As many of our cities and older inner-ring suburbs are being renovated and revitalized, the great challenge of our time -- far bigger than urban renewal was in decades past -- is to remake our many shoddily-built, far-off exurbs into denser, more- connected, more livable communities. Some of them -- the ones that were built as much to keep the building boom going as because people needed to live in them -- might be fated to shrink back into small towns or disappear altogether.

In my travels across the country, I've heard from people who are in the process of resetting their lives. Young people just out of college tell me that they don't want their parents' suburban lifestyle; they'd prefer to find an affordable rental apartment in a city they love where economic opportunities are better. They don't want to go into hock buying a big house and a big car, just so they can endure a long commute.

Young parents tell me they've had to defer their dream of buying a bigger house with a backyard, either because they can't afford it or don't qualify for a mortgage. Instead, they've decided to stay put and renovate their city apartment or fix up their small house in an older, closer-in suburb. Empty nesters tell me they've decided to sell the big house, sometimes for a lot less than they could have gotten for it a few years ago, and buy a smaller condo or house closer to their kids in the city.

These shifts, brought on by economic exigencies, are already adding up to a gradual but enduring change in the way we live -- one that will prove every bit as consequential as the move towards suburban living was in the 1950s and 1960s.

Gradually, our great complexes of cities and suburbs are being knit into mega-regions -- giant city-states that are home to millions upon millions of people and generate billions and in some cases trillions of dollars of economic activity. Driving this is not just our individual choices and preferences but the very logic of economic development. Geographic concentration and clustering speeds the transmission of new ideas, increases the underlying productivity of people and firms, and generates powerful economies of scale.

This new economic landscape and emerging way of life won't come together completely on their own. All of this must be underpinned and supported by new kinds of infrastructure -- from more efficient living patterns to more effective, less car-dependent transportation systems that run the gamut from more bicycle paths and sidewalks to improved mass transit and high speed rail. Just as government programs and policies underpinned the rise of suburbia in the 1950s and 1960s (think of all those subsidized highways), new public policies toward rental and affordable housing, alternative transport, and more sustainable energy will help encourage this shift today.