That's kind of funny to hear, because I was going to use real-world Chicago and Seoul as practical examples of this already existing in cities today.
Chicago Loop has a dual-layer approach (where faster moving cars are on the ground floor, but a "second street level" is directly above them, for pedestrians + buses). It's not an exact match (cars can drive on both levels, those lanes should all exist on one level), but it's pretty close to this idea already in practice.
On the opposite side, Seoul has a "pedestrian highway" slung above 8ish lanes of car traffic below, which is a cheaper (although less effective) version of the same idea.
