Some sources make a distinction between roundabouts and traffic circles, stating that roundabouts have certain "modern" design elements such as a yield on entry to circulating drivers, and deflection from a straight path of approaching vehicles. 

The roundabout, or traffic circle is contracted at intersections and requires drivers to circulate to reach their intended exit. One problem is that if the diameter of the roundabout is less than about 45 feet large trucks cannot make 270 degree left turns around it. In the UK this is overcome by making some (small roundabouts) or all (mini-roundabouts) of the roundabout able to be driven over by the wheels of a truck. In the UK however drivers are used to circulating roundabouts. In the US this may lead some drivers to go straight across a roundabout, if it is physically possible. In fairly low traffic situations roundabouts can still be used by trucks which will make left turns in front of the roundabout. 

A photo of a typical small (approximately 25 feet in diameter) roundabout is shown below.

As can be seen it provides an aesthetic improvement to the road and reduces the perception of a long straight road. I have used a roundabout in a temporary form in the US (see photo) on a collector road carrying approximately 6000 vehicles per day.

The intention is to reduce traffic speeds and divert traffic to a parallel arterial (somewhat as an alternate to speed humps). Two more roundabouts will be installed on the collector (approximately one third mile in length) to give a better trial of roundabouts. Therefore at this time I do not know if they will slow and divert traffic. I am however in favor of the increased use of roundabouts particularly as an alternative to four way stops. Their aesthetics (when permanent) and the need to slow down to negotiate them makes them a useful traffic calming tool.



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