No matter where you stand on a roundabout there is always a carriage way heading in the opposite way to your right with a physical barrier separating you so a roundabout fits the definition of a duel carriageway better than a single carriageway with exception of mini roundabouts which i would except are single carriage ways and any road that forms a loop could be considered a so called circular carriageway. So is the M60 a single carriageway by your rules? It also make more common and practical sense. if you set every roundabout to 60 a satnav may start informing a driver that the speed limit has changed every time they end and leave a roundabout which is silly.