Caltrans deserves some kind of award for "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 "fly over" the congestion caused by the roundabout.
If Caltrans wants to study how well a roundabout works with drivers unaccustomed to such things, they need only to watch the traffic around the circle in the middle of the city of Orange, in which the traffic moves very inefficiently.