Until we get our flying cars, one way to take advantage of all of that unused airspace above our heads might be to build a bunch of tiny 'pocket airports' in local parks and fill them with futuristic flying SUVs.
There's really no good reason why medium-distance transportation has to stay on the ground, and a lot of good reasons why moving it all up in the air would be a way better idea. You can fly where you're going in a straight line. There are virtually unlimited numbers of lanes. Accidents don't cause delays (presumably because aerial roadways are self-clearing). And you don't have to waste time, space, and ludicrous amounts money building more roads.
So why aren't we all flying where we need to go? Two reasons: firstly, it's a huge pain in the ass to get to the airport, and secondly, it's damn expensive. Both of these problems, though, would be solved by using lots of little local airports that can quickly fly you to other little local airports in efficient robot aircraft without all the hassle.
These 'pocket airports' would be really, really small. We're talking like two acres, which is only about two football fields. At this size, they could be stuffed into existing urban and suburban areas without too much trouble, and the more of them you can squeeze in, the more appealing the whole system becomes, since it becomes easier to get closer to where you're going. They'd be cheap to construct, too, especially considering that a new freeway costs upwards of $20 million per mile.
The whole concept hinges on having aircraft that can actually use the pocket airports, of course. The vision for Suburban Air Vehicles (SAVs) includes a capacity of two to four passengers, a cruising speed of 120 mph, the ability to take off in 100 feet, gas mileage 200 of mpg or better, a parachute, and noise emissions of less than 60 decibels from 125 feet away, which works out to be less average noise than street traffic. Oh yeah, and they'd be autonomous, and a computer would do all the air traffic control.
A prototype SAVs is supposed to be flying by 2015 or so, although the infrastructure changes will of course take much longer. It seems like realistically, by the time all of this stuff gets put into place, we'll all have Ferraris that take off from our driveways instead.