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Dec252010

National Transport Dilemmas + Opportunities

National Transportation Dilemmas and Opportunities

 

A Transportation Agenda for American and Australian Cities



In recent decades, environmental and health issues have received considerable research attention within sustainable urban transportation literature. Research agendas surrounding the concerns for the long-term environmental impact of urban transportation have included urban air pollution from vehicular emissions, global climate change, overconsumption of energy resources, destruction and disruption of natural habitats and green space from infrastructure expansion, and noise pollution [cf. Black, 1997, 1998; Greene and Wegener, 1997; Litman 2005a; see also Journal of Transport Geography 1997, Vol. 5(1), entire issue]. Research has also focused on inequitable distributions of costs and benefits in maintaining environmentally sustainable transportation systems (Black, 1998), the transportation-based environmental injustices upon poor and minority urban communities (Schweitzer and Valenzuela, 2004), or the public health crises of increased obesity correlated with urban sprawl growth patterns and automobile dependence in U.S. cities (Ewing et al., 2003; Lopez, 2004).

However, the extent to which the research literature explores the social dimensions of sustainable transportation in the U.S. urban context is less explicit. This is particularly significant considering the many ways transportation-based barriers contribute to social injustices and socio-spatial inequities in U.S. urban areas, especially along lines of race and class. For example, inequitable access to employment exists in U.S. cities as low-skilled, low-wage, and minority workers are often more likely to experience problems of inadequate transportation to overcome the spatial separations between their residential location and places of work opportunities, resulting in higher levels of unemployment, more costly commutes, or compromised wages (Holzer, 1991; Ihlanfeldt and Sjoquist, 1998; Preston and McLafferty, 1999). These problems of accessibility may become exacerbated for lower-wage workers who commute by automobile, as the rising cost of fuel poses greater financial hardships (Ball, 2004; Foss, 2006), and individuals who are unable to absorb rising commuting costs must renegotiate issues of mobility and access to employment. Furthermore, the aftermath of Hurricane Katrina in August 2005 brought to light the mobility issues of inner-city poor populations (Canellos, 2005; Hess, 2006; Litman, 2006), highlighting the greater threat to vulnerability among the nation’s urban car-less minority poor in the event of emergency evacuations (Lui et al., 2006; Renne, 2006). Just as transportation creates critical concern for environmental protection, the existent social disparities in transportation necessitate concerted attention.

The broad concept of sustainable development and sustainability has become widely popular in research, political, and community agendas, especially since the Brundtland Report (WCED, 1987) identified the declining environmental condition and associated human factors as a global problem. With emphasis placed upon resolving the global environmental crises, the equal centrality of environment, economic, and social dimensions in the conceptual understandings of sustainability has been lost. If sustainability seeks to balance three interlinked objectives, to protect the environment while promoting economic vitality and social equity, then more research is needed to understand the social dimensions of sustainability. Of this sustainability triad (environment, economic, and social dimensions), this article focuses exclusively on the social. In doing so, it is not assumed the social takes precedence over the significance and urgency of the environmental concerns. But if all the dimensions are to be balanced, it is important to also examine the implications of those sustainability dimensions which receive less attention.

The purpose of this article is to explore the current status of research on social sustainability that is particularly relevant to urban transportation. It focuses on the dimensions of socially sustainable urban transportation (SSUT) within the context of U.S. cities. Here, SSUT is understood as transportation that provides equitable access to urban opportunities, minimizes social exclusion, and improves or does not overly diminish an individual’s quality of life. Because transportation is a crucial mechanism by which individuals maintain a livelihood in the urban context, when balancing the environmental, economic, and social dimensions of transportation sustainability, the enabling of individuals to meet their needs and aspirations for a better life is crucial for sustaining livelihoods. This literature-based preliminary definition will be further elaborated in subsequent sections of the article.

Specifically, the article seeks to highlight the social dimensions of sustainable urban transportation as an important topic. It suggests that to some degree, the social is explicitly addressed in sustainable transportation literature but more often appears implicitly in broader transportation literature. It also offers a preliminary research agenda specific to the scale(s) and context of individual livelihoods in U.S. cities.

The next section examines the broader social aspects of sustainability and then explores the social dimensions of sustainable urban transportation. A review of relevant literature suggests that SSUT is most critically researched at the scale of individual livelihoods and is implied in transportation research through the elements of social equity, social exclusion, and quality of life. The research progress in each of these areas is reviewed. In the final section, urban sustainability and transportation in the U.S. urban context is explored. It is argued that the well-established research literature on urban form, accessibility, and planning for sustainability offer potentially fruitful theoretical, methodological, and applied insight for ongoing future research on socially sustainable urban transportation.

2. PLACING SOCIALLY SUSTAINABLE URBAN TRANSPORTATION

2.1. Recovering the “Social” in Sustainability

Contemporary sustainability and sustainable development discourse has been highly influenced by the emergence of the Brundtland Report (WCED, 1987). This significant document was driven by growing concerns about expanding global populations, increased resource consumption, environmental degradation (including depletion of natural resources, global climate change, and destruction of the ozone layer), increased poverty, squalor, and the inability of the masses to meet their basic human needs. In the wake of the Brundtland Report, sustainable development and sustainability gained remarkable exposure not only in political programs but also in a wide array of scientific and scholarly literature. In fact, bibliometric evidence has shown that between 1987 and 2001, the Brundtland Report was cited in more than 2400 scholarly research publications in science and social science journals (Schubert and Laacuteng, 2005). Central to the Brundtland Report’s conceptualization of global sustainability was the necessity to protect the environment while also promoting economic vitality and social equity, thus presenting the challenge in implementing balance between the environmental, the economic, and the social concerns of sustainability:

  • The environmental concern for resource depletion and overconsumption; air, water, and soil pollution; and energy crises.
  • The economic concern for the ability to promote economic growth, expansion, and long-term prosperity that is socially and environmentally viable.
  • The social concern for eradicating widespread poverty and hunger, meeting basic human needs, and addressing the growing social and economic disparities.

However, universally agreed conceptualizations and definitions of sustainability and sustainable development remain elusive (Lele, 1991; Redclift, 1992; Wilbanks, 1994; Beatley, 1995). To define sustainability, many research publications draw upon a single passage from the Brundtland Report, “…development that meets the needs of the present generation without compromising the ability of future generations to meet their needs” (WCED, 1987, p. 8), suggesting intergenerational environmental equity. Yet as emphasis is commonly placed upon the environmental dimensions of sustainability, many argue the social dimension of sustainability in the Brundtland Report is overlooked (Yiftachel and Hedgcock, 1993; Pinfield, 1994; Polegravese and Stren, 2000; Low and Gleeson, 2003; Foladori, 2005). Some suggest social dimensions of sustainability are explicitly incorporated (Foladori, 2005) within the Brundtland Report’s vision of sustainability, if not at the heart of it (Pinfield, 1994). Others argue sustainability is not limited to natural resource management for future generations, but also implies intragenerational socioeconomic equity (Bailly et al., 2000; Stren and Polegravese, 2000; Maloutas, 2003). Without a doubt, the urgency of preserving the environment for future generations has been a primary driving force in sustainability. But in meeting environmentally sustainable objectives, the Brundtland Report outlined how sustainability should also seek to meet social objectives of intragenerational equity that “enhance both current and future potential to meet human needs and aspirations” for a better life (WCED, 1987, p. 46). Thus, it is argued here that the social is integral to the whole understanding of sustainability. With this in mind, research is needed to explore the parameters of social sustainability in the context of urban transportation.

2.2. Social Dimensions of Sustainable Urban Transportation

Although it has been argued that research on the social dimensions of sustainable urban transportation is quite limited (Black et al., 2002), particularly in the context of the United States, there are some exceptions. For example, the STAR/STELLA transatlantic network focused on sustainable transport research in Europe and the United States between 1999 and 2005. The collaborative network sought scientific research knowledge exchange to better understand the underlying forces influencing mobility for broad benefit of the research community, policymakers, and industrial organizations (Button and Nijkamp, 2004). Focus Group 3 (Society, Behavior, and Public/Private Transport) made explicit the transatlantic perspectives of social and behavioral components of sustainable transport (Donaghy et al., 2004; Donaghy et al., 2005; see also Transport Reviews 2004, Vol. 24(1), entire issue).

In many instances, however, the social dimensions of sustainable urban transportation are more implicit than explicit in the literature. For example, although Sanchez et al. (2003) do not use the terminology of sustainability, the research focus on the inequitable effects of surface transportation policies in the United States certainly implies dimensions of social sustainability. Similarly, Bullard and Johnson (1997) and Bullard et al. (2004) examine the race- and class-based transportation injustices in the United States but do not use a framework of sustainability.

To further explore the status of the literature on this, we examined the literature to answer the following question: In what ways can urban transportation be socially unsustainable? That is, what barriers in allowing humans to meet their basic needs and aspire toward a better life are created or augmented by urban transportation systems? The literature suggests that SSUT is most appropriately examined at localized scales, the level at which activities of individual livelihoods occur. Further, social equity, social exclusion, and quality of life emerge as three integral and interconnected qualities illustrative of socially sustainable urban transportation in the context of U.S. cities. Although none of these are unambiguous in their definition,1 they provide a platform from which to conceptualize SSUT. A common thread among all three is the goal of social justice, understood as the distribution of society’s benefits and burdens (Johnston et al., 2000). Admittedly a contested term (Harvey, 1996), social justice encompasses the entirety of issues embedded in socially sustainable urban transportation including the inequitable distribution of the benefits and burdens of transportation in U.S. cities, geographic patterns of social exclusion, and diminished quality of life among individuals. The remainder of this section more specifically outlines these literature contributions that imply the social dimensions of sustainable urban transportation.

2.2.1. Scale and Context

Concepts of sustainability that emerged from global-scale reports (e.g., WCED, 1987; UN, 1992; NRC, 1999) offered visions of sustainability related to concerns of global change (e.g., population growth, resource consumption, hunger/poverty, and the viability of economic growth). But many argue that the level at which these processes of global change occur and are researched should be addressed at more localized scales (Wilbanks, 1994; Bailly et al., 2000; CUPR, 2000). CUPR (2000) identified a specific scale tension in definitions of urban sustainability:

  1. A global scale, big-players’ version in which sustainability is synonymous with sustainable development and its management, embracing the agenda of the market, top-down planning, and scientific, technological, and/or design-based solutions; and
  2. A local-scale version in which sustainability is synonymous with sustainable livelihoods and in which local context can lead to different and locally contingent perspectives on the meaning of and conditions for sustainability and the means to achieve it (Hanson and Lake, 2000, p. 2).

By shifting the scale of sustainability research to the locality, social sustainability concerns of meeting human needs and satisfying aspirations for a better life can more appropriately be addressed and researched within context. Looking specifically at the locality, the objective of sustainability becomes the establishment and maintenance of livelihoods (CUPR, 2000). Sustainable livelihoods, “processes of social and ecological reproduction situated within diverse spatial contexts” (CUPR, 2000, p. 7), enable individuals, households, and neighborhoods to construct a living that meets basic human needs. Within this localized level, sometimes the barriers to sustaining livelihoods are transportation-based.

Transportation infrastructures of urban regions are essential for individuals to construct a livelihood in the city, providing mobility and access to places of opportunity across the geographic landscape. Socially sustainable transportation systems are adequate, efficient, effective, and crucial to alleviating poverty by providing access to markets, employment, education, and basic services (UN, 1992; World Bank, 1996) and “sustain the progress of the society towards prosperity, freedom, and justice for all…” (Low and Gleeson, 2003, p. 12). It must further be considered that local manifestations of sustainability may be location specific (CUPR, 2000) and context dependent (Maloutas, 2003), suggesting that the meaning of and barriers to sustainable livelihoods is not uniform for all cities but is contingent upon the local conditions.

2.2.2. Social Equity

Equity is one common general understanding for the social dimensions of sustainability within the literature. Equity refers to fairness of distribution, not equality. Everyone in society does not have equal need; thus, with equity, the distribution of resources is based upon need. Equity and social justice in the distribution of transportation benefits has been theorized through three key philosophical perspectives: egalitarianism, horizontal equity, and vertical equity (Khisty, 1996; Litman, 2005b).

  • Egalitarianism: All persons are created equal and should be treated as such. Thus, regardless of socioeconomic status (or geographic location), all of society should receive equal benefit of transportation service. In this approach, the net benefits of transportation should be distributed equally within society.
  • Horizontal equity: There should be equal treatment for people in unequal positions. This “users-pay” perspective presupposes impartial treatment across society where individuals and groups receive and use what they pay for in transportation.
  • Vertical equity: The distribution of benefits across socioeconomic classes should provide the greatest benefit (at the least cost) to the most disadvantaged. This is closely aligned with Rawls’ (1971) theory of justice that favors equal distribution unless it is necessary to create an unequal distribution that ultimately favors the least advantaged in society; here priority of equity is placed above efficiency.

Considerations of equity in urban transportation literature frequently analyze disparities in accessibility to urban opportunities, disparities in cost-benefit distributions, or disparities in environmental justice related to transportation systems. Traditional concepts of accessibility refer to the opportunities and activities available in geographic space. Geographic context enables or constrains access to opportunities through the spatial arrangement of activities; the design, efficiency, and availability of the transportation infrastructure; and personal availability of transportation (Hanson, 1995; Hanson, 1998; Couclelis, 2000). Socially sustainable urban transportation examines the equitable distribution of access to those opportunities that are fundamental to meeting human needs such as employment, social/public services, affordable housing, education, health care, recreational/open space, shopping (Bailly et al., 2000; Burton, 2000; Stren and Polegravese, 2003; Arend, 2004; Richardson, 2005) and is especially concerned with the equitable distribution of opportunities for disadvantaged populations. For example, research has shown that in situations of city growth and urban sprawl, the levels of accessibility among the elderly and children tend to be compromised (Gilbert, 2002). Research has also noted many distributional inequities of the costs and benefits of urban transportation (Hodge, 1995; Deka, 2004; Bae and Mayeres, 2005). For example, Deka (2004) found that the percentage of household expenditures on transportation has increased most for low-income households. Further, many argue that the negative environmental consequences of urban transportation systems disproportionately affect minority and disadvantaged populations, whose neighborhoods experience higher levels of vehicle emission air pollution, traffic congestion, noise pollution, and loss of land to highway construction (Bullard and Johnson, 1997; Sanchez et al., 2003; Bullard et al., 2004; Schweitzer and Valenzuela, 2004). The inherent tension within transportation planning between social equity and total system efficiency (Khisty, 1996; Deka, 2004; Bae and Mayeres, 2005) illustrates a key challenge of implementing socially sustainable urban transportation.

2.2.3. Social Exclusion

Urban transportation is socially unjust when the lack of benefits or unfair distribution leads to the social exclusion of individuals or groups in society. Social exclusion refers to the “situation where certain members of society are, or become, separated from much that comprises the normal ‘round’ of living and working in society” (Johnston, 2000, p. 751). Generally, this may involve exclusion from participation in democratic governance, decision-making, or production processes that otherwise lead to empowerment of disadvantaged groups (Foladori, 2005). Patterns and processes of social polarization, segregation, and inequality (Maloutas, 2003) may also reflect forces of social exclusion. Transportation systems can cause persons or groups to become socially excluded as a result of spatial, temporal, financial, or personal obstacles, such as cost-prohibitive transportation, fear for personal safety, restrictive transit schedules, or unserviced locations (Solomon, 2003).

Pickup and Guiliano (2005) identify three key factors where unsustainable urban transportation exacerbates the conditions of social exclusion. For one, poor access to services, places of employment, education, shopping, or amenities/recreation is a plausible indicator of social exclusion. Continual isolation resulting from persistent transportation-based barriers to opportunities ultimately fosters a lack of hope for the future among isolated individuals and groups. Also, concentrated social segregation (Stren and Polegravese, 2000; Burton, 2000) and polarized and fragmented communities emerge within metropolitan landscapes as a result of transportation-based social exclusion. For many, creating livable cities that enable construction of livelihoods requires social cohesion (Arend, 2004), social diversity (Yiftachel and Hedgcock, 1993; Stren and Polegravese, 2000), and social integration (Burton, 2000). Currently, a consistent definition of social exclusion and the linkages to urban transportation remains elusive (Solomon, 2003; Pickup and Giuliano, 2005). Further, measurable benchmarks have yet to be developed (Solomon, 2003). But the literature illustrates the significance urban transportation can play in the exclusion of individuals from society’s benefits.

2.2.4. Quality of Life

The effect of transportation on the quality of life (QoL) of an individual or a group is another aspect from which research literature considers the social sustainability of urban transportation. Quality of life is “a multi-dimensional construct, and may be defined as the extent to which important values and needs of people are fulfilled” (Steg and Gifford, 2005, p. 62). Used as a public policy goal for reducing inequities, QoL indicators help examine the conditions for seeking happiness and fulfilling need and has been recognized as more conducive than adhering to strict economic goals (Helburn, 1982). In transportation research, such indicators help to measure the extent to which an urban transportation system contributes to diminished quality of life. Although this area conceptually overlaps with social equity and social exclusion, it focuses more upon the individual experiences of daily life, acknowledging that people are perhaps ultimately less concerned about issues of social justice (as in the collective economic, social, and physical conditions of people in a community) than about their own quality of life (Khisty, 1996).

Quality-of-life has been used extensively in urban-related research (Pacione, 2003; Randall and Morton, 2003) and in sustainable transportation research (Steg and Gifford, 2005; Djist and Kwan, 2005). Poortinga et al. (2004) developed a useful list of 22 quality of life indictors based on the “needs, values, and human well-being in relation to sustainable development” (p. 73). Common among the 22 QoL indicators is an essence of meeting individual needs. Several indicators reflect the direct influence of urban transportation systems on meeting needs including equal access to opportunities, the ability to find employment, and having sufficient leisure time. Other QoL indicators are influenced by urban transportation systems only to the extent to which adequate access to facilities, services, and opportunities are impeded (e.g., education, nature/biodiversity, spirituality/religion, health care). Steg and Gifford (2005) subsequently adapted this list to analyze QoL and sustainable transportation and policy implications.

Djist and Kwan (2005) have directly explored social dimensions of urban transportation by examining quality of life through individual accessibility approaches using time geographic perspectives. Recognizing drastic changes in social trends (e.g., family composition, employment scenarios, gender roles, overall lifestyle changes), their research found significant insight in understanding the influences upon individual choices, constraints, and access to opportunities in the urban environment. For one, individuals in low-density suburbs experienced a decreased realization of activities than did residents in more concentrated urban environments. Also, the accessibility of individuals is more dependent upon their own personal and household constraints and less dependent upon their geographic context. In essence, as a matter of quality of life, families create their own cities through their specific criteria of destinations (Djist and Kwan, 2005).

There are noted methodological complications with the use of QoL for future research. Suitable indicators of QoL need to be established, as well as how QoL will be accurately measured (Helburn, 1982; Steg and Gifford, 2005). Also, quality of life is clearly a subjective metric. Individuals seeking to meet their needs will exhibit a willingness to offset deteriorations in one QoL indicator given plausible improvements in other QoL aspects (Steg and Gifford, 2005). This variability in compensation is potentially a methodological problem in research. Further, transportation systems that are sustainable require balancing collective interests with individual interests. When considering the implications of quality of life, obvious conflicts with individual short-term interests (e.g., convenience and privacy of personal automobile) and collective social interests may prove difficult in policy and research design (Steg and Gifford, 2005).

3. RESEARCH POTENTIALS IN THE CONTEXT OF U.S. CITIES: THEORETICAL, METHODOLOGICAL, AND APPLIED ISSUES

3.1. Sustainability Research and the U.S. Context

Within the United States, there is a general shortage of—and a call for more—urban sustainability research (CUPR, 2000; Cutter et al., 2002; Portney, 2003). Basic literature searches reveal a scarcity of U.S. urban sustainability research compared with the quantity of empirical research on European and Asian cities. As urban sustainability is not a major research agenda, there is a lack of hypothesis-driven research and, as such, a lack of theoretical understandings of sustainable cities in the U.S. context (Portney, 2003; see also CUPR, 2000; Cutter et al., 2002). Policy concern for some social dimensions of transportation, however, has received recent attention in the United States (Schweitzer and Valenzuela, 2004). For example, President Clinton’s 1994 Executive Order for environmental justice (Office of the President, 1994) addressed the adverse health and environmental effects (partly caused by transportation systems) that were disproportionately distributed among minority and low-income populations. Federal legislation Transportation Equity Act for the 21st Century (U.S. Congress, 1998) and the follow-up Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (U.S. Congress, 2005) target transportation-based equity in environmental quality and access to opportunity for low-income and welfare residents. Despite these recent policies, a cohesive research agenda that develops theoretical understandings, methodological insight, and issues of application specific to the U.S. urban context are needed (Sanchez et al., 2003) and could help establish the social dimensions more explicitly.

Urban regions in the United States provide an ideal context for developing socially sustainable urban transportation research agendas, as transportation-based problems of social inequities and social polarizations are pervasive. Cities in the advanced capitalist world face an array of social disparities that obstruct equity in the achievement of livelihoods. In U.S. cities, socio-spatial processes are serious contributors to these problems. A critical political-economy view of the socio-spatial urban structure suggests that uneven development in metropolitan areas is a result of capital flows to areas with greatest potential for returns. As such, cities in capitalist economies have become mosaics of disparity, exhibiting varying socio-spatial patterns of poverty, social polarization, and powerlessness among disadvantaged residents (Harvey, 1973; Pacione, 2003). Most prominently, U.S. cities experience metropolitan structures of social polarization that create spaces of concentrated social exclusion and spatial isolation from opportunities, preventing sustainable livelihoods from flourishing. The obstacles to achieving livelihoods within U.S. cities are many: poverty and socioeconomic disparities; exclusion of marginal and disadvantaged individuals, groups, or neighborhoods; social segregation and metropolitan polarization; and urban decay, including social turmoil of violence, crime, drugs, homelessness, and joblessness (cf. Yiftachel and Hedgcock, 1993; Bailly et al., 2000; Stren and Polegravese, 2000). Urban transportation, as an integral component to the totality of urban systems, functions as a mechanism to move and integrate social capital across the metropolitan landscape, to facilitate the attainment of individual livelihoods, and ultimately influences the structures of urban opportunities. The significance of urban transportation to many of these socio-spatial problems in U.S. cities are examined in the urban form, accessibility, and city planning literature and provide insight for enriching future SSUT research.

3.2. Urban Form

Studies of urban form suggest that depending upon the spatial structures of urban opportunities and the available transportation systems, patterns of accessibility barriers in particular urban areas may exist. In metropolitan landscapes in the United States, patterns and processes of urban sprawl dominate. Growing at the edges of central cities, sprawl is characterized as scattered low-density residential, commercial, and retail development, with poor accessibility (Ewing, 1994). With urban sprawl has come the decentralization of industrial and manufacturing employment into the metropolitan periphery (Kasarda, 1989) and the disparate concentrations of retail, commercial, and high-skilled and service-sector jobs in metropolitan edge cities (Garreau, 1991). This polycentric or dispersed urban spatial structure (Giuliano and Small, 1991) has created metropolitan landscapes of social inequality and social polarization (Levine, 2000; Squires, 2002). Specifically, with the outward growth of scattered low-density development of opportunities in the suburbs, individuals throughout the United States have become increasingly dependent upon the private automobile (Newman and Kenworthy, 1999) to reach suburban destinations of opportunity, especially employment.

This growing dependence upon automobiles creates geographies of social inequities and polarization in metropolitan areas along socioeconomic lines. Racial minorities and the poor are disproportionately reliant upon inefficient public transportation systems that provide limited spatial and temporal service (Fielding, 1995; Pucher and Renne, 2003; Pucher, 2004). In many sprawling cities in the United States, the large employment markets in the metropolitan periphery remain inaccessible via public transportation, leaving large populations of central-city residents without employment. These transportation-based barriers to employment led to the formation of urban ghettos or inner cities (Harvey, 1973), resulted in social isolation (Wilson, 1987) and concentrations of poverty among African Americans (powell, 2002) in metropolitan urban core areas.

Several urban form-related research areas may prove useful for theoretical development in socially sustainable urban transportation research agendas. The spatial mismatch hypothesis (Kain, 1968; Ihlanfeldt and Sjoquist, 1998; Preston and McLafferty, 1999) has argued that higher unemployment rates or longer commutes of disadvantaged populations can be attributed to socio-spatial processes, such as housing discrimination, and to deficiencies in adequate transportation. Jobs-housing balance research (Cervero, 1989; Giuliano, 1991) has identified how suburban communities are unbalanced, with residences concentrated in neighborhoods and jobs clustering in office parks and retail centers in the edge cities. Together these research agendas identify how urban spatial structures and transportation influence access to employment opportunities and could provide theoretical insight for future socially sustainable urban transportation research.

Spatial planning with alternative urban forms suggests potential social advantages of more concentrated urban structures. In opposition to patterns of sprawling, low-density, auto-dependent urban growth, the planning strategy of compact cities believes more concentrated environments create better access to facilities and opportunities; increase the overall equity in access; reduce commute time/length and the need to travel; provide greater opportunities for social interaction and social contact leading to social cohesion; and increase capacity to meet the housing needs of the population (Jenks et al., 1996; Williams, 2000; Masnavi, 2000). These characteristics of more concentrated urban form increase the quality of life by creating “safer and more vibrant urban areas, support for local businesses and services, greater social equity and social interaction, and better accessibility to facilities” (Jenks et al., 1996, p. 65). Similarly, the New Urbanism (Boarnet and Crane, 2001; CNU, 2001; Falconer Al-Hindi and Till, 2001) planning movement suggests that more dense urban environments with mixed land-uses reduces automobile dependency through transportation alternatives and increased walk-ability to daily urban opportunities—including employment. Thus, by maximizing access and mobility across social groups, theoretically a more humane and livable community environment is created. Although it remains debated whether compact cities are more socially just (Burton, 2000) or if socially disadvantaged groups actually benefit from the New Urbanism (Falconer Al-Hindi, 2001), future research on the social sustainability of urban transportation could build upon the theoretical benefits of mobility, access, and social integration in concentrated urban forms.

3.3. Accessibility

Equity in access to opportunities (e.g., employment, services, shopping, education, health care, and amenities) that contribute to meeting basic human needs and aspirations for a better life has been argued as central to understanding SSUT. The well-established research field of accessibility offers useful theoretical and methodological insight for future SSUT research. In urban transportation research, accessibility measures the potential opportunities or interactions within a set geographic space—traditionally measured within a certain distance or travel time threshold. Two distinctive approaches to accessibility, location (place) accessibility and individual (personal) accessibility (Hanson, 1995; Kwan, 1998; Couclelis, 2000; Kwan et al., 2003; Horner, 2004), have unique contributions for future SSUT research.

In the place-based approach, emphasis upon the relative accessibility of a location is commonly measured using aggregated data of geographic space to identify potential interactions between places or to identify the relative prominence or importance of a place based on available sets of opportunity. This approach can be used to develop theoretical understandings of urban structures of opportunity and interaction, to understand and geographically visualize actual spatial structures, or to analyze potential maximization (minimization) of benefits (costs) of transportation network structures (Song, 1996). In planning for accessibility, location-allocation models seek to minimize barriers to access (i.e., time or distance) and have been used extensively for evaluating the accessibility of public transit systems or in applying public service distributions (Kwan et al., 2003). Spatial interaction models have been used to understand metropolitan patterns of accessibility and the significance of places by modeling intraurban commuting using aggregated commuting and employment data to understand the spatial variations of job accessibility (Wang, 2000). These methodological techniques of location-allocation and spatial interaction models could help future SSUT research better understand the interactions between places, measure the accessibility levels of locations, and visualize urban structures and spaces of inclusion/exclusion.

The individual accessibility approach uses disaggregate data (at the individual level) to examine what opportunities are available to an individual given spatial-temporal context, personal constraints, and geographic context. Methodologically, location accessibility assumes homogeneity of opportunity distribution and travel behavior within zones; individual accessibility, on the other hand, understands the travel patterns of individuals based on individual activity and opportunity sets (Hanson and Schwab, 1987). Although distance from opportunities is a significant factor to accessibility, it can only be the first approximation (Kwan, 1998, 1999; Kwan et al., 2003). Characteristics of individual identity, personal constraints, or household structure can greatly influence travel behaviors of individuals. When such is the case, rather than locational proximity, space-time accessibility measures are advantageous as they incorporate the effect of interactions between a person’s constraints and identity, with the available urban opportunities. The methods of space-time accessibility enable exploration into the variations of accessibility based on individual differences (Kwan and Weber, 2003). For example, Kwan (1999) found that women have lower levels of access to urban opportunities than do men, suggesting the significance of gender-based differences in domestic responsibilities, as well as unique individual constraints in travel behavior. Further, individual accessibility is more contingent upon individual and household characteristics than the effects of geographic context (Weber and Kwan, 2003). As an approach, space-time accessibility “reflects interpersonal differences associated with [the] contingencies of everyday life” (Kwan, 1999, p. 212) and is well suited for incorporating the unique realities of individuals in future SSUT research. This type of accessibility research approach could prove useful by contributing theoretical insight on human travel behaviors and uncovering specific socioeconomic inequities in access, quality of life issues, and places of access or exclusion.

3.4. Planning for Sustainability

In the planning literature, the social dimensions of sustainable urban transportation is generally not an explicit factor in sustainability planning in U.S. communities but rather is implied through sustainability indicators such as reducing automobile use, increasing general social equity, and reducing overall commuting time/distance (perhaps a quality of life indicator). At the same time, planning for SSUT has been more prominent outside the United States. For example, the extensive and comprehensive sustainable development initiative of the Swiss government includes indicators for SSUT that provides equal opportunities and fair distribution of wealth, integrates the less fortunate, and meets the needs of people (SFSO, 2004). Similarly, the PROSPECTS2 project developed sustainability indicators for European city transportation systems, including equity and social inclusion and livable streets and neighborhoods.

Implementing local plans for sustainability in the U.S. urban regional context has been difficult (Berke and Conroy, 2000; Conroy and Berke, 2004) and poses challenges for potential SSUT empirically based research. Unlike other countries where sustainability planning is part of the national legislation, planning for sustainability in the United States is driven by local initiatives (Berke and Conroy, 2000). As a result, local plans for sustainability either lack power for implementation or become mired by the multitude of sustainability interest groups (e.g., numerous city municipalities and jurisdictions, citizen organizations, planning organizations, and nonprofit groups) with conflicting agendas. In the case of Seattle, Washington, the grassroots Sustainable Seattle Coalition successfully developed a broad-based set of community sustainability indicators (AtKisson, 1996; Portney, 2003; Miller, 2003), but ultimately lacked the power to implement their plans for sustainability (Wheeler, 2000).

Another obstacle facing planning initiatives for sustainable communities in the United States is the challenge to strike a balance between environmental, economic, and social sustainability objectives (Campbell, 1996; Wheeler, 2000; Berke and Conroy, 2000; Portney, 2003). Portland, Oregon, gained notoriety for its innovative and ambitious regional agenda in planning for sustainability by implementing an urban growth boundary and developing a light-rail transit system (Ozawa, 2004). However, it remains debated whether Portland’s attempts to thwart urban sprawl has subsequently caused rising housing costs thus reducing the city’s supply of affordable housing, increased traffic congestion, or reduced the level of automobile usage (Lang and Hornburg, 1997; Downs, 2002; Richardson and Gordon, 2002; Ozawa, 2004).

Despite these obstacles, the 135 identified sustainability planning initiatives in U.S. urban area communities (Conroy and Berke, 2004) provide extensive opportunities for comparative empirical research on developing SSUT theories and examining application issues. Such analyses of sustainable urban planning initiatives could reveal insight for constructing a conceptual framework of appropriate indicators of SSUT and address fundamental issues of the importance of scale and context in sustainability research. Further, it would be fruitful to explore the underlying political processes of sustainability planning to understand why SSUT is generally not an initiative in U.S. communities.

4. CONCLUSIONS

This article has explored the dimensions of SSUT and highlights research progress and potentials. Although the social is a critical dimension in the overall conceptualization of sustainability (in addition to the environmental and economic), there are many possible reasons why the social dimensions of sustainable urban transportation are less explicit in the research literature. For one, sustainability research has generally focused upon the dire environmental protection concerns and has not widely encompassed the social dimension of sustainability; this may also stem from the elusiveness of clear a definition of sustainability. Additionally, as social sustainability is difficult to measure in quantitative terms (Kennedy, 2002) and any plausible indicators are normative in nature (Yiftachel and Hedgcock, 1993), greater difficulty in establishing a widely accepted meaning is subsequently created. Further, if sustainability is location specific and context dependent (CUPR, 2000; Maloutas, 2003), any absences in the literature may also result from conceptualizations of sustainability that remain entrenched in global-scale thinking; simultaneously, specificity in context and location may hinder the development of useful generalizable theories about SSUT.

Despite these reasons, it does seem, however, that the social dimensions of sustainable urban transportation are implied within the transportation literature, particularly through the elements of social equity, social exclusion, and quality of life. The literature does suggest that in the context of advanced capitalist cities, SSUT creates equitable access to opportunities necessary for enabling individuals to achieve sustaining livelihoods, establishes livable cities of social cohesion and inclusion, and overcomes the problems of polarized, fragmented, and socially segregated metropolitan communities.

Given the breadth of literature, it is important to highlight some eminent questions for future SSUT research. The NSF Workshop on Urban Sustainability (CUPR, 2000) provides a crucial outline of the pressing needs for a comprehensive research agenda. More specific to SSUT, two key research questions should be addressed:

What are the parameters of the social dimensions of sustainable urban transportation? In addition to the need of developing a widely accepted concept of sustainability (CUPR, 2000), it is crucially important to similarly construct an accepted meaning of the social dimension of sustainability and how it is influenced by urban transportation. One way to begin is by delimiting the research to a specific context/place and scale. For example, as reviewed in this article, the scale and context of individual livelihoods within U.S. cities is one specific parameter by which SSUT can be conceptualized and problematized. Establishing such parameters will facilitate the development of focused research questions and the design of projects, which leads to a second key question.

What are the most appropriate epistemological and methodological approaches for conducting research on the social dimensions of sustainable urban transportation? It is also important to identify the most useful and appropriate methods, data sources, measurements, indices, and ways of representation specific to the research questions that emerge. In particular, a mixed-method approach holds promise to simultaneously use highly relevant secondary data in combination with detailed primary data collected specifically for the developed research questions. The capabilities of geographic information systems (GIS) (e.g. data integration, complex geocomputation, multiscale analysis, geovisualization) could support rigorous spatial and statistical analysis to explore underlying patterns and processes, provide insight to explain relationships, and offer generalizations about the conditions of social sustainability. At the same time, qualitative-based research could help understand individual experiences as well as the underlying social structures that are complexly entwined with SSUT. Given the subjective and contingent nature of the social, qualitative data collection may be essential to elucidate information necessary to construct appropriate quality of life indicators specific to each research context. Similarly, as social networks3 are often fundamental to meet individuals’ daily transportation needs, qualitative methods may be most appropriate for analyzing the complexities of these structures and their relevance to individual livelihoods and subsequent impacts on sustainability (Axhausen, 2005; Kwan, 2007).

Together these questions could help establish a concerted research effort to advance the field, knowledge, and ongoing theoretical research, as well as potentially influence policy at local, regional, national, and international levels.

ACKNOWLEDGMENTS

The helpful comments from three anonymous reviewers are greatly appreciated.

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