Urban sprawl is the outward expansion of a town or city into previously undeveloped, agricultural, rural, or environmentally sensitive land, usually at relatively low population density. It is different from urban growth in general because the defining issue is the spatial pattern and efficiency of expansion rather than simply an increase in population. A city can grow through compact development, infill, redevelopment, or vertical construction without sprawling substantially.
Sprawl commonly produces extensive residential districts containing detached or semi detached houses, separated commercial areas, industrial estates, large roads, parking areas, and other land intensive uses. UN Habitat describes sprawl as rapid geographic growth over undeveloped land, commonly involving low density housing, remote residential areas, extensive roads, limited walking and cycling infrastructure, and dependence on private vehicles.
One major cause of urban sprawl is rising demand for housing combined with the availability of comparatively inexpensive land at the urban fringe. When land close to an established centre becomes expensive or scarce, households and developers may seek larger plots farther from the centre. Developers can construct housing estates on peripheral agricultural or undeveloped land where parcels can be assembled relatively easily.
Lower land prices can permit larger homes, gardens, driveways and lower building densities, making peripheral development attractive to some households. The process can become self reinforcing because new housing increases the demand for shops, schools, roads and other infrastructure, which then makes additional peripheral land more accessible and attractive for development. UN Habitat notes that pressure on land prices can contribute both to peripheral informal occupation and to leapfrog development associated with sprawl.
Transport technology and infrastructure are particularly important causes. Widespread private car ownership reduces the practical importance of living close to workplaces, shops, schools and services because households can travel longer distances relatively quickly. Roads, motorways, bypasses and interchanges can open previously inaccessible peripheral land to development. Once a major road is constructed, land near junctions may become commercially valuable, encouraging warehouses, retail parks, offices and housing.
The resulting development can create further traffic, which may produce pressure for still more road construction and further outward expansion. UN Habitat identifies automobile use as one of the major forces behind outward urban expansion, particularly when it is combined with weak planning and land speculation. Sprawl therefore involves not merely housing preferences but the interaction between transport systems, land markets and planning decisions.
Planning systems can also produce or intensify sprawl. Zoning may separate residential, commercial, industrial and institutional activities into different districts, increasing the distance between everyday destinations. Minimum plot sizes, minimum parking requirements, restrictions on apartment construction, low density zoning and development rules that favour detached housing can reduce the amount of housing that can be built within established urban areas.
If development is easier on peripheral greenfield sites than through redevelopment or infill, developers have an economic incentive to expand outwards. Fragmented governance can make this more difficult because a metropolitan area may contain many local authorities whose planning decisions affect one another. A municipality may permit development on its edge while neighbouring authorities experience the additional traffic and infrastructure costs. UN Habitat has linked inappropriate planning, weak development control and poorly coordinated expansion with inefficient land consumption.
Population growth can contribute to sprawl, but population growth alone does not automatically create it. When a growing population is accommodated through compact apartments, urban infill, mixed use development and redevelopment of vacant land, the physical footprint of a city may expand relatively slowly. Conversely, a stable or slowly growing population can occupy substantially more land if households become more dispersed. Household size also matters because demographic change can increase the number of households faster than population growth.
Rising incomes may increase demand for larger dwellings, gardens, private parking and suburban amenities, while changes in household composition can increase the number of separate homes required. These factors interact with land prices and transport access, so the same population increase can produce very different spatial outcomes depending on the housing and planning system.
Land speculation is another important mechanism. Investors may purchase undeveloped land because they expect future roads, utilities, population growth or planning permissions to increase its value. Anticipated development can therefore influence land prices before construction begins. Developers may also assemble large peripheral parcels because they can be easier to develop than fragmented inner city sites containing existing buildings, complicated ownership patterns or contaminated land. Public infrastructure can unintentionally increase private land values by making remote locations accessible and serviced.
Where planning authorities allow development beyond the existing built up area without requiring developers to bear its full infrastructure and environmental costs, the market price of peripheral land may not reflect its wider social costs. The resulting pattern can encourage leapfrog development, in which new construction appears beyond existing built up areas while undeveloped land remains between the old city and the new development.
Consumer preferences can contribute to sprawl, although preferences themselves are shaped by economic and institutional conditions. Some households prefer suburban locations because they offer larger dwellings, private gardens, quieter streets, perceived safety, schools, parking and access to open space. Retail development can reinforce these preferences through shopping centres, retail parks and large supermarkets designed around automobile access. Employment decentralisation can also reduce the need to commute into the historic central business district.
When offices, logistics facilities, universities, hospitals and entertainment venues move or develop outside the traditional centre, residential growth may follow. The resulting metropolitan structure can become polycentric rather than centred on one downtown, although decentralisation and sprawl are not identical. A well planned network of compact suburban centres can support relatively efficient urban form, whereas dispersed development connected mainly by roads tends to consume more land and increase travel distances.
The consequences help explain why the causes of sprawl are important. Low density outward growth generally requires longer roads, utility networks, drainage systems and service routes per resident than compact development. Longer journeys can increase vehicle kilometres travelled, congestion, fuel consumption and transport emissions, while extensive impermeable surfaces can increase stormwater runoff and reduce groundwater infiltration. Conversion of farmland and natural habitats can fragment ecosystems and reduce open land.
Social effects can include greater dependence on car ownership, reduced accessibility for people unable to drive, and differences in access to employment and services. However, sprawl is not caused by a single factor and its severity varies greatly between cities. Policies such as compact development, mixed land uses, public transport investment, brownfield redevelopment, urban growth boundaries, coordinated metropolitan planning and infrastructure pricing can alter the incentives that produce dispersed expansion. UN Habitat specifically associates sprawling development with inefficient land use, environmental impacts and reduced urban sustainability.
