Keck Circle in Montpelier Vermont is the first roundabout on a State road in the U.S. The roundabout performs very well. It has three street approaches and one driveway approach.
There is a school in the upper left of the picture. School kids reported no problems to MIST when we visited the roundabout in winter of 2005 and again in winter of 2006.
This photo shows an entering vehicle encountering a circulating vehicle. U.S. guidelines suggest that the number of entering vehicles (E) plus the number of circulating vehicles (C) in front of a particular entry point should not exceed 1,400 in an hour. In other words, anyone can do a rough planning exercise to determine if a modern, single lane roundabout would work at a particular location by counting cars and marking turning movements. 
As long as E + C is 1,400 or under, a single lane roundabout will work. If the number is over 1,400, you do not have to automatically rule out the roundabout (as some planners and engineers are apt to do), but you do have to figure out ways to reduce or reroute traffic, or accept that there will be some congestion (which is OK too).
Another view of the Keck Circle roundabout. The landscaped inner circle is about 30’ in diameter, the inner circle truck apron about 20’ wide and the circulating travel lane is about 18’ wide.
This totals 106’. Some Scandinavian research has suggested that 90’ to 100’ is the safest diameter for a single lane roundabout with respect to pedestrians and cyclists.
This is a close-up of school kids crossing the single lane roundabout. Single lane roundabouts are considered very safe for pedestrians (including children, elders, and the mobility challenged) 
because speeds are low, visibility is excellent and down at street level, and motorists only have to do one thing at a time.
This is a closer photo of the truck apron. In this example, the apron is not too distinguishable from the circulating lane. In the lower right of the photo you can see the 3 inch granite, roll over curb. This small curb keeps cars in the circulating lane while allowing the rear wheels of big trucks (the biggest trucks are called WB-67’s) to roll over the curb if necessary. 
Three inches is a kind of ‘magic’ number for the height of a truck apron curb. If the height of this curb is shorter then cars are apt to go straight over it (thus retaining higher speeds than going around it). If the height is taller then truck loads can shift excessively as rear wheels go up and over the curb.