One thing of importance that letters on roundabouts miss is the area necessary to set one up. The one not far from where we lived in Glasgow, Scotland, took up at least an acre and serviced only three busy roads. Are we willing to give up that much land to make them work efficiently? And the roundabout at Hermiston Junction, where M9 joins the City of Edinburgh Bypass, takes up about three acres, perhaps five, and has stoplights.
Roundabouts are quaint but not very efficient in high traffic.