions for noncompliance that include loss of federal highway funds. Legislation similar to the CAAA has not been passed recently in the energy area. However, transportation's share of petroleum consumption in the United States has steadily increased during the past decade, and dependence on foreign oil sources has reached levels exceeded only in 1979. Thus energy officials are seeking ways to improve fuel efficiency and reduce vehicle travel to cut back energy consumption.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use OVERVIEW The CAAA require close scrutiny of metropolitan area transportation improvement programs (TIPs), particularly projects, such as highway capacity additions, that could stimulate new motor vehicle travel and thus increase vehicle emission levels. The conformity process required by regulation is the primary instrument by which transportation agencies in nonattainment areas and maintenance areas1 must demonstrate the compatibility of TIPs with state implementation plans (SIPs) for meeting air quality standards by attainment deadlines. The conformity process as it is currently interpreted in Environmental Protection Agency (EPA) regulations places heavy demands on the modeling and analytic capabilities of metropolitan planning organizations (MPOs). Until SIPs are revised and approved by EPA,2 areas that have not attained national standards must demonstrate that (a) projects in regional plans and TIPs3 will not result in motor vehicle emission levels higher than those in a 1990 baseline year and (b) by building these projects, emissions will be lower in future years 4 than if the projects are not built (i.e., the build–no-build test). Once new SIPs are approved, with new budgets established for motor vehicle emissions, the conformity test changes.5 The build–no-build test is no longer required, but nonattainment areas must demonstrate through regional emissions analyses using network-based transportation demand models that the TIP will not produce aggregate emission levels in excess of the motor vehicle emissions budget in the approved SIP (Federal Register 1993, 62,193–62,194, 62,249).6 The tests required by the conformity regulations, particularly the build–no-build analysis mandated by the interim conformity process, require a precise assessment of whether specific investments in highway capacity at specific locations in metropolitan areas will result in a net gain or a net loss in regional air quality. This assessment would be difficult to make under the best of circumstances. Not only does it involve estimating initial changes in vehicle emissions from changes in traffic flow characteristics as a result of the new capacity, it also requires the long-term responses by transportation system users to be forecast. Travel time savings and the improved access provided by new capacity will influence travel demand.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use MPOs, states, federal agencies, and courts that must exercise oversight responsibility are being asked to make judgments on the basis of their interpretations of the available scientific evidence. This study was motivated in large part by the practical needs of policy makers and decision makers in meeting regulatory requirements. The study task was complicated by the recognition that the concern for the environmental effects of highway building is part of a larger debate over the appropriate direction of metropolitan development and the role of transportation investments in shaping such development. There is an emerging consensus that a less polluting, more energy efficient transportation system is desirable (Deen and Skinner 1994, 11). Yet there are strongly held differences of opinion concerning the speed with which this goal should be pursued, the emphasis that should be placed on mobility and economic growth vis-à-vis environmental protection, and the extent of government regulation and intervention that is appropriate (Deen and Skinner 1994, 11). The benefits of the nation's highway system are well documented. Highways are a major component of the infrastructure base that has supported the growth and development of the nation's metropolitan areas. An extensive highway network affords a high degree of mobility for people and freight at low out-of-pocket costs. These benefits are not without costs. Highway vehicles, the dominant form of passenger and freight transport, are a major source of pollutants and a major user of fossil fuels. Highways have supported the decentralization of U.S. metropolitan areas, increasing reliance on motor vehicle travel with the associated emissions and energy use, although many other factors have influenced dispersed urban settlement patterns. The private cost of motor vehicle transport does not adequately capture the full social costs of congestion, air pollution, and energy use. Public policies could change travel patterns and location preferences in ways that reduce trips and encourage travel by less polluting modes. Such policies include pricing motor vehicle travel to more fully recover its social costs from users and encouraging more coordinated land use to cluster development. The current debate turns on how large these changes should be, how feasible they are to introduce, and how quickly they will bring about substantial changes in travel behavior.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Although massive investments in new highway infrastructure are unlikely in the post-Interstate era, pressure for expanding highway capacity will continue in the growing suburban and exurban portions of large metropolitan areas. These locations are where most new road construction or expansion is expected and where the potential for increasing emission levels and energy use is greatest. Thus, this study has reviewed what is known and what is not known about the likely size and direction of these effects. In the following sections the crosscutting issues are reviewed, committee findings are presented for each impact area, and the net effects of highway capability expansion on air quality and energy use are assessed. Recommendations for research and data collection that could advance the state of knowledge are then provided. Concluding observations address the current focus of the CAAA. CROSSCUTTING ISSUES At the conclusion of its review, the study committee identified several issues that had complicated the study task: Changes in highway capacity affect the urban transportation system in complex ways that are not always obvious. The primary effect of adding highway capacity is to reduce travel time on the specific facility with increased capacity, but because of the network character of the transportation system, a reduction in travel time from expanding a particular highway facility affects travel on other routes and modes. The spatial extent of these changes varies from case to case but is related to the magnitude of the highway capacity addition and traffic flow conditions on the larger network. These complex network adjustments must be considered in assessing the overall impact of highway capacity additions. Users of the transportation system will respond to improvements in the system by changing their travel behavior when it is to their advantage to do so. These responses will change not only the travel volumes on different facilities but also total travel on the highway network because of mode shifts and changes in the number and length of trips. In addition, users may change the

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use time of day they travel to more preferred travel times. These adjustments must be taken into account in an overall assessment of the impact of highway capacity additions. Changes in highway capacity must be assessed with reference to conditions on the rest of the urban transportation system. The impacts will differ depending on local factors–congestion levels, automobile ownership levels, and land use densities. The effects of not adding highway capacity or of making alternative investments must also be compared with the build option to determine net effects. This involves an assessment of whether congestion will worsen if the new capacity is not added, whether there is adequate capacity in the system so that congestion can be averted if travelers shift their routes and times of travel, or whether investments in other modes (e.g., transit or bicycle facilities) can accommodate travel needs. Estimating these effects is also difficult. The effects of major highway capacity additions vary over time. User responses to travel service changes are likely to be more significant in the long run than initially as highway capacity additions influence long-term decisions about the location of development in a metropolitan area and automobile ownership. A key uncertainty is at what point, or whether at any point, the emission increases from the development and traffic stimulated by the capacity addition will offset initial emission reductions from smoothing traffic flows. Estimates of overall net effects on emissions and energy use depend on the length of time over which the effects are analyzed and the value placed on long-term versus more immediate effects. Forecasting the long-term impacts as well as the initial impacts of highway capacity additions requires modeling a complex sequence of interrelated changes in land use, travel behavior, traffic patterns, vehicle emissions, and energy use. There is considerable uncertainty about the quantitative outcome of many of these components. The predicted impacts for each component in the modeling system typically are small and have relatively large variances. These uncertainties are likely to be compounded through the entire sequence of models and for longer forecast horizons.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use The effects of highway capacity additions on emissions are highly dependent on the state of vehicle design, automotive and motor fuel technology, and emission controls. Major changes in any of these factors that reduce the overall level of vehicle emissions or the emissions response to traffic flow variables will proportionally reduce the positive or negative effects of highway capacity additions on vehicle emission levels. Changes in emission levels from major alterations in vehicle design, motor vehicle technology, and emissions regulations will occur gradually (over 10 to 15 years) because of the slow pace of vehicle turnover. Other changes, such as the introduction of less polluting motor vehicle fuels (e.g., reformulated gasoline and low-sulfur diesel fuels) will have more immediate effects. FINDINGS FOR INDIVIDUAL IMPACT AREAS After reviewing what is known from theory, empirical research, and modeling about the relationships among highway capacity additions, emissions, air quality, and energy use, the study committee summarized its findings for each of the three impact areas. Initial Impacts on Traffic Flow Characteristics The committee focused its attention first on the initial effects of highway capacity additions. It attempted to isolate the effects of changes in traffic flow patterns on emission levels and energy use from longer-term effects on travel demand and location decisions. The primary impacts of interest are changes in traffic flow patterns, particularly speed levels and speed variability. The effects of these changes on vehicle emissions and energy use, both positive and negative, depend on changes in the level and daily pattern of congestion experienced before and after the project, the traffic volume affected by the capacity addition, and the amount of traffic diverted from other heavily congested facilities. To determine net effects on emissions and energy use, these changes must also be compared with changes in traf-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use fic flow patterns that would have occurred had the highway capacity addition not been made. To clarify what is known about the initial impacts of highway capacity additions, net increases in highway use attributable to capacity expansions are assumed to be negligible. (This assumption is relaxed in the following section.) This is a reasonable assumption for initial conditions, because initial increases in traffic volume on expanded highway links typically reflect diversions from other routes or from travel at other times of day. Finding 1: Although considerable research and vehicle testing have been performed, few definitive and comprehensive conclusions can be reached about how highway capacity additions and their effects on traffic flow characteristics change vehicle emission levels. Virtually all vehicle emissions testing has been based on a limited set of driving test cycles whose representativeness for specific traffic flow conditions is doubtful. Finding 2: Current emission models rely on average trip speed as the sole descriptor of traffic flow. Variability in speed, roadway grade, and other factors that strongly influence emissions are not explicitly dealt with. This one-dimensional approach cannot adequately describe the variety of traffic flow conditions that occur for different facility types and levels of congestion. People do not drive at average speeds. Under heavily congested traffic conditions, vehicle speeds are not only low but also highly variable; many stops, starts, accelerations, and decelerations occur. Under moderate to light levels of congestion, vehicle speeds are higher and less variable; there is more opportunity for cruise-type driving. These differences, particularly speed variations that produce large variations in emission levels, are not adequately reflected in current emission models. Finding 3: Current emission models cannot predict with confidence net changes in emission rates for a wide range of changes in average trip speed that can be expected from many highway capacity additions. The predicted changes are not significantly different from zero.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Finding 4: Smoothing traffic flows reduces travel speed variation and the incidence of vehicle accelerations, which place heavy loads on the engine, thereby initially reducing emissions of carbon monoxide (CO), volatile organic compounds (VOCs), and oxides of nitrogen (NOx). The effect is most pronounced for emissions of CO and VOCs, which increase dramatically when the vehicle operates in a fuel-rich mode in response to sharp accelerations. Finding 5: Highway capacity additions that provide relief from heavily congested traffic by smoothing traffic flows should initially reduce emissions of all three pollutants from vehicles traveling on the improved facility. Congested conditions are most common on urban arterial roads, but high levels of congestion are also found on some freeways, particularly at freeway-to-freeway interchanges. Finding 6: The emissions reductions produced by capacity additions that smooth traffic flows (i.e., reduce acceleration variance but do not raise average speeds) will be diminished if the additions lead to average speed increases. There is considerable uncertainty, however, about the exact average speeds at which the initial reductions are lost. NOx emissions will increase gradually as speeds increase and the acceleration variance is reduced. At higher speeds (i.e., free-flow, freeway speeds), the increased power demands on the engine will cause CO and VOC emissions to increase. Finding 7: Highway capacity additions that result in increased speeds and cruise-type driving will initially increase NOx emissions from vehicles traveling on the improved facility. However, the magnitude of the increase cannot be predicted reliably at present. The effect of such capacity increases on CO and VOC emissions is currently not well understood. High-speed, cruise-type conditions are most likely to occur when capacity is added to freeways, which operate at relatively high average speeds for a broad range of service conditions. Finding 8: The overall initial effect of highway capacity additions on emission levels depends on changes in the distribution and daily flow of traffic on all affected network facilities, which in

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use turn depend on the magnitude of the highway capacity addition and on local traffic conditions. All else being equal, the emissions effects are greater the more traffic is diverted from other congested roads, the greater the level and duration of congestion experienced before the capacity addition, and the greater the total volume of affected traffic. Finding 9: Diesel emissions from heavy-duty vehicles have not received the same scrutiny as emissions from gasoline-powered, light-duty vehicles, but it is likely that, with the exception of particulate exhaust emissions, the former can be predicted with greater certainty than emissions from gasoline-powered automobiles. The reasons are as follows: (a) diesel engines do not yet use any exhaust aftertreatment, so modeling their emissions does not require predicting catalyst efficiency; and (b) diesel engines require little cold start or acceleration enrichment, which significantly increases the variation in emission levels for gasoline-powered vehicles. Finding 10: Current emission models predict that highway capacity additions will initially reduce all diesel emissions from heavy-duty trucks with speeds up to about 56 to 64 kph (35 to 40 mph). At higher speeds NOx and VOC emissions increase, with VOC emissions increasing more sharply. The certainty of these results must be qualified because they are based on test data from a small sample of 1979 model year, in-use diesel heavy-duty trucks. Finding 11: Detailed data on diesel particulate exhaust emissions as a function of speed are unavailable. Trends in particulate exhaust emissions may follow those for VOC emissions because both result from incomplete combustion of motor fuels. The lack of data on particulates is troublesome because exhaust emissions from diesel-powered, heavy-duty vehicles are the primary highway vehicle source of particulate exhaust emissions and are the most sensitive of all regulated diesel emissions to deterioration and poor performance of vehicle components from inadequate maintenance or tampering. High concentrations of particulates over extended periods are a risk factor for lung cancer.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Finding 12: The initial effects on air quality of improved levels of service from highway capacity additions vary by pollutant. Localized CO hot spots will be relieved by capacity additions that eliminate bottlenecks where congestion occurs. Ozone nonattainment areas sensitive to NOx precursor emissions could be adversely affected if capacity expansions result in high-speed, cruise-type traffic operations. Finding 13: The initial effects on energy use from highway capacity additions can be predicted more reliably than effects on emissions because fuel economy is not as sensitive to traffic flow conditions, particularly speed variation, as are emissions. Finding 14: Current fuel economy models suggest that highway capacity additions that increase vehicle speeds from low speeds [32 kph (20 mph)] to about 56 to 72 kph (35 to 45 mph) for automobiles and about 80 kph (50 mph) for heavy-duty trucks should improve fuel economy. [Of course, all improvements in fuel economy will be accompanied by corresponding reductions in emissions of carbon dioxide (CO2), the principal greenhouse gas.] At higher speeds, fuel economy degrades rapidly. The certainty of these results must be qualified because they are based on a sample of vehicles that is not representative of the current fleet mix. Impacts on Travel Demand Over time, by reducing travel time and improving the reliability of travel, highway capacity additions can induce increased highway use by encouraging more frequent trips, longer trips, and shifts from other modes. Because net increases in regional trips and travel mean increased aggregate emission levels and higher total energy use, the central concerns are to determine the conditions most conducive to stimulating new highway use and the likely magnitude of the increase. It is important to distinguish shifts in travel, such as changes in travel routes or the time of day of travel, from new travel because the former generally do not result in a net increase in highway system use.7 It is also important to distinguish increases in highway use that occur

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use because of population or regional economic growth from increases attributable to the supply addition, the primary focus of this study. Finding 1: Highway capacity additions that reduce travel time and the day-to-day variability in travel time will induce increased highway use as long as travel times are shorter and the reliability of motor vehicle travel is improved, all else being equal. Finding 2: Highway use also increases in response to population growth, rising personal income, increased automobile ownership, regional economic growth, effective reductions in fuel prices, and land use policies that favor dispersed development patterns. Expansion of highway capacity interacts with these factors to expedite the growth in highway use and channel the location of growth within the metropolitan area. It is difficult to separate these effects because changes in development and other factors contributing to highway use often occur together with, and may either influence or be influenced by, additions to highway capacity. Finding 3: The greatest effect on travel demand from added highway capacity is to shift traffic from other routes and other times of day to the newly expanded facility at peak travel periods. Some shifts in travel destination and mode of travel may also occur. New trips and increased automobile ownership induced by the capacity expansion are likely to be modest initially; over the long run both may increase. The greatest net increase in highway use occurs when there are large time savings from the new capacity during both peak and off-peak travel times. When capacity additions reduce only peak-period travel times, most of the perceived increase in highway use is the result of shifts in traffic from other times of day or other routes rather than a net increase in highway system use. The potential for mode shifts from transit to automobile is greatest in large metropolitan areas with extensive rail transit systems serving commuter travel in the same corridor as the expanded highway facility. Shifts from other travel modes such as walking and bicycling will occur to the extent that the highway capacity addition makes motor vehicle travel more attractive relative to those other modes. This effect is minor compared with the others discussed in this finding.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use pacity additions on air quality. For example, in nonattainment areas where ozone formation is NOx limited, speed-enhancing highway capacity additions that increase NOx emissions will exacerbate the conditions leading to ozone formation. On the other hand, in areas with localized pollution problems, such as high concentrations of CO on particular corridors, highway capacity additions are likely to relieve the hot spots, at least initially, by eliminating bottlenecks where congestion is heavy and traffic speeds are highly variable. Consequences of alternative scenarios: The effect of highway capacity additions also depends on the likely outcomes of alternative futures. For example, not adding highway capacity has consequences. If congestion is predicted to grow in the absence of the project, the effects of the capacity addition could become more valuable over time in comparison with the no-build scenario. Alternatively, highway funding could be used to support other investments such as improvements in transit service. In this case, a comparison of the relative effects of the two alternatives would entail making judgments about such issues as the substitutability of and elasticity of demand for automobile and transit trips. Assumptions about the future growth of the region also affect predicted outcomes. Would growth occur without the new highway? Would that growth take place in central locations in the region or in more distant exurban areas, or would the growth occur in other, less congested metropolitan areas? Certain combinations of conditions suggest where highway capacity additions are more or less likely to reduce emissions and energy use. For example, traffic flow improvements that alleviate bottlenecks in developed areas may reduce some emissions and energy use by smoothing traffic flows with limited risk of offsetting increases from major new development and traffic growth. The cumulative effect of multiple small improvements in traffic flows, however, may attract increased traffic even in developed areas, at least in the vicinity of the improvements. Major highway capacity additions in less developed parts of metropolitan areas, where most growth is occurring, pose a greater risk of increasing emission levels and energy use in those areas. If devel-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use opable land is available and other growth conditions are present, new capacity is likely to attract more development and related traffic to the location of the improvement. Corresponding increases in emission levels and energy use in these areas are likely. Because of the durability of current metropolitan spatial patterns, however, it may be years before these changes make a significant difference in regional emission levels and air quality. RECOMMENDATIONS FOR RESEARCH, MODELING IMPROVEMENTS, AND DATA COLLECTION Better models and data supported by research can help increase the certainty with which the impacts of highway capacity additions can be predicted and can narrow the gap between regulatory requirements and analytic capabilities. Two key areas are identified where further work could significantly improve the current level of knowledge. Emissions Modeling Regulatory requirements of the CAAA have placed great importance on emissions modeling to demonstrate conformity, yet as discussed in the previous sections, current models are not up to this task. The most pressing needs are for the development of an emission rate model that is sensitive to a wider range of typical driving patterns than current models and is truly representative of the range of vehicles on the road today, and for the collection of vehicle activity data to use as input to this model. Several efforts are under way to achieve this end, but with the possible exception of a state-funded effort, a sense of urgency is not apparent. Funding levels are not adequate to produce results in a timely manner to meet regulatory requirements. Perhaps the most comprehensive effort is the state-initiated research project to develop and verify a modal emissions model within 3 years, supported by state-pooled funds through the National Cooperative Highway Research Program (NCHRP 1994).9 EPA, through its Office of Research and

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Development, and the California Air Resources Board are also conducting research on drive cycles leading to development of modal emissions models, although fully operational models may be more than 3 years from completion. The Federal Highway Administration (FHWA) has funded the Oak Ridge National Laboratory to improve the accuracy of emissions (and fuel consumption) estimates used in traffic models.10 Finally, a multimodal traffic simulation model, sponsored by FHWA, is being developed at the Los Alamos National Laboratory. The model is expected to be capable of providing second-by-second data on individual vehicle movements that can be combined with correspondingly detailed emission rates to determine emission levels. However, this system of integrated models is also several years from being fully operational. Federal leadership and funding are required to accelerate and better coordinate these efforts. Model development requires extensive and expensive vehicle testing to develop emission rates representative of the current vehicle fleet and typical vehicle operations. The cost of the testing could be substantially reduced if the research is coordinated and well focused. Incorporation of the models into the regulatory process can only be achieved by close coordination between the transportation and the regulatory communities early in the model development process. Travel Demand and Land Use Forecasting Models Improvement in the ability to forecast likely behavioral responses to highway capacity additions and corresponding changes in travel service characteristics is essential to assessing environmental and other consequences of supply changes. Development of better forecasting capability requires a sound theoretical understanding of household and corporate location decisions as well as of the determinants of personal and freight travel. Substantial new understanding of household location and passenger travel has developed during the last decade. Incorporation of this understanding into state-of-practice models requires considerable effort, both to refine the relevant behavioral linkages among the many travel and travel-related decisions made by households and to esti-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use mate the appropriate analytic relationships among them. The effort requires (a) collection of substantially enhanced data about the location and travel behavior of households and individuals, (b) estimation of enhanced models that incorporate more of the behavioral linkages among decision elements, and (c) test applications in one or more metropolitan regions including the development of implementation software. The data collection effort should incorporate advances from the development of activity-based travel analysis approaches. Such approaches take account of household structure and roles of household members, assignment of tasks to household members, and linkages among different aspects of travel-related decisions (e.g., the interrelationships among mode choice, destination choice, and automobile ownership). Development of more sophisticated models for freight transportation in metropolitan areas requires a substantial effort because of the current state of freight modeling practice. The main limitations are the lack of data on freight and truck movements in metropolitan areas and the complexity of freight demand estimation and truck trip modeling. Because of these limitations, efforts should be focused at first on developing corridor-scale rather than comprehensive regional freight models. Longitudinal studies are required to examine the long-term effects of transportation investments on land use and urban form. Although the inherent methodological and data problems are formidable, this effort is needed to understand the effects of investments that accumulate and unfold over decades. Cross-sectional comparisons of trip making as a function of different land use patterns, however, may provide partial answers more quickly as long as the analysis controls for the effects of other factors, such as automobile ownership, income, household size, and other demographic characteristics. Support for all these activities could be provided, at least in part, from the funds granted to MPOs for their expanded planning responsibilities under the Intermodal Surface Transportation Efficiency Act of 1991. The models used by MPOs and state departments of transportation should be updated with the results of these efforts. In addition, where good historical data are available, they should be used to calibrate both travel forecasting and land use models and to “predict” current travel and land use patterns so that the models' forecasting

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use capabilities can be validated. Finally, continued improvement in the linkages between land use and transportation forecasting models should be sought so that the long-term effects of transportation policies, such as highway capacity additions, on land use and related travel behavior can be examined comprehensively and with greater accuracy. CONCLUDING OBSERVATIONS Despite the considerable uncertainties in predicting the effects of expanding highway capacity on air quality and energy use, policy makers and planners must comply with current regulatory requirements and make decisions on the basis of the best available information. Thus, the committee thought it should provide its best judgment of the likely payoffs of pursuing current policies. In its opinion, the current regulatory focus on curbing growth in motor vehicle travel by limiting additions to highway capacity is an indirect approach to achieving emission reductions in metropolitan areas that is likely to yield small changes, positive or negative, in metropolitan air quality by attainment deadlines. Historically, measures to control traffic demand or improve traffic efficiency have had limited effects (Apogee Research, Inc. 1994).11 According to estimates from local studies using current emission models, these traditional transportation control measures (TCMs), which include traffic flow improvements among others, are likely to yield changes of 1 to 2 percent individually in regional emissions of key pollutants by current attainment deadlines (DOT and EPA 1993, 9). (The effects of traffic flow improvements could be positive or negative depending on offsetting increases in traffic.) These are small changes on a declining base; EPA projects continuing reductions of highway vehicle emissions from transportation sources over this period (Nizich et al. 1994, 5-4–5-6). Market-based TCMs—such as increased parking charges and time-of-day tolls—have greater potential for emission reductions than more traditional TCMs. However, the political feasibility of some measures is untested (Apogee Research, Inc. 1994, 43–44). Curtailment of all highway capacity expansion that has any potential for increasing emissions risks pitting environmental against economic concerns. The conditions with the greatest potential for stim-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use ulating development and traffic and thus increasing motor vehicle emissions and energy use are likely to occur at those locations where most new capacity additions are being proposed: rapidly growing suburban areas. Yet it is precisely in these locations where the greatest need is perceived for highway capacity additions to support regional economic growth and competitiveness and where the pressures to provide highway capacity to support development are most intense. In the past, when environmental goals have been in conflict with economic objectives, the response has been to delay or reassess environmental regulations. Given the high economic stakes associated with major highway capacity additions combined with the difficulty of determining precisely the magnitude of likely adverse effects on regional air quality, similar pressures may emerge again. A more constructive approach in the committee's view is to look for solutions that reconcile air quality with economic goals in metropolitan areas. Historically, most reductions in motor vehicle emissions have come from changes in vehicle technology (e.g., the catalytic converter and fuel-injection technology) and fuels (unleaded gasoline) rather than from controlling motor vehicle travel. Incremental improvements in technology alone (preheated catalytic converters, engine power enrichment regulations), fuels (oxygenated fuels, alternative fuels), and maintenance and inspection programs (in-use emissions testing) should result in further emission reductions and air quality improvements over the next two decades. EPA has projected that continued fleet turnover combined with implementation of CAAA standards will result in decreases in highway vehicle emissions of VOC, CO, and NOx of 31, 26, and 13 percent, respectively, from 1990 baseline levels by 2010 (Nizich et al. 1994, 5-4–5-6).12 Market solutions also have promise, although the feasibility of some approaches is untested. For example, new highway capacity could be added where congestion levels dictate, but restrictions on its use could constrain the demand-inducing potential of the capacity additions. Time-of-day tolls (i.e., congestion pricing) using electronic toll collection technologies could be introduced, traffic growth monitored, and toll levels varied to control travel demand. This managed capacity approach, if applied in a limited setting, would not require major changes in current highway finance patterns. It would allow capacity to be provided where it is most needed—that is, in highly congested

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use corridors—but could mitigate some of the negative effects on emissions from travel growth. Managed highway capacity additions may keep travel growth in check to meet near-term conformity requirements in some nonattainment areas. However, more aggressive measures may be necessary in the long run to reduce air pollution and conserve energy. Pricing motor vehicle travel to ensure that motorists pay the full social costs of their trips, including air pollution, could have a pronounced effect on the overall level of travel demand. Areawide time-of-day tolls could help ensure that existing metropolitan highway capacity is used more efficiently by spreading motor vehicle travel more evenly throughout the day and even eliminating some trips. A recent National Research Council report on congestion pricing (NRC 1994) has examined in depth the technical and political feasibility of this approach. Local land use and zoning measures that increase building density and encourage mixed-use development also have the potential to reduce automobile travel and thus improve air quality, although these changes are likely to occur gradually. They should have more significant effects if they are implemented concurrently with pricing measures. More radical technological advances—such as the “clean” car13 that would significantly reduce pollution if not congestion from motor vehicle travel, and the automated highways and advanced vehicle control systems of the intelligent transportation system program that could dramatically improve the efficiency of existing highway facilities14—are on the horizon. However, these advances are at least a decade away from large-scale commercial use (TRB 1991, 33; Williams 1994, 28; PNGV 1994), and they will require massive investments and long lead times before they become widespread.15 Pricing strategies and major technological advances offer more direct ways of reducing motor vehicle emissions and energy consumption, but they demand a level of public acceptance that has not been evident in the past. Technology changes require massive investments, consumer acceptance of new products, and long lead times before changes are implemented in the marketplace. Pricing and land use measures require major changes in public attitudes and institutional arrangements (more coordinated regional institutions to implement areawide pricing schemes and comprehensive land use strate-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use gies) to have significant impacts. However, in the judgment of the committee, as long-run alternatives to current policy they offer far greater promise of reconciling environmental and economic interests and making significant improvements in metropolitan air quality and energy conservation. NOTES 1. Currently the regulations apply to nonattainment and maintenance areas. However, a recent court ruling, which EPA is expected to appeal, may require the agency to issue criteria and procedures for determining conformity in attainment areas within 9 months (AASHTO Journal 1995). 2. EPA approval of SIPs was required by late 1993 for carbon monoxide, by late 1995 for ozone, and by late 1997 for particulates–approximately 12 months from the respective submittal deadlines for those plans, which is the maximum federal review period provided under the CAAA. 3. The final conformity regulations require a regional emission analysis for any “regionally significant” project, that is, for any facility with an arterial or higher functional classification, or any other facility that serves regional travel needs and would normally be included in the modeling for the transportation network (Federal Register 1993, 62,211). 4. The analysis years are the first milestone years (1995 in carbon monoxide nonattainment areas and 1996 in ozone nonattainment areas) and the attainment year for the area or, if the latter is the same as or earlier than the first milestone year, the second analysis year is at least 5 years after the first analysis year (Federal Register 1993, 62,244). 5. During the transition period after SIPs have been submitted but have not been approved by EPA, MPOs must demonstrate that TIPs pass the build–no-build test and also that the build scenario does not exceed the emissions budget for motor vehicle emissions contained in the submitted SIP (Federal Register 1993, 62,191). 6. If EPA disapproves all or portions of the SIP, states are notified, and transition period conformity regulations are in effect. If EPA finds that SIP revisions are incomplete or that states have failed to submit revisions, the conformity status of the transportation plan and the TIP will lapse 120 days after EPA's final disapproval and such a finding starts the nondiscretionary sanctions clock ( Federal Register 1993, 62,191–62,193). 7. Route shifts can increase or decrease highway system use as measured in vehicle miles traveled by increasing or decreasing the circuity of trips to reach the new or expanded facility, but these effects are relatively minor.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 8. There are no operational models, however, that formally analyze the impact of transportation improvements on aggregate regional growth, in part because of the lack of a theoretical basis for determining the relationship between transportation improvements and population and economic growth. 9. The objective of the research is a model that accurately reflects the impacts of speed, engine load, and start conditions on emissions under a comprehensive variety of driving characteristics and vehicle technologies (NCHRP 1994, 1). 10. Oak Ridge National Laboratory will collect modal emissions and fuel consumption data for a limited sample of vehicles and develop a model to estimate emissions and fuel use for alternative traffic patterns. 11. In the past, lack of funds at the local level to develop and implement competitive alternatives to automobile travel was seen as the primary impediment to achieving larger emission reductions from transportation control measures. However, the Intermodal Surface Transportation Efficiency Act of 1991 addressed this gap by expanding planning and research funds, establishing a new Congestion Mitigation and Air Quality Improvement Program, and allowing state and local governments to use traditional funding sources more flexibly to achieve air quality goals (DOT and EPA 1993, 2). 12. Although the overall level of emissions is predicted to be lower in 2010 than in 1990, highway vehicle emissions of CO are forecast to turn back up by 2005, and VOC and NOx emissions are forecast to turn back up by 2008, because of rising emissions from growth in vehicle miles traveled (EPA 1994, 5-4–5-6). 13. The “Big Three” U.S. automobile manufacturers and the U.S. government have committed to a 10-year Partnership for a New Generation of Vehicles (PNGV) to develop a passenger vehicle with up to 3 times the fuel efficiency of today's midsized sedan. The vehicle is to cost no more to own or drive than today's comparable automobile (adjusted for economics) and is to meet or exceed current safety, emission, and performance standards. The PNGV expects to narrow the technology choices by 1997 and to develop a concept vehicle or vehicles by 2000 and a production prototype or prototypes by 2004 (PNGV 1994). 14. In the absence of other policies to control demand, advanced vehicle control systems could have the undesired effect from an air quality perspective of encouraging more motor vehicle travel and further decentralization of metropolitan areas by significantly improving the efficiency of highway transport. 15. Some low-emission vehicles (LEVs) will be introduced before the turn of the century. In California, for example, 2 percent of the light-duty vehicles marketed in 1998 must be zero-emission vehicles. EPA has approved

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use a similar program for the northeastern states beginning with model year 1999 (Federal Register 1995) but gives the states flexibility to provide emission reductions equivalent to a LEV program. The automobile makers have proposed an alternative program that would require low-emission, but not zero-emission, vehicles (i.e., electric vehicles) to be marketed in the 49 states on a phased-in approach beginning in the Northeast. EPA and state environmental officials are doubtful that the industry alternative as it is currently structured will achieve air quality objectives (AASHTO Journal 1994). REFERENCES ABBREVIATIONS AASHTO American Association of State Highway and Transportation Officials DOT U.S. Department of Transportation EPA Environmental Protection Agency NCHRP National Cooperative Highway Research Program NRC National Research Council PNGV Partnership for a New Generation of Vehicles TRB Transportation Research Board AASHTO Journal. 1994. EPA Approves Low-Emission Vehicle Programs for Northeast. Dec. 30, pp. 11–12. AASHTO Journal. 1995. Court Extends Conformity to Attainment Areas. Feb. 24, pp. 8–10. Apogee Research, Inc. 1994. Costs and Effectiveness of Transportation Control Measures (TCMs): A Review and Analysis of the Literature. National Association of Regional Councils, Bethesda, Md. Deen, T.B., and R.E. Skinner, Jr. 1994. A Paradigm for Addressing Change in the Transportation Environment TR News, No. 174, Sept.–Oct., pp. 11–13. DOT and EPA. 1993. Clean Air Through Transportation: Challenges in Meeting National Air Quality Standards. A joint report from DOT and EPA pursuant to Section 108(f)(3) of the Clean Air Act, Aug. 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, pp. 62, 188–62, 253. Federal Register. 1995. Final Rule on Ozone Transport Commission: Low Emission Vehicle Program for the Northeast Ozone Transport Region. Vol. 60, No. 15, Jan. 24, pp. 4, 712–4, 739.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Hartgen, D.T., A.J. Reser, and W.E. Martin. 1994. State of the Practice: Transportation Data and Modeling Procedures for Air Quality Emissions Estimates. Center for Interdisciplinary Transportation Studies, The University of North Carolina at Charlotte, Charlotte, N.C., July. NCHRP. 1994. Development of a Modal-Emissions Model. Research Project Statement. NCHRP Project 25-11, Transportation Research Board, National Research Council, Washington, D.C., Oct. 6. Nizich, S.V., T.C. McMullen, and D.C. Misenheimer. 1994. National Air Pollutant Emission Trends, 1900–1993. EPA-454/R-94-027. Office of Air Quality Planning and Standards, Research Triangle Park, N.C., Oct., 314 pp. NRC. 1994. Special Report 242: Curbing Gridlock: Peak-Period Fees to Relieve Traffic Congestion, Volumes 1 and 2. Transportation Research Board and Commission on Behavioral and Social Sciences and Education, National Academy Press, Washington, D.C. PNGV. 1994. Program Plan. U.S. Department of Commerce, July, 37 pp. TRB. 1991. Special Report 232: Advanced Vehicle and Highway Technologies. National Research Council, Washington, D.C., 90 pp. Williams, R.H. 1994. The Clean Machine. Technology Review, Vol. 97, No. 3, April, pp. 20–30.