What is the difference between a traffic circle, a rotary, and a roundabout?

Traffic Circles
     Neighborhood traffic circles are circular intersections built on local streets. These circles are utilized to lower the speeds on residential streets. Neighborhood circles generally vary from stop control use to no control at all on one or more entries. They are characterized by no raised channelization to guide the drivers onto the circulatory roadway. Consequently, some of the neighborhood traffic circles allow left-turning movements. Some traffic circles still exist in the USA, however safety and operational problems caused many of them to fall out of favor in the 1950s and 60s.

Rotaries
     Rotary is a term used in Eastern U.S. for the older-style circular intersections. Rotaries generally require the circulating traffic to yield, therefore giving the entering traffic the right of way. Rotaries typically have diameters greater than 300 feet, which allow for higher circulatory speeds. This results in a varying speed differential between the circulating and entering traffic. However, it also provides the space for a higher number of entering approaches and lanes. Some older rotaries also allow pedestrian traffic to access the central island.

Roundabouts
     The modern roundabout is now recognized nationally as an intersection type and traffic control treatment capable of providing unique and significant operational and safety benefits over a wide range of traffic volumes and conditions. It was developed in the United Kingdom in an attempt to rectify problems associated with neighborhood traffic circles and other circular intersections typically referred to as rotaries. Key features of the modern roundabout include yield control of entering traffic, channelized approaches, and geometric curvature that prohibits excessive speeds.

Modern Roundabouts are different than traffic circles in the following ways

• Traffic circles can involve stop signs or stop signals
• Traffic circles can be very large or very small
• Traffic circles can operate at higher speeds and often require motorists to move from one lane to another

Modern Roundabouts are not Traffic Circles

• Modern roundabouts follow a yield at entry rule, which requires approaching vehicles to wait for a gap in the circulating traffic before entering the roundabout
• Modern roundabouts involve low speeds for traffic entering and driving through the roundabout
• Modern roundabouts use deflection to slow entering traffic and enhance safety
• Vehicles in the modern roundabout have the right of way.

The distinguishing differences between traffic circles and modern roundabouts are:

• Modern roundabouts have a smaller diameter than most traffic circles, resulting in safer conditions and lower speeds.
• Vehicles can enter modern roundabouts much easier than traffic circles due to flared approaches, entry angles, slower speeds on the circulating roadway and the fact that vehicles entering roundabouts always yield to circulating traffic.
• Properly planned modern roundabouts are designed using rigorous standards based on specific turning volumes. Traffic circles are typically sized based on land availability and or road distance needed for accomplishing high speed weave movements.

Many of us have experience with a large, old style traffic circle (also called rotaries in some locations). The DuPont Circle in Washington D.C. for example is a traffic circle. Traffic circles are large in diameter, have high circulating speeds and can require some merging and weaving between lanes to exit. Traffic circles exhibit poor operations and high crash rates. Driving through traffic circles can be unnerving to the uninitiated driver, heck in some instances driving through traffic circles can be unnerving to even initiated drivers!

People unfamiliar with the modern roundabout often assume they cause similar problems since they are circular too, plus that term roundabout was tossed around in Great Britain in the 1920s so naturally lots of us are/were confused by that.

Here is a brief history about traffic circles and modern roundabouts:

• In 1966, research in Great Britain led to a yield at entry rule. The yield at entry rule ended a locking problem in the modern roundabout, improved capacity, reduced crashes and created a complete change in philosophy of roundabout design and operation.
• In the mid–1970s research continued and engineers came up with another variable that introduced a revised design that recommended a curved vehicle path or deflection be added to prevent vehicles from taking too straight a path into the intersection.

The term Modern Roundabout really took root because by the year 1984 the modern roundabout came into being with three principal features:

• yield to the traffic in the circle
• deflection at entry and
• low design speed.*

*Roundabout Studies in Kansas E.R. Russell, G. Luttrell. M. Rys