Urban sprawl is the extension of low-density residential, commercial, and industrial development into areas beyond a city's boundaries that occurs in an unplanned or uncoordinated manner. It is generally characterized by:
- low-density development that is dispersed and situated on large lots (sometimes greater than one acre)
- geographic separation of essential places such as work, home, school, and shopping
- high dependence on automobiles for travel
- increased impervious surface area due to pavement, which interferes with groundwater recharge.
- habitat fragmentation and degradation
Urban sprawl combines low-density housing and single-use development (housing separated from shopping). This leads to fragmentation of habitats, increases the average travel distances for daily trips, and hinders a shift toward less energy-intensive transportation modes.
The sprawling nature of cities is critically important because of the major impacts that are evident in increased energy, land and soil consumption. These impacts threaten both the natural and rural environments, raising greenhouse gas emissions that cause climate change, and elevating air and noise pollution to levels often exceeding the agreed human safety limits. Thus, urban sprawl produces many adverse impacts that have direct effects on the quality of life.
If communities are not walkable or bikeable, people are dependent upon automobiles. As a result, populations become more sedentary. Residents of sprawling communities are less likely to walk during leisure time and obesity as well as associated health issues become more prevalent. A sedentary lifestyle increases the risk of overall mortality, cardiovascular disease, and some types of cancer. The effect of low physical fitness is comparable to that of hypertension, high cholesterol and diabetes.
Consumption of Energy
A consequence of the increasing consumption of land and reductions in population densities as cities sprawl is the growing consumption of energy. Generally, compact urban developments with higher population densities are more energy efficient. Evidence from 17 cities around the world shows a consistent link between population density and energy consumption (Figure below). Specifically, high energy consumption rates are associated with the lower population densities of sprawling environments.
Transportation-related energy consumption in cities depends on a variety of factors including the nature of the rail and road networks, the extent of the development of mass transportation systems, and the split between public and private transport. Evidence shows that there is a significant increase in travel-related energy consumption in cities as population densities fall. Essentially, the sprawling city is dominated by relatively energy inefficient car use, as the car is frequently the only practical alternative to more energy-efficient, but typically inadequate, public transportation systems. Increased transport-related energy consumption is in turn leading to an increase in the emission of CO2 to the atmosphere. Urban sprawl therefore poses significant threats to the commitments to reduce GHG gas emissions.
Using fossil fuels also results in the emission of other gases and particulates that degrade air quality (note that commuters generate emissions of air pollution, which lowers the ambient quality of the air in areas they pass through and causes health problems for other people). Longer transportation distances intensify traffic congestion, resulting in lost productivity, and increase the need for more extensive infrastructure (such as more highways) that negatively impact the environment by increasing the amount of impervious cover and by requiring more natural resources. Finally, traffic congestion and air pollution from driving contribute to an estimated 900,000 fatalities per year worldwide.
Natural and Protected areas
The impact of urban sprawl on natural and protected areas is exacerbated by the increased proximity and accessibility of urban activities to natural areas, imposing stress on ecosystems and species through noise and air pollution. These are known as edge effects and the urban edge which is the boundary between development and natural ecosystems is increasingly impacted by sprawling development. Other impacts include the loss of agricultural and natural land as well as the fragmentation of forests (Figure below), wetlands and other habitats. Urban land fragmentation can lead to the disruption of migration corridors for wildlife species, isolate these populations and reduce natural habitats to the extent that the minimum area required for the viability of species populations is no longer maintained.
From a social perspective urban sprawl generates greater segregation of residential development according to income. Consequently, it can exacerbate urban social and economic divisions. The socio‑economic character of suburban and peripheral areas is typified by middle and upper income families with children, who have the necessary mobility and lifestyle to enable them to function effectively in these localities. However, the suburban experience for other groups, including the young and old, who lack mobility and resources can be very different and can reduce social interaction. Furthermore, large segments of urban society are excluded from living in such areas.
On the social sustainability side, we can look at social capital otherwise defined as the “connectedness” of a group built through behaviors such as social networking and civic engagement. Greater social capital has been associated with healthier behaviors, better self-rated health, and less negative results such as heart disease. However, social capital has been diminishing over time. Proposed causes include long commute times observed in sprawling metropolitan areas, as well as the lack of mixed-use spaces that would allow for greater social and physical activity.