A flying car would not be useful because:
1. Everywhere you want to go is still on the ground.
2. You won’t be able to go exactly in a straight line from A -> B.
There will need to be some form of lanes and traffic control to prevent crashes.
Since the “lanes” will be laid out based on the landscape below (because it’s about where you want to go),
it’s most likely that they will be in the exact same topology as the existing road.
Considering you need extra runway to take off / land, the air lanes would probably mirror the highway,
and use the on / off ramp space for converting between air and ground transportation.
The overall effect will be the same as adding more lanes to the highway.