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.
