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Federal Highway Administration Research and Technology
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This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: Autumn 1995|
Issue No: Vol. 59 No. 2
Date: Autumn 1995
All images of British and Australian roundabouts have been deliberately reversed to show right-side driving so that most Public Roads readers can better comprehend the traffic pattern within the roundabout.
The superior safety record of modern roundabouts is well-known in Western Europe and in most British-influenced countries around the globe. Still, many in North America question whether drivers who are unfamiliar with this type of intersection can safely adapt to it.
But many American highway engineers have become advocates for modern roundabouts, and they are designing and building roundabouts to reduce accidents and increase capacity. Modern roundabouts have recently been built in California, Colorado, Florida, Maryland, Nevada, and Vermont. (1-4) Additional modern roundabouts are proposed for freeway interchanges in Maryland and California, and one modern roundabout interchange was built this summer on Interstate Highway 70 in Colorado.
This article takes a look at the roundabout accident experience of America, which began building modern roundabouts in 1990, and of some European countries, where as recently as five years ago modern roundabouts were relatively new. But first ...
The era of modern roundabouts began in the United Kingdom in 1956 with the construction of the first "yield-at-entry" roundabouts. In 1966, a nationwide yield-at-entry rule launched the modern roundabout revolution. Australia and most other British-influenced countries soon built modern roundabouts. Countries such as the United States, where people drive on the right side of the road, were slower to follow, but many of these countries have been rapidly catching up. For example, roundabouts have greatly increased in number in France since the adoption of the yield-at-entry rule on national routes in 1983.
Yield-at-entry is the most important operational element of a modern roundabout, but it is not the only one. Deflection of the vehicle path and entry flare are also important characteristics that distinguish the modern roundabout from the nonconforming traffic circle, which does not have these characteristics. (See side bar page 48). Other features include splitter islands at all approaches (to control entry speed and deter left turns), good sight distance, good lighting, good signing, no crosswalks across the circulatory roadway, yield lines downstream of the pedestrian crossings, and no parking in the roundabout.
All of these design characteristics must be met for a traffic circle to qualify as a modern roundabout. For example, figure 1 illustrates changing an old traffic circle to conform to a roundabout design by installing yield signs at all entries and forcing a deflection at the northern entry to the circle.
Roundabouts are designed in different sizes to serve various objectives and conditions. Even mini-roundabouts (with a diameter of 25 meters or less) are effective at reducing speed and improving safety. Small to medium roundabouts are 25 to 40 m in diameter. The larger roundabouts (with a diameter greater than 40 m) provide greater separation of traffic and a higher capacity.
The primary characteristics of the modern roundabout reduce many of the safety hazards of traditional intersections and nonconforming traffic circles.
The physical configuration of a modern roundabout, with a deflected entry and yield-at-entry, forces a driver to reduce speed during the approach, entry, and movement within the roundabout. This is contrary to an intersection where many drivers are encouraged by a green or yellow light to accelerate to get across the intersection quickly and to "beat the red light" and contrary to old traffic circles where tangent approaches also encourage, or at least allow, high-speed entries.
Another important safety factor is that the only movement at an entry and an exit of a roundabout is a right turn, thus reducing the potential frequency and severity of accidents compared to accidents typically occurring during left turns and when traffic crosses an intersection in perpendicular directions.
DeAragao reported on the history of roundabouts. It is thought that one-way circular intersections were invented by a French architect, Eugene Henard, in 1877.(5) During the same period, the American architect William Eno was also proposing his plan for small circles to alleviate traffic congestion in New York City. Since the adoption of a yield-at-entry regulation in 1966 by Great Britain and in 1983 by France, there has been overwhelming interest and research in roundabouts because of the simplicity of their design and operation and particularly because of their safety.
Enthusiasm for the safety and high capacity of roundabouts has resulted in a huge increase in the number of roundabouts. By contrast, as growing traffic demand causes nonconforming traffic circles to fail, they are converted to other types of intersections.
The Netherlands experienced spectacular growth of roundabouts beginning in the late 1980s.(6) In only six years, approximately 400 roundabouts were built. The reasons given are: a drastic reduction in serious crashes; lower driving speeds through the roundabouts; improved pedestrian crossing facilities; elimination of the need for traffic signals, thus reducing the costs of maintenance and enforcement; and high capacity more than 2,000 motor vehicles and several hundred bicycles and mopeds per hour in one-lane roundabouts.
Norway installed yield signs at the entries of all roundabouts in 1985, thus improving traffic flow and reducing accidents.(7) The number of roundabouts in Norway increased to 500 in 1992 (about one roundabout per 8,000 persons) from 350 in 1990.(8) In 1980, there were only 15 roundabouts in Norway.
Switzerland adopted the yield-at-entry rule in 1987. (7) The number of Swiss roundabouts grew to 220 in early 1992 from 19 in 1980. (6) Five hundred roundabouts were under study in 1992.
By 1987, more than 500 roundabouts had been built in the Brittany and western regions of France. Thereafter, new yield-at-entry roundabouts started "popping up" everywhere in France in new construction and in changing signalized intersections. In 1991, the growth of implementation was at the rate of 1,000 roundabouts per year.
In Portugal, the implementation of old-design traffic circles was driven by planning, architecture, and aesthetics of public areas, not necessarily for traffic capacity reasons.(9) Traffic circles marked the transitions and limits of the city of Lisbon. Similar to France, they were designed according to Henard's concept. With the uncontrollable increase in vehicular flow, larger and older circles in Lisbon became very congested, forcing the authorities to install signalization. Conversely, in medium- and small-size towns, modern roundabouts were successful because they were economical and did not require signalization.
The first modern American roundabouts were built in the spring of 1990 in Summerlin, a rapidly growing planned community on the west side of Las Vegas. (See figures 2 and 3.) With rapid growth of the surrounding community, daily traffic has increased from very low flows to about 7,000 vehicles in the north roundabout and to about 11,000 vehicles in the south roundabout. Only four accidents have been reported at the two roundabouts over their five-year history.
The first modern roundabout on the California state highway system was installed by the city of Santa Barbara in October 1992. The roundabout replaced an intersection of five two-lane streets regulated by stop signs. The old intersection averaged four accidents per year. Since installation of the roundabout, accidents have averaged 2.1 per year, with only five accidents reported in a 28-month period.(2)
Maryland's first roundabout was built in April 1993 in Lisbon. The 31.5 -mdiameter roundabout replaced a lightly traveled four-leg intersection regulated by a flashing beacon. The former intersection had averaged eight accidents with eight personal injuries per year.(1) Two accidents occurred in the first three months after construction of the roundabout, resulting in two personal injuries. For the following 21 months, there were no reported accidents.
Unlike nonconforming traffic circles, which often permit traffic to enter tangentially at speed, modern roundabouts deflect and slow entering traffic. At this roundabout in England, in the year before it was converted to a roundabout, there were eight serious injury accidents and two fatalities; in the year after conversion, there were no serious injury accidents.
The California Department of Transportation (Caltrans) converted the old nonconforming Long Beach traffic circle to a modern roundabout on June 30, 1993. (See figure 4.) Yield signs, stripes, and legends were added to all entries, which were widened to three and four lanes. The circulatory roadway was opened to one unstriped lane 11 m wide in front of the three-lane entries and 15 m wide in front of the four-lane entries. Accidents fell 36 percent compared to the average rate of the previous three years. Accidents with injuries fell 20 percent.
Around the world, accident rates are falling as roundabouts spread.
The Netherlands achieved a 95-percent reduction in injuries to vehicle occupants as many conventional intersections were replaced by modern roundabouts.(10)
The fatality rate in the United Kingdom is about half the rate in France 5,000 in the United Kingdom compared to 10,000 in France.
The difference is partially attributed to the use of roundabouts since the French and British population and their number of motor vehicles are about the same. (11)
In France, where roundabouts were installed mostly in urban areas and their suburbs including residential areas, the safety of roundabouts was generally superior to signalized intersections except where the roundabouts were large with wide entries or where there was extensive bicycle traffic. The accident rate on rural roads was clearly better for roundabouts than for major/minor junctions regulated by stop or yield signs. The average rate of reported injury accidents per 100 million vehicles entering major/minor junctions was 12. This was three times higher than the rate for roundabouts where there were only four accidents per 100 million vehicles.(12)
Numerous one- or two-lane roundabouts have been built recently in Germany, but several large old-style traffic circles remain. Researchers investigated 14 circular intersections and 14 non-circular intersections near the circular intersections. The numbers of accidents per million vehicles were: (13)
|Old traffic circles||Signalized intersections||Smaller roundabouts|
The most extensive roundabout accident analysis in Norway was conducted in 1990. Accident records from 1985 to 1988 at 59 roundabouts and 124 signalized intersections were examined. The comparative accident rates, in numbers of reported accidents per hundred million vehicles, are given below: (8)
Besides safety benefits, other advantages to roundabouts were demonstrated. Speed reduction, moderation of traffic flows in favor of through traffic, use of the central island to mark the transition from one class of road to another, and improved capacity are "products" of roundabouts.
Studies of British mini-roundabouts, which often have two- or three-lane entries even though the central islands are less than 4 m in diameter, indicate that larger roundabouts are generally safer.(14) However, recent studies of mostly one-lane mini-roundabouts in continental Europe found a lower accident rate at mini-roundabouts than at larger roundabouts.
Studies in Switzerland and France identified the following benefits of mini -roundabouts:(15,16)
While modern roundabouts have long been considered safe for pedestrians, the record for bicycles and motorcycles has been mixed.
According to one study in the United Kingdom, 15 percent of all intersection accidents in 1984 involved at least one bicyclist, but 22 percent of all roundabout accidents involved at least one bicyclist. (17)
In contrast, British mini-roundabouts do not appear to be particularly dangerous for bicyclists. A survey in 1989 of mini-roundabouts in England, Scotland, and Wales found that the crash-involvement rates of motorcycles and bicycles in 50-km/h (kilometers per hour) speed zones was about the same for four-leg mini-roundabouts as for four-leg signalized intersections. However, the rate for cars at the mini-roundabouts was much lower than at the intersections:
(per 10 million of vehicle type)
The flared entries of modern roundabouts give them a high capacity in compact space. At this multilane entry, a vane island between the second and third lanes deflects the trajectories of vehicles in the outer lanes.
Contrary to the British experience, a recent study in the Netherlands of 181 mini-roundabouts that were converted from three- and four-leg intersections found injuries to bicyclists decreased on average from 1.30 casualties per year to 0.37 casualties per year a 72-percent reduction. (10)
In Europe, bicyclists at roundabouts were handled in one of three ways: bicyclists mix with motor vehicles, bicyclists have a separate lane, or bicyclists have a separate bike road. The bike road provided the best protection for cyclists, and, perhaps surprisingly, the bike lane was the least safe option because this design requires the motorists and bicyclists to cross paths. (See figures 5 and 6.)
In 1990, 202 accidents were investigated at 179 urban roundabouts in France. Table 1 shows the relative frequency of the different causes of these accidents.(18)
The major design recommendations derived from the above study are:
Compared to signalized diamond interchanges, modern roundabout interchanges are safer and more efficient. A modern roundabout on each side of the freeway keeps the traffic flowing, eliminating the need for storage lanes. Thus, fewer lanes are needed on the most expensive element of the interchange - the overcrossing.
A study of roundabout lighting in France found that nighttime accidents are relatively rare and most accidents involve property damage only. The study recommends that the lighting design should be based on a perception process: remote perception at about 250 m, approaching perception at about 100 m, and entry perception at the entry.(19)
Modern American roundabouts have produced remarkable safety records. Since this experience is similar to the roundabout experience reported in other parts of the world, the safety of roundabouts compared to signalized intersections and old traffic circles has been well established. As a result, the number of roundabouts in the United States is expected to increase geometrically in the next decade.
(1) E.J. Myers. Press release on Lisbon, Md., roundabout; Hurst-Rosche Engineers, Cockeysville, Md., April 13, 1995.
(2) P. Wessel. Information transmittal from Santa Barbara Public Works Department to Peter I. Doctors, March 16, 1995.
(3) J. Goodway. Report by the Las Vegas Public Works Department, May 4, 1995.
(4) L. Ourston. "Nonconforming Traffic Circle Becomes Modern Roundabout," Unpublished report to Caltrans, October 1994.
(5) P. DeAragao. "Circles and Roundabouts: An Historic Review," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Institute of Transportation and Planning, Swiss Federal Institute of Technology, Lausanne, Switzerland.
(6) P.H. Bovy. "Spectacular Growth of Roundabouts in Switzerland: From 19 to 720 Roundabouts in 15 Years," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Institute of Transportation and Planning, Swiss Federal Institute of Technology, Lausanne, Switzerland.
(7) A. Dagersten. "Roundabouts in Switzerland and Sweden, Thesis 72," Department of Traffic Planning and Engineering, Institute of Technology, University of Lund, Lausanne, Switzerland, 1992.
(8) T. Giaever. "Application, Design, and Safety of Roundabouts in Norway," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology.
(9) F. Nunes da Silva. "The New Roundabouts in Portugal," Actes du Seminaires "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Graduate Technical Institute, Lisbon, Portugal.
(10) C. Shoon and J. Van Minnen. "The Safety of Roundabouts in the Netherlands," Traffic Engineering and Control, March 1994, pp. 142-148.
(11) Lauer. Center for Urban Transportation Studies, France.
(12) T. Brenac. "Roundabouts in France: Development, Level of Safety," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted at the National Institute of Research on Transportation and Safety, Provence, France.
(13) W. Brilon and B. Stuwe. "Roundabouts in Germany: Recent Results Regarding Capacity and Safety," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992.
(14) G. Maycock and R.D. Hall. Accidents at 4-Arm Roundabouts , Transport and Road Research Laboratory, United Kingdom, 1984.
(15) P. DeAragao. "Mini-Roundabouts in Switzerland," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted at the Institute of Transportation and Planning, Swiss Federal Institute of Technology, Lausanne, Switzerland.
(16) B. Guichet. "Mini-Roundabouts in France," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Center for Technical Studies of the Western Facilities, Nantes, France.
(17) R.E. Layfield and G. Maycock. "Pedal-Cyclists at Roundabouts," Traffic Engineering and Control, June 1986, pp. 343-349.
(18) B. Guichet. "Classification of Accidents on Urban Roundabouts," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Center for Technical Studies of the Western Facilities, Nantes, France.
(19) J. Menard. "Lighting of Roundabouts," Actes du Seminaire "Giratoires 92," Nantes, France, Oct. 14-16, 1992. Research conducted by the Center for Technical Studies of Facilities, Normandie, France.
Leif Ourston is president of Ourston & Doctors, a highway engineering firm in Santa Barbara, Calif., that specializes in modern roundabouts. He designed America's first modern roundabouts (in Las Vegas), Maryland's first modern roundabouts (in Lisbon and Gaithersburg), America's highest capacity modern roundabout (in Long Beach, Calif.), and America's first modern roundabout interchange (in Vail, Colo.). He wrote Caltrans' "Modern Roundabout Guidelines," and he is advising the Florida Department of Transportation on a modern roundabout program.
Joe G. Bared is a professional civil (transportation) engineer in the Design Concepts Research Division of the Federal Highway Administration. He manages research contracts and conducts staff research. Currently, he is developing accident prediction models for the Interactive Highway Safety Design Model. He is also pursuing a doctorate in transportation engineering at the University of Maryland.
Unlike nonconforming traffic circles, modern roundabouts conform to modern roundabout guidelines. Among other important new features, modern roundabouts have yield-at-entry, deflection, and (often) flare, as illustrated below.
Entering traffic yields to circulating traffic.
Nonconforming Traffic Circle
Entering traffic cuts off circulating traffic.
Entering traffic aims at the center of the central island and is deflected slowly around it.
Entering traffic aims to the right of the central island and proceeds straight ahead at speed.
Upstream roadway often flares at entry, adding lanes.
Lanes are not added at entry.
Since its installation in November 1992, the Five Points roundabout in Santa Barbara, Calif., has reduced accidents, confusion, and delay.