I definitely like the idea of this but it seems like the tendency is always the opposite because it's much cheaper to build a pedestrian overpass than a car one, even though they're also an accessibility nightmare and usually kind of a terrible experience and aesthetic blight in their own right.
Most roundabouts of that type I know have replaced similarly sized all asphalt intersections. Now there is more green. When planned ahead there really isn’t that much extra real estate required if you are willing to have steeper grades for pedestrians only for example.
https://www.google.com/maps/place/Airborneplein,+6828+KP+Arn...
It's extremely efficient.