Fly like George Jetson? As his boss, Mr. Spacely, might say, "Are you out of your orbit?" The 1960s cartoon "The Jetsons" popularized the fantasy of a future filled with flying cars, but the idea still hasn't taken off in the real world. Of course, that hasn't stopped tinkerers from trying.

Canadian-born inventor Paul Moller infamously has been working since the 1960s and has reportedly spent more than $200 million to develop his Skycar. Nothing has gotten off the ground, despite his company's claim that "the automobile is only an interim step on our evolutionary path to independence from gravity."

Other flying cars in development include MACRO Industries' SkyRider X2R, Urban Aeronautics' X-Hawk and StrongMobile's Magic Dragon.

Aside from the monumental challenge of creating a personal vehicle that navigates the skies and roads equally well, their developers also face the reality of our crowded world.

Think of the heavy traffic you face on your commute to work each day, said Motor Trend technical editor Frank Markus.

Now imagine all of those cars in the air at once, in addition to commercial air traffic.

Getting cut off by a guy in the next lane on the road is bad enough, but how about when he's coming at you from above or below? And watch out for that incoming flight from Cincinnati!

The third dimension adds tremendous complexity for navigation, making traffic control in big cities unfeasible, said St. Paul futurist Joel Barker. And don't forget having to license everyone as pilots and drivers, he added, or the impracticality of flying short distances.

There's nothing wrong with wanting to be like Icarus, Barker said.

"We've always wanted to fly," he said. "But flying cars are impractical."