The ~dozen or so roundabouts near where I live (northern suburb of Atlanta, Georgia) have all been 4-way stop retrofits. And all in the last 10 years or so.
LA and southern California could use some roundabouts. There's so many places with stop lights almost stacked on each other.
No one except beautiful Portland, Maine. And probably loads of other towns in New England.
Carmel, Indiana has been retrofitting extensively.