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.

Photo: Penn State.

This 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 recent history.

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."

The 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.

The authors 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 surprises there.

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.

Better 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:

As 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.

Critics of smart growth efforts or rail and transit investments often wave off the potential gains from building differently by noting that so much of the current housing stock is of the sprawling, single-family home, auto-oriented sort. Convincing the people who currently live in such places to give that up for something different, they say, is sure to be an extremely difficult sell.

But that’s not the issue. No one is suggesting we rip down all of suburbia. Rather we, or at least I, am pointing out that between now and mid-century, the country will very nearly have to build itself all over again to accommodate population growth. In addition to the 100 million homes now in America, somewhere between 62 and 105 million more will be built.

The critical question is what the balance of that new construction will look like. The TRB report suggests that if 75 percent of this new construction is of a more compact variety, that emissions could be reduced 10 percent or more from the baseline scenario (and that is not taking into consideration the deployment of cleaner electricity generation and other potential sources of savings).

Ed Glaeser argued — and this is kind of hard to believe — that land use shifts from building high-speed rail between Dallas and Houston would not provide much in the way of benefits, since, he guessed, only 100,000 or so people in each city would move from the suburbs to the central city. But this entirely misses the point.

Houston and Dallas may each double their current housing stock between now and 2050. Where are those homes going to go, with what climate impacts? That’s the critical question.

Demographic shifts and changes in energy prices are sure to encourage some households that are currently living at low densities to move to more compact developments, and that’s a good thing. But that’s not the main reason to begin focusing on the significant available savings from smarter land use decisions.

The main reason is the growth that America will continue to face. It’s difficult to imagine that the nation can double its housing stock while building in a sprawling fashion without facing major environmental costs and economic difficulties. Land use patterns will need to change. And as this report documents, there will be considerable advantages to facilitating that change.