Given sufficient road width, takeoff/landing length, and road "quality" (smoothness, etc), why wouldn't an aircraft be able to takeoff from and land on to a highway? I mean, what is the actual significance of this?
A-10's high mounted jet engines are up there for a reason. It can operate from almost anything resembling a runway.
They clean the road from FOD [0] before such operations.
[0] Foreign object debris, can cause foreign object damage. https://en.wikipedia.org/wiki/Foreign_object_damage