If we ever have some sort of easily cost effective flying "car" like machine... its still a LONG way off.
main problems:
Fuel <----- it would have to be cost effective, which leaves out fossil fuels, which will start becoming more and more scarce soon enough. And it would also have to have alot of energy over a small amount of space/weight, so that the size of the cars could be manageable.
Lift <----- something along the lines of VTOL (vertical take off and landing) seems feasible, except for the huge amount of energy that wastes, if there were any "lanes" then some sort of way to quickly switch between moving and stopping, while remaining airborn, would have to exist.
Control <----- it would have to be easy enough for the majority of the public to understand how to control the thing, and with precision so that crash's would be minimalized.
Dependability <----- engine dies out and your a few hundred feet in the air, you can kiss your life goodbye. Just the insurance for owning one of these things would be outrageous. Weekly check up or some sort quality standard would need to be high above todays standard.
Law <----- how to catch criminals when they can get in away in 3 directions, not just 2.
Roads <----- there would have to be some sort of monitered path that you would only be allowed to drive. With boundaries that you would HAVE to adhere too, so that chaos and crashing wouldn't be a problem. See how many planes you see in the skys? those are already carefully monitered so that they have enough area to not have any sort of collision, can you imagine how it would be if public transport would have full flying privilages?
hmm... good hypothesis on my part =)