I think there's another problem -- ease of driving. When driving a car, you don't have to make many choices. Any directional choice is binary: do I turn right here or not? As long as you stay on a road, the choices are limited for you. Driving doesn't tax the mind. In an airplane, you have 360 degrees of choice in addition to your climbing or descending angle. Drivers of air cars will have to take a cue from airline pilots today, and set a heading, and then correct it later. This requires more effort and planning than driving down Rt. 1 until the third stoplight, than turning left. It's more of a mental load than many drivers are willing to take.<p>Also, when driving a car, you can use the brakes as a safety valve -- just hit them if anything goes wrong. On any road, at any time, you can stop the car and deal with the problem. There's nothing like that in an airplane; you can't just land anywhere you want, anytime (there might even be other airplanes below you, so you can't even descend).<p>The first problem could be solved by having intelligent autopilot -- GPS with control over the system. The second could be solved by having air cars that could hover.