There would need to be a decent buffer between the lowest altitude for planes and the highest for plaars (i'm calling these flying cars plaars), unreal do you have any suggestions on that?
That's fine and good in cruise flight, but what about if a plane wants to climb/descend? Transitioning through scores of "plaars" would be horribly dangerous.

Most people believe that flying cars can be a reality because they look up and see nothing but blue skies. Not many planes, right? Believe me, the skies are crowded enough as it is.