Think of future traffic will be 10 times worse even with double the lanes
Driverless Flying Cars won't need to build huge highways, air is free
Most of the land used for huge overpasses could then be reclaimed
Yeeeesss.....
Small problem there. We need an AI that is efficient enough and safe enough to do so.
We're talking a fully-functional, completely automatic system that can navigate in three dimensions, in a seamless grid. It's not like just plugging in a GPS and say that's that.
It has to be completely self-enclosed so the users can't interfere with it (otherwise 9/11 will look like a 4th of July show), it has to be completely hacker-proof (same reason), and yet it has to be software-updateable for the entire lifespan of the car.
Guess what, we don't have a clue how to do that.
Most PCs/consoles/smartphones have an average life-expectancy somewhere between 2-5 years, after which people will exchange them with new models. So any glitches (of which there are usually bucketfuls) can be fixed in the next model, and the next iteration of the OS.
Can't do that with something that has a life-span of 20-30 years.
It's already a huge problem just keeping something like hospital-machinery operating, simply because they were built with e.g. WinXP. Trying to fix the exploits and also keeping the machinery capable of talking to all the other systems that have since been upgraded, without spending millions of pounds on new hardware is what can only be described as a major headache.
One they still haven't solved, and this is fixed equipment we're talking about here.
Not to mention that air most certainly isn't free. There are massive problems already trying to cram the current air-traffic into place. The airspace over London is running at about 98% ATC capacity.
Even assuming that flying cars would stay below 1,000ft (which seems reasonable), you'd still need an infrastructure capable of handling massive amounts of traffic, which again would have to be automated.