Indicator HH.1.g Residential density
Why Is This An Indicator Of Health and Sustainability?
Urban sprawl is a term that has been used to describe a “pattern of uncontrolled development around the periphery of a city.”a Urban sprawl development is generally characterized by the reliance on automobiles to get from one place to the next and historically has not been suitable for more active transportation options such as walking, biking or mass transit. Negative health implications have been associated with urban sprawl. Research has found that people living in counties with sprawling development are less likely to walk, weigh more and are more likely to suffer from high blood pressure than those living in more compact counties.b Walking for utilitarian purposes, such as going to work, shopping, and school, is more prevalent in dense, mixed-use neighborhoods which supports a more active lifestyle for residents, thereby helping to prevent obesity, cardiovascular disease and other chronic illnesses associated with physical inactivity.c Additionally, metropolitan areas with lower degrees of urban sprawl have been found to have an increased prevalence of cycling.d
People in sprawling areas drive more. Vehicle miles traveled are directly proportional to air pollution and greenhouse gas emissions. Air pollutants, including ozone and particulate matter are causal factors for cardiovascular mortality and respiratory disease and illness. Greenhouse gases contribute to climate change and may increase heat-related illness and death, health effects related to extreme weather events, health effects related to air pollution, water-borne and food-borne diseases and vector-borne and rodent-borne disease. Areas with high levels of vehicle miles traveled per capita also tend to have higher accident and injury rates.
The impact of increased highways, roads and parking lots can also take away from trees and green space which improve the physical environment by removing air pollution from the air and mitigating the urban heat island effect produced by concrete and glass. Water resources are also negatively impacted by urban sprawl. As sprawling communities increase impermeable surfaces, such as highways and roads, the more difficult protecting the quantity and quality of water supplies becomes. Watersheds with as little as ten percent impervious surfaces can experience impaired water resources. Water resources are impacted by the runoff; emissions generated by travel; use of chemicals in landscaping; and construction activities.e
To quantitatively define a city’s level of sprawl, researchers have developed the metropolitan sprawl index, which uses 22 variables to define four factors of sprawl. The four defining factors of the sprawl index include residential density; neighborhood mix of homes, jobs and services; strength of activity centers and downtowns; and accessibility of the street network.f Higher density neighborhoods help support the availability of local retail options, schools and transit services.
Density can be calculated in a number of ways. Population density is calculated by dividing the total population within a census tract by the total acreage in that tract. Residential density can be calculated by 1) dividing the total number of housing units by the total acres zoned for residential development (thus not zoned for commercial or industrial development) within that tract or 2) dividing the total number of housing units by total acres within the census tract. For the purposes of this indicator, residential density is calculated by dividing the total number of housing units within a census tract by the total number of acres in the census tract (the U.S. Census terms this housing density).
Certain areas of San Francisco that have lower residential density do not necessarily indicate urban sprawl. Residential density as low as six or seven houses per acre can still support the existence of local services, retail and transit that help promote mix land use and decrease reliance on driving.d Many areas of low residential density in the city as shown in the map represent areas that are preserved as park space, natural habitat or are largely industrial, and therefore do not have high population densities.
Resnik DB. 2010. Urban Sprawl, Smart Growth, and Deliberative Democracy. Am J Public Health. 100(10);1852-1856.
McCann B. and Ewing R. 2003. Measuring the health effects of sprawl: A national analysis of physical activity, obesity and chronic disease. Smart Growth America, Surface Transportation Policy Project.
Ewing, R, Schmed, T, Killingsworth, R, et al. 2003. Relationship Between Urban Sprawl and Physical Activity, Obesity, and Morbidity. Am J Health Promot. 18(1):47-57.
Rashad, I. 2009. Associations of Cycling With Urban Sprawl and the Gasoline Price. Am J Health Promot. 24(1): 27-36.
United States Environmental Protection Agency (EPA). Protecting water resources with smart growth. Last accessed online August 30, 2007 from http://www.epa.gov/smartgrowth/pdf/waterresources_with_sg.pdf
Ewing R, Pendall R, Chen D. 2002. Measuring Sprawl and Its Impacts. Smart Growth America. Available at : http://www.smartgrowthamerica.org/documents/MeasuringSprawl.PDF