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A rotary, sometimes nicknamed a traffic circle, is a type of intersection that directs both turning and through traffic onto a one-way circular roadway, usually built for the purposes of traffic calming or aesthetics. Contrary to a roundabout, where entering traffic always yields to traffic already in the circle and merges in directly, the entrances to traffic circles are three-way intersections either controlled by stop signs, traffic signals, or not formally controlled. Colloquially, however, roundabouts are sometimes referred to as circles.
In the United States, traffic engineers typically use the term rotary for large scale circular junctions between expressways or controlled-access highways. Rotaries typically feature high speeds inside the circle and on the approaches.
In New England, traffic circles are generally called rotaries and the traffic that is already driving in the rotary always has the right of way. For examples of where this is specified, in Massachusetts "Any operator of a vehicle entering a rotary intersection shall yield the right-of-way to any vehicle already in the intersection.". In Rhode Island entering vehicles "Yield to vehicles in the roundabout." 
Distinct from roundabouts, traffic circles and rotaries may also have an interior lane that requires traffic on it to change lanes in order to exit the circle.
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 or 48 km/h require circulating vehicles to yield, often stopping, which lowers capacity and increases crash rates compared to 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 were not 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 USA 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.
Starting in the 1990s the USA saw a revival of mostly smaller traffic circles, termed "roundabouts". The modern roundabout finally arrived in the United States in 1990 in Summerlin, a major Las Vegas residential subdivision. As of December, 2015 there are about 4800 of these modern roundabouts in the United States. As an example, Washington State contains about 120 roundabouts as of October 2016, all having been built since 1997 with more planned.
Examples of traffic circles
Rotaries in parts of New England
Traffic circles are referred to as "rotaries" in Massachusetts as well as parts of Connecticut, New Hampshire & Vermont.
Traffic circles in other parts of the US
- Washington, D.C.: numerous traffic circles; see List of circles in Washington, D.C.
- New Jersey: multiple current and former traffic circles; see List of traffic circles in New Jersey.
- Berkeley, California: traffic circles (usually paired with stop signs) throughout the city; Marin Fountain Circle, one of few named circles, is the intersection of six streets ( ).
- Indianapolis, Indiana: Monument Circle, surrounding the Soldiers' and Sailors' Monument in the downtown area.
- New Orleans, Louisiana: Lee Circle.
- 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
- Maryland has numerous traffic circles, especially at state highway junctions. Notable ones include: State Circle and Church Circle in Annapolis, Washington Monument Circle on Charles Street in Baltimore, and Millenium Park Circle in Towson.
- 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
- Mecom Fountain Circle, in the Houston Museum District, intersection of Main Street, Montrose Boulevard, and Herman Park Drive, in Houston, Texas.
- New Braunfels, Texas: Intersection of Seguin Avenue/Texas SH 46 Business and San Antonio Avenue downtown at Main Plaza.
- 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 eastern end of Eastern Parkway in Louisville, Kentucky, intersecting with Cherokee Road at an entrance to Cherokee Park. All accesses are controlled with stop signs.
- R A Long Square in Longview, Washington, where Washington and Olympia Ways meet.
- Illinois Route 58 known as Golf Road, in Des Plaines, Illinois. Two streets leading to U.S. Highway 14 (Northwest Highway).
- A small, apparently unnamed traffic circle at the intersection of Rancho California Road and Anza Road, just east of Temecula, California.
- North end of Palisades Interstate Parkway in Fort Montgomery, New York, at junction with US 9W and US 202.
- The intersection of 41st Street and Avenue B in Austin, Texas.
- The intersection of Washington Avenue, Westcott Street, and Arnot Street in Houston, Texas.
- Roosevelt Boulevard, a six-lane artery which carries US-1 through Northeast Philadelphia, formerly had two complex traffic circles in which the inner three lanes in each direction went under the circle, while the out three lanes remained at grade and entered the circle. In addition, those outer three lanes had a "through path" which effectively cut a chord across the circle, with the flow controlled by multiple traffic lights. As of 2016 only Oxford Circle preserves this configuration, with Pennypack Circle having gone extensive modifications which removed the circle aspect entirely.
- Remedios Circle, Malate, Manila
- Philippines–Thailand Friendship Circle, Santa Ana, Manila
- Place Charles de Gaulle (Arc de Triomphe), Paris
- Praça do Marquês de Pombal, Lisbon, which comprises two concentric traffic-light controlled circles for motor vehicles, separated by a green space with foot and cycle paths
- "Online TDM Encyclopedia - Traffic Calming".
- 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.
- U.S. Department of Transportation: Safety Aspects of Roundabouts presentation
- U.S. Department of Transportation: Technical Summary: Roundabouts
- Commonwealth of Massachusetts, Registry of Motor Vehicles. "Sharing the Road: A User's Manual for Public Ways". Retrieved 2009-05-13.
- "Index - Roundabout: An Informational Guide, June 2000 - FHWA-RD-00-067".
- Shashi S. Nambisan, Venu Parimi (March 2007). "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
- History of the Modern Roundabout, RoundaboutsUSA http://www.roundaboutsusa.com/history.html
- History of the Modern Roundabout, RoundaboutsUSA http://www.roundaboutsusa.com/history.html
- Washintong's Roundabouts, WS-DOT website http://www.wsdot.wa.gov/safety/roundabouts/washingtons.htm
- William Kenny, New intersection could uproot Boulevard for two years, Northeast Times http://www.northeasttimes.com/2012/oct/10/6074-new-intersection-could-uproot-boulevard-for-two-years.html/
- Media related to Traffic circles at Wikimedia Commons