INTRODUCTION Background Definitions -Traffic Circles - Roundabouts HISTORY AND EVOLUTION OF ROUNDABOUTS The Old Traffic Circle The Beginnings of the "Modern Roundabout"

BACKGROUND

Although the United States was home to the first one-way rotary system in the world (implemented around New York City's Columbus Circle in 1904), traffic circles had fallen out of favor in this country by the 1950s. Older traffic circles, located primarily in the northeastern states, encountered serious operational and safety problems, including the tendency to lock up at higher volumes. The modern roundabout, although following different design principles from those of the old circles, has been notably less popular in the United States than abroad, in part because of this country's experience with the traffic circles and rotaries built in the first half of the 20^th century.

Since 1990, however, there has been an emergence of the modern roundabout in some parts of the United States. The strong interest expressed in this type of intersection in recent years is partially due to its success in several countries in Europe and in Australia, where the modern roundabout has changed the practice of intersection design.

DEFINITIONS

"NONCONFORMING" TRAFFIC CIRCLES

The early traffic circles often incorporated one or several problematic operational or design elements that would not be permitted in a modern roundabout. For example:

1. Entering traffic had right-of-way - At higher volumes this locks up the circle
2. Entries were regulated by stop signs or traffic lights - This reduces fluidity and capacity.
3. Entries were tangential to circle - This encourages high entering speeds and reduces the safety benefits.
4. Pedestrians crossed onto the central island - This is unsafe for pedestrians and disruptive for drivers.
5. The through road cut through the circle - Capacity, fluidity, and safety benefits are lost by the need to signalize the central intersection.
6. Circulating traffic was controlled by a traffic signal or stop sign - This decreases the fluidity of circulating traffic and can lock up the circle.
7. Parking was permitted in the circle - This reduces the capacity and safety of the circle by adding friction and conflicts.

1. Dupont Circle in Washington, D.C. - where entries are regulated by a mixture of traffic lights, stop and yield signs, the circle includes a weaving section, and pedestrians walk onto the central island;
2. Columbus Circle in New York City - where traffic lights control the entries, the circle is cut by through traffic, and pedestrians walk onto the central island; and
3. Place Charles de Gaulle in Paris, formerly know as Place de l'Etoile - where entering traffic has priority, and police officers regulate traffic in the circle.

DEFINITION OF MODERN ROUNDABOUTS

The term modern roundabout is used in the United States to differentiate it from the nonconforming traffic circles or rotaries that have been in use for many years, primarily in the Northeast. Modern roundabouts are defined by two basic operational and design principles:

1. Yield-at-Entry; Also known as off-side priority or yield-to-left rule, yield-at-entry requires that vehicles in the circulatory roadway have the right-of-way and all entering vehicles on the approaches have to wait for a gap in the circulating flow. To maintain fluidity and high capacity, the entry control is a YIELD sign. As opposed to nonconforming traffic circles, modern roundabouts are not designed for weaving movements, thus allowing smaller diameters. Even for multi-lane roundabouts weaving movements are not considered a design or capacity criterion.
2. Deflection for Entering Traffic: No tangential entries are permitted, and no traffic stream gets a straight movement through the intersection. Entering traffic points toward the central island, which deflects vehicles to the right, thus causing low entry speeds.
3. Flare: To provide for increased capacity, the modern roundabout often incorporates flares at the entry by adding lanes before the yield line, and has a wider circulatory roadway.

HISTORY AND EVOLUTION OF ROUNDABOUTS

The history of the modern roundabout, and in particular its evolution from the old traffic circles and rotaries built in the first half of the 20^th century, explains to a large degree its current status in the United States, and particularly the negative perception of roundabouts held by many traffic engineers and the general public.

No consistent right-of-way rules were adopted in those years. In New York City, for example, the north-south and south-north traffic had priority of east-west and west-east traffic. Practices differed in other places in the United States. Some U.S. courts decided that the "first-in" rule would be the most practical.  In general, the right-of-way rule was not too critical in the early days because traffic volumes were fairly low. Wisconsin, in 1913, was the first state to adopt the yield-to-right rule, meaning entering vehicles had the right-of-way. The yield sign, however, was unknown in the United States until the early 1950s.

As traffic volumes increased, however, more and more traffic circles locked up. At the Ellisburg traffic circle in New Jersey, traffic would lock up at hourly volumes ranging from 4,400 to 5,600 vehicles, and traffic often remained at a standstill until the police intervened. This circle has an elliptical shape with outside diameters of 130 m and 99 m (436 ft and 325 ft). Reluctant to reverse the right-of-way rule, the highway department installed a $270,000 computerized signal system yielding an hourly capacity of 4,400 vehicles. Other traffic circles, such as the Hawthorne Circle in Westchester County, New York, were replaced with grade-separated interchanges.

In France, the large sizes of the circles, the desire to maintain relatively high speeds, and the priority to the right became major impediments to safety and high capacity. The original gyratory at the Place Charles de Gaulle (formerly the Place de l'Etoile) became the symbol of traffic congestion in Paris.

THE BEGINNINGS OF THE "MODERN ROUNDABOUT"

Progress in roundabout design began early in Great Britain, where one-way streets and gyratory systems had existed since the mid-1920s, partially as the result of the consulting work by Eno. It was also in Great Britain where the term "roundabout" was officially adopted in 1926 to replace the term "gyratory." In the 1950s, British traffic engineers started questioning the American practice of large circles, arguing that long weaving sections, combined with higher speeds made possible with the large radii, were detrimental to high capacities. The American view that weaving volumes in excess of 1,500 hourly vehicles were impractical was challenged in Great Britain, although British traffic engineers continued analyzing roundabout capacity in terms of weaving capacity.

The off-side priority rule was officially adopted for roundabouts in Great Britain in 1966. From then on, roundabout design changed from larger circles with emphasis on merging and weaving to smaller roundabouts where the driver’s task was to accept a gap in the circulating flow. Capacities of large roundabouts were increased by 10 to 50 percent by reducing the size of the central island, bringing the yield line closer to the center of the circle, and widening the entries to the roundabout. In some cases, the roundabout capacity was increased to the degree that the capacity of the links between the intersections became the limiting factor for the network capacity. The first design guidelines for off-side priority roundabouts were issued in 1971 by the British Ministry of Transport, followed by revised guidelines in 1975, 1984, and 1993. Roundabouts were "exported" to Australia and some communities in France in the 1970s, and then to a larger number of countries in the 1980s.  In 1984, the French government adopted the off-side priority rule for roundabouts on national highways, meaning that entering traffic had to yield to traffic in the circle, even if the entering road was a national highway. This represented a major shift in French driving laws where "priority-to-the-right" had always been a basic rule.

As of mid-1997, there are about 15,000 modern roundabouts in France. Other European countries have also adopted this form of intersection as a standard design solution. In addition to the popularity in Great Britain and France, roundabouts are very common in Germany, Switzerland, the Benelux countries, the Nordic countries, Spain, and Portugal. Outside of Europe the modern roundabout is a standard feature in Australia, and is becoming more common in New Zealand, South Africa, and Israel.