A traffic circle or rotary is a type of intersection that has a generally circular central island. Entering traffic must typically alter direction and speed to avoid the island, creating a circular flow in one direction. In most applications, traffic circles replace the stop lights and traffic signs that regulate flow in other intersections.
In English-speaking countries other than the United States, circular junctions or intersections are typically in the form of roundabouts which give priority to circulating traffic and which are physically designed to slow traffic entering the junction to improve safety, so that the roads typically approach the junction radially; whereas rotaries are frequently designed to try to increase speeds, and thus have roads that enter the traffic circle tangentially.
In the U.S., traffic engineers use the term roundabout for intersections in which entering traffic must yield to traffic already in the circle, reserving the term traffic circle for those in which entering traffic is controlled by stop signs, traffic signals, or is not formally controlled. Colloquially, however, roundabouts are often referred to as circles. 
Design criteria include:
- Right-of-way—whether entering or circling vehicles have the right of way. The New Jersey Driver's Manual recommends that, in the absence of flow control signs, traffic yields based on "historically established traffic flow patterns", and there are no set rules. In New England, Washington, D.C. and New York State, entering traffic yields, as is the norm in virtually all countries outside of the U.S.
- Angle of entry— Angles range from glancing (tangential) that allow full-speed entry to 90 degree angles (perpendicular).
- Traffic speed—High entry speeds (over 30 mph / 50 km/h) require circulating vehicles to yield, often stopping, which lowers capacity and increases crash rates than modern roundabouts.
- Lane changes— Allowed or not
- Diameter—The greater the traffic, the larger the circle.
- Island function—Parking, parks, fountains, etc.
French architect Eugène Hénard was designing one-way circular intersections as early as 1877. American architect William Phelps Eno favored small traffic circles. He designed New York City's famous Columbus Circle, which was built in 1905. Other circular intersections were subsequently built in the United States, though many were large diameter 'rotaries' that enabled high speed merge and weave maneuvers. These designs were doomed to failure for two primary reasons:
- It takes a large diameter circle to provide enough room for merging at speed. Although some of these circles were huge (many were in excess of 100 meters or 300 feet in diameter), they weren't large enough for high-speed merging.
- Giving priority to entering traffic means that more vehicles can enter the circulatory roadway than it can handle. The result is congestion within the circle which could not clear without police intervention.
The experience with traffic circles and rotaries in the US was almost entirely negative, characterized by high accident rates and congestion problems. By the mid 1950s, construction of traffic circles and rotaries had ceased entirely. The experience with traffic circles in other countries was not much better until the development of the modern roundabout in the United Kingdom during the 1960s.
Examples of traffic circles
Traffic circles are referred to as "rotaries" in Massachusetts as well as parts of Connecticut, New Hampshire & Vermont; see Rotaries in Massachusetts
- Monument Circle, surrounding the Soldiers' and Sailors' Monument in downtown Indianapolis, Indiana
- Lee Circle in New Orleans, Louisiana
- Columbus Circle, Frederick Douglass Circle, and Duke Ellington Circle in Manhattan, New York City
- Park Circle, Grand Army Plaza, and Bartel-Pritchard Square in Brooklyn, New York City
- Dupont Circle in Washington, D.C.
- MacArthur Drive Traffic Circle in Alexandria, Louisiana
- U.S. Bypass Highway 1 at U.S. Highway 4/New Hampshire Route 16, and interstate 95 in Portsmouth, New Hampshire (Referred to locally as the Portsmouth Traffic Circle)
- Weatherford Circle, U.S. Highway 377 at Texas State Highway 183 in Benbrook, Texas (Referred to locally as a Traffic Circle)
- Shirlington Circle (Interstate 395 & Virginia State Route 402), Shirlington, Virginia, has off ramps inside the rotary and multiple at-grade streets.
- Union Square (aka "the Oval"), Milford, New Hampshire
- Benjamin Franklin Parkway in Philadelphia
- Los Alamitos Circle in Long Beach, California
- Centre Square in Easton, Pennsylvania, where access is controlled by traffic lights.
- U.S. Highway 1 and Maine Highway 236 in Kittery, Maine
- U.S. Highway 302 and U.S. Highway 202 in Windham, Maine
- U.S. Highway 202, U.S. Highway 201, and Maine state highway 17 in Augusta, Maine, called Cony Circle
- U.S. Highway 202, Maine state highway 104 and Maine state highway 27 in Augusta, Maine, called Memorial Circle
- At the western intersection of U.S. Highway 34 and U.S. Highway 63 in Ottumwa, Iowa. Three approaches; includes right turn bypass lanes.
- Quezon Memorial Circle, Elliptical Road, Quezon City, Metro Manila (Philippines)
- Arc de Triomphe, Paris
- U.S. Department of Transportation: Roundabouts: an Informational Guide para 1.5
- Johnson, Jeffrey A. (6 August 2012). "Salem Four Corners traffic circle to start rounding into shape". The Day.
- Commonwealth of Massachusetts, Registry of Motor Vehicles. "Sharing the Road: A User's Manual for Public Ways". Retrieved 2009-05-13.
- Modern Roundabouts, an Informational Guide
- Shashi S. Nambisan, Venu Parimi. "A Comparative Evaluation of the Safety Performance of Roundabouts and Traditional Intersection Controls". Institute of Transportation Engineers. Retrieved 2007-11-27.
- P. M. Wolf, Eugene Henard and the Beginning of Urbanism in Paris, 1900–1914, International Federation for Housing and Planning, The Hague, 1969, cited by Ben Hamilton-Baillie & Phil Jones, Improving traffic behaviour and safety through urban design, Proceedings of ICE – Civil Engineering, volume 158 Issue 5 May 2005 p. 41 http://www.hamilton-baillie.co.uk/papers/ICE_paper_April05.pdf