Urban decay (also known as urban rot and urban blight) is the sociological process by which a previously functioning city, or part of a city, falls into disrepair and decrepitude. It may feature deindustrialization, depopulation or deurbanization, economic restructuring, abandoned buildings and infrastructure, high local unemployment, increased poverty, fragmented families, low overall living standards and quality of life, political disenfranchisement, crime, elevated levels of pollution, and a desolate cityscape, known as greyfield or urban prairie. Since the 1970s and 1980s, urban decay has been associated with Western cities, especially in North America and parts of Europe (mostly the United Kingdom and France). Since then, major structural changes in global economies, transportation, and government policy created the economic and then the social conditions resulting in urban decay.
The effects counter the development of most of Europe and North America; on other continents, urban decay is manifested in the peripheral slums at the outskirts of a metropolis, while the city center and the inner city retain high real estate values and sustain a steadily increasing populace. In contrast, North American and British cities often experience population flights to the suburbs and exurb commuter towns; often in the form of white flight. Another characteristic of urban decay is blight--the visual, psychological, and physical effects of living among empty lots, buildings and condemned houses.
Urban decay has no single cause; it results from combinations of inter-related socio-economic conditions--including the city's urban planning decisions, tight rent control, the poverty of the local populace, the construction of freeway roads and rail road lines that bypass--or run through--the area, depopulation by suburbanization of peripheral lands, real estate neighborhood redlining,, immigration restrictions, or even just heat cost.
During the Industrial Revolution, from the late eighteenth century to the early nineteenth century, rural people moved from the country to the cities for employment in manufacturing industry, thus causing the urban population boom. However, subsequent economic change left many cities economically vulnerable. Studies such as the Urban Task Force (DETR 1999), the Urban White Paper (DETR 2000), and a study of Scottish cities (2003) posit that areas suffering industrial decline--high unemployment, poverty, and a decaying physical environment (sometimes including contaminated land and obsolete infrastructure)--prove "highly resistant to improvement".
Changes in means of transport, from the public to the private--specifically, the private motor car--eliminated some of the cities' public transport service advantages, e.g., fixed-route buses and trains. In particular, at the end of World War II, many political decisions favored suburban development and encouraged suburbanization, by drawing city taxes from the cities to build new infrastructure for towns.
The manufacturing sector has been a base for the prosperity of major cities. When the industries have relocated outside of cities, some have experienced population loss with associated urban decay, and even riots. Cut backs on police and fire services may result, while lobbying for government funded housing may increase. Increased city taxes encourage residents to move out.
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Rent controls are often enacted due to public pressure and complaints regarding the cost of living. Proponents of rent controls argue that rent controls combat inflation, stabilize the economic characteristics of a city's population, prevent rent gouging, and improve the quality of housing. Economists have documented that rent control affects the supply and demand relationship in housing markets which can contribute to urban blight and does not provide the benefits its proponents advocate. Rent control contributes to urban blight by reducing new construction and investment in housing and deincentivizing maintenance. If a landlord's costs to perform maintenance consume too large a proportion of profit, that is revenue minus costs, from rent, the landlord will feel pressure to drastically reduce or eliminate maintenance entirely. This effect has been observed in New York City, a 2009 study by a lobbying firm found 29% of rent-controlled buildings were categorized as either deteriorated or dilapidated in contrast with 8% of non-rent-controlled housing .
Historically in the United States, the white middle class gradually left the cities for suburban areas due to African-American migration north toward cities after World War I. American cities often declare blighted status once determined that urban renewal strategies are the most appropriate means to encourage the private investment for reversing deteriorating downtown conditions.
Some historians differentiate between the first Great Migration (1910-1930), numbering about 1.6 million Black migrants who left mostly Southern rural areas to migrate to northern and midwestern industrial cities, and, after a lull during the Great Depression, a Second Great Migration (1940-1970), in which 5 million or more African-Americans moved, including many to California and various western cities.
Between 1910 and 1970, Blacks moved from 14 states of the South, especially Alabama, Louisiana, Mississippi and Texas to the other three cultural (and census-designated) regions of the United States. More townspeople with urban skills moved during the second migration. By the end of the Second Great Migration, African Americans had become an urbanized population. More than 80 percent lived in cities. A majority of 53 percent remained in the South, while 40 percent lived in the North and 7 percent in the West.
From the 1930s until 1977, African-Americans seeking borrowed capital for housing and businesses were discriminated against via the federal-government-legislated discriminatory lending practices for the Federal Housing Administration (FHA) via redlining. In 1977, the US Congress passed the Community Reinvestment Act, designed to encourage commercial banks and savings associations to help meet the needs of borrowers in all segments of their communities, including low- and moderate-income neighborhoods.
Later urban centers were drained further through the advent of mass car ownership, the marketing of suburbia as a location to move to, and the building of the Interstate Highway System. In North America this shift manifested itself in strip malls, suburban retail and employment centers, and very low-density housing estates. Large areas of many northern cities in the United States experienced population decreases and a degradation of urban areas.
Inner-city property values declined and economically disadvantaged populations moved in. In the U.S., the new inner-city poor were often African-Americans that migrated from the South in the 1920s and 1930s. As they moved into traditional white neighborhoods, ethnic frictions served to accelerate flight to the suburbs.
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Major cities like Glasgow, the towns of the South Wales valleys, and some of the major industrial cities like Manchester, Liverpool and Newcastle upon Tyne, along with East London, experienced population decline due to counterurbanization, with large areas of 19th-century housing experiencing market price collapse. Some seaside resort towns have also experienced urban decay towards the end of the 20th century, due to the popularity of package holidays to the continent.
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Large French cities are often surrounded by areas of urban decay. While city centers tend to be occupied mainly by upper-class residents, cities are often surrounded by public housing developments, with many tenants being of North African origin (from former French colonies Morocco, Algeria and Tunisia), and recent immigrants.
From the 50s to the 70s, publicly funded housing projects resulted in large areas of mid to high-rise buildings. These modern "grands ensembles" were welcomed at the time, as they replaced shanty towns and raised living standards, but these areas were heavily affected by economic depression in the 80s.
The banlieues of large cities like Lyon, especially the northern Parisian banlieues, are severely criticized and forgotten by the country's territorial spatial planning administration. They have been ostracised ever since the French Commune government of 1871, considered as "lawless" or "outside the law", even "outside the Republic", as opposed to "deep France" or "authentic France", which is associated with the countryside.
In November 2005, the French suburbs were the scene of severe riots sparked by the accidental electrocution of two teenagers in the northern suburbs of Paris, and fueled in part by the substandard living conditions in these areas. Many deprived suburbs of French cities were suddenly the scenes of clashes between youngsters and the police, with violence and numerous car burnings resulting in huge media coverage.
Today the situation remains generally unchanged; however, there is a level of disparity. Some areas are experiencing increased drug trafficking, while some northern suburbs of Paris and areas like Vaulx-en-Velin are undergoing refurbishment and re-development.
Some previously mono-industrial towns in France are experiencing increasing crime, decay, and decreasing population. The issue remains a divisive issue in French public politics.
In Italy, one of the most well known case of urban decay is represented by the Vele di Scampia, a large public housing estate built between 1962 and 1975 in the Scampia neighbourhood of Naples. The idea behind the project was to provide a huge urban housing project, where hundreds of families could socialise and create a community. The design included a public transportation rail station, and a large park area between the two buildings. The planners wanted to create a small city model with large parks, playing fields, and other facilities.
However, various events led to tremendous urban decay inside this project and in the surrounding areas. It all started with the 1980 earthquake in Irpinia, which led many families, left homeless, to squat the flats inside the Vele. Things were made worse by the total lack of police presence, resulting in a deep bundling of the Camorra inside the area, which now controls drug trafficking, illegal street racing, gangs, and fencing operations.
In South Africa, the most prominent urban decay case is Hillbrow, an inner city neighborhood of Johannesburg. A formerly affluent neighborhood, at the end of apartheid in 1994 many middle-class white residents moved out, and were replaced by mainly low-income workers and unemployed people, including many refugees and illegal immigrants from neighboring countries. Many businesses that operated in the area followed their customers to the suburbs, and some apartment buildings were "hi-jacked" by gangs who collected rentals from residents but failed to pay the utility bills, leading to termination of municipal services and a refusal by the legal owners to invest in maintenance or cleaning. Occupied today by low-income residents and immigrants, and heavily over-crowded, the proliferation of crime, drugs, illegal businesses and decay of properties became prevalent.
Many east German towns such as Hoyerswerda face or are facing significant population loss and urban shrinkage since the reunification of Germany. Hoyerswerda's population has dropped about 40% since its peak and there is a significant lack of teenagers and twenty-somethings due to the declining birthrates during the uncertainty of reunification. Part of the blight in east Germany is due to the construction and preservation practices of the socialist government under the German Democratic Republic. To fill the housing needs, the East German state quickly built many prefabricated apartment buildings. In addition, historic preservation of pre-war buildings varied; in some cases, the rubble of buildings destroyed by the war were simply left there while in other cases the debris was removed and an empty lot remained. Other standing historical structures were left to decay in the early GDR as they did not represent the socialist ideals of the country. 
The main responses to urban decay have been through positive public intervention and policy, through a plethora of initiatives, funding streams, and agencies, using the principles of New Urbanism (or through Urban Renaissance, its UK/European equivalent). Gentrification has also had a significant effect, and remains the primary means of a natural remedy.
In the United States, early government policies included "urban renewal" and building of large-scale housing projects for the poor. Urban renewal demolished entire neighborhoods in many inner cities; in many ways, it was a cause of urban decay rather than a remedy. These government efforts are now thought by many to have been misguided.
For multiple reasons including increased demand for urban amenities, some cities have rebounded from these policy mistakes. Meanwhile, some of the inner suburbs built in the 1950s and 60s are beginning the process of decay, as those who are living in the inner city are pushed out due to gentrification.
In Western Europe, where undeveloped land is scarce and urban areas are generally recognised as the drivers of the new information and service economies, urban renewal has become an industry in itself, with hundreds of agencies and charities set up to tackle the issue. European cities have the benefit of historical organic development patterns already concurrent to the New Urbanist model, and although derelict, most cities have attractive historical quarters and buildings ripe for redevelopment.
In the inner-city estates and suburban cités, the solution is often more drastic, with 1960s and 70s state housing projects being totally demolished and rebuilt in a more traditional European urban style, with a mix of housing types, sizes, prices, and tenures, as well as a mix of other uses such as retail or commercial. One of the best examples of this is in Hulme, Manchester, which was cleared of 19th-century housing in the 1950s to make way for a large estate of high-rise flats. During the 1990s, it was cleared again to make way for new development built along new urbanist lines.
The construction of the Gowanus Parkway, laying a concrete slab on top of lively, bustling Third Avenue, buried the avenue in shadow, and when the parkway was completed, the avenue was cast forever into darkness and gloom, and its bustle and life were forever gone.
Recommended restrictions should include provision for the following . . . Prohibition of the occupancy of properties except by the race for which they are intended . . . Schools should be appropriate to the needs of the new community and they should not be attended in large numbers by inharmonious racial groups. Federal Housing Administration, Underwriting Manual: Underwriting and Valuation Procedure Under Title II of the National Housing Act With Revisions to February, 1938 (Washington, D.C.), Part II, Section 9, Rating of Location. "Archived copy". Archived from the original on 29 December 2008. Retrieved 2007.CS1 maint: archived copy as title (link)
Ninety-five percent of cities with populations greater than 100,000 people in the U.S. lost population between 1970 and 1975.
"(In Chicago) while whites were among those uprooted in Hyde Park and on the North and West Sides, urban renewal in this context too often meant, as contemporaries noted, "***** removal". Between 1948 and 1963 alone, some 50,000 families (averaging 3.3 members) and 18,000 individuals were displaced."