Sustaining What Is Unsustainable: A Review of Urban Sprawl and Urban Socio-Environmental Policies in North America and Western Europe

: Urban sprawl and its economic, social, and environmental consequences are central issues for approaching more sustainable forms of life and production. This review provides a broad theoretical exploration of the main features of urban sprawl but also of sustainable urban policies in Western Europe and North America. Urban sprawl can be observed in both continents, as the search for higher standards of economic, social, and environmental sustainability is also an essential feature of urban governance in the last years. Urban sprawl has been slightly weaker in Western Europe, as its are cities generally more compact. Moreover, in Western Europe, urban sprawl has sometimes been confronted with ex-ante preventive policies. However, in North America, urban sprawl from the 1950s has been an essential element of the social ordering and, thus, of the American way of life. In both cases, urban sprawl has generated successive rounds of accumulation of built capital, which is currently managed in sustainable ways essentially through ex-post and palliative measures, that is, trying to “sustain what is unsustainable”. In other words, the idea is to make urban sprawl more sustainable but without altering its main morphological elements.


Introduction
The world population is nowadays over 7.7 billion. The United Nations projections maintain a growing trend with global population close to 10 billion by 2050 [1]. Additionally, the greatest concentration of population around cities since the middle of the 20th century can be observed. This concentration occurs essentially in large cities, which, in the era of globalization, become centers of productive and financial control [2]. Thus, more than 4200 million people currently live in cities and following the estimations of the Department of Economic and Social Affairs of the United Nations, 68% of the world population will live in cities in the year 2050 [3].
Such concentration of population and economic activities in cities has been explained on the basis of the concept of External Economies, which, in the urban sphere, takes two forms: Localization Economies and Urbanization Economies [4]. The Localization Economies insist on the reduction of production costs associated with the concentration of productive, commercial, and financial activities. Thus, it is associated with a decrease in corporative costs and an increase in the quality of labor, intermediate goods, and knowledge [5,6]. The economies of urbanization insist on the benefits offered by the proximity of the final markets, which allows increasing the contact with clients and final consumers. In consequence, they consider the size and social diversity of urban centers as essential aspects for understanding the dynamics of business concentration [7].
The operation of the localization and urbanization economies has been essential in the form in which the cities have been historically organized, dimensioning their spaces and activities. In the last 70 years, the transformations in urban morphology have accelerated and deepened in parallel with the increase in mobility of capital, information, people, and goods [8,9]. As shown in Figure 1, the present review will focus on the presentation of the main features of the phenomena of urban sprawl in the context of Western Europe (WE) and North America (NA), trying to synthesize both its causes and consequences, mainly in terms of sustainability. This, in turn, will allow an approach to urban policies that have been recently applied using sustainability as a reference. The emphasis on explaining the European and North American cases does not mean that the relevance of urban sprawl in many other territorial contexts, such as Latin America [10], Asia [11,12] or Africa [13] is not recognized. However, in this article, it is understood that there is a set of common causes and effects that justify a specific revision in the European and North American cases. Without going into the content of the discussion that follows, it can be anticipated that in both WE and NA the urban sprawl has as its origin an excess of accumulation in the primary circuit (goods and services) that results in an intense transformation of the territory. Expressed in easier terms, urban sprawl originates from an economic, but also from a territorial overflow of wealth. However, in developing countries, in general, and in practically the totality of areas such as Africa and Latin America, the origins of urban sprawl are very different. Thus, these processes are not derived as much from an overflow of wealth (although this element is also present), but rather from the opposite process, an overflow of poverty that materializes in massive processes of rural-urban migration, land takeovers, and the more or less unlawful building of large sections of popular neighborhoods. In other cases, such as those observable in Asia, there are mixed processes, accompanied by strong state intervention [11,12]. Before properly entering in this review, it is necessary to insist that there is no homogeneous and shared definition of urban sprawl phenomena. Nevertheless, a set of specific elements is frequently listed [14,15]. In any case, the academic literature frequently approaches the urban sprawl from a morphological characterization that emphasizes low density, low proximity, and low centrality. Low density is understood as a space generated essentially from single-family dwellings that have a relatively low population in relation to the space they occupy, producing a territorial model featured by a high consumption of land per inhabitant [16,17]. This is related with high dispersion, understood as a strong distribution of the population across relatively large areas. Low centrality and proximity are understood as a spatial distance between suburbs and central areas [14,18]. However, as will be seen in the following sections, what low density and low centrality exactly meant differs significantly in WE and NA.
Urban sprawl is a central element in the present socio-economic and territorial organization. In this sense, a large number of studies and academic reviews have been conducted to expose its main features and consequences. Some of these reviews are referred to NA [19], to WE [20,21], and even to both cases [9,22]. In this context, this review presents two main new features. Firstly, it was carried out using interpretative frameworks based on a Political Economy approach, which made possible to connect urban sprawl with a wide range of theoretical concepts. Among them, elements such as Secondary Circuit of Accumulation, State's Rescaling, Public Private Partnership, Financialization, Foreclosures, Rur-urbanization, Greenbelts, Brownfields, Traffic Oriented Policies, Green Areas Provision, Environmental Justice, or Green Gentrification have had a special relevance in the analysis. This has allowed obtaining not only a wider vision of urban sprawl dynamics but also to deepen some of the elements on which its future evolution depends. Secondly, this article focuses on the different ways in which the sustainability discourse is shaping urban sprawl. Namely, although it is recognized (and it is demonstrated) that the sustainability discourse is not preventing the advance and much less reverting urban sprawl, the different forms in which "sustainable" public policies are modifying and interacting with urban transformation patterns are analyzed. In addition, this is done by considering a wide range of policies and planning programs.
This review is organized as follows. First the methodology is presented. In the following section, the main economic, social, and institutional elements that explain urban sprawl processes in both NA and WE are presented. In the fourth point, some economic, social, and environmental consequences of these processes are analyzed. In the fifth point, some of the various urban policies that have tried to limit or correct some of the most negative impacts of urban sprawl in terms of sustainability are introduced. Finally, in point six, some conclusions are obtained.

Methodology
The present text analyzes the evolution of the phenomenon of urban sprawl as well as its causes and consequences from a structural and holistic perspective, based on an extensive review of the academic literature. It also reviews some of the initiatives that have been used in the recent past to confront these phenomena, both from ex-ante (preventive strategies) and ex-post (palliative strategies) approaches. The review performed is undertaken from a previously defined structure based on the next phases that we can see in Figure 2. The first phase consists of framing a set of interrelated questions related with the topic analyzed. Particularly, three different questions are stated. What are the main reasons which explain urban sprawl? What are the main consequences of urban sprawl in terms of economic, social, and environmental sustainability? What are the main policies addressed to make urban sprawl more sustainable?
The second phase consists of identification and selection of the most relevant works according to the above questions. For this purpose, academic databases and scientific literature search engines (Web of Science, Scopus, Google Scholar) were used. The article covers a broad historical period, from 1950 to the present. However, the reviewed academic literature focuses on the period 2000-2020, with some previous references, essentially to seminal contributions on the analyzed topics. Many of the articles reviewed conducted long-term analyses, thus making it possible to obtain sufficient references to the phenomena reviewed. Therefore, a large and relevant number of bibliographic references were obtained, which were selected using both quantitative (number of citations) and qualitative (reference authors, breadth of treatment of the topic, etc.) criteria.
In the early 1950s, the phenomena of urban sprawl were already clearly perceptible in the United States (US). Research by Nechyba, and Walsh (2004) shows that, in 1950, of the total population living in US cities, 65% resided in the centers and 35% in suburban areas [22]. The transformations in the dominant urban model in the US can be put in relation to five interrelated elements: (a) The formation of the middle classes and the gradual consolidation of the model of mass consumption (b) The generalization of the automobile and the explosion of motorized mobility (c) The expansion of infrastructure associated with public investment programs (d) The abundance of natural resources at low prices: Oil and territory (e) The economic functionality of urban sprawl in the dominant accumulation model At this stage, we were witnessing the formation of a middle class characterized by levels of training, income, and job security that were much higher than those observed among working classes. These middle classes, on the one hand, could devote a large part of their income to consumer goods and services that go beyond basic needs. However, in addition, a part of these incomes were allocated to the improvement of their living conditions, with the acquisition of housing being a central element in this model. This socially prevalent behavior caused major changes in urban models and implied a strong growth of real estate markets. Those processes were additionally supported in the US by the Federal Government's policy of encouraging demand through, for example, the financial promotion of the construction of affordable housing for the veterans of the Second World War [33][34][35].
Moreover, the increase in income levels was associated with a growing desire for social differentiation [36]. Thus, there was a demand for housing in areas far from the old central districts, characterized by being perceived as more natural, less urbanized and, therefore, with easier access to not yet exploited parts of natural capital [37,38]. This generated a cultural pattern (the American way of life) in which the diffuse city was considered, at least in the minds of many of its new inhabitants, close to the ideal of city-garden [39,40]. This reinvention of the urban also absorbed important economic resources. Thus, the cities were not only instituted as places oriented to production but also as wide, extensive and diverse markets in which the capacity to consume new public and private services and new forms of leisure, is generalized [41,42] The automobile, the development of its industry, and the extension of its market, was the second motor of the urban sprawl. It was also closely associated with the growth of the middle classes. Hence, middle class families were first able to have one vehicle, and then moved towards the model of one vehicle for each adult family member. This development of the automotive industry has been related to the generation of mobility needs being urban sprawl and the use of the private vehicles mutually reinforcing processes [43,44]. The provision and improvement of road networks, through the policy of public investment in new roads, was the third element that explains the relevance of urban sprawl processes [45].
It should also be noted that the intensity of the urban sprawl phenomena was also related to the abundance of two natural resources, which in this period were offered not only massively but also at relatively low prices. First, the availability of cheap oil was an essential element in the intensive car use [45]. In this sense, it should be noted that the price of gasoline was particularly low throughout this period in the US. Even in 1978, after the strong oil shock, the average price of a liter of gasoline in the US was $0.18 [46]. This meant that the physical costs of a model of motorized mobility were not translated into household economies [47]. Second, the urban sprawl model was a strong consumer of territory, which, at this stage, was incorporated into the market at relatively low prices.
Finally, it should be stressed that the urban sprawl model was fully functional to the dominant accumulation logics. Already at the beginning of the 1960s, a situation of excess of capital that could not be absorbed by the ordinary production of goods and services was observed. In this context, the explosion of urbanization and the affirmation of the suburban model was able to absorb a high share of the existing capital surpluses. Thus, the phenomenon that Harvey (1990) called "the urbanization of capital" was generated, which was essential in continuing the expansive economic dynamics that marked the North American and European economies until the mid-1970s [28].
Therefore, urban sprawl was, from the beginning, based on an intensive consumption of natural resources (energy, materials, space) associated with strong problems of unsustainability. However, at that time, many of those problems were not perceived as such. Certain consumption patterns, as car use, were not yet a generalizable element at the global level. The issue of space was not yet associated with objective limitations in its provision either. On the contrary, the model of urban sprawl allowed not only a greater quality of life, but the perception of a greater proximity to nature [39,40].
WE followed the suburban patterns dominant in the US, but with a number of distinctive features. Although there was a growth in suburban areas, this was slower because the same causes that promoted urban sprawl in the US also existed in WE, but to a lesser extent. For instance, there was also strong development of the middle classes, but their income levels were significantly lower, especially in the immediate post-World War II period. This, in turn, influenced the rates of motorization. Private car did not spread in Europe the way it did in the US [19,48]. Thus, in the middle of the 20th century, the number of cars per thousand inhabitants in WE only reached 66% of that of the US [48].
The price of oil was also higher and, in most European countries, it was subject to high levels of taxation, becoming a very relevant source of tax revenue. Considering again 1978 as a reference, the average price of gasoline in Germany was 0.46 dollars, more than 2.5 times the price in the US. Likewise, WE does not have the same abundance of territory. This favored vertical construction dynamics. Thus, houses were offered to incipient middle classes at more affordable prices. Additionally, European cities promoted to a greater extent forms of public transport which, in many cases, already had a significant level of development before the Second World War: Train, tram, underground, bus, etc. These facilitated intra-urban displacements without being always necessary the use of the automobile [22]. Accordingly, the existing tradition of more compact cities was not excessively weakened. Notwithstanding, there are important differences by country, with metropolitan areas generally more dispersed in the North Atlantic and more concentrated in Central Europe and the Mediterranean areas [20,49,50].
3.2. The Process of Urban Restructuring and its Effect on Spatial Morphology: The Restructuring of the Diffuse City (1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985) In the mid-1970s, with the end of the growth process that had been a feature of the European and North American economies since the end of World War II, urban realities were strongly impacted, albeit to different degrees, by the phenomena of urban sprawl. However, urban sprawl did not act in the emptiness but superimposed to previous rounds of capital accumulation featured by other morphologies and forms of city [51]. Thus, the majority of the urban agglomerations in WE and NA have, at this moment, a mixed character. In general terms, there was a division between a more or less compact inner city, and an increasingly extended suburban periphery.
In a context of economic crisis and restructuring of social and production relations, this differentiation became increasingly important. In this sense, the majority of the economic and social problems were located in the inner city, gradually inhabited by social groups with lower levels of income. Thus, in the case of US, more than 17% of the population in those areas was below the poverty line. This ratio was reduced to 7% in the suburbs [22]. Accordingly, the urban centers became the "heart of the crisis" with an appearing increasingly widespread urban discontent [52].
All this happens in a context where there is a strong destruction of capital [53]. Not only does this affect the industries themselves, but it also has a strong effect on land markets, causing dynamics of declining prices in important parts of the urban tissue. In addition, the industrial crisis goes together with the disappearance of traditional jobs and with the fall of the working classes' living conditions. As a result, many of the neighborhoods located in the central city entered into accelerated processes of degradation [20,54,55]. This was exacerbated by the public policies implemented, which eliminated much of the existing subsidies and protection networks, thus aggravating the recessive trends in many of those areas [56]. As a result, there was a sharp increase in insecurity and crime in many of the main cities, with New York perhaps being the most prominent example. This, in turn, affected bank guarantees and generated tensions that undermined the stability of the system as a whole [57].
However, the new, mostly middle-class suburbs largely eluded many of these problems. They had neither industrial activities to restructure nor populations based on sectors particularly affected by the crisis. Moreover, suburbs increasingly located a whole new set of tertiary activities [58]. Urban sprawl was reinforced by a new round of migration of middle-class families [59]. These migrants seek safer social environments, but also a better provision of public services (education in particular). In this sense, the flight from the central city reflects, to a great extent, the "every man for himself" concept, that is, the individual capacity to avoid the costs of the crisis [59,60].
Suburbs became particularly attractive for real estate investment because they were not only protected from the dynamics of property depreciation but also increasingly demanded. This is particularly relevant in a context in which the first elements of financialization were appearing, making credit access essential for financing certain consumer goods [61][62][63]. However, credit provision required, among other factors, guarantees, becoming real estate in the suburbs a central piece to this machinery [64]. Moreover, real estate became a fundamental element in the form taken by processes of capital accumulation in which differences in wealth stock were not only wider but also had growing socio-economic relevance [65].
The city, in this context, was under great financial pressure. There was a context featured by the first elements of the neoliberal model that tends to restrict the financing of social programs and other existing protection institutions, in the phase that some authors have called "rolling-back" [29]. In the face of the lack of funds, a series of transformations took place in the system of urban governance, with the aim of increasing the possibilities of regeneration based on the active involvement of private capital. In this way, there was a transition from a managerialist approach (centered on the planning and management of sufficient and relatively ordered public funds) to an entrepreneurialist approach (centered on the possibility of attracting and committing private capital to urban transformation projects) [27]. Entrepreneurialism implies that local authorities are responsible for raising funds and generating a "business-friendly" environment. It, therefore, involves the existence of individual projects of cities competing for investment, that is, the institutionalization of a model of competition among territories [66].

Urban Sprawl and the Neoliberal-Globalized Urban Model
The widespread practice of entrepreneurialist approaches has been one of the basic elements in the affirmation of a neo-liberal urban model. From the mid-1980s, there is a new stage, which some authors call "rolling out", in which a set of devices that allow a sustained accumulation of real estate capital are instituted [29]. Among these institutional devices, three independent, but closely related, aspects should be highlighted: The State's rescaling, the generalization of the Public-Private Partnership (PPP), and financialization [67].
Since the 1970s, there has been a strong reconfiguration of State activity both in WE and NA [68]. This led to a transformation not only in the forms of management but also in the competencies attributed to different State levels, as well as in the way these levels interact with private capital. In this way, the transfer of relevant powers can be observed at the supranational level (see the case of the European Union), while regional and local governments gain in "autonomy" not only in management but also in the attraction of resources. This largely affects the urban terrain, which becomes the subject of different interventions at different scales [69,70]. The result is a growing diversity of government agencies responsible for urban management. Thus, some emergent scales are added as Metropolitan Areas, Association of Municipalities, Public-Private Partnerships, etc. In this sense, the State apparatus diversified into a complex set of organizations with competences that often overlap each other, in what some authors have called "Statehood" [68]. This profusion of institutions that regulate and promote urban growth has led to a model that favors urbanization. For example, in Spain in the period 1996-2007, very permissive regulations facilitated sprawled models of urbanization [67,71,72].
The State's rescaling goes in tandem with the generalization of PPP. The justification is that Statehood's expansion does not involve a cost to the taxpayer because these new institutions are self-financing from the investment they help to mobilize [68]. As the territory is scarce in many areas, this model ensures that private capital has an influence on urban regulation and planning, which is essential to guarantee their business profitability [73]. PPP has been progressively extended to different areas and, for example, is now increasingly important in concessions for the construction and operation of infrastructures as motorways, railways, metro systems, etc. as it will be analyzed in the epigraph 4.2.1 [74].
The mobilization of private resources for real estate also has a strong financial component. In part, developers and construction companies can mobilize their own accounting resources. However, most of the funds they mobilize originate from indebtedness to banking institutions or to direct access to capital markets. Thus, financial operations became increasingly relevant. This centrality of financial issues in broader dynamics of capital accumulation led to the affirmation of a financialized model of capital accumulation [75]. Such a model relies on the existence of a chronic excess of liquidity that seeks profitable investment opportunities worldwide in order to avoid depreciation. Those excesses of liquidity have different origins. It derives in some cases from the concentration of income derived from the exploitation of certain commodities, such as oil [76]. Moreover, it has its origin in the processes of financial deregulation, which have multiplied the forms of private money creation [64,77,78]. Finally, it is also explained, especially since the 2007 crisis, by the intense creation of monetary base both by the European Central Bank and the U.S. Federal Reserve [79][80][81].
Financialization is not only characterized by a chronic excess of liquidity, but also by the fact that the financial flows are particularly unstable. Thus, there are situations of high liquidity and easy realization of profit and other phases of lack of financing because of sudden credit crunches [82,83]. These situations are recurrent, generating a plethora of bubbles and financial crises. However, far from weakening the system, the succession of financial crises allows sudden (and sometimes temporarily short) fall in prices of real estate assets, which can multiply the levels of profitability of financial agents [84]. This model has been described as a "casino economy" because of this instability in asset prices and this erratic behavior of liquidity [85]. Additionally, financialization has a major impact on mortgage markets [64]. State's rescaling, PPP and financialization have resulted in the creation of new coalitions of pro-urban sprawl agents.
In the case of the inner city, the new model of neoliberal urbanism implies a valorization of some previously degraded areas. This has different aspects, generating a wide range of phenomena: Development of financial and cultural districts, gentrification, touristification, etc. [86,87]. However, this does not mean that suburban forces have been weakened. On the contrary, it can be observed that urban sprawl is consolidating. In this way, some metropolitan areas grow to occupy a whole regional space. This is the case of some European cities, such as London, which covers a considerable part of the British south-east, or the Paris region. Both are now genuine urban regions [88,89]. In addition, PPP and the availability of financial resources have often favored the complexization of many residential areas which have been included commercial and leisure facilities, particularly in the form of shopping centers or malls. There has also been a tendency to locate productive installations such as technology parks [58].
Furthermore, the urban sprawl extends to areas where its presence was still reduced. This would be the case of the Mediterranean city which, especially from the second half of the 1980s, is going to be strongly impacted by the phenomena of urban sprawl [50,90]. In this sense, Arellano and Roca's research shows that it is "the western European periphery is where the greatest processes of urban sprawl have taken place" [91]. This coincided with strong investment in road networks from the 1980s onwards related with the application of the European Union' s structural funds [92].
Moreover, urban sprawl phenomena are increasingly influenced by the extension of second residencies. In relatively warm environments, this means an increasing occupation of the coastline. Here, the popularization of sun and beach tourism converges with the massive construction of apartments and single-family dwellings [93]. Frequently, owners are not only nationals but also residents of other countries. In this sense, practically all European countries with a Mediterranean fringe (France, Italy, Spain, Portugal, or Greece) have seen their coastlines strongly affected by the proliferation of summer-use dwellings.
Finally, an important process of housing construction in rural areas is observed, especially but not exclusively, in areas with a colder climate. These processes are especially evident in the areas with a high landscape and heritage value, which can be associated with the idea of "rural idyll" [94]. As a result, the so-called rur-urbanization processes are spreading, which can be understood as the outcome of the extension of sprawl dynamics beyond conventionally associated borders of metropolitan areas. Rural idyll and rur-urbanization are strongly observed in both NA and WE. In the US, it affects areas of high scenic beauty, located in relatively sparsely populated areas, such as the Northwest, sometimes accompanied by high levels of social segregation [95][96][97]. Similar phenomena are observed in WE, also with strong class dynamics in countries such as the UK, France, or Spain [98][99][100]. In addition, particularly in the European case, many of these rur-urbanized areas have strong relationships with specific metropolitan areas. This is, for example, the case of Empordanet and Barcelona [101].

Main Socio-Economic Effects of Urban Sprawl
The urban sprawl groups a complex set of processes with multiple effects that can be approached in terms of sustainability [20]. However, the concept of sustainability is polysemic, therefore, it is necessary to clarify its content. Thus, different dimensions are recognized, considering economic (

Economic Effects
As it has been pointed out, the Agglomeration Economies are one of the theoretical foundations of urban phenomena. Thus, when densities strongly diminish, some of the key economic forces which explain the existence of cities are weakened [103]. However, the operation of Agglomeration Economies is strongly influenced by the unequal distribution of income and the reduced monetary cost of some modes of transport. Thus, in the case of the US, the fact that suburban communities are characterized by substantially higher levels of income means that, in general, there are no problems associated with the provision of goods and services. In fact, the areas with greater deficiencies in the provision of, for instance, healthy and nutritious food, the so-called "Food deserts" (FD), are mostly concentrated in depressed areas usually located in the inner city [104].
FD is a concept with a strong metaphorical component that has been defined in many different ways [105]. Thus, references have been made to elements, such as the number of stores [106], the characteristics of the food demand [107], or the proximity, diversity and variety of supermarkets [108]. The existence of FD has been verified in countries such as US [109], Canada [108] or UK [110]. FD must be understood as the result of evolutionary processes closely associated with the phenomena of urban sprawl [111]. In this sense, the growing migration of middle-class families to the suburbs tends to reduce the average income of inner-city inhabitants and, thus, the potential market for businesses. As a result, there is in the long term a reduction in the inner-city food supply and a parallel increase in the suburbs. Moreover, new suburban supermarkets have a wider and better-quality offer, generally at lower prices [110]. In this sense, the extension of the suburban stain has in the US a significant correlation with the appearance of FD areas [111]. However, in the cases in which low-income social groups inhabit suburban spaces, the problem of access to certain goods can be accentuated [112].
There is also a range of costs that have a dual economic and ecological status. The ecological dimension, derived from their impacts on the general operation of ecosystems, will be analyzed in more detail in Section 3.3. However, as those costs are often non-internalized in monetary terms, it is also possible to speak of non-recognized economic costs or, in other words, of negative external economies (or externalities). One of these negative externalities is associated with travel costs. This has a strictly economic dimension, partly correctly internalized and partly not. Expenditure on fuel consumption, the need for the systematic renewal of cars, or the cost of their maintenance is internalized. In fact, these elements account for a significant share of household budgets. However, an important part of the ecological costs derived from greater pollution and the depletion of resources is not internalized, generating important external diseconomies. Moreover, the economic translation of many social costs is not considered either [113], for example, the time drivers have to spend traveling to their workplaces. On average, every American lost a week of their time in 2017 as a result of congestion in metropolitan areas, which involved extra spending of 21 gallons of gasoline per vehicle [114]. In 1982, with less energy-efficient vehicles, this extra expense was only five gallons. In aggregate terms, the extra expense in 2017 accounted for 3.3 billion gallons [114]. In monetary terms, congestion costs amounted to $19.5 billion. This is 19% higher than the costs estimated five years earlier in 2012 and almost 48% higher than in the last decade. Each US driver had to pay, on average, $1080 per year in costs due to congestion. Congestion times also tend to increase with the size of the metropolitan areas and, therefore, the distance between residential, work, and leisure spaces [71,115].
Another economic effect is the conversion of non-urban to urban land. Here again, there is an interplay between directly accountable economic benefits and hidden negative external economies. The monetary income generated is associated with the monetary revaluation of the land. The change (growth) in the price of land once it is oriented to residential or business use is an important source of monetary income, which generates economic activity and employment in housing and infrastructure construction. However, there are also external dis-economies, as a result of the shift in agricultural production sometimes towards less fertile lands [116]. Therefore, the final consequence of the urban sprawl, will be the loss of rural spaces that could have productive uses to generate food or raw materials [71].
Finally, it is necessary to mention the strong association between urban sprawl and financialization. On the one hand, urban sprawl is closely related with strong growth in real estate wealth, due to the combination of two phenomena: The intense construction dynamics and the expansive trend of prices [117]. This housing wealth is largely household ownership, although there is a growing presence of financial funds and corporative investors [79,118]. On the other hand, this goes together with a dramatic growth of household debt that is mainly explained by the increasing cost of housing [79,119].
"From 1999 to 2007, household mortgage debt in the U.S. doubled in real terms to almost $11 trillion (see Figure 1), at the time, more than all U.S. government debt (including local and state), and about 30 percent of all domestic nonfinancial sector debt . . . " [120] (p.284) The growth of mortgage debt is not exclusive to the US. The percentage of mortgage debt over GDP was 64.5% in the US at the beginning of the housing crisis, that is, in 2009. However, this indicator reached 111% in the Netherlands, 89.7% in Denmark, 72.5% in the United Kingdom (UK), 52.7% in Ireland and 52.4% in Germany. However, there were some European countries, such as France (26.2%) or Italy (14.5%), where mortgage debt remained at relatively low values [121]. Moreover, the growth in prices is causing many families to be excluded from mortgage credit, despite its increasing ease of access [122]. Attempts by neoliberal institutional structures to include a larger percentage of the population in mortgage credit through the development of subprime segments have clearly failed, producing a chain of effects that led to the credit crunch of 2007 [123]. In addition, the general increase in housing prices has not been reflected in the improvement of housing conditions [124].
The inherent instability of financialization means that there is an additional cost between the strictly economic and social fields. The periodic eruption of financial crisis and credit crunch situations not only affects the value of assets but also causes massive defaults on mortgage payments, foreclosures, and auctions, which ultimately operate as intensive mechanisms of accumulation by dispossession [82,125]. In this sense, foreclosures have been massive in countries like the US, especially affecting lower-income groups and racialized population, with a special impact among Latin Americans and African Americans [126,127]. Foreclosures have also strongly affected various European countries, such as Ireland and Spain [128,129].

Social Effects
Urban sprawl is associated with increased social segregation and the generation of undiversified communities. In this respect, there is a strong relationship between urban sprawl and migratory movements, which try to socially and spatially escape from poverty. Moreover, on occasions, disadvantaged social groups tend to settle in remote, isolated, and poorly communicated spaces [14]. In this context, public planners and decision-makers try, in some cases, to avoid the provision of infrastructure and equipment in suburban areas, which could generate environments in which poverty is concentrated [130].
There is also a relationship between urban sprawl and racial segregation. There is relative evidence that until the late 1970s, urban sprawl in the US was based on strong levels of racial discrimination, underpinned, among other elements, by differentiated access to bank credits for racial minorities [131]. In this sense, the presence of racial minorities, in general, and of African-Americans, in particular, was associated with higher risks of property depreciation. This "technically justified" the systematic denial of credit for homeownership in certain areas [132]. This led to the widespread perception of white suburbs as opposed to racialized urban areas, although suburbs occupied by racialized populations existed from the beginning [133]. In this sense, the abandonment of the central city by those who have the financial capacity to do so turned many of its areas into ghettos. In fact, Glaeser and Kahn conclude that in many areas of the centers there were only those who cannot afford to have a vehicle and cannot buy or rent in the expanding suburban areas [19,134].
Since the 1980s, levels of racial segregation have been decreasing, although it can be considered a structural trait of the NA urban system [135,136]. In this regard, studies conducted with microdata (over 240,000 households and 650,000 individuals drawn from 39,000 census blocks) in the San Francisco Bay Area by Bayer, McMillan, and Rueben demonstrate the influence of racial factors in the choice of place of residence. These factors explain 95% of segregation among Latin Americans, 50% among Asians, and only 30% among White and African-American households [137]. However, there are other events that have increasingly contributed to the persistence of significant levels of urban segregation. One of them is the growing importance socially attributed to food, combined with a proliferation of FD. Thus, FD have been increasingly spatially identified, becoming an increasingly significant socio-political element [138].
Moreover, the processes of urban sprawl are associated with a decrease in social participation. This is essentially due to the distance and lack of communication resulting from dispersion that limits the possibilities of socialization and implies the loss of the notion of neighborhood and mutual trust [103]. Social segmentation is aggravated, especially by limiting the possibilities of interaction of those who do not have a car [47]. However, it also affects those who have a vehicle, because the time of displacement limits their possibilities of socialization. Thus, Putnam established that for every 10 minutes added to driving time on habitual journeys, civic participation decreased by 10% [139].
There are also impacts on people's own health [140][141][142][143][144]. The most perceptible effects of urban sprawl on health are closely associated with intense car use, especially in terms of the risks of accidents [144]. Using a structural equation model, Ewing et al. studied these factors and found that, more compact urban morphologies showed a lower accident rate because of the shorter distances and the lower speeds [145]. In contrast, suburbs show a higher volume of serious accidents resulting in death. In the USA, 12.4 people per 100,000 inhabitants die annually as a result of traffic accidents [146]. In the EU (28), with more compact cities, and with greater use of public transport, this ratio is 4.9 people per 100,000 people [147]. This ratio is particularly low in countries such as Germany (4.0), Spain (3.9), Denmark (3.0), or the UK (2.8) [147].
Urban sprawl is also associated with a drastic reduction in walking tours because of the spatial distance from places of work, residence, and consumption, as well as the usual lack of pavements. The urban morphology of the urban sprawl, where spaces are fragmented and functionally distanced and where the car culture has not left spaces for pedestrians, means that the private vehicle is practically the only way by which people can move around [148,149]. In the US, for example, the use of the bicycle is very residual (1%), and only 9% of journeys are made on foot. A quarter of all journeys in the US are less than a mile long. However, 75% of them are made by car [144]. Thus, suburbanization is increasingly associated with an increase in sedentariness, which is one of the main causes of overweight and obesity, a prominent risk factor for cardiovascular diseases, colon cancer, apnea, or osteoarthritis [142]. Beyond sedentariness, the composition of the diet and, in particular, the increasing reliance on "empty calories" is also behind the majority of these pathologies. In this sense, high levels of morbidity, mortality, and adverse health outcomes are related, among other elements, to diet and, in turn, to the existence of few alternatives to promote healthy nutrition. Consequently, they are associated with the existence of FD [104,111,138].

Land Use by Urban Occupation
Suburban expansion extraordinarily broadens the dimension of cities [103]. Thus, for example, in the US, the impact in terms of land was more than proportional to the increase in the population. If the population in low-density suburban areas doubled, the extension of land they occupied doubled by four [150]. In Europe, a similar phenomenon occurs even though urban morphologies are more compact and there are some institutional constraints.
The expansion of the cities' size has a set of particularly negative externalities when the land affected has a production model focused on supplying food to local markets. This means that the loss of agricultural land, often under traditional, ecological or integrated farming, limits food sovereignty and exposes these areas to imports [151]. This aspect is particularly relevant in a moment in which, both in WE and in NA, there are strong movements involved in the development and upgrading of short food chains, based on local quality productions [152,153].
Not only are there considerable reductions in the amount of soil available, but also in its quality. Firstly, frequent processes of soil salinization can be observed as a consequence of the sealing by asphalt [154]. In this regard, the coastal areas are frequently the most [155,156]. Secondly, there is sometimes a deterioration of the soil due to erosion processes (piping and gullies) caused by urbanization [157]. Erosion, in the long term, is also associated with desertification processes [158]. Finally, soil deterioration is also related with growing risks of flooding [159], which, in densely populated areas, has not only important environmental but also social costs [156].
At the end of the 2000s, 9% of the European territory had its soil covered with impermeable material [154,160]. It is easy to conclude that the creation of artificial land was a consequence of urbanization, which reduced the natural drainage network, affecting runoff and flooding. However, the damage is greater when flood zones are developed [161]. For example, urban growth in areas at risk of flooding is, on the Spanish southeast coast, greater than anywhere else in Europe. "...the number of properties and the built-up surface area within flood-prone areas have increased to 255.02% and 258.35% (1975 = 100) . . . " [162] (p.311). The research carried out in the US about the Urbanization impacts on surface runoff shows interesting results [159]. Not only urban intensification, but also urban expansion led to a ten-year increase in the surface runoff between 2001 and 2011. The counties with low runoff decreased and with medium and high runoff increased (by 21.8% and 23.3%, respectively). Where there was greater urban expansion, runoff and flood risks grew significantly [159,163].
The impact of urban sprawl in terms of soil loss is especially relevant when areas of high natural and landscape value are affected. Firstly, it is necessary to emphasize that outstanding landscapes revalue those houses that occupy privileged spaces around them. Therefore, in the absence of regulation, certain spaces of high heritage value are quickly occupied: The front of the coastline, locations with beautiful views of the mountains, valleys, etc. [164]. However, the extension of suburbanization sometimes causes a loss, or at least a deterioration of these natural and landscape values, and therefore, losses in the value of the properties [93]. It also has a direct impact on the fauna, which loses habitats in which it used to feed, but also suffers from noise and anthropogenic pollution [165].
In this context, especially in certain European countries, increasingly demanding regulations have been established to preserve cultural and natural heritage [166,167]. However, these regulations do not prevent the progression of suburbanization, despite limiting and shaping it [168]. In this sense, environmentally and scenically privileged spaces are increasingly reserved for the enjoyment of social groups with high levels of economic, cultural, and social capital [169].

The Disruption of Water Cycles
Urban expansion, in general, also has a significant impact on water use and management. Urbanized areas do not always have sufficient water resources to meet a growing water demand [170]. However, in WE and NA, access to the resource is guaranteed because of both heavy public investment and the development of different institutional mechanisms [171,172]. There are cases, such as the city of Las Vegas (US), where water demand is covered by the use of non-renewable water resources that are difficult to replace [173].
Water use inside the homes is closely associated with the family size but also with the extent of property (gardens, swimming pools, fountains, and other ornamental uses) [174,175]. In this sense, the demand for water grows substantially with middle-class suburbanization because their homes are frequently accompanied by swimming pools, gardens, or even sometimes integrated into environments characterized by high water consumption, such as golf clubs [176]. This sometimes involves the construction of differentiated drinking and non-drinking (for irrigation and other uses) water supply networks. For instance, in Spain, Alicante was, after Madrid and Barcelona, the place of greatest urban expansion. This urban expansion was associated with the use of second homes (residential tourism), which was mostly oriented towards northern Europe's residents. In the northern area of the coast, there are single-family dwellings with the largest garden areas, resulting in water consumption being substantially higher than in the southern area, 588 liters/house/day, as compared with 372 liters [160].
The availability of a private swimming pool is closely associated with the garden and both are associated with higher levels of household income and higher value of the dwellings [175,177]. Likewise, there may be an associated factor of a seasonal nature, which precisely shows the need to use a greater amount of water at certain times of the year (for garden irrigation, swimming pools, etc.) [178], or even that said seasonality shows the factor of second residence, in which case, both consumption for internal uses and for external uses increases considerably. However, there is also an educational and cultural element that tends to limit these expansive trends in water consumption. Studies carried out in Hillsboro Oregon (USA), show, in these suburban spaces, that high levels of education are associated with more conscious behavior in a contained use of water, fundamentally in times of drought [178]. In this way, even in the driest places, the models associated with the urban sprawl are present. Hence, in the USA, lawns and swimming pools are shown as decorative elements in architecture and are also very present in arid places. The studies of Harlans 2009, in Phoenix, showed a pattern of water consumption associated with income levels. Thus, those with greater purchasing power had single-family dwellings with high outdoor water consumption, giving rise to an optical impression of oases in the midst of arid spaces [179].
This leads to relatively frequent use of groundwater. In this sense, urban sprawl in these environments also has an effect on aquifers. In some cases, this is aggravated because the strong demand for water in certain metropolitan areas (particularly coastal areas) implies strong detractions of this resource through transfers that could contribute to salinize the aquifers of the coastal border [180,181].

Material Consumption and Environmental Pollution
Urban sprawl patterns increase the movement of people, and consequently, the flow of vehicles, increasing both energy consumption and pollutant gas emissions [22,182]. Research by [183] highlights this association between energy consumption and urban morphologies in the US. Thus, per capita fuel sales are significantly higher in extremely low-density states, such as Wyoming or North Dakota. At the other extreme is the District of Columbia, which, with a higher density and an adequate public transport network, shows far lower per capita gasoline consumption [183]. The differences between those living inside and outside metropolitan areas corroborate this correlation between population density and energy consumption. Thus, per capita gasoline sales are 41.2% higher outside metropolitan areas [183]. Similarly, Brownstone and Golob found that a thousand fewer homes per square mile means driving 1200 more miles a year, and therefore, consuming 65 more gallons of gasoline [182].
Vehicle traffic and congestion also have an impact on anthropogenic water pollution. The asphalt road surfaces on which vehicles drive show significant amounts of zinc. Runoff eventually carries the zinc into the aquatic environment. Corrosion of the galvanized metal and abrasion of tires and brakes are the origin of zinc and other metals as iron, manganese, copper, antimony, barium, and zirconium [184,185]. Although technological innovations and the regulations are already alleviating pollution problems in large cities, particularly in terms of CO2, they will have a limited impact on metal erosion and tire abrasion. [19]. Other studies even find polluting metals, such as cadmium, copper, lead, zinc, and nickel, in high-traffic road networks [186].
In the US, 28.5% of greenhouse gases are generated by transport [183]. The movement of vehicles on US roads generates 80% of the total transport's carbon footprint. The remaining 20% is caused by airplanes, ships, and railways. In fact, the air polluted by nitrogen oxides and hydrocarbons is caused, in one-third of cases, by road traffic associated with combustion engines (cars and trucks). In metropolitan areas, the share of road traffic can be as high as 50% [144]. Polluted air affects ecosystems and generates and aggravates respiratory problems, particularly for people suffering from asthma, acute respiratory infections, or other cardiopulmonary diseases, the elderly and children. In the spaces most polluted by high ozone levels, there are more deaths associated with cardio-respiratory diseases, as well as higher absenteeism from work and school [144,187]. The investigations in five German cities on the days of the transport strike show growth in morning peak hours (entry into work) of between 13.3% and 14.8% in emissions of polluting particles. In fact, recently, hospital admissions have increased due to respiratory problems by 13%, mainly in children under five years of age, whose admissions increased by 34% [188].
In the US, the work of Brown, around 100 metropolitan areas shows that higher population density and more compact models show lower emissions of polluting gases [189]. Similarly, studies by Liu et al. on commercial transport vehicle traffic and the emission of its pollutants for the US, conclude that a more compact city model would allow a reduction of at least 3% in the emission of air pollutants and 36% in deaths [190]. Moreover, building models have an impact on greenhouse gas emissions. Vertical construction, associated with more compact and denser areas, tends to have smaller dwellings with shared walls. The energy consumption of this type of housing is consequently more efficient. Research by Brown et al. shows that buildings with five or more dwellings consume 38 percent of the energy required by single-family homes [191].

Sustainable Socio-Environmental Policies: A Synthesis of Selected Experiences in Western Europe and North America
As has been indicated, urban sprawl is the result of both the molecular dynamics of capital and the intervention of public authorities [32]. In this sense, a reduced planning capacity often results in an aggravation of socio-environmental problems and in depreciation of real estate capital. In this context, the increasing State's rescaling in the context of neoliberal governance is particularly problematic [192]. On the one hand, the design and management of socio-environmental policies cannot be subject to fragmented decisions in differentiated spheres of competence with disparate or even conflicting criteria [20,193]. On the other hand, there is growing complexity in the generation of multiple and complex socio-environmental relations that require the participation and coordination of multiple agents in the context of eco-governmentality [194,195].
In this scenario, there are countless initiatives associated with a search for more sustainable patterns of urbanization, in what has been called green urbanism [196,197]. Green urbanism is not only more widespread but more intensively applied in WE [196]. According to [197], green urbanism has three main strands: Energy and Materials, Water and Biodiversity, as well as Urban Planning and Transport. In this review, it is impossible to cover all those elements, so we are going to focus on the last one (Urban Planning and Transport) because it is the most directly related to urban sprawl. Anyway, Urban Planning and Transport include such heterogeneous aspects as the bronwfields, the greenbelts, the areas of residential priority, infrastructures, and equipment for sustainable mobility, etc. [198].
The approaches to sustainable urban planning that address the effects of urban sprawl are based on two main approaches. On the one hand, there are those proposals that try to conduct, content, and limit urban sprawl through ex-ante approaches based on the anticipation, planning, and regulation of land uses. On the other hand, there are those proposals that do not try to limit urban sprawl but to take measures that could palliate and correct its main economic, social, and environmental problems [92]. The aim is, in this last case, to manage the urban sprawl by promoting initiatives that facilitate greater multifunctionality of spaces, that enable alternative forms of mobility and transport or that advocate for greater efficiency in land use [199,200].
Practical applications of both models are observed in both WE and NA. In any case, both views are compatible with the dominant neoliberal urban policies which they not only do not directly question the role of real estate in broader dynamics of capital accumulation, but also prevent its hypothetical depreciation. In this sense, they can be included within the variegated forms of ecological modernization [201][202][203].

The Conservation of Peri-Urban Environments: Greenbelts and Urban Containment Policies
One of the most serious approaches to trying to contain or at least conduct the processes of urban sprawl is the establishment of physical limits to urban growth, prohibiting construction in specific areas protected by different legal devices. This would be the case of the so-called greenbelts [204]. The idea is, therefore, to create a protected area around the urban centers that serve as a border with the surrounding rural environment, contributing to the maintenance of the agricultural, landscape and natural wealth of the surrounding ecosystems.
There are many different approaches to greenbelts that can encompass different kinds of interventions, such as green corridors, green roads, green strips, or peri-urban parks [205,206]. Although there are some examples of their application in the 19th century, their use became popular in the 1950s, especially in Europe [198]. They were especially expanded in Great Britain with fourteen cities where they have reached a significant dimension [206]. It has also been applied in many other places with Anglo-Saxon influence, such as  [198].
Initially, the greenbelts were understood in a very rigid way, even limiting the maximum dimensions and densities of the different areas around them [206,207]. Their rationality, particularly of the initially considered immovable urban limits, was also questioned [206]. However, the greenbelt concept has eventually evolved, framing a multiplicity of changes. Firstly, the greenbelts, by territorially restricting the sphere of the urban contributed to generating a series of limits of meaning and to modeling social preferences. In fact, in some cases they could propitiate an increase in the price of housing in the centers [208]. On the other hand, the greenbelts by themselves could not avoid migrations of urban population to the periphery and, thus, the territorial expansion of the suburban stain [209]. In this way, the phenomenon known as "jump" is produced [204]. This would be the case of Great London [210]. In the British experience in general, greenbelts by itself were not able to restrict growth in suburban spaces, although they were able to create an accessible and extensive network of green areas around the central city. Suburbanized spaces beyond the greenbelt became almost exclusively for residential use. Greenbelts were thus trapped between two spaces of strong urbanization pressure with different characteristics, the central subject to a high real estate revaluation and the expanding suburban spaces [211].
In the case of Germany, however, greenbelts seem to have been more successful in containing the urban stain. In fact, practically 60% of its regions have applied measures of greenbelts accompanied by others addressed to maintain a compact urban morphology and to avoid the disorderly processes of urban expansion [204]. To this effect, fractal analyses show that, considering 20 European cities, the German city of Frankfurt has the highest concentration of its periphery, followed closely by Stuttgart [212]. Frankfurt's greenbelt, approved in 1991, was designed to restrain urban sprawl, while protecting natural areas around the city. This was coupled with the specific declaration of numerous cultural, historical and environmental sites [198]. There are other outstanding examples of greenbelts. One example is the city of Copenhagen, whose greenbelt model, called the Finger Plan, shows morphological characteristics different from the circular one. In this case, on the ground as the palm of an open hand. The palm is the central zone and the fingers the spaces of expansion towards the outside. Between these fingers, there are a set of natural spaces. Thus, a radial model was designed in order to facilitate the availability of nearby green areas and the operation of the lines of public transport [213].
As indicated, greenbelts do not have a high level of development in the US. However, this does not mean that the authorities are entirely renouncing to regulate the territorial expansion of cities. Its main instrument is the establishment of temporary restrictions on construction through Urban Containment Policies (UCP). UCP limit the space that can be developed during a certain period of time. Beyond this limit, the land must be reserved for agricultural work. The aim of this instrument is not to prevent urban expansion or to restrict urban development, but to ensure that there are not too many urban spaces left without construction, while protecting agricultural spaces that could be of strategic importance. Likewise, UCP estimate the space requirements in order to have sufficient land supply for undertaking urban development projects [214][215][216].

Use of Disused Spaces: The Brownfields
If the objective of the greenbelts is to contain the processes of suburbanization and guarantee the provision of green areas around the central city, the objective of the so-called brownfields is to increase the supply of land, usually in degraded central areas, generating alternatives that could prevent migrations to suburban zones. In this sense, brownfields rose in response to the processes of post-industrial transition [91]. The phenomenon has been especially important in the US, Canada, and some European countries that, strongly affected by deindustrialization processes, had important bags of land situated in degraded industrial areas at relatively central locations [217]. The recovered spaces can be of various kinds, from the largest ones, which would include entire blocks or neighborhoods (houses in a state of disrepair, squares, streets, gardens), to specific structures or megastructures: Abandoned factories, disused shopping centers, buildings in substandard housing, train and bus stations, disused train tracks, warehouses, inactive port areas, etc. The recovery of these spaces can also have different characteristics, requiring different types of action, urban regeneration, environmental regeneration, or both simultaneously.
The definition of the concept of brownfield is not homogeneous either. Following Ferber and Grimski, brownfield land is that which has vestiges of a previous use, contrasting with the term greenfield, which has not been previously used. This is essentially the European meaning of the term in which brownfield land is not necessarily contaminated [218]. However, in the US, a much more restrictive meaning of the term brownfield is used, as long as it requires the presence or possible presence of a contaminant or hazardous substance, discarding, of course, those considered as highly contaminating [219].
Having made these remarks, it should be noted that brownfield recovery processes have been particularly intense in the UK. Already in 1998, the UK had set a target: Six out of ten new homes to be build in the next ten years should be on brownfield land [220]. In 2005, the data from the National Brownfield Strategy, approved two years earlier, showed that 73 percent of new houses occupied brownfield space. Moreover, 62 percent of land used for new housing construction was brownfield space. Thus, there is a remarkable and high-density use of these spaces [219]. Most brownfields are located in densely populated places with a strong industrial tradition such as London, Liverpool, Manchester, Newcastle, Birmingham, Leeds, Portsmouth, or Sheffield [219]. On the contrary, in the US, brownfield spaces are concentrated in areas with lower income levels. In this sense, the different meaning of brownfields in the US has a considerable downward effect on prices [221].

Models for Limiting Vehicle Traffic and Developing Sustainable Forms of Transport
Regarding transport and mobility, different policies have been used in recent years to try to reduce congestion, energy consumption, and pollution. These policies shown in Figure 6 can be grouped under the generic name of Traffic Oriented Developments (TOD). They aimed to influence both negative environmental and social effects of congestion and high social segregation that are characteristic of the dispersed city. TOD encompasses the construction of a wide range of infrastructures, but they are also associated with other economic and institutional measures [222]. Firstly, they include some actions directed at discouraging the use of the car, especially when they are related with local regulations. They comprise issues such as the establishment of speed limits in residential areas, road modifications, or changes in parking facilities. In this last sense, the reduction of parking spaces in city centers and their parallel increase in other locations related to the use of other means of transport is promoted [223]. Secondly, TODs promote the use of public transport. This is associated with the establishment of an appropriate structure of prices and tariffs which must be affordable for users, while ensuring sufficient resources to maintain high-quality standards. The development of integrated systems in which the offer and timetables of the different means of transport are coordinated is also promoted [224]. Moreover, the promotion of public transport often involves the construction of new infrastructures, such as metropolitan railways, light rails, trams, which will be analyzed below. Finally, one of the central elements of TOD is to increase the attractiveness of walking and cycling, which, in turn, is related to another set of actions, such as the increase in the size of sidewalks and the facilities associated with them (benches, trees, etc.), the promotion of pedestrian areas, the construction of bicycle lanes, or the integration of the bicycle with other means of transport [225].
It should be stressed that although there is, especially since the early 1980s, a general movement of promotion of public transport, the starting situation in the different countries (or even in the different urban areas) was quite different. As indicated in Section 3, the US is objectively much further away from the conditions needed for the optimal operation of sustainable mobility patterns, with its cities characterized by higher levels of dispersion [226]. This limits the potential population that can be served by any form of public transport. In addition, as it has also been indicated, the price of gasoline has historically been lower in the US. Something similar can be said of the historically higher roadway provision or the rate of motorization [46]. In Europe, by contrast, the existence of more compact cities, a certain tradition of urban sprawl regulation (especially in some countries, such as Germany) and more developed public transport systems created more favorable conditions for changing mobility models [224].
Moreover, these are transformations that cannot be understood without considering the scalar dimension that we have referred to several times. Some elements, for example, the financing of certain projects, have a Central State and, sometimes, even a supranational dimension. Others, such as the reduction of maximum speeds or the reduction of parking spaces in central areas, are essentially local responsibilities. The most successful cases of the development of sustainable urban transport systems, such as Germany, are characterized by strong coordination of action at different scales [227]. However, local success stories also matter because they demonstrate that Sustainable Transitions are not only possible, but desirable and associated with higher levels of life quality. For instance, Freiburg has sometimes been symbolically considered the "environmental capital" of Germany [228].
Although the transition to sustainable transport models implies, as we have seen, a multiplicity of changes, the aspects that, on the one hand, are most noteworthy and that, on the other, have the greatest capacity to promote a broader set of transformations (from limiting vehicle speeds, to increase the size of sidewalks or the pedestrianization of certain areas) are associated with certain mega-projects [229]. In recent years, the extension or ex-novo construction of metropolitan railway and tram networks has been particularly relevant [230]. There is a fuzzy boundary between traditional urban rail, light rail and tram, so that models are often developed that partially integrate one and the other [231]. Having made this clarification, it can be said that light rail has had the highest level of development in recent years in both NA and WE. For instance, in the case of the US, "Over 25 billion dollars were spent between 1970 and 2000s in 14 major cities of the United States on the construction of new rail transit lines" [232] (p. 155).
These massive investments had, following the multiscalar character previously mentioned, strong support from the Federal Government, at least until the middle of the 2000s, through two main instruments: The Intermodal Surface Transportation Efficiency Act (ISTEA) of 1991 and the Transportation Equity Act (TEA21) of 1998 [233,234]. Some of the main metropolitan areas that developed light train systems were Cleveland, San Francisco, San Diego, Portland, Sacramento, San Jose, Los Angeles, Baltimore, St. Louis, Denver, Dallas, or Salt-Lake City [235]. In any case, this is not an exhaustive list citing the literature other cases such as Miami [236], Seattle [237], or Washington DC [232].
In general, the construction of these infrastructures has not resulted in an appreciable modification of mobility patterns in the US that continue to be based on the use of private vehicles. This is partly explained because of the highly dispersed morphology of the cities in the US in which many of the suburbs are often only reachable by car. In this context, it can be understood that the levels of use of public transport not only did not increase but fell. This reduction was from 12% to 6% between 1970 and 2000. Furthermore, the negative social bias of those who use public transport (those who do not have the resources to essentially acquire and maintain a private vehicle) did not change either. In fact, the participation of suburban residents in public transport was particularly low [232]. Moreover, this negative assessment of people using public transport contributes if not to a certain resistance, at least a lack of interest in the development of public transport systems from many middle-class suburbs. Not surprisingly, since the mid-2000s, the Federal Government canceled the privileged funding lines. Therefore, it is possible to speak of a failed policy.
Additionally, the effects of these public investments in terms of the revaluation of the affected areas were ambivalent. Sometimes, processes of real estate revaluation associated with gentrification are observed. This would be the case of San Francisco or Washington DC [232,235]. Other times, however, there are processes of loss of property value and counter-gentrification, which follow the logic by which public transport attracts poverty. This would be the case of Los Angeles or Portland [232,235]. This uncertainty in the results undoubtedly limits the involvement of private capital in the currently prevailing strongly financialized context.
In the case of WE, there has also been a strong process of construction of Light Rail Systems in an institutional environment also marked by multi-scalarity in which both the European Union [238] and the different national, regional, and local governments have a key role. The construction of Light Rail Systems has been especially relevant in countries such as the UK with projects with a strong social and political projection in cities like Newcastle, Manchester, or Nottingham. These infrastructures had strong financial support from the British State, in the framework of the so-called Ten Years Transport Plan of 2000, which, however, was withdrawn in 2004 [239]. The construction of light trains, which sometimes change tracks and are transformed into trams, has also been particularly numerous in a large number of German, French, and Spanish cities. In the German case, the aim was to recover a previously lost tradition: The operation of trams that existed until the 1950s and 1960s. Nowadays, there are outstanding cases of development in cities such as Berlin, Bremen, Cologne, Dortmund, Düsseldorf, Frankfurt, Munich, or Stuttgart. In France and Spain, these infrastructures are being created almost ex-novo. Some French cities with a presence of light rail systems are Angers, Bordeaux, Grenoble, Le Havre, Lille, Lyon, Marseille, Montpellier, Nice, Paris, or Toulouse. It has also been developed in Spanish cities such as Alicante, Barcelona, Bilbao, Madrid, Seville, and Saragossa. There are also prominent examples of development in other countries, such as Switzerland or Portugal [230]. The construction of light trains has often been associated with urban development projects that have resulted in a revaluation of the areas under intervention. This has favored the emergence and popularization of public-private partnership schemes, which are very present in the case of Spain, for example [240].
It is also very relevant to stress the strong development of cycling in some European countries. In this sense, the bicycle is an important means of transport that accounts for more than 10% of urban travels in Germany, Denmark, and the Netherlands. The extensive construction of bicycle lanes and parking facilities, the integration with other means of transport and the restrictions on vehicle traffic seem to be among the reasons for this increase [225]. However, in other European countries, such as UK, the importance of bicycle travel is still very low. Finally, it should be noted that this range of initiatives has an effect on changing actual urban mobility patterns in some European countries. The most paradigmatic case would be Germany, which has managed to significantly reduce the share of car travel. These reductions are also occurring in a number of major European cities. Thus, between the first half of the 1990s and the first half of the 2010s, car share travels were reduced by 13 points in Vienna, 10 in Paris, 9 in Copenhagen, and 8 in Amsterdam [222]. There is also stagnation or even a slight reduction in the rate of vehicle ownership in countries such as Germany [241].

Provision of Green Areas
As previously mentioned, one of the fundamental elements of Green Urbanism is the promotion of compact city patterns, enabling to curb the trend towards suburbanization. This implies confronting the image of greater proximity of the suburbs to nature, as opposed to the greater artificiality of the central city [242,243]. The generation of an extensive and close network of green spaces plays a central role for this purpose [244]. These networks of green spaces have a particularly strong multifunctional element especially valued in the current context of post-modern living [245]. In this way, the creation and expansion of green areas are related to a particularly wide range of positive outputs such as: (a) Reduction of environmental risks, such as flooding [246] (b) Adaptation to climate changes by, for example, controlling the effect of urban heat islands [247,248]. (c) Improving key elements of urban ecosystems: Enhancement of biodiversity [249], capture of carbon [250], or increase of air quality [251]. (d) Promotion of leisure, recreation, and sport [252] (e) Fostering social relations [253,254] (f) Improving social care actions, especially for aging populations [255] (g) Positive health effects: Reduction of mortality rate [256], cardiovascular diseases [257] or mental health problems [258] Such greening policies have often concentrated on the provision (according to more or less ambitious schemes) of green areas in large sectors, usually in the central city, on the basis of three fundamental indicators: Park proximity, park acreage, and park quality [259]. Although the provision of green areas is a policy with a long tradition in both WE and NA, differentiated policy orientations can be observed [243,260]. However, in both cases, the studies coincide in highlighting the existence of strong social differences in access to green infrastructures.
In the American case, there is a better and more abundant supply of green areas in the case of middle-class suburbs, compared to the inner city [242,243]. However, beyond this general statement, the extensive existing academic literature coincides in indicating elements of race, education, and income in order to explain different degrees of access to green areas [261][262][263]. In this sense, some approaches have mostly focused on racial elements [261], while others on educational and income differences [263]. In this context, many cities have embarked on programs to increase the provision of urban green spaces in poorly endowed central areas with, in some cases, significant levels of socio-environmental degradation. These policies have led to the conversion of remnant urban lands and the reuse of spaces of obsolete or underused transport infrastructure [262]. There are specific studies on the impact of this type of actions in cities such as Atlanta [261], Detroit [245], New York [264], or Portland [265].
Those policies are the result of different factors, partially contradictory to each other as we can see in Figure 7. On the one hand, these are the products of social demands voiced by distinct urban movements, based on a generic demand for equity, under the form of environmental justice [243,262,266]. On the other hand, improvements in habitability, environmental quality, and social relations are often associated with processes of green gentrification [267]. Green gentrification has been observed in cities such as Atlanta [268], Chicago [269], New York [270], or Vancouver [271]. However, it was not confirmed in the case of Portland [272]. Moreover, green gentrification must be understood in the context of a strongly financialized urban model, where, as was seen in Section 2, real estate wealth is a central element in the actually existing forms of social structuring. In this sense, many of these developments involve significant costs, which often tend to be financed, at least partially, by private capital and have an effect on housing prices [273]. WE follows similar patterns to those observed in NA in a parallel socio-political context [260]. However, there are some differences to emphasize. Firstly, the affirmation of the schemes of green urbanism has the reference of more compact cities, therefore, what is really claimed is densification of quality. In this sense, the deregulated operation of real estate markets in WE sometimes produces density increases that lead to the reduction of green spaces [274]. Furthermore, from an urban morphology perspective, the provision of green areas in inner-city is not independent of greenbelts analyzed in Section 4.1, which are more developed in the European case. Greenbelts are particularly relevant from an ecological point of view as they make it possible to connect inland urban areas with adjacent rural areas, increasing the complexity of urban ecosystems. Finally, it should also be noted that there is great heterogeneity in Europe, not only among countries, but also among cities. In any case, historically, the provision of green areas has been, on average, higher in northern Europe [275,276].
Consequently, in the case of WE, there is a prevalence of especially contested processes because green areas must compete with other uses, such as facilities for educational, social and community purposes, alternative means of transport, etc. [244]. In general terms, the research conducted showed that, between 1990 and 2000, there was no significant expansion of green areas in the European Union as a whole, despite the growing strength of urban sustainability discourse. On the contrary, in the period between 2000 and 2006, there was a relevant increase in green areas in both northern and southern Europe [276].
In the context of quality densification, but also of increasing income differences and financialization, the growing provision of environmental services in certain central areas has also been often accompanied by processes of green gentrification. Such processes can be observed in cities both in northern (Berlin or Ghent) and southern Europe (Barcelona), sometimes making the conciliation of greening and environmental justice problematic [277][278][279].

Discussion and Preliminary Conclusions
The search for sustainable models has become one of the aims of public policies in recent years. Accordingly, several elements have been increasingly stressed. One of them has been the role of the local level in promoting specific forms of sustainable transition [280]. Another has been the relevance of cities in those processes, as it is not possible to conceive sustainability without the urban [281,282].
In this context, the academic literature agrees that urban sprawl is inextricably associated with major sustainability problems. In the previous lines, many of them have been seen. Purely economic aspects (the high hidden costs that are not internalized or the growing family debt, for example), but also social (such as the high levels of racial and social segregation or the effects on health) and environmental aspects (land use and degradation or disruption of water cycles) are included [119,132,140,144]. Therefore, it could be argued that urban sprawl is the outcome of production and consumption patterns with relevant hidden costs that are neither computed, nor allocated, nor charged. However, despite this, there are two main elements that make it impossible to think of urban sprawl reversion, at least in the short term.
On the one hand, after at least 70 years of urban sprawl, an important built capital has been generated both in NA and WE. In market economies, this capital must maintain its value in order not to provoke situations of economic recession [57]. Thus, it is currently unthinkable to massively abandon territorially dispersed suburbs, frequently formed by houses with a high real estate value, which are an important part of the wealthy people's lives [51]. Given these conditions, there is practically no social basis for a radical change in urban morphologies, beyond more or less rhetorical calls to "recover" cities' compactness.
On the other hand, there has been a particularly strong real estate component in the neoliberal-globalized model that is observed since the 1980s. Institutional changes, such as the growing relevance of entrepreneurialism, the strong processes of State's rescaling, or financialization, have led to a real estate hypertrophy [27,68,72]. The historical conjunction of these processes has caused not only suburban booming but also recurrent situations of real estate financial bubbles. In fact, the early 2000s saw the generation of what is possibly the largest bubble, at least in the last century. Thus, its bursting had strong effects on both NA (with California or Florida as prominent cases) and WE (see the cases of Spain or Ireland) [82,128,129]. The socio-economic consequences of this kind of bursts mean to avoid, as much as possible, risky situations. This excludes "radical" policies of containment or reversal of urban sprawl.
Additionally, the non-reversal and non-containment of urban sprawl processes is not only explained by macro-economic and macro-social reasons. Moreover, individuals in their decision-making autonomy are relevant. In this sense, despite the strong questioning that the irruption of the concept of urban sustainability has implied, broad groups of people continue to look for housing models based on low-density open spaces separated from the city centers. In other words, the city-garden ideal generated at the beginning of the last century continues to be a socially relevant benchmark. This capacity of "seduction" is particularly effective because it affects individuals with high purchasing power and access to credit. Consequently, it tends to be emulated by social groups with lower levels of income [36].
Moreover, some of the deficiencies initially associated with urban sprawl models have been attenuated through the years. For instance, the commercial, leisure, and productive facilities of many of suburban areas have grown significantly. In fact, in many cases, they are currently better than those observed in inner-city areas [111]. Moreover, areas with less pollution, higher water quality, etc. are increasingly valued. This is generating green gentrification processes that, in many cases, reinforce the diffuse suburban model [68]. In this context, there are academic positions that assume that the sum of these individual benefits could compensate for the costs that they may generate, especially if technologies are progressively improved (less polluting vehicles, cleaner energies, more efficient management of integral water cycles, new ways of organizing work, etc.). [19,22].
Nevertheless, on the other side, since the mid-seventies of the last century, a strong discourse has emerged, first of environmental protection, and later of sustainability. This discourse was not merely rhetorical but was associated with important transformations in the forms and ways of making cities [20,102]. The discourse of sustainability has had different impacts. On the one hand, it has transformed, in a certain sense, individual decision models by introducing a whole set of elements, such as greening, urban ecosystems, walkability, etc. [102]. This process, however, has not been generalized. In this context, a struggle can be observed between the old (the consumption of large extensions of territory on the basis of the massive use of the car and the imaginary of the garden-city of the early 20th century) and the new (the new approaches of green urbanism). This confrontation has social groups, political visions, and segments of capital behind them.
In addition, the sustainability discourse has had a strong impact on many public policies. This can be explained, in part, by the extension and generalization of the urban sprawl and its impacts. The extensive use of land, the loss of agricultural production spaces, the loss of spaces of landscape heritage value, the destruction of ecosystems, the extension of sealed soils, run-offs, intensification and congestion associated with mobility, inefficient uses of water and energy, health problems due to a lack of physical mobility, stress, air pollution, etc. are insistently listed by the academic literature. In this context, an increasing number of public interventions have tried if not to stop, at least to regulate and control some of these processes.
Thus, a large number of policies have been implemented, including some directed at containing urban sprawl and others at reducing its most damaging elements [196,197]. In the first case, previous preventive measures were required. In the second case, however, palliative accompanying measures were taken. The degree to which these policies have been implemented also varies greatly. In any case, as has been seen in the previous pages, a substantially greater application can be observed in WE. This can be explained by several factors.
On the one hand, there is, especially in some Northern European countries, a greater ecological awareness, which leads individuals and also public authorities to make other decisions. Furthermore, the implementation of sustainable policies has been favored by the morphological features of many European cities, which have traditionally been more compact, and have experienced lower levels of social segregation. Likewise, the application of sustainable policies has been reinforced by various institutional elements, such as a tradition of greater regulation and urban planning, or the existence of better and more extensive forms of public transport [20]. Finally, in the case of some cities of WE, sustainable urban policies have been associated with processes of intense monetary revaluation of some centers with attractive historical heritage, in which strong processes of gentrification and touristification can be observed [35]. On the other hand, in the context of NA, many of these policies have also been implemented, but in some cases, such as transport, their results have been modest and even disappointing. In this sense, the more dispersed character of cities in NA, as well as other elements of their social structure, is causing sustainable transition processes to take different forms than those observed in WE.
In any case, the urban policies to be applied cannot be generalized because the realities are diverse. In other words, urban sustainability must be considered, in any case, as a socio-environmental construction. However, for the reasons mentioned above, the socio-environmental constructions to be implemented, despite being important, cannot reverse or even contain the urban sprawl. In the face of this reality, there remains only the possibility of reducing its negative consequences as much as possible. In other words, this means making as sustainable as possible, what is, for the reasons mentioned above, structurally unsustainable. In other words, sustainability is a relevant element in the current morphology of cities, but not because it reduces or contains the urban sprawl but because it has a growing and paradoxical capacity for shaping it. Summarizing, the only available option is to try to "sustain what is unsustainable".
Given these parameters, a set of urban policies are explored which, without aspiring to radically modify urban phenomena, do try to give cities another face, from an approach which, in general terms, can be framed within ecological modernization [201][202][203]. Thus, following patterns of green urbanism, attempts have been made to limit urban expansion (through greenbelts), to use degraded areas for residential purposes (brownfields), to promote walkability, cycling and public transport (Transport-Oriented Developments), or to substantially increase infrastructures such as green zones [206,220,222,259]. In many cases, those policies have had a significant urban impact, contributing to the generation of new social demands about housing and habitability, and also new forms of business. Thus, the initiatives of ecological modernization result in generating the necessary socio-political space for the irruption of new rounds of real estate capital accumulation (in this case, more ecological) that are superimposed on the previous rounds [57]. In other words, sustainability becomes a basic argument to understand the forms that the processes of creative destruction of the built capital assume in the first decades of the 21st century.