Dueling headlines cropped up in my newsreader today:
> More People, Less Driving: The Imperative of Curbing Sprawl (Smart Growth America)
> Forget Curbing Suburban Sprawl: Building denser cities would do little to reduce CO2 emissions, a new NAS report concludes (MIT Technology Review)
Both articles refer to the same study, conducted by a blue-ribbon panel for the U.S. Department of Energy. And both are pretty much right: the study does affirm the link between sprawl and carbon emissions, but it also strongly suggests that attacking sprawl is a tough way to curb energy use.
Despite the common-sense link between density and driving, establishing cause and effect in a rigorous manner remains difficult. The sheer number of variables tend to overwhelm the analysis. Nevertheless, empirical evidence does support a link between land use and vehicle miles traveled (VMT). The MIT Technology Review summarizes the study’s conclusion thusly:
> Even if 75 percent of all new and replacement housing in America were built at twice the density of current new developments, and those living in the newly constructed housing drove 25 percent less as a result, CO2 emissions from personal travel would decline nationwide by only 8 to 11 percent by 2050, according to the study. If just 25 percent of housing units were developed at such densities and residents drove only 12 percent less as a result, CO2 emissions would be reduced by less than 2 percent by 2050.
In other words, an enormous shift in land development trends yields a roughly 10% cut in emissions over 40 years. A more modest shift in land development trends yields a fairly anemic drop in emissions over 40 years. Vehicle fuel efficiency provides a much bigger lever for lowering emissions from transportation. This is what I was getting at the other day when I said, “If your car runs on electricity, and your electricity comes from the sun, and your McMansion is built to the Passive House standard, then your suburban lifestyle is suddenly looking a lot more benign.”
All that being the case, the study sensibly recommends a variety of policies to promote compact, mixed-use development. There are a number of good reasons to pursue smart land use policies, even if attacking sprawl isn’t likely to save us from climate change in the near- or even medium-term.
The first is that many anti-sprawl measures are just good policy in their own right. Carbon taxes, gas taxes, transit development, better zoning laws, etc., all make sense regardless of their immediate payback in reduced gasoline consumption. And, of course, sprawl has environmental impacts beyond just VMT.
The second is that we’re stuck with our built environment for a very long time, so if we want the future to look different than the past, we had better get started making some changes. As Ryan Avent reminds us, “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.” Examples such as Portland show that it is possible to buck the dominant trend in development. But it takes a few decades for the results to really show.