Healthy Development Measurement Toolkit

Indicator HH.1.h Residential density

Data Source

Applied Geographic Solutions, Inc. Spring 2007 Update: Current Year Estimates. Methodology available at: http://www.appliedgeographic.com/library.html.

Maps and tables prepared by City and County of San Francisco, Department of Public Health, Environmental Health Section using Geolytics software. Map data is presented at the level of the census tract, which was calculated by assigning census block data to census tracts based on spatial location. The map also includes planning neighborhood names, in the vicinity of their corresponding census tracts.

Table data is presented by planning neighborhood. While planning neighborhoods are larger geographic areas than census tracts, census tracts do not always lie completely within a planning neighborhood. SFDPH used ArcGIS software and a 'centroids within' methodology to convert census blocks to geographic mean center points. We then assigned census blocks to planning neighborhoods based on the spatial location of those geographic mean center points and calculated the planning neighborhood totals for the table.

Detailed information regarding census data, geographic units of analysis, their definitions, and their boundaries can be found in the HDMT at the following links:

http://www.thehdmt.org/etc/Geographic_Units_of_Analysis.pdf

http://www.thehdmt.org/data_map_methods.php

Explanation and Limitations

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).

Housing or residential density is one measure of urban sprawl. Urban sprawl is a term used to denote the continued growth of suburban neighborhoods which have sprawled from large cities into more rural farmlands, or "natural" areas. Urban sprawl can be associated with a number of negative environmental and public health issues. Two of the largest issues include the loss of natural habitat for flora and fauna or farmland, and the impact of increased motor vehicle travel. As distances increase between housing, work and daily necessities, there is an increase in cars, roads and parking spaces (resulting in more loss of natural- or farm-land), traffic, air and water pollution, and increased temperatures. All of which result in negative health implications, such as increased respiratory disease and vehicle collisions, and decreased social interactions, and physical activity.

San Francisco, at the tip of a peninsula, has a limited amount of land for development and therefore efficient use of space is critical to limit urban sprawl. High residential densities can allow for more housing units to be built on a given piece of land and can potentially lower the cost of construction and the cost of housing. Density and transportation needs also have a direct relationship. For example, in lower density areas, there is often limited public transit and individuals are more dependent on cars, increasing the household cost of transportation and increasing the need for parking in both residential and commercial areas.

Why is this a Community Health Indicator?

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 less sprawling counties.^a

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.

Notably, many individuals who do not reside within city limits in San Francisco on any given day (i.e., commuting employees, students, visitors and tourists). The San Francisco Injury Center estimates that in addition to the 469,000 vehicles registered to San Francisco residents and businesses, there are an estimated 435,000 vehicles that come in and out of San Francisco daily. In addition, a significant amount of time is also spent driving; the average U.S. resident spends 443 hours in a car each year. This represents time that could otherwise be spent in productive or leisure activity.

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.^b