The successful airports of the 20th century were serious economic engines of the first order. Starting life as airstrips in the 1920s, they eventually became major travel hubs, typically surrounded by industrial buildings, convention hotels and even office complexes. Unlike the cities they serviced, however, they were distinctively suburban in nature: auto-oriented land hogs, built on the assumption that land was abundant and cheap.

The successful airports of the 21st century will be different. For one thing, they will have to operate within a crowded and increasingly urban fabric. It's unlikely that any brand-new airports will be built. Only two have been constructed in the past 40 years -- in Dallas-Fort Worth and Denver. The future is likely to focus on incremental expansion -- cramming more things in and around airport properties and working more closely with surrounding communities and jurisdictions to make airports compatible.

Which raises an interesting question: Why are airports so auto-oriented? After all, it's one of the few situations in American society where absolutely everybody leaves their car behind. All airline passengers spend a great deal of time as pedestrians. At an increasing number of airports, they become rail passengers, too, just to get from their gate to the terminal.

So, tens of thousands of people are traveling around and to airports without their cars every day. This ought to be a New Urbanist dream. And, according to airport experts, this is probably how airports will evolve in the decades ahead.

John Kasarda, the University of North Carolina business professor who coined the term "aerotropolis," says that in order to be economically successful in the future, airports cannot afford to follow the "spontaneous, haphazard" development pattern of the past few decades. Because airports are congested and running out of land -- and because more and more of their patrons will be arriving without cars -- these new, high-end business centers will have to be nodal and mixed-use. They may lack housing because of noise considerations, but they are going to have to provide air travelers with many business services in a compact setting -- and these services will have to be easily accessible without a car. In other words, the successful 21st-century airport will be a New Urbanist economic engine.

As airports expand in crowded conditions, they are going to have to work more closely with their neighbors. The key will be doing more comprehensive land use planning that involves areas within the airport's boundaries, as well as on property known in the airport business as "outside the fence."

Most airports are focused on "doing a plan that meets FAA regulations inside the fence," says consultant Mark Bowers, who has been working on a commercial development plan for Dallas-Forth Worth International Airport. DFW's plan was done in collaboration with four surrounding cities and is increasingly focused on the smart-growth approaches planners love. The DART Orange Line light rail to DFW is scheduled to open in 2013, and contains an important stop -- Belt Line Road -- just outside the fence. Bowers says that DART was originally imagining Belt Line as primarily a park-and-ride location, but now is reconsidering the possibility of a mixed-use center, one that would be easily accessible from the airport terminals, as well as from other nearby business centers such as Las Colinas, which also will be on the Orange Line.

DFW itself will focus on centralized business centers that provide "valet services," such as auto repair and dry cleaning. That way, travelers can drop their car and use their legs and various rail-transit lines to run their errands and get to the terminal.

The "aerotropolis" notion is partly dependent on the assumption that demand for air travel will continue to increase and there will be an ongoing need to expand airports in the future. Not everybody believes that this pressure will materialize. The recent climb in fuel prices has clearly dampened travel demand and put many airlines in a tough spot. Either they can increase prices significantly, thus cutting ridership, or they can eat the fuel cost, thus eliminating profitability.

It may well be that air-travel demand will level off as fuel prices continue to rise. But even if it flattens, airports will continue to serve as critical business hubs for the nation's economy. And that means the New Urbanist airport is likely to emerge.