I imagine with car AIs communicating with each other and automating most of the process, you could squeeze in 4 additional vertical lanes over the ground one within the 20 meter altitude range. That would effectively square the number of potential traffic participants on a freeway, and improve city traffic by an even wider margin. So yeah, it would solve congestion for generations to come. Parking space would be problematic within existing cities though. The only option I can predict is cars dislodging passengers, then pulling their noses up until they're perpendicular to the ground, and stacking themselves in parking spots like sardines. The entire city street could become a parking zone, making non-flying wheeled cars a thing of the past.