I set out the other day to find the outer edge of the Philadelphia suburbs and ended up in a Chester County subdivision called Oakcrest. Located 45 miles west of Center City, just outside Coatesville, Oakcrest has a network of immaculately paved streets, glossy utility boxes, and an active sales office. What it does not have is a lot of houses.

Oakcrest was laid out for 169 single-family homes, but only 32 were completed when the builder went belly up after the 2007 housing crash. The result is a kind of zombie subdivision: Oakcrest is outfitted with all the necessary infrastructure, but it lacks the pulse of human life. Numbered signs resembling grave markers have been jammed into the earth to identify the available house lots. Since being taken over by a new developer last year, only three more Oakcrest houses have found buyers.

Is Oakcrest a sign that the region's suburban sprawl has finally reached its limit, or is it just a casualty of the housing bust?

Back then, experts maintained that the relentless march of suburbia would resume just as soon as the overstock of houses was exhausted. But five years after the market seized up, planners and economists aren't so sure, and they've begun to ponder a previously unthinkable notion: The heyday of the suburbs may be over.

Not for every suburb, of course. The original, close-in, commuter suburbs, such as those on the Main Line, aren't likely to lose their luster anytime soon. The next ring of suburbs can probably survive, too, if they make some structural adjustments, such as adding more townhouses and apartments. It's low-density, fringe exurbs like Oakcrest, beyond the orbital pull of the big city, that may not have much of a future.

The demise of the Great American Exurb was heralded this fall in a New York Times op-ed by University of Michigan planning professor Christopher B. Leinberger. He argues that "a profound structural shift" has begun to reverse the residential patterns set in the 1950s. Cities are rising, while suburbs are going into decline. His views are shared by many developers, who are shifting to townhouse and apartment projects.

Not surprisingly, Leinberger's essay set off a firestorm in the blogosphere. Demographer Joel Kotkin, a champion of suburban development, denounced Leinberger's conclusions as the wishful thinking of a condo-dwelling elitist. Kotkin notes that the single-family home on a spacious lot remains the residence of choice for Americans. He also points out that plenty of urban condos sit unsold.

So, who's right? Their dueling data sets can seem to support both points of view.

Leinberger isn't claiming that cities are out of the woods yet, only that the trend lines are moving in their favor. A recent survey by the National Association of Realtors found that the majority of buyers - 56 percent - preferred houses located in traditional cities or in walkable suburbs that have good transit connections to downtown. The percentage is even higher among two influential groups, the retiring baby boomers and the mid-career millennials, who are now in their 20s and 30s.

Surveys tend to be suspect, sale prices are not. When Leinberger analyzed sales data in different markets, it showed that single-family houses in outer-ring suburbs - like Oakcrest - are hemorrhaging value. Many now sell for less than their replacement cost.

In contrast, urban housing has generally held its value during the bust. The priciest housing on the market can be found in cities and commuter suburbs, suggesting people really do prefer to live in those neighborhoods. This is a "reversal of what took place in the 1950s, when drivable suburbs boomed and flourished as center cities emptied and withered," Leinberger concludes.

Leinberger's essay topped off a year in which cities - or, at least, the idea of cities - rose to new heights of hipness, with an outpouring of books, blogs, and films. The phenomenon was seen at packed showings of the documentary Urbanized, a frenzied survey of issues facing the world's cities by independent filmmaker Gary Huswit. Such is the hunger for information that the Atlantic magazine spun off an online daily magazine, the Atlantic Cities, that exclusively covers urbanist issues.

The most unlikely plug for cities came from a free-market economist from Harvard. In his new book, Triumph of the City, Edward Glaeser celebrates urban density, arguing that it's the most efficient way to create new ideas and jobs. In the past, such free-market advocates tended to defend unfettered suburban development as the embodiment of consumer preference. Glaeser's view supports the opposite conclusion, that the market, if left to its own devices, would produce high-rise cities.

One problem in discussing the future is that economists have an annoying habit of conflating the city and its denser neighbors into one vague urban blob. In his book, Glaeser sometimes opts for city to refer to traditional central cities such as Philadelphia. Other times, he considers everything within the Metropolitan Statistical Area - which includes suburbs in three states - to be urban. By that definition, Valley Township, where Oakcrest is located, is part of the city.

"The word suburb has lost its meaning," acknowledges Alan Berube, a fellow at the Brookings Institution.

The real competition, he suggests, is no longer between cities and suburbs, but between places that have density and good transit connections and those that don't. By that definition, North Philadelphia could be well-placed for a comeback, but the city's suburbanized Northeast may struggle. The success of Collingswood has everything to do with the proximity of PATCO trains and its small house lots.

Both Berube and Glaeser, along with other urban economists, are convinced that those less-dense places - whatever they're called - will have to embrace greater density to survive. That means adding mid-rise apartment buildings so aging homeowners can retire in place and allowing mixed commercial and residential buildings along commercial boulevards.

Such construction would not only energize the suburbs, it would also bring more tax revenue. That's important because the cost of providing services and paying pensions is skyrocketing. As taxes fall off, some towns have had to turn off streetlights to balance their budgets. In home-rule states such as Pennsylvania and New Jersey, which comprise many small towns, we're likely to see mergers of micro-burbs to form larger, more urban entities.

Despite such pressures, you can count on many suburbs to resist the forces of urbanization. We saw that impulse last year during the debate in Lower Merion over rezoning City Avenue to accommodate apartment towers. Some residents said they objected to high-rises because the form didn't fit their image of suburbia.

Rising energy prices may force them to reconsider. The fastest-growing suburban house form, developers say, is the townhouse. People are willing to live in smaller homes if the location is convenient, says Joe Molinaro, who studies smart growth for the National Realtors Association.

Maybe that means Oakcrest does have a future. After all, Coatesville's Amtrak station is 15 minutes down the road.

Changing Skyline:

Inga Saffron blogs about Philadelphia architecture at www.philly.com/skylineEndText