I think a lot has to do with the size of the roundabout.

Kings Beach in California has 3 smallish roundabouts that are going in and had to be reviewed by state DoT - troy corliss is one of the artists and could probably tell you the state regulations.

In LA area, Freyja Bardell and Brian Howe completed this one - as you can see the artwork is ground level and up, but the roundabout itself is quite large: http://www.publicartinpublicplaces.info/faces-of-elysian-valley-2017-by-freyja-bardell-and-brian-howe

Faces of Elysian Valley - PUBLIC ART IN PUBLIC PLACES www.publicartinpublicplaces.info Public Art "Faces of Elysian" (2017) by Freyja Bardell and Brian Howe