It was just a fleeting moment amid the hours of presidential debate that have taken place through this longest of election cycles, but it nonetheless warmed my heart. No-longer-a-candidate Bill Richardson, in response to a question on climate policy, said of the fight against climate change:

It’s going to take a transportation policy that doesn’t just build more highways. We have to have commuter rail, light rail, open spaces. We’ve got to have land-use policies where we improve people’s quality of life.

The remark was all but ignored by the Democratic front-runners, and was greeted by pundits with praise or disdain, depending upon their ideological stripe, before being once again set aside in favor of discussion on sexier issue areas.

But Richardson had hit upon a truly pressing matter, one which deserves the attention of federal policy makers. Transportation accounts for a third of all carbon dioxide emissions in this country. Moreover, concerns about gas prices, congestion, housing costs, and other related urban ills loom large in the lives of Americans, if not necessarily in political debates. We should be having a discussion about the way in which we build and grow our cities, the costs of our current approach, and what the federal government can do to fix what’s broken.

The great shift towards sprawl began early last century with the growth of streetcar suburbs and the rise of personal automobile ownership, but it accelerated significantly during the massive road-construction boom of the 1950s and 1960s. At that time, outward migration was primarily driven by shifts in transportation costs. The growth of new highways allowed commuters to quickly and easily travel from their home in the suburbs to a central business district. Suburbanization fed on itself, as outward migration bled urban tax bases dry, leading to crime, failing public services, and bankruptcy for once-proud central cities.

In recent decades, sprawl has been driven forward primarily by housing cost pressures. As economist Ed Glaeser has convincingly shown, older, denser neighborhoods and cities tend to regulate new housing construction tightly, restricting supply growth and pushing up housing prices. New neighborhoods built on pristine land in outlying counties are subject to far looser rules. In many exurban counties, new neighborhoods require little more than the purchase of a large parcel of farmland and the rubber stamp of a small government development board. It’s easy to add new housing supply on the margins of great cities, which keeps housing prices relatively low and attracts a steady flow of migrants from high cost areas of the country.

Sprawl is nonetheless supported by transportation infrastructure. It’s more difficult to build infrastructure of any kind in dense areas than in exurbs; land is harder to obtain and more expensive, and potential opponents more numerous. This dynamic is reinforced by federal funding priorities. Highway money is plentiful and comes with few strings attached, while transit funding is limited and contingent upon approval by the Federal Transit Administration, which rigorously reviews and critiques — and frequently dictates revisions to — applicant plans.

The result is a world where exurban, low-density growth is cheap, easy, and incredibly common. The outcome of this state of affairs is striking. The nation’s fastest growing counties are overwhelmingly located in the exurbs of large cities — places like Loudoun County (Washington, D.C.), Kendall County (Chicago), Rockwall County (Dallas), and Pinal County (Phoenix), all of which have grown in population by more than 50 percent since the 2000 census. Metropolitan areas where growth almost exclusively follows the suburban form have experienced population explosions in recent years. Since 2000, Atlanta, Dallas, Houston, and Phoenix have all added between 750,000 and 1 million people each. And the regional implications are extreme. Since the 2000 census, the southern and western portions of the country have grown by nearly 17 million people. The Northeast and Midwest, by contrast, added just under 3 million.

This population shift has been viewed as a triumph by many economic observers and political leaders. Empty land and sleepy, economically stagnant small cities have given way to metropolitan juggernauts. States that once emptied their pockets in attempts to lure northern manufacturers to their underemployed towns now find themselves awash in jobs, many of them good ones. Northern families hit hard by the decline of rust belt manufacturing have experienced new economic lives in the south and west, and northern households that couldn’t afford to buy homes near their jobs despite excellent salaries find themselves in expansive estates on the outskirts of places like Charlotte, Atlanta, and Houston.

These booming states, and the nation as a whole, are beginning to understand that these massive migrations are not without significant costs. Budgets in the south and west have been stretched tight with the burden of massive new infrastructure construction. Heavy dependence on automobile commuting has led to epidemic congestion. And recent implosions in some local housing markets, particularly those in southern California and Florida, have forced some to question the strength of the economic fundamentals underpinning the population boom.

Ultimately, those concerns could prove to be small potatoes relative to other potential challenges. The sun-belt shift has meant millions of new people in areas subject to extreme drought, wild fires, hurricanes, and flooding, at precisely the time when scientists expect such events to become less predictable and more severe. The population shift itself isn’t helping, as these migrations have generally meant the relocation of families from denser areas with better transit options to low-density neighborhoods where transit is practically unheard of, leading to increased fuel use and carbon dioxide emissions. On a number of key traffic indicators (PDF), including delays per traveler and wasted fuel per traveler due to congestion, cities like Dallas, Houston, Atlanta, and Phoenix perform far worse than cities like Chicago and New York, despite significantly smaller populations.

And while commuters in New York and Chicago can shift (and have shifted, impressively) from driving to transit as gas prices rise, residents of autocentric towns in the south and west cannot, and are therefore forced to swallow high fuel costs. If the U.S. manages to adopt carbon limiting rules, as it should, long automobile commutes will become more expensive still; as such, the massive southward migration based on low-density development will make emission reductions more difficult and more painful. It may also make them less likely, since consumers will have a strong incentive to fight new costs they can’t easily avoid.

The problem, then, is not the migration itself but some of the chief forces driving it. Mobility based on better economic opportunities is important for the economic health of the country. Mobility based on inefficient housing supply restrictions, on an inefficient distribution of transportation funding, and on underpriced carbon emissions, congestion, and environmental risks is a different beast altogether. It’s an unhelpful mobility that is creating and will continue to generate large future costs. It’s a mobility that the government should address with improved land-use and transportation policies.

Substantial improvements in federal policies aren’t all that difficult to imagine or implement. I would recommend the following steps:

  1. First and foremost, the US should adopt a nationwide plan to price carbon emissions. Emission pricing would reduce the incentive to build and live in homes and cities which depend upon long automobile commutes.
  2. The federal government should eliminate the distinction between funding for highways and funding for transit. Doing so would significantly increase the pool of money available for transit, and it would make it easier for cities and states to allocate transportation money to transit priorities. A better distribution of funding will enable dense growth to continue where it’s bound by inadequate infrastructure.
  3. The above steps, while necessary, will nonetheless do little to change the regulatory framework on the ground, which makes it easy to build homes in exurbs and difficult to add housing units in dense cities. To alter this dynamic, the federal government should make transportation and housing funding dependent upon growth in density. This would encourage local governments in highly regulated cities to confront interests opposed to new dense growth by arguing that density must be increased in order to qualify for needed federal funds. At the same time, local governments in loosely regulated cities should be hesitant to authorize massive new low-density developments, since responsibility for new infrastructure funding will then fall exclusively on local budgets. Sunbelt towns can continue to grow and thrive so long as new developments aren’t exclusively outward oriented and automobile dependent.

The goal of these proposals is not to impede the free movement of labor, but to ensure that when jobs and people move from place to place, the driving force is not low costs made possible by cheap, dirty, and unsustainable uses of land. Competition between metropolitan areas should not be based around an environmental race to the bottom.

It’s incredibly unfortunate that land-use and transportation issues are given such short shrift on the left and are merely fodder for jokes on the right. These are forces leading to the movement of tens of millions of Americans, completely altering the urban geography of the country. Those alterations have led to billions of dollars of losses due to congestion, massive new vulnerabilities to natural disasters, steady increases in carbon emissions from transportation and in the expected future cost of reducing those emissions. It’s serious, serious business, and we leave these matters on the political sideline at our peril.