The guidelines and restrictions for roundabouts (traffic intersections) are set by the governing agency that maintains the roadway. In most cases, the government will have very specific guidelines for you to follow. Our area is unique in that we have adjoining roundabouts, each controlled by a different entity, Town, County State CA DOT. None can provide a clear recipe to follow.
Some policies include guidelines for artwork, some do not, so, I would recommend establishing a good relationship with your local (representing) agency. They will be the ones who can best advocate for your project. Here is a link to CalTrans for example. (just to highlight the bureaucratic irony, the photo pictured on the CalTrans site is of my project in a Town of Truckee roundabout…not a CalTrans roadway)