I think the idea (if we're taking notes from the Jetsons) is to have a car that hovers. 'Plane cars' have to pick up speed to become airborne. You can't have stop and go (air) traffic in planes... Not in mid air. Unless science produces a sort of anti-gravity vehicle.
Besides, that would be chaotic anyways. There'd be nothing to guide people where to go. Unless there was some sort of invisible grid or GPS was enabled to enforce any sort of boundaries.