"IF" these "Plaars" were ever implimented, they wouldn't be ran until we had a lot of things, as listed below.
1. Extremely advanced tracking systems and GPS. This would have the vheacle so that it wouldn't have a "manual" function.
2. Sensors that keep you at least 50' away from a car in every direction, including 50' off the ground.
3. Floating lane markers to keep the sensors in line, so that they can measure themselves in the air to which altitude level that they need to be, in case of a ditch or something in the way.
4. Strict laws concerning flight plans and manual controls.
5. Gas sensors that will sense if your vheacle has a 1/4th tank it'll land you in a clear spot on the ground determined by the above markers and not allow you to turn on your vheacle without more fuel, as a safety measre.