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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Executive Summary The Clean Air Act Amendments of 1990 (CAAA) and complementary provisions of the Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) introduced new constraints on the transportation sector to help ensure that transportation activities do not delay expeditious attainment of national health standards for air quality. As a result highway projects that expand capacity have come under particular scrutiny in many metropolitan areas for their potential to increase motor vehicle traffic and emissions—a primary source of air pollution. The issue is already at the center of legal challenges or threats of litigation in several metropolitan areas, potentially stalling local highway construction programs. For years transportation agencies have responded to traffic growth by expanding highway capacity to maintain reasonable levels of service. Capacity expansions ranging from small-scale signal-timing improvements to construction of major highways were expected to relieve congestion without substantial negative effects on air quality. In fact, capacity enhancements that raised travel speeds and smoothed traffic flows were believed to reduce vehicle emissions and improve
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use energy efficiency. Further, it was widely accepted that new highway capacity was essential to the continued economic growth and competitiveness of major metropolitan areas. These views are now being challenged. Some analysts and environmental groups argue that adding highway capacity will result in more traffic, higher emission levels, and greater energy consumption in the long run by stimulating motor vehicle travel and encouraging dispersed, automobile-oriented development. In addition they see continued highway expansion as antithetical to a more environmentally oriented and resource-conscious future that stresses the revitalization of older urban and inner suburban neighborhoods and supports transit and nonmotorized forms of transport. These issues are part of a larger debate over the appropriate direction of metropolitan growth and the role of transportation in that process. This debate involves value judgments about the relative importance of mobility, economic growth, environmental protection, and energy conservation. It considers a broad range of policies, from investments in transportation supply to demand management and pricing strategies. This study is focused on a more specific topic: the effects of investment in highway capacity on air quality and energy use in metropolitan areas. Its primary audience is metropolitan planning organizations (MPOs), state officials, legislators, and courts with oversight responsibilities. These agencies and officials are being asked to meet the regulatory requirements of the CAAA by making judgments about the environmental effects of highway capacity expansion on the basis of their interpretation of the best available evidence. Energy issues do not convey the same urgency or require the same regulatory analysis, yet transportation 's increasing consumption of the nation's petroleum resources is of concern. To the extent that energy efficiency and energy use are affected by changes in traffic flow characteristics and travel volume from highway capacity expansion, these effects are considered in this study. The purpose of this study is to review the current state of knowledge, evaluate the scientific evidence, and narrow the areas of disagreement about the impacts of highway capacity additions on traffic flow characteristics, travel demand, land use, vehicle emissions, air quality, and energy use. The state of modeling practice is also exam-
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use ined to assess the reliability of forecasting tools available to planning agencies; research, modeling improvements, and data collection are recommended to help narrow the gap between regulatory requirements and analytic capabilities. International experience relevant to the study charge was considered, and alternatives to highway capacity expansion, such as “traffic calming,” were examined to the extent they shed light on the effect of changes in traffic flow characteristics on vehicle emissions and energy use. In most cases, however, the experience of other countries, particularly European countries, is not directly applicable to the United States because of considerable differences in land availability and cost, population density, mode choice, pricing structures, and institutional governance of regional growth. Wholesale adoption of European transport strategies might produce some reductions in vehicle energy use and emissions and concomitant improvement in metropolitan air quality, but, in the committee's judgment, it would also impose substantial social and economic costs and raise questions about institutional and political feasibility. REGULATORY CONTEXT Under the CAAA the U.S. Department of Transportation (DOT) and MPOs are directly accountable for demonstrating the compatibility of transportation investments with timely attainment of national air quality standards. According to the current interpretation in Environmental Protection Agency (EPA) regulations, the act provides a grace period for states to revise their air quality attainment plans, which set emissions limits by source. In the meantime, transportation agencies in metropolitan areas that have not attained national standards must meet strict interim conformity requirements to prevent air quality degradation. Specifically, nonattainment areas are required to prove that (a) projects in regional transportation improvement programs and plans will not lead to motor vehicle emissions higher than in a 1990 baseline year and (b) by building these projects, emissions will be lower in future years than if the projects are not built (the “build–no-build” test).
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Once EPA approves new state air quality attainment plans the conformity test changes: the emissions that result from carrying out regional transportation improvement programs and plans in nonattainment areas must not exceed target emissions caps for transportation sources established in the EPA-approved state air quality plans. The conformity test still requires a regional emissions analysis, but the criterion for comparison is less demanding: predicted regional emissions from transportation sources must be within state-determined emissions budget caps. The most analytically demanding comparison—estimating changes in emission levels in future years from marginal expansions of regional transportation networks relative to maintaining the status quo—would no longer be required. The committee did not limit its examination of the effects of highway capacity additions to a specific time frame. However, particular attention was paid to the 20-year time frame established by the CAAA for attainment of air quality standards, because this represents the planning and forecasting horizon within which local planning agencies are required to make judgments about the air quality effects of highway projects. ANALYTIC REQUIREMENTS Providing a precise assessment of the net effects of expansions of highway capacity on air quality and energy use is not straightforward. Addressing the questions raised in this study requires modeling a complex sequence of interrelated events—from initial impacts on traffic flows to longer-term consequences on travel demand, automobile ownership, and residential and business location in a metropolitan area. It also requires modeling the emissions generated by the predicted travel impacts and forecasting their effects on air quality at or near the points of emission and throughout the region. There is significant uncertainty in predicting precise quantitative outcomes at each stage in the analysis. In addition, different levels of analysis are required to distinguish project-level from regional effects. Determination of initial effects is complicated by the network character of the transportation system. The ability of drivers to change their route, time, and mode of travel means that adding highway ca-
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use pacity and thus reducing travel time at one location in the system will affect other locations as users take advantage of the new capacity. Modifications in traffic flows and volumes on this broader system of facilities, and their effects on emissions and energy use, must be taken into account. User responses also change over time. The response to large reductions in travel time is likely to be greater in the long run than initially, as highway capacity additions influence decisions about location and automobile ownership. The key uncertainty is at what point, or whether at any point, the emissions increases from the new development and traffic growth stimulated by the capacity addition will offset the initial emission reductions gained from smoothing traffic flows. Assessment of net project effects on emissions and energy use depends on the length of time over which the effects are analyzed and the value placed on long-term versus more immediate effects. The longer the prediction period the more likely other, often unpredictable factors such as changes in demographic or economic conditions are to intervene and diminish forecasting accuracy. Forecasts of the net effects of adding highway capacity also involve comparisons with alternatives. Not investing in highway capacity or undertaking other investments or demand management strategies has consequences for air quality and energy use. A comparison of outcomes requires estimating how sensitive users are to changes in travel time and cost and to what extent travel by transit or other modes can be substituted for automobile travel. It also requires understanding how growth would be distributed both within and outside the metropolitan area if highway capacity expansions were restricted. Paradoxically, because of past and ongoing efforts to reduce vehicular emissions, it will become more difficult to discern differences in emissions resulting from different transportation and demand management strategies. These alternative strategies all represent small changes to a declining base of emissions from highway sources. FINDINGS After examining the considerable literature on the relationships among transportation investment, travel demand, and land use as well
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use as the current state of the art in modeling emissions, travel demand, and land use, the committee finds that the analytical methods in use are inadequate for addressing regulatory requirements. The accuracy implied by the interim conformity regulations issued by EPA, in particular, exceeds current modeling capabilities. The net differences in emission levels between the build and no-build scenarios are typically smaller than the error terms of the models. Modeled estimates are imprecise and limited in their account of changes in traffic flow characteristics, trip making, and land use attributable to transportation investments. The current regulatory requirements demand a level of analytic precision beyond the current state of the art in modeling. The state of emissions modeling illustrates the problem well. In theory the initial effect of a highway capacity addition on traffic flow characteristics and the resulting changes in vehicle emissions should be measurable for the current fleet. Despite considerable research and vehicle testing, however, no definitive and comprehensive conclusions can be reached. Virtually all motor vehicle testing has been based on a limited set of driving test cycles that inadequately represent current urban driving conditions. In addition, current emission models rely on average trip speed as the sole descriptor of traffic flows. Variability in speed, road grade, and other factors that strongly influence emissions is not explicitly incorporated into the models. Changes in vehicle emission rates thus cannot be predicted reliably for a wide range of changes in average trip speeds, many of which are in the range of average speed changes expected from highway capacity additions. Current emissions models were developed to estimate motor vehicle emissions at the regional level. They cannot be appropriately applied to estimate the emissions effect of changes in traffic flow patterns at specific highway locations. Even for regional estimates, current models significantly underpredict emissions of some pollutants. Models can be developed that are more sensitive to vehicle operations and traffic conditions; some research and testing have already begun. However, development and incorporation of new models into the regulatory process will take substantial time and investment and require close coordination between the transportation and the regulatory communities. The initial effects on energy use from highway capacity additions can be predicted more reliably than effects on emissions because fuel
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use economy is not as sensitive as emissions to traffic flow conditions, particularly speed variation. However, energy and emissions estimates must both be linked with reliable data on the likely impacts of highway capacity additions on traffic, travel demand, and location decisions. Here, too, the available travel demand and land use models provide imprecise and limited estimates of likely outcomes. Improvements in modeling capability will also require substantial investment. More research and improved models can help narrow the gap between regulatory requirements and analytic capabilities. The conformity tests required by current regulations will themselves change as the build–no-build test is phased out once state air quality plans are approved by EPA. However, the complex and indirect relationship between highway capacity additions, air quality, and energy use, which is heavily dependent on local conditions, makes it impossible to generalize about the effects of added highway capacity on air quality and energy use, even with improved models. On the basis of current knowledge, it cannot be said that highway capacity projects are always effective measures for reducing emissions and energy use. Neither can it be said that they necessarily increase emissions and energy use in all cases and under all conditions. Effects are highly dependent on specific circumstances, such as the type of capacity addition, location of the project in the region, extent and duration of preexisting congestion, prevailing atmospheric and topographic conditions, and development potential of the area. Nevertheless, some general findings do apply. Within developed areas, traffic flow improvements such as better traffic signal timing and left-turn lanes that alleviate bottlenecks may reduce some emissions and improve energy efficiency by reducing speed variation and smoothing traffic flows without risking large offsetting increases from new development and related traffic growth. However, the cumulative effect of multiple small improvements in traffic flows may attract increased traffic, at least in the vicinity of the improvements. In lessdeveloped portions of growing metropolitan areas—where developable land is available and most growth is occurring—major highway capacity additions such as a freeway bypass or a major interchange reconstruction are likely to attract further development to these locations and increase motor vehicle travel, emissions, and energy use in these areas. Whether these outcomes lead to a net increase in regional
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use emissions and energy use depends on whether the highway expansion redistributes growth that would have occurred elsewhere in the region or whether it stimulates productivity gains that result in net new growth. CONCLUDING OBSERVATIONS Despite the considerable uncertainties in predicting the effects of expanding highway capacity on air quality given the current state of knowledge and modeling practice, policy makers and planners must comply with current environmental regulatory requirements and make decisions on the basis of the best available information. Thus, the committee thought it should provide its best judgment on the likely payoffs of pursuing current policies. In its opinion, the current regulatory focus on curbing growth in motor vehicle travel by limiting highway capacity is at best an indirect approach for achieving emissions reductions from the transportation sector that is likely to have relatively small effects, positive or negative, on metropolitan air quality by current attainment deadlines. Historically, measures to control traffic demand have had limited effects (Apogee Research, Inc. 1994). 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 or 2 percentage points individually in regional emissions of key pollutants by 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, given EPA projections of continuing emission reductions for carbon monoxide and volatile organic compounds (DOT and EPA 1993, 9). In the committee's opinion, major highway capacity additions are likely to have larger effects on travel and to increase emissions in the affected transportation corridors in the long run unless some mitigating strategy is implemented in conjunction with the capacity addition. However, because of the large investment implicit in current metropolitan spatial patterns, it may be years before changes in land use and related traffic patterns induced by the added capacity make a significant difference in regional emission levels and air quality.
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Curtailment of all highway capacity expansion that has any potential for increasing emissions risks pitting environmental against economic concerns. In the past, when environmental goals have conflicted with economic objectives, the response has been to delay or reassess environmental regulations. It is easy to envision these pressures building again. In addition, requiring policy decisions to hinge on uncertain model outputs leaves the entire process vulnerable to error and manipulation. In the committee's view, a more constructive approach is to look for ways to reconcile air quality with economic goals. The committee believes that technology improvements can yield more significant benefits for air quality relative to the current focus on curbing travel growth. Catalytic converters, electronic fuel injection, and unleaded gasoline have resulted in substantial reductions in vehicle emissions in the past 20 years (Nizich et al. 1994). EPA predicts further emission reductions for major pollutants on the order of one-quarter to one-third from 1990 baseline levels by attainment deadlines simply from continued vehicle fleet turnover and implementation of CAAA-required vehicular and fuel standards and enhanced vehicle inspection and maintenance programs (Nizich et al. 1994, 5-4, 5-6). Market solutions also have promise, although the feasibility of some approaches is untested. For example, potential increases in traffic from new highway capacity such as added expressway lanes might be reduced by imposing tolls varied by time of day (i.e., congestion pricing) and collected electronically to control travel growth on the expanded facility. A recent National Research Council report on congestion pricing (NRC 1994) has examined in depth the technical and political feasibility of this approach. Applied in a limited setting, it would not require major changes in current highway finance patterns. It would allow highway capacity to be provided where it is needed but could mitigate negative effects on emissions from travel growth. In the long run, stronger measures, such as pricing motor vehicle travel to better reflect the full social costs of highway travel and the introduction of areawide, time-of-day tolls, may be necessary to provide direct incentives for reducing or shifting travel demand in ways that use highway capacity more efficiently and with less cost to the environment. Local land use and zoning measures that increase building density and support mixed-use development could be introduced
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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use more widely. Land use measures may reduce areawide automobile travel and emissions, but the changes are likely to occur gradually and will have more significant effects if they are implemented in conjunction with pricing measures. Finally, radical advances in vehicle technology could produce cleaner transportation, substantially reducing the level of vehicle emissions. These more radical solutions are neither new nor easy. Major technological improvements require substantial investment and time to produce results. Changes in pricing or land use policies require significant institutional changes—such as more powerful regional institutions to coordinate areawide pricing schemes and land use strategies—and more public acceptance than has been demonstrated in the past. In the judgment of the committee, however, as long-run alternatives to current policy, they offer better prospects for reconciling economic and environmental goals. REFERENCES ABBREVIATIONS DOT U.S. Department of Transportation EPA Environmental Protection Agency NRC National Research Council 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. DOT and EPA. 1993. Clean Air Through Transportation: Challenges in Meeting National Air Quality Standards. Aug. 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.
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