Density calculations: Urban-area population and residential density are based on population counts for city and suburban blocks, obtained from the US Census Bureau (for 1990 and 2000) and the Census of Canada (for 1986, 1991, 1996, 2001, and 2006). For each urban area, Sightline first estimated the population at each location in a 30-by-30 meter grid, based on the populations of the census blocks in which those grid cells are located. Then, the population density of each grid location was calculated as the density of the smallest circle that contained at least 500 residents -- a rough proxy for a neighborhood. Grid locations are then assigned to a neighborhood density "class": rural, low-density suburban, medium-density suburban, compact/transit-oriented, or compact/pedestrian-oriented.
Definitions: Sightline defines rural areas as places with a local population density lower than 1 person per acre. Compact neighborhoods have at least 12 residents per acre, a modest target for compact neighborhoods; most of the older "streetcar neighborhoods" that were built before World War II have at least that many residents per acre. Pedestrian-oriented neighborhoods have densities of at least 40 residents per acre.
In some respects, these thresholds are arbitrary. Some suburban areas with large-lot zoning are classified as rural under this definition; and some neighborhoods with 12 or more people per acre have poor access to transit and services. Indeed, some researchers have suggested that local densities above 12 people per acre may be required to support transit cost-effectively. Still, the 12-person-per-acre marker provides a convenient, if not absolute, reference point for transit-oriented neighborhoods, as a robust body of international research suggests that below 8 to 12 people per acre, a city’s land use patterns are inherently car dependent. See Peter Newman and Jeffrey Kenworthy, Sustainability and Cities: Overcoming Automobile Dependence (Washington, DC: Island Press, 1999), and Jeffrey R. Kenworthy et al., An International Sourcebook of Automobile Dependence in Cities, 1960–1990 (Boulder: University Press of Colorado, 1999).
See also: "Slowing Down: Greater Vancouver's Smart-Growth Leadership Slips," Sightline Institute, May 2008, www.sightline.org/publications/reports/slowing-down; and "The Portland Exception: A Comparison of Sprawl, Smart Growth, and Rural Land Loss in 15 US Cities," Sightline Institute (formerly Northwest Environment Watch), October 25, 2004, www.sightline.org/research/sprawl/res_pubs/portland04.
(Sources updated January 2009.)