Roundabouts vs. Traffic Circle:
Three basic principles distinguish the modern roundabout from a traffic circle.

1. Modern roundabouts follow the "yield-at-entry" rule in which approaching vehicles must wait for a gap in the circulating flow before entering the circle. Many traffic circles in the United States require circulating vehicles to grant the right of way to entering vehicles though few, if any, of these type circles exist in New York State. Some traffic circles also use stop signs or signals to control vehicle entry.

2. Modern roundabouts involve low speeds for entering and circulating traffic, as governed by small diameters and deflected (curved) entrances. In contrast, traffic circles emphasize high-speed merging and weaving, made possible by larger diameters and tangential (straight) entrances.

3. Adequate deflection of the vehicle entering a roundabout is the most important factor influencing their safe operation. Roundabouts should be designed so that the speed of all vehicles is restricted to 30 mph (50 km/h) or less within the roundabout. This is done by adjusting the geometry of the entrance alignment, splitter island, center island, and exit alignment to ensure that "through" vehicle paths are significantly deflected.
https://www.dot.ny.gov/main/roundabouts/background

First, I'd like to explain the differences between a roundabout and a traffic circle (rotary). The first traffic circle in the United States used for traffic control was Columbus Circle in New York, which opened in 1905. Traffic circles are circular or elliptical islands, with a central island 300 to 600 feet wide. They are designed for vehicles to enter, merge, circulate, change lanes and exit at relatively high speeds -- 30 to 50 mph.

Early in the 20th century, when vehicle speeds were slower, drivers were patient and traffic volumes were low, circles were an efficient and relatively safe form of traffic control. As traffic volumes and vehicle speeds increased, congestion and crashes increased significantly
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Then came roundabouts. The first roundabout in America was built in 1992 at an intersection in Gainesville. Although traffic circles and roundabouts use a circular design, they operate very differently. Traffic circles are very large and are designed for high-speed vehicle operation. Roundabouts are designed as small as possible, 16 to 180 feet wide, and operate at 15 mph to 25 mph.

The design of roundabouts forces drivers to slow as they approach them, then limits drivers' circulating and exit speed. It is difficult to pass through a well-designed roundabout above these design speeds.

In addition to slow vehicle speeds, modern roundabouts require drivers to slow and select gaps in the circulating traffic before entering the roundabout at low speed.
http://www.heraldtribune.com/article/20090621/COLUMNIST/9062...