Concerns about vehicle use and carbon dioxide emission numbers and their possible implications for climate change issues are having an impact on the Greening of large residential developments, as reported in a late 2007study by the National Association of Home Builders (NAHB).
Green development and building is the practice of increasing the efficiency of homes and other buildings as developers and builders utilize energy, water, and materials. Green development and building also enhances overall building impacts on human health and the environment through better siting, design, construction, operation, maintenance, and removal; the complete building life cycle.
The compactness of a residential subdivision is defined as how close together the homes are built.
According to the Energy Information Administration, carbon dioxide has the largest impact on global warming of any of the monitored greenhouse gases. About 33 percent of total U.S. greenhouse gas emissions are generated from the transportation sector, and among these, carbon dioxide emissions represents 95 percent of the greenhouse gas emissions from mobile transportation sources.
These concerns have prompted states such as California, Massachusetts and Washington to require that real estate developers quantify greenhouse gas emissions from vehicle-use in large residential projects they are planning. However, these states typically do not provide any guidance on how to perform the calculations, and there is currently no well-established, verifiable method for estimating carbon dioxide emissions or vehicle miles traveled (VMT) for households in a particular development.
The NAHB study estimates household gasoline consumption and associated carbon dioxide emissions using the 2001 National Household Travel Survey (NHTS), which is conducted by the Federal Highway Administration (FHWA) within the Department of Transportation. Standard statistical methods were used to estimate gasoline consumption as a function of the geographic and household characteristics available in the NHTS data. Housing units per block group was used as a standard for the compactness of a residential subdivision (how close together the homes are built).
1.56 to 4.69 housing units per acre translates into about 0.21 to 0.64 acres per home, which is a fairly typical lot size for new construction. About 31 percent of single family detached homes completed in 2006 were built on lots falling into this size range. Nearly 80 percent are on a lot that was 0.64 acre or smaller. However, lot sizes will generally be smaller than acres per housing unit measured over a block group or subdivision. This is because a subdivision will also include roads and other public spaces.
Because carbon dioxide emissions are computed as a simple ratio of gasoline consumption, carbon dioxide emissions also decline as the subdivision becomes more compact.
According to the study, it was not possible to control for every conceivable factor with a possible impact on gasoline consumption in a statistical model based on the NHTS data. Characteristics such as nearby concentrated employment centers and their proximity to a particular subdivision, for example, would impact overall gasoline consumption.
The research shows little relationship between efficiency of vehicles owned, such as hybrids, and subdivision compactness. The exception is that residents in the least dense subdivisions tend to own less efficient vehicles. On the other hand, there is a clear relationship between subdivision compactness and the average speeds vehicles are driven. As the subdivision becomes more compact, the estimated results show that vehicles are driven fewer miles, but they tend to be driven slower which is a a less efficient speed. However, the congestion effect (less efficient driving speeds) is not strong enough to completely offset the effect of reduced emissions.
So, on balance, households in more compact developments still tend to use less gasoline and thus generate fewer carbon dioxide emissions.