Experience with case studies has made it clear to many urban
planners and environmentalists that to maximize the benefits of transit
investments, and to slow growth in traffic congestion, vehicle miles
traveled (VMT), and carbon emissions, you have to focus on land use.
knowledge has begun working its way into the policymaking world, to the
extent that local and state legislatures are beginning to craft rules
that explicitly factor the carbon impact of land use effects into
decisions about new development and infrastructure construction. In a
few years time, the federal government may follow.
But there's not as much in the way of hard studies of the
effects of land use as we might like -- mainly because it's been a
non-issue, so far as most of the country is concerned, for much of
Aiming to address this (and acting under a
congressional mandate), the Transportation Research Board recently
completed a study that has now resulted in a very large report: "Driving and the Built Environment: The Effects of Compact Development on Motorized Travel, Energy Use, and CO Emissions."
report is actually five mini-papers, and at nearly 200 pages long it
makes for a lot of reading. But the findings reported in the
introduction give an idea of what it's all about.
conclude that compact development is likely to reduce VMT: "The effects
of compact, mixed-use development on VMT are likely to be enhanced when
this strategy is combined with other policy measures that make
alternatives to driving relatively more convenient and affordable." No
Finding No. 2 is: "The literature suggests
that doubling residential density across a metropolitan area might
lower household VMT by about 5 to 12 percent, and perhaps by as much as
25 percent, if coupled with higher employment concentrations,
significant public transit improvements, mixed uses, and other
supportive demand management measures."
They note that were
you to move the residents of Atlanta to an area built like Boston,
you'd lower the Atlantans' VMT per household by perhaps 25 percent.
land use results in reductions in energy use and carbon emissions, the
authors report, from both direct and indirect causes. (Direct causes
would be a reduction in VMT; indirect include things like longer
vehicle lifetimes from reduced use and the greater efficiency of
smaller or multi-family housing units.)
But one of the crucial pieces of data included in the report is this:
many as 57 million new housing units are projected to accommodate
population growth and replacement housing needs by 2030, growing to
between 62 and 105 million units by 2050 - a substantial net addition
to the housing stock of 105.2 million in 2000.