When people hear about my support for Personal Rapid Transit, they sometimes compare it to the Jetsons or wonder about Flying Cars. Unless there is a pretty significant technological breakthrough, I'm pretty skeptical that flying cars will ever happen.
Let's pretend for a moment that the breakthrough occurs and a car can be made to take off and hover without the huge energy expenditure that doing that today requires. (This might be a breakthrough in anti-gravity, but it's more likely a new, very cheap, very portable source of energy.). This doesn't change the need for roads, although the new technology would allow them to be in the sky. Presumably this would mean that they could be built very cheaply, and that they don't interfere so much with other construction. If more capacity is needed, they could be easily widened or extra decks added. They are still plainly finite and subject to traffic jams. Many science fiction movies with flying cars, such as Blade Runner, Star Wars, etc., have recognized this and show nearly all the cars moving in neat rows. The action of the movie often involves a breakdown in this order but if such disorder were commonplace, flying cars would either crash into each other constantly, or have to stop to avoid such collisions almost as often.
NASA and the FAA have been working for some years on a "NextGen" instrument navigation system, sometimes called "Highway in the Sky". Early stages of this amount to technological improvements to present instrument flight systems--automated radio frequency handoffs, altitudes and headings transmitted directly to "glass cockpit" navigation systems, etc. If limited to present instrument routes, it wouldn't have much impact on system capacity, but it would substantially reduce workload and errors for instrument pilots. But over the long term, it would allow many more routes to be managed. But without cheap VTOL and hover, the runway would remain exactly the bottleneck it is with aviation today.
Cheap VTOL changes everything. The "road" could be above the scene, and when you need to stop, you simply leave the road and drop down to whatever your destination is. Present VTOL and hover consume a tremendous amount of energy. Present technologies, which are aerodynamic, also require substantial space between vehicles.
Interestingly, Personal Rapid Transit allows a bit of an approximation: As with flying cars, the "roads" are elevated, so they don't consume much real estate on the ground, and can easily go over or around obstructions and other routes. They're a lot cheaper than other forms of railed transit, although not so cheap as simply reserving a corridor of air. And you can add a station anywhere you want, without interfering with traffic. Transportation with PRT is a lot more like taxi service than having a private car. Like the movie versions of flying cars, the vehicles can run very close together and move a lot of people. And because they are on fixed guideways, it's very difficult to make them collide.
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