How Urbanism Slows Global Warming, Part 2

In a previous posting, I scratched the surface in describing how traditional, mixed-use urban neighborhoods help slow global warming. I provided a link to a slideshow by Hank Dittmar, a great map on cities and CO2 production from the Center for Neighborhood Technologies, and promised to provide summaries of some research on the topic, much of it detailed in the LEED-ND Report on Public Health and the Built Environment.

So here's that summary:
The Role of Compact, Connected, Mixed-Use Neighborhoods: What the Research Says

Transportation-related benefits: Studies show people drive less as the density and complexity of their neighborhoods increase, thereby reducing their contribution to greenhouse gas accumulation. A 1990 study by John Holtzclaw used odometer readings to study driving habits in various urban and suburban San Francisco neighborhoods and concluded household driving miles (VMTs) dropped 20-30 percent every time density doubled. At the extremes, the Nob Hill area was found to have 32 times greater household density, much better public transit and 200 times more local shopping (retail and service employees per acre) than San Ramon, while only about one-fourth the household auto ownership and VMT.

In a peer-reviewed follow-up study published in Transportation Planning and Technology in 2002 that used odometer readings recorded at annual auto-emission test to compare driving patterns across metro areas, Holtzclaw, Robert Clear, Hank Dittmar and others showed that doubling of density in Chicago, Los Angeles, and San Francisco produced VMT reductions of 32%, 35%, and 43% respectively. (LEED-ND report, p. 17.) It's important to consider that density alone does not lead to VMT reductions. In these regression studies, it tends to serve as a reliable proxy for the other conditions that accompany it in traditional urban neighborhoods — a connected grid of walkable streets, a mix of uses, reliable transit service.

Chart from Location Efficiency, Holtzclaw et al, Transporation Planning and Technol., 2002)

Here’s one way to understand these numbers: In moving from a typical exurban neighborhood with 2 units per acre to a neighborhood of townhouse homes and and loft apartments like the one shown at right (34 units per acre), a household would expect the amount of driving it does to be about one-quarter what it formerly was. (Image: Chatham Square, Alexandria, VA. Credit: Boris Feldblyum for Lessard Group, Inc.). With a shift to other modes of transportation (transit, walking, biking), transportation-related emissions decline along with VMTs, although frequency of trips needs to be considered along with total miles driven since proportionally more emissions are generated starting a "cold" car than driving a warm one. (Updated 7-21-07).

Empirical studies have shown that neighborhoods with retail services within walking distance of houses have higher levels of non-motorized trips than do purely residential areas. One such study in King County, Washington found that the average distance per trip driven by residents of mixed-use neighborhoods was half that of people in single-use areas. And residents of mixed-use neighborhoods took non-motorized trips 12.9% of the time, compared to 3.9% of the time for those in single-use communities. (LEED-ND report, p. 21)

Heating-cooling related benefits: Whereas single-family homes predominate in sprawling subdivisions, urban neighborhoods have a much greater variety of housing types, including townhouses and apartment buildings. These urban forms are more efficient to heat and cool, leading on average to reduced building-generated emissions. According to a study by Michael Phillips and Robert Gnaizda published in CoEvolution Quarterly in 1980, a typical apartment in a building near downtown San Francisco used just 20 percent of the heating fuel consumed by a new tract house in sprawling Davis down the road. Of course, green techniques can improve building performance in both locations.

A report prepared in preparation for New York Mayor Michael Bloomberg’s PlaNYC recognized that density, transit-use and building types play a powerful role in making city residents low producers of greenhouse gas emissions. While the average person U.S. is responsible for generating 24.5 metric tons of these emissions per year, residents of New York account for just 7.1 metric tons per person, below even San Francisco, at 11.2 metric tons. According to the New York Times, “The report said that this is because less energy is needed to heat, light, cool and fuel buildings in the city because they are more densely packed and homes are below average in size. In addition, the city’s public transit system allows fewer New Yorkers to drive.”