Understanding the difference between roundabouts and traffic circles:

They might share a circular shape, but any similarity ends there. Modern roundabouts operate much differently than other deservedly oft-maligned traffic circles and rotaries. A modern roundabout requires entering traffic to yield the right-of-way to traffic already in the roundabout. This keeps the traffic in the roundabout constantly moving and prevents much of the gridlock that plagues rotaries, for example. Modern roundabouts are also much smaller than rotaries and thus operate at safer, slower speeds. The design of a modern roundabout allows capacities comparable to signals but with generally a higher degree of safety.

	Traffic Circles	Modern Roundabouts
Capacity	Low	Very high
Speed	High (20-40+)	Low (15-25 mph)
Use	No priority, "free for all"	Clear priority, "orderly"
Crashes	Many, some serious	Very few, most slight
Diameter	200-300' or more	5'-120''
* from the Barry Crown Report

Perhaps the most significant difference between traffic circles and roundabouts is the way vehicles enter the intersection. Traffic circles force entering vehicles to merge at a high speed with vehicles already in the circle. At 40 mph, a merging distance of 240' would ideally be required, but traffic circles are rarely large enough to provide adequate merging distance, and leads to their poor functionality and safety record. Because the speeds in modern roundabouts are much lower, their operation is based on gap acceptance, rather than merging. Drivers entering the roundabout must yield to those in the center, and then determine when the approaching gap is large enough to enter at the lower speed of 10-25 mph. The lower speed means the acceptable gap size is much smaller, improving the operation of the intersection.

The geometric differences may seem subtle, but result in major differences in performance and safety track records:

	Traffic Circles	Modern Roundabouts
Size	Very large	Very small-medium
Traffic weaves	Yes	No
Yield lines	No	Yes
Fast entries	Yes	No
Fast circle speed	Yes	No
Priority rule	No	Yes

Well-designed modern roundabouts are easy and safe to drive and incur minimal crashes. Those crashes that do occur generally have low severity, because the geometry is specifically designed to force all vehicles to drive slowly through the intersection. And, if an entering vehicle does crash into a circulating vehicle, the angle of impact is generally very shallow, typically 20-30 degrees, rather than the 90 degree angle of a 'head to side' crash experienced at traffic signals.

The design of the old traffic circles was based on speed - a high-speed entry, a high-speed merge, and a high-speed exit, all very intimidating to the driver and prone to accidents. Modern roundabouts are generally designed as small as possible for each unique environment in which they are being installed.

The modern roundabout, though, should also not be confused with modern "neighborhood traffic circles" - design devices typically used within a traffic calming plan for neighborhoods with very small diameter circles. These circles function differently and with different purpose than the modern roundabout, whose primary purpose is as an intersection control device and acts secondarily as a traffic calming device.