Caltrans deserves some kind of award for the Most Shallow Research when it comes to the roundabouts such as are common in England. I have just returned from four weeks in England, during which I drove several thousand miles and have been through hundreds of roundabouts in the country and in cities of all sizes.
The roundabouts work fine when traffic is light, but when the traffic is moderate, they become major bottlenecks. For heavily traveled roundabouts, the English have installed traffic lights to control the flow of traffic into and within the roundabout. They have even resorted to rickety one-lane bridges to permit some traffic to "flyover" the congestion caused by the roundabout.