Mike - That's why I think that autonomous "drone" vehicles most likely will be used for a specific transportation purpose and not private transportation. And like I mentioned earlier, these "drones" would be allocated to a specific corridor (min/max altitude and specific route) as well as a timed "enter/exit/spacing" algorithm that the "drones" follow - no user input available. Think of it kind of like an aerial "People Mover".
In a private application, there are just too many people and too many things that can go wrong. The futuristic idea of air-mobile private vehicles belongs in the realm of the Jetson's and the Fifth Element.
As it is, places like New York, London and such are relatively congested with vehicle traffic until you compare them to the brutal traffic of Beijing, Singapore and any large city in India. There's just no way that the the idea of multi-level traffic with air-mobile vehicles is possible.
I suspect that in the next several decades, private vehicles (of any form - except bicycles or similar) will be phased out in favor of mass transit (of various means) for any of the large cities of earth. This of course excludes rural areas.