5

Land Use and Urban Form

As described in the previous chapter, a highway expansion can induce traffic increases by releasing suppressed demand or attracting travelers away from other modes. The concern of this chapter is whether, over the long term, increased highway capacity results in development that fosters additional motor vehicle travel and hence degrades air quality and increases energy consumption (Figure 5-1). In the first section of this chapter the debate is framed against the backdrop of the influences on metropolitan decentralization. In the next three sections an overview is given of the current knowledge —from theory, empirical research, and modeling—about transportation's effect on land use. In the fifth section the efforts to change population densities through land use and transportation policies are discussed, and the possible effect of these changes on regional air quality and energy consumption is examined. In the final two sections the chapter is summarized and recommendations are made for research to improve the ability to estimate the land use, air quality, and energy consumption consequences of expanded highway capacity.



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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 5 Land Use and Urban Form As described in the previous chapter, a highway expansion can induce traffic increases by releasing suppressed demand or attracting travelers away from other modes. The concern of this chapter is whether, over the long term, increased highway capacity results in development that fosters additional motor vehicle travel and hence degrades air quality and increases energy consumption (Figure 5-1). In the first section of this chapter the debate is framed against the backdrop of the influences on metropolitan decentralization. In the next three sections an overview is given of the current knowledge —from theory, empirical research, and modeling—about transportation's effect on land use. In the fifth section the efforts to change population densities through land use and transportation policies are discussed, and the possible effect of these changes on regional air quality and energy consumption is examined. In the final two sections the chapter is summarized and recommendations are made for research to improve the ability to estimate the land use, air quality, and energy consumption consequences of expanded highway capacity.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use FIGURE 5-1 Impacts of highway capacity additions on land use and urban form.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use BACKGROUND Arguments Against Capacity Additions Arguments about the adverse air quality and energy effects of shifts in land use patterns related to expanded highway capacity can be distinguished at two levels of impact. The initial impact is at the project level, at which the capacity expansion on an individual link improves the access between points at either end. Over time, the improved access may facilitate more economic activity and more development, which will result in more traffic. At the general level, it can be argued that increased roadway capacity, especially projects serving the periphery of urban areas, supports continued low-density land development. Such development further encourages automobile use and ultimately increases air pollution and energy consumption (Figure 5-2). Between these two extremes the potential for a network effect also exists. New capacity on a link serving two points will improve access to those points from other locations that are connected by the road system [although in some cases improvements in the flow between two points in a complex network can actually worsen system performance because of Braess's paradox (Bass 1992)]. As access is improved between any two points in a network, economic growth may follow, which would ultimately result in more traffic on adjoining segments of the network because of the improved link. As noted earlier in this report, the scale of capacity enhancements ranges from modest (such as improved signalization) to major (such as new links or new beltways). The potential land use consequences of these varied levels of expansion increase with scale. Research Questions The air-quality arguments against expanding capacity must be framed as research questions in which the net traffic, air quality, and energy impacts are considered. The questions that arise can be grouped into the following categories:

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use FIGURE 5-2 Highway capacity effect on land use and air quality? (Toles © 1989 The Buffalo News. Reprinted with permission of Universal Press Syndicate. All rights reserved.) Direct effect: Will the increased traffic and emissions between Points A and B as a result of new development negate the air-quality benefits of improved traffic flow? Will the period of net benefits outweigh the period of net costs? If so, over what period (e.g., 5 years, 10 years, 20 years)? General effect: Do capacity expansions result in or contribute to low-density land use developments that spawn automobile-oriented life-styles, increased traffic, higher energy consumption, and additional air pollution? The counter-factual case: Will congestion worsen in the absence of an expanded link or new facility because of growth (indepen

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use dent land use changes) at both A and B that increases traffic between A and B? Is this scenario better or worse for air quality and energy use than building the facility? Direction of causality: Does the improved link between A and B cause the growth between them or is the pressure to improve the link caused by expectations of or demand for growth? As will be described in the following sections, few of these questions can be fully answered using available theory and research. The main reason is that the research must address a very complex and indirect process. Answering the question raised in this study—what are the long-term effects of adding highway capacity on air quality and energy use—requires first an examination of the impacts of capacity additions on land use and development patterns; then identification of the related impacts on travel patterns; and finally an assessment of the effects of changes in travel patterns on vehicle emissions, air quality, and energy use. There are extensive literatures on many of these individual topics (e.g., the linkage between highway improvements and land use patterns and between transportation investments and regional growth) and numerous models developed to estimate these relationships (e.g., location theory). However, only a limited number of studies are available that examine directly the link between expansion of highway capacity, land use, and air quality. In addition, many methodological problems arise in trying to address these questions. For example, as described later, although the provision of transportation is a requirement for suburbanization, the trend toward suburban development is influenced by far more than transportation capacity itself; the pressures for suburbanization themselves may create the demand for highway capacity. In addition, development spawns new and often unanticipated demands for transportation. Thus transportation and land use are interconnected. Moreover, land use changes take many years, if not decades, to unfold. Before turning to the theories, studies, and models that have been used in an attempt to answer some of these questions, the context of land use changes in metropolitan areas is reviewed to better appreciate the complexity of the process and transportation's role in development and economic growth.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Context of Transportation–Land Use Relationship The questions listed earlier assume a direct and discernable relationship between transportation investments and land use. This assumption appears well founded in light of the history of urban development and changes in transportation technologies. When walking and horse-drawn vehicles were the principal modes of transportation, cities supported high population densities with a dense network of narrow streets (Garrison and Deakin 1992). With the advent of technological innovations such as the streetcar, residential development began to shift away from the congested inner core. This was particularly evident in Philadelphia between 1840 and 1880 (Putman 1977). The early electrified rail transit systems of the era between 1870 and 1910 further expanded the process of decentralization. Finally, the advent of the automobile permitted the dispersed low-density suburban development characterizing contemporary metropolitan areas. Throughout this period, land use policies were limited to serving the interests of developers and landowners. Where zoning was practiced its scope was limited to minimizing conflicts between incompatible uses (Garrison and Deakin 1992). Both transit systems and road building served suburbanization. Many analysts have argued that this process was accelerated after World War II by federal policies such as mortgage underwriting standards and support for road construction. These policies are believed by some to have facilitated the development of the detached, single-family homes so typical of suburbia. The evidence regarding decentralizing land uses around cities, however, suggests a more complicated set of forces. Metropolitan areas in the United States and abroad have been decentralizing, as measured by population density, since at least the middle of the 19th century (Mills and Tan 1980; Meyer and Gomez-Ibanez 1981; Mieszkowski and Mills 1993). Cities became increasingly dense during the early stages of the Industrial Revolution but then began decentralizing, particularly as resident incomes and transportation facilities improved. The decentralization of cities can be measured using a density gradient. Density is measured by the number of people per unit of land and the gradient is a measure of the declining density away from the

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use center. Cities are typically denser at the middle than at the periphery. At least since records have been kept, population densities at the urban center have declined, as have densities on the periphery. With some notable exceptions such as Tokyo, which does not have inexpensive land for expansion, this change has occurred independently of changes in transportation technology or degree of automobile use (Meyer and Gomez-Ibanez 1981).1 There is no clear evidence that the federal and state housing and transportation policies of the postwar period accelerated this trend. The period of most rapid decentralization occurred between 1920 and 1950 (Mieszkowski and Mills 1993). In addition to the transportation-related explanations, various causes have been advanced to explain this process in the United States. Various urban fiscal and social stresses have been cited as having encouraged substantial proportions of the urban population to leave for suburbs that are racially and ethnically homogeneous, have lower taxes, and offer better schools (Mieszkowski and Mills 1993).2 Rising incomes in the urban population also play a major role in suburbanization. As incomes rise, individuals seek larger residences, which are typically less expensive at the periphery because of lower land costs (Gomez-Ibanez 1985). Mieszkowski and Mills (1993) point out that rising incomes are influential in both the transportation-related and fiscal and social stress-related classes of explanations for decentralization. Suburbanization is unquestionably facilitated by state and federal support of road building and by local land use policies encouraging low-density development. However, density gradients in the United States and abroad suggest that much decentralization would have occurred in any event. Moreover, many argue that the policies of road building and support for detached single-family housing are strongly favored by the population at large (Downs 1993). Thus it is open to question whether the policies have shaped the preferences or vice versa. Before turning to the research itself, it is important to acknowledge the difficulty of disentangling transportation's role in land use from this mix of fiscal and social stresses, rising incomes, and public preferences. Net Growth and Redistributed Growth The effects of transportation in facilitating physical development are not necessarily the same as its effects on economic growth. Part of the

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use argument made against expanded highway capacity is that by facilitating increased economic activity in a region, the expanded capacity will encourage more traffic and more air pollution. If a region is growing economically, development will occur somewhere within or near it. Combined with the effects of land use and zoning policies, transportation investments influence the location of growth, but they alone do not cause the growth. A number of highway and transit projects that were built to facilitate development, which did not subsequently occur, could be cited as evidence. Although transportation capacity is necessary for growth, it is not sufficient. Other conditions must be present —such as better access to labor, agglomeration economies (in which similar industries cluster together), or previously inaccessible natural resources—for a transportation project to contribute to net economic growth. Transportation investments may affect growth, when other conditions hold, but it is important to distinguish between net new growth and redistributed growth. A highway project providing access to undeveloped land may encourage a shift of future development from a developed to an outlying area, but this may merely be a redistribution of growth that would have occurred elsewhere in the region. Redistributed growth at low densities raises concerns about increased traffic, but this is not the same as the traffic that might be generated by net new growth. Transportation investment, however, may result in productivity advantages associated with a location change, which could lead to net new growth. For example, a new factory located on the periphery of an urban area may be able to reorganize its work flows in more productive ways, or the employer may be able to hire labor at lower costs. If the owner of the factory is able to sell more of its product, the factory may be expanded. Thus transportation investments can influence private productivity, which in turn can stimulate new growth. A debate has emerged over the stimulative effects of public infrastructure investments, of which transportation is a major component. In a series of articles Aschauer (1989a, 1989b, 1991) has argued that national public infrastructure investments (highways, transit, airports, and water and sewer supply) have a major stimulative effect on private productivity and output. Aschauer's application of a production function model at the national level implies that a $1 increase in pub-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use lic infrastructure investment will increase private output by roughly 40 cents. Munnell (1990, 1991) obtains similar results. The national effects of public investments, as estimated by these approaches, are much larger than the multistate, state-level, or metropolitan estimates developed by Munnell and others (Hulten and Schwab 1991). For example, Eberts's (1988) study of metropolitan areas found that a $1 increase in public investment increased private output by 7.5 cents. Munnell (1991) argued that one should not make too much of the size of these estimates; the pertinent fact is that they are all positive and statistically significant. Hulten and Schwab (1993), however, show that plausible corrections to the statistical procedures used by Aschauer, Munnell, Eberts, and others can alter both the sign and the statistical significance of the relationships they report. Whereas all of these authors would agree that individual projects can influence private output if other conditions hold, there is little agreement about their effects in general. Thus the highway capacity expansions of interest in this report cannot be assumed to stimulate growth, although they do when other conditions, such as the presence of agglomeration economies or improved access to labor or materials, hold. Demographic Influences on Growth Metropolitan decentralization has created a demand for transportation capacity not merely because of redistributed growth but also because many regions, particularly those in the South and West, are growing even as they decentralize. Population growth in the United States increased by approximately 10 percent between 1980 and 1990, but almost all of this increase occurred in metropolitan areas (Bureau of the Census 1994, Table 41). During that period population increased by 11.8 percent in metropolitan areas but by only 2.7 percent in non-metropolitan areas (Table 5-1). The growth in metropolitan population is unevenly distributed around the nation, with metropolitan areas in the South and West growing much faster than those in the Northeast and Midwest (Table 5-1). Changes in the location preferences of firms and individuals across regions appear to create much of the demand for new transportation capacity. The transportation capacity is not itself attracting population

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use TABLE 5-1 Percentage Change in U.S. Population, 1980–1990 (Bureau of the Census 1994, Table 41)   METROPOLITAN AREAS NON-METROPOLITAN AREAS United States 11.8 2.7 Northeast 3.2 5.2 Midwest 2.7 −2.0 South 17.7 3.0 West 24.1 12.0 and employment to the South and West, nor is it attracting growth to metropolitan areas. As described in later sections, however, transportation capacity affects the location and shape of development. THEORY LINKING TRANSPORTATION AND LAND USE In the early 1900s Von Thunen and Ricardo recognized that agricultural land values depended in large measure on the accessibility of the land. Farmers valued land located closer to markets or linked by superior road access more highly than land located farther away. These insights were applied to residential land values in urban areas by Alonso (1964), who advanced the theory that residential land values closer to the center would be valued more highly than those on the periphery. Residents would balance housing costs against travel costs, and when travel costs (principally in the form of time) were high, residents would pay a premium for housing near the center. Alternatively, transportation improvements serving the periphery would reduce transportation costs and increase the value of land there while reducing the value of land in the central area. Reductions in commuting costs therefore would tend to cause cities to spread out. The shape of cities, of course, is also heavily influenced by the location preferences of commercial entities. Giuliano (1989a) groups commercial location theories into three categories: (a) retail and service, (b) industrial, and (c) business location. Retail and service enterprise location is based on central place theory, which predicts

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use location on the basis of population characteristics and the willingness of residents to travel for different kinds of goods (Christaller 1966; Giuliano 1989a). According to this theory, a reduction in transport cost would result in larger, more dispersed centers, because consumers would be willing to travel farther to shop and consume services. Conversely, an increase in travel cost would result in smaller, more concentrated centers, all things being equal. (Giuliano 1989a) Industrial location theory traces to Weber's early work during the 1920s, which is focused on the location advantages of regions. Firms are postulated to locate where the costs of transporting inputs to production and serving markets are minimized. Business location theory is an extension of residential location theory; it holds that expanding transportation capacity to the periphery will increase the value of land at the periphery because of shifts in residential demand. The shifts will reduce the relative value of land at the center, which businesses can then use to their advantage, but the attractiveness will depend on whether the business is labor-or land-intensive. Thus, this theory predicts that reductions in transportation cost will have a decentralizing effect for residents, a centralizing effect for businesses seeking agglomeration economies or access to labor with specialized skills, and a decentralizing effect for businesses seeking access to land or lower-cost labor (Giuliano 1989a). Several insightful reviewers have pointed out the limitations and weaknesses of location theory (Meyer and Gomez-Ibanez 1981; Gomez-Ibanez 1985; Giuliano 1989a; Giuliano 1989b; Garrison and Deakin 1992; Moore and Thorsnes 1994). Their conclusions are summarized in this section. It is acknowledged at the outset that all theories tend to oversimplify, for example by treating only a single mode or type of trip; assuming a homogeneous housing stock; or dealing with residential location while holding business location constant or vice versa, although the two are logically related. Perhaps most significant, most theories assume a monocentric city whereas current metropolitan areas—at least those that are growing—are polycentric.3 Industrial location theory has been criticized for focusing on cost minimization rather than consumer demand and for ignoring other

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use TABLE 5-5 Mode Shares: LUTRAQ Western Bypass Study Area (Cambridge Systematics et al. 1992 and Parsons-Brinckerhoff in Giuliano 1994)   MODE SHARE (PERCENT) ITEM BYPASS LUTRAQ I LUTRAQ II Home-based work trips Walk 2.5 3.5 3.5 Solo drive 75.1 72.7 63.9 Carpool 13.6 13.8 19.7 Transit 8.8 10.0 12.8 All Trips Walk 3.7 4.5 4.5 Automobile 89.0 87.6 87.0 Transit 7.3 8.0 8.6 II would reduce total automobile travel in the afternoon peak by 13.6 percent (Giuliano 1994, 30). It is difficult to predict how much pedestrian- and transit-oriented residential designs in newly developing or redeveloping locations in a metropolitan area might affect automobile travel and how these changes, in turn, would affect regional air quality. Density benefits in terms of reduced automobile travel are partially offset by increased emissions due to traffic congestion. Modeling results from the Denver metropolitan area, for example, suggest that restricting almost all future development to transit corridors would substantially increase population density and transit use in these corridors by 2010, but no net improvement in regional air quality would result (May and Scheuernstuhl 1991). According to these estimates, carbon monoxide emissions would increase because of increased congestion, whereas other emissions would be largely unchanged. This study, however, assumed that pedestrian trips would not replace automobile trips within the transit corridors, which could bias the results. Nor did the study model the possible effects of complementary travel demand policies such as pricing parking. More important to understanding the implications for regional emissions is the low density of the existing metropolitan area. As with most western and southern metropolitan

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use areas where population is growing, Denver's urbanized area has a relatively low population density [687 persons per square kilometer (1,780 persons per square mile)]. Even for the forecast year of 2010, 70 percent of Denver's population and 60 percent of employment locations are already in place. Thus substantially increased density, which at current growth rates would not be reached until well after 2010, would be required for these shifts in transportation and land use policies to substantially affect mode choice and trips at the regional level. The regional consequences of added highway capacity and its effects on transportation, land use, and air quality are being estimated in many metropolitan areas because of the requirements in the Clean Air Act Amendments of 1990 (CAAA) and the Intermodal Surface Transportation Efficiency Act of 1991 that state air quality plans and transportation capital investment plans conform with one another. The San Francisco Bay Area's transportation investment plan has been extensively analyzed for its potential effects on air quality. These analyses were generated in part by litigation to stop the expansion of highway capacity in the region (Harvey and Deakin 1991). The new traffic that could be generated by the plan was estimated, and its effects on land use and regional air quality were forecast. The forecasts indicated that capacity increases internal to the region resulted in net reductions in emissions, whereas capacity increases in the fringe areas resulted in net increases in emissions. The overall effects of the build scenario were small, however; the transportation investment plan was estimated to reduce regionwide CO and VOCs by about 1 percent by 2010. These results are well within the forecast error of the models. The results could become negative if plausible changes in assumptions, such as more or faster automobile travel, are made (Harvey and Deakin 1991). 8 Current zoning and development restrictions also affect the results. When these restrictions are relaxed, specific communities can experience significant increases or decreases in density because of shifts in development and employment within the region (ABAG 1991), but the overall regional effects, positive or negative, remain small. Similar results have been obtained in the conformity analyses conducted by other metropolitan planning organizations (Hartgen et al. 1993; Hartgen et al. 1994). The modeling procedures used to calculate these effects, although state of the art in some re-

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use gions, are fairly crude in others. Concerted research on such topics as travel behavior and modeling improvements is required to enhance the ability to estimate the consequences of highway capacity expansion on land use and air quality. SUMMARY Highway capacity expansions are criticized because they are thought to lead to suburban sprawl, greater automobile use, and increased energy consumption and air pollution. The factors causing the decentralization of urban areas, however, are far more complex than just the expansion of highway capacity. Highway access to outlying areas is certainly necessary to support decentralization, but rising incomes, home buyers ' desire for larger homes and more land, and flight from problems in central cities have also influenced decentralization. It remains an open question whether the provision of road capacity helped cause the decentralization of urban areas or whether the demands for decentralization caused the roads to be built or expanded. Highway expansions, combined with prevailing land use and automobile pricing policies, have influenced the form of metropolitan development. Most metropolitan areas have grown in population and spread that population over a larger land area. This has been particularly true in the south and west over the last decade. Highway expansions have helped redistribute this growth. Capacity expansions tend to redistribute growth that would have occurred anyway, but they can result in net new growth if they stimulate productivity gains in the private sector. This can happen if an expansion allows firms to improve access to labor, capital, or markets. Theory predicts that a reduction in transportation costs due to an expanded highway will have a decentralizing effect on residential development and both centralizing and decentralizing effects on businesses. Businesses that serve individuals tend to follow them. Businesses exporting goods and services out of the urbanized area may or may not decentralize depending on the importance of agglomeration economies and the extent to which they are labor or land intensive. The net effect of reduced transportation costs, however, will be toward decentralization.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use Early case studies indicated that highway investments had powerful effects on land values and suburban development. More recent studies of major highway and transit investments do not indicate large effects. Analysts have advanced a number of reasons why transportation investments in recent decades have small effects on land use. Part of the problem is methodological. The cause-and-effect relationships are difficult to distinguish, even with sophisticated statistical techniques. Moreover, the impacts affect subsequent development for many decades, but most studies examine short-term consequences. There are practical reasons why highway capacity expansions have relatively minor effects, at least over a 5- to 10-year period. The out-of-pocket cost of transportation is low (and falling), which reduces the importance of location relative to other preferences. It could be that once a basic level of accessibility is established, other preferences—for housing, neighborhood, or schools—take precedence (Giuliano 1989b). In addition, the transportation infrastructure in large urbanized areas is extensive and affords a high degree of personal mobility. Even substantial new investments in capacity will make only incremental changes in the infrastructure and will therefore have small effects on regional land use. (The effects within specific corridors, however, can be substantial.) By contrast, past changes in transportation technology that influenced urban form were dramatic (Meyer and Gomez-Ibanez 1981,105). Shifting from walking to horse-and-buggy to streetcar to automobile produced sharp changes in travel speed, which facilitated a dispersal of residential development around metropolitan areas. These changes helped shape land use when they were introduced, but no similar changes have occurred in recent decades (Gomez-Ibanez 1985; Meyer and Gomez-Ibanez 1981, 105) or are anticipated in the near term. Given the extensive highway network and the low-density built environment around it, even large expansions of highway and transit capacity are unlikely to have more than marginal consequences on land use in a 5- to 10-year time frame. It follows that small changes in capacity, such as improved traffic flow from signalization, will have negligible effects on land use. Integrated models have been developed to examine the transportation-land use relationship. These models are far from perfect, but they provide a sense of the direction and magnitude of change. They confirm that highway capacity expansions on the periphery of an

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use urban area lead to decentralization as the theory predicts, but they also indicate that the effects on density, and hence air quality, are small over a 20-year forecast period. Conformity analyses of metropolitan areas to meet the requirements of the CAAA, for example, indicate that the differences between the build and no-build scenarios are typically within the forecast errors of state-of-the-art models. Because highway expansions and low-density development patterns determine land uses over many decades, the effects of even marginal increases in highway capacity in any given year are likely to compound beyond the 20-year forecast used in most analyses. Policies that encourage higher residential densities are being pursued in some metropolitan areas to reduce automobile use. Given the prevailing use of automobiles—and the subsidies that encourage their use (e.g., subsidized parking)—it would take large increases in density for trip making to shift from automobiles to other modes. Modest increases in density can reduce the amount of automobile travel, but the density of the average urbanized area would have to more than double to significantly affect mode choice. Moreover, most development is occurring on the periphery of urbanized areas, where the existing densities are far lower than within the urbanized area. Current policies and practices in most metropolitan areas make changes in density sufficient to affect mode choice and regional air quality difficult to achieve. Neighborhoods resist measures to increase density or to provide mixed uses, and close-in suburbs impose growth restrictions or use zoning practices that force development farther out. Efforts to reduce automobile use and improve regional air quality will have more potential if society is willing to make major changes in land use planning at the regional level and to fully price the use of the highway system. RECOMMENDATIONS FOR IMPROVING THE KNOWLEDGE BASE Behavioral Research An improved understanding of behavioral responses to transportation capacity expansions, among other influences shaping location preferences, is fundamental to efforts to better predict the land use and environmental effects of increased highway capacity. Because land

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use uses change very slowly, behavioral responses to changes in highway capacity must be measured over forecast periods of 20 to 40 years to fully appreciate the implications of the capacity change. Longitudinal studies are required to address these issues. The main methodological problem to be solved is how to control for and isolate the effects of other influences shaping location preferences so that the effects of transportation investment can be studied. Methodological problems are compounded by the lack of adequate historical data on the many components to be measured. Finally, the cost and lack of funding for ongoing longitudinal studies make it difficult to mount a research program that might provide additional insights into the issues. The committee recognizes the significant methodological and empirical problems in addition to the costs of performing such analyses. Nevertheless, longitudinal studies are necessary to understand how the effects of changes such as highway capacity additions accumulate and unfold over decades. Research is needed on appropriate methodologies and data collection strategies. Cross-sectional domestic and international comparisons of trip making as a function of land use patterns (e.g., density, land use mix, and urban design)—with controls for life-style, demographic, and household characteristics—should also be pursued to better understand how travel behavior can be affected through land development policies. Modeling Suggestions for improving both the theoretical framework and the application of land use models have recently been made by other scholars (Wegener 1994; Deakin 1991) and are not repeated here. One of the key deficiencies is the lack of practical models that can be applied in metropolitan planning practice for policy analysis. Existing land use models could be improved by validating their ability to forecast. This could be done by first calibrating the models on existing historical data and then testing their ability to “predict” the present. In a few metropolitan areas the data exist to make such analyses possible. The linkages between land use and transportation forecast models should be improved so that the long-term effects of transportation

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use policies, such as adding highway capacity, on land use and related travel behavior can be examined comprehensively and with greater accuracy. NOTES 1. See especially the arguments advanced by Meyer and Gomez-Ibanez in Chapter 2 and Appendix A of their book, which builds upon the paper by Mills and Tan (1980). 2. Mieszkowski and Mills (1993) provide a critical review of the different explanations of suburbanization. Whereas crime, educational attainment, and taxes were not found to influence the decentralization of U.S. cities, race was found to be influential. Comparisons between Canadian and U.S. cities, which were found to be decentralizing at the same rate between 1950 and 1970, were unable to distinguish the influence of the more centralized land use planning and transit-dependent policies on the higher densities of Canadian cities from social stresses such as crime and schooling. 3. Moore and Thorsnes (1994) point out that the monocentric model provides important and useful insights that inform the development of a polycentric model, but the polycentric model is much more complicated because it is multidimensional. 4. For simplicity of exposition, this discussion assumes that increased regional automobile travel will have similar emissions consequences across regions. Because of the variability in meteorological conditions, the effects of increased travel on ozone formation will vary considerably across regions (NRC 1991). 5. The effects on air quality of jobs following residents in a decentralized pattern are complex. They include the consequence of less transit use but, as discussed in Chapter 3, are also greatly influenced by the automobile speed of the trips they replace. Depending on speed, speed variability, traffic volume, and local climatic conditions, this could be better or worse for air quality. 6. Ewing et al. (1994) obtain similar results for the effects of density on travel and trips in a microlevel analysis of travel in six South Florida communities of widely varying densities. 7. Urbanized area is a Bureau of the Census term defined by total population, presence of a central city, and population density. There must be a total population of 50,000 or more, and population density must exceed 386 people per square kilometer (1,000 people per square mile). An urbanized area can be parts of counties, unlike metropolitan areas, which are defined by county boundaries.

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EXPANDING METROPOLITAN HIGHWAYS: Implications for Air Quality and Energy Use 8. The effects of capacity expansions on total traffic and emissions were also influenced by existing constraints on development. Growth controls in inner suburbs, for example, restrict the ability to increase density (and thereby reduce automobile use) as a result of expanded capacity in the urban core. By removing these restrictions on development, “the [modeling] results would have been somewhat different specifically, highway investment in the core would lead to some densification ” (Harvey and Deakin 1991). REFERENCES ABBREVIATIONS ABAG Association of Bay Area Governments NRC National Research Council ABAG. 1991. Assessing the Future: A Sensitivity Analysis of Highway and Road Improvements on Growth in the San Francisco Bay Area. Working Paper 91-4. Oakland, Calif. Alonso, W. 1964. Location and Land Use: Toward a General Theory of Land Rents. Joint Center for Urban Studies Publication Series. Harvard University Press, Cambridge, Mass. Anas, A. 1983. Discrete Choice Theory, Information Theory, and the Multinomial Logit and Gravity Models. Transportation Research B, Vol. 17, pp. 13–23. Anas, A. 1984. Discrete Choice Theory and the General Equilibrium of Employment, Housing and Travel Networks in a Lowry-Type Model of the Urban Economy Environment and Planning A, Vol. 16, pp. 1489–1502. Aschauer, D. 1989a. Is Public Expenditure Productive? Journal of Monetary Economics, Vol. 23, No. 2, March, pp. 177–200. Aschauer, D. 1989b. Does Public Capital Crowd Out Private Capital? Journal of Monetary Economics, Vol. 24, No. 2, Sept., pp. 171–188. Aschauer, D. 1991. Public Investment and Private Sector Growth: The Economic Benefits of Reducing America's “Third Deficit.” Economic Policy Institute, Washington, D.C. Atash, F. 1993. Mitigating Traffic Congestion in Suburbs: An Evaluation of Land-Use Strategies. Transportation Quarterly, Vol. 47, No. 10, pp. 507–524. Bass, T. 1992. Road to Ruin. Discover, May, pp. 56–61. Berechman, J., and K. Small. 1988. Modeling Land Use and Transportation: An Interpretive Review for Growth Areas. Environment and Planning A, Vol. 20, pp. 1285–1309.

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