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A roundabout is a type of circular
intersection
In mathematics, the intersection of two or more objects is another object consisting of everything that is contained in all of the objects simultaneously. For example, in Euclidean geometry, when two lines in a plane are not parallel, their i ...
or junction in which road
traffic
Traffic comprises pedestrians, vehicles, ridden or herded animals, trains, and other conveyances that use public ways (roads) for travel and transportation.
Traffic laws govern and regulate traffic, while rules of the road include traffic ...
is permitted to flow in one direction around a central island, and priority is typically given to traffic already in the junction.
[''The New Shorter Oxford English Dictionary,'' Volume 2, Clarendon Press, Oxford (1993), page 2632]
Engineers use the term modern roundabout to refer to junctions installed after 1960 that incorporate various design rules to increase safety.
Both modern and non-modern roundabouts, however, may bear street names or be identified colloquially by local names such as rotary or traffic circle.
Compared to stop signs, traffic signals, and earlier forms of roundabouts, modern roundabouts reduce the likelihood and severity of collisions greatly by reducing traffic speeds and minimizing
T-bone
The T-bone and porterhouse are steaks of beef cut from the short loin (called the sirloin in Commonwealth countries and Ireland). Both steaks include a "T"-shaped lumbar vertebra with sections of abdominal internal oblique muscle on each side ...
and
head-on collisions. Variations on the basic concept include integration with tram or train lines, two-way flow, higher speeds and many others.
For pedestrians, traffic exiting the roundabout comes from ''one'' direction, instead of ''three'', simplifying the pedestrian's visual environment. Traffic moves slowly enough to allow visual engagement with pedestrians, encouraging deference towards them. Other benefits include reduced driver confusion associated with perpendicular junctions and reduced queuing associated with
traffic light
Traffic lights, traffic signals, or stoplights – known also as robots in South Africa are signalling devices positioned at intersection (road), road intersections, pedestrian crossings, and other locations in order to control flows of traf ...
s. They allow U-turns within the normal flow of traffic, which often are not possible at other forms of junction. Moreover, since vehicles that run on gasoline averagely spend less time idling at roundabouts than at signalled intersections, using a roundabout potentially leads to less pollution.
[ When entering vehicles only need to give way, they do not always perform a full stop; as a result, by keeping a part of their momentum, the engine will produce less ]work
Work may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
** Manual labour, physical work done by humans
** House work, housework, or homemaking
** Working animal, an animal tr ...
to regain the initial speed, resulting in lower emissions. Research has also shown that slow-moving traffic in roundabouts makes less noise than traffic that must stop and start, speed up and brake.
Modern roundabouts were first standardised in the UK in 1966 and were found to be a significant improvement over previous traffic circles and rotaries. Since then, modern roundabouts have become commonplace throughout the world, including Australia, the United Kingdom and France.
History
Origins and demise of traffic circles
Circular junctions existed before roundabouts, including:
* 1768 England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
: the Circus
A circus is a company of performers who put on diverse entertainment shows that may include clowns, acrobats, trained animals, trapeze acts, musicians, dancers, hoopers, tightrope walkers, jugglers, magicians, ventriloquists, and unicyclist ...
in the city of Bath, Somerset
Bath () is a city in the Bath and North East Somerset unitary area in the county of Somerset, England, known for and named after its Roman-built baths. At the 2021 Census, the population was 101,557. Bath is in the valley of the River Avon, ...
was completed. This was constructed based on architectural considerations and not for traffic purposes.
* 1780 (ca.) France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
: the Place de l'Étoile
Place Charles de Gaulle (), historically known as the Place de l'Étoile (), is a large road junction in Paris, France, the meeting point of twelve straight avenues (hence its historic name, which translates as "Square of the Star") including t ...
around the Arc de Triomphe
The Arc de Triomphe de l'Étoile (, , ; ) is one of the most famous monuments in Paris, France, standing at the western end of the Champs-Élysées at the centre of Place Charles de Gaulle, formerly named Place de l'Étoile—the ''étoile'' ...
in Paris
* 1791 US: Pierre Charles L'Enfant
Pierre "Peter" Charles L'Enfant (; August 2, 1754June 14, 1825) was a French-American military engineer who designed the basic plan for Washington, D.C. (capital city of the United States) known today as the L'Enfant Plan (1791).
Early life an ...
(who came to America in 1776) designed a plan of Washington D.C. which was then being planned. This design contained several large places/street crossings. Many of them had a rectangular outer outline, but within each was a place, around which the streets should be built, thus reducing the number of arms/legs of each crossing.
* 1821 US: the Governor's Circle (later renamed Monument Circle
The Indiana State Soldiers and Sailors Monument is a tall Neoclassicism, neoclassical monument built on Monument Circle, a circular, brick-paved street that intersects Meridian and Market streets in the center of downtown Indianapolis, Indiana. ...
) of Indianapolis
Indianapolis (), colloquially known as Indy, is the state capital and most populous city of the U.S. state of Indiana and the seat of Marion County. According to the U.S. Census Bureau, the consolidated population of Indianapolis and Marion ...
, Indiana
Indiana () is a U.S. state in the Midwestern United States. It is the 38th-largest by area and the 17th-most populous of the 50 States. Its capital and largest city is Indianapolis. Indiana was admitted to the United States as the 19th s ...
(which gave the city the nickname "The Circle City");
* 1877 France: The French architect Eugène Hénard
Eugène Alfred Hénard (22 October 1849 – 19 February 1923) was a French architect and a highly influential urban planner. He was a pioneer of roundabouts, which were first introduced in Paris in 1907.
Hénard advocated several major urban p ...
was designing one-way circular intersections.
* 1879 Netherlands
)
, anthem = ( en, "William of Nassau")
, image_map =
, map_caption =
, subdivision_type = Sovereign state
, subdivision_name = Kingdom of the Netherlands
, established_title = Before independence
, established_date = Spanish Netherl ...
: the Keizer Karelplein in Nijmegen
Nijmegen (;; Spanish and it, Nimega. Nijmeegs: ''Nimwèège'' ) is the largest city in the Dutch province of Gelderland and tenth largest of the Netherlands as a whole, located on the Waal river close to the German border. It is about 6 ...
;
* 1899 Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
: Brautwiesenplatz (Bride Meadow('s) Place) in Görlitz
Görlitz (; pl, Zgorzelec, hsb, Zhorjelc, cz, Zhořelec, :de:Ostlausitzer Mundart, East Lusatian dialect: ''Gerlz'', ''Gerltz'', ''Gerltsch'') is a town in the Germany, German state of Saxony. It is located on the Lusatian Neisse River, and ...
;
* 1904 US: Columbus Circle
Columbus Circle is a traffic circle and heavily trafficked intersection in the New York City borough of Manhattan, located at the intersection of Eighth Avenue, Broadway, Central Park South ( West 59th Street), and Central Park West, at the so ...
in Manhattan
Manhattan (), known regionally as the City, is the most densely populated and geographically smallest of the five boroughs of New York City. The borough is also coextensive with New York County, one of the original counties of the U.S. state ...
, New York;
* 1905 US: American architect William Phelps Eno
William Phelps Eno (June 3, 1858 – December 3, 1945) was an American businessman responsible for many of the earliest innovations in road safety and traffic control. He is sometimes known as the "Father of traffic safety", despite never hav ...
favored small traffic circles.
He re-designed New York City's famous Columbus Circle, which was finished in 1905.
* 1907 US: architect John McLaren designed one of the first American traffic circles for both autos and streetcars (trams) in the Hanchett Residence Park in what is now San Jose, California
San Jose, officially San José (; ; ), is a major city in the U.S. state of California that is the cultural, financial, and political center of Silicon Valley and largest city in Northern California by both population and area. With a 2020 popul ...
.
* 1909 United Kingdom: The first British circular junction was built in Letchworth Garden City
Letchworth Garden City, commonly known as Letchworth, is a town in the North Hertfordshire district of Hertfordshire, England. It is noted for being the first garden city. The population at the time of the 2011 census was 33,249.
Letchworth ...
.
Although some may still be (somewhat confusingly) referred to as 'roundabouts', the operating and entry characteristics of these traffic circles differed considerably from modern roundabouts.
Circular intersections were built in the United States, though many were large-diameter 'rotaries' that enabled high-speed merge and weave manoeuvres. Older-style traffic circles may control entering traffic by stop signs or traffic lights. Many allow entry at higher speeds without deflection, or require a stop and a 90-degree turn to enter. Because these circumstances caused a lot of vehicle collisions, construction of traffic circles and rotaries ceased around the world in the 1950s, and some were removed.
File:South-Yarmouth-MA-US rotary plaque.JPG, National Register of Historic Places
The National Register of Historic Places (NRHP) is the United States federal government's official list of districts, sites, buildings, structures and objects deemed worthy of preservation for their historical significance or "great artistic v ...
plaque on the first traffic circle in the United States, at the intersection of River and Pleasant streets in Yarmouth, Massachusetts
Yarmouth ( ) is a town in Barnstable County, Massachusetts, United States, Barnstable County being coextensive with Cape Cod. The population was 23,793 at the 2010 census.
The town is made up of three major villages: South Yarmouth, West Yarmou ...
File:Thomas-Circle Washington-DC 1922.jpg, Thomas Circle in Washington, D.C., 1922
File:Place Charles-de-Gaulle from the Arc de Triomphe, July 2001.jpg, Traffic ten abreast traverses the ''Place de l'Étoile
Place Charles de Gaulle (), historically known as the Place de l'Étoile (), is a large road junction in Paris, France, the meeting point of twelve straight avenues (hence its historic name, which translates as "Square of the Star") including t ...
''. This traffic circle surrounds the ''Arc de Triomphe
The Arc de Triomphe de l'Étoile (, , ; ) is one of the most famous monuments in Paris, France, standing at the western end of the Champs-Élysées at the centre of Place Charles de Gaulle, formerly named Place de l'Étoile—the ''étoile'' ...
'' at the intersection of ten two-way and two one-way streets. It has no lane markings.
1960s development of modern roundabouts
Widespread use of the modern roundabout began when the UK's Transport Research Laboratory
TRL Limited, trading as TRL (formerly Transport Research Laboratory) is an independent private company offering a transport consultancy and research service to the public and private sector. Originally established in 1933 by the UK Government a ...
engineers re-engineered and standardised circular intersections during the 1960s. Frank Blackmore
Frank Blackmore Order of the British Empire, OBE Distinguished Flying Cross (UK), DFC (16 February 1916 – 5 June 2008) was a British airman and traffic engineering (transportation), traffic engineer. He led the development of the offside prio ...
led the development of the "priority rule" and subsequently invented the mini-roundabout to overcome capacity and safety limitations. The priority rule was found to improve traffic flow by up to 10%. In 1966, the United Kingdom adopted a rule at all circular junctions that required entering traffic to give way to circulating traffic. A Transportation Research Board
The Transportation Research Board (TRB) is a division of the National Academy of Sciences, Engineering, and Medicine, formerly the National Research Council of the United States, which serves as an independent adviser to the President of the Unite ...
guide reports that the modern roundabout represents a significant improvement, in terms of both operations and safety, when compared with older rotaries and traffic circles. The design became mandatory in the United Kingdom for all new roundabouts in November 1966. Australia and other British-influenced countries were the first outside the UK to build modern roundabouts.
Spread in Europe and North America since 1970s
* In the 1970s France
France (), officially the French Republic ( ), is a country primarily located in Western Europe. It also comprises of Overseas France, overseas regions and territories in the Americas and the Atlantic Ocean, Atlantic, Pacific Ocean, Pac ...
and Norway
Norway, officially the Kingdom of Norway, is a Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of Jan Mayen and t ...
adopted the modern roundabout.
* In 1980 Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
had 19 roundabouts.
* In 1980 Norway had 15 roundabouts.
*In the early 1980s, single-lane roundabouts (or mini-roundabouts) were also introduced in the Netherlands
)
, anthem = ( en, "William of Nassau")
, image_map =
, map_caption =
, subdivision_type = Sovereign state
, subdivision_name = Kingdom of the Netherlands
, established_title = Before independence
, established_date = Spanish Netherl ...
. It began in the relatively sparsely populated northern and eastern Netherlands because of fears that the roundabouts would not be able to cope with the traffic density of the Randstad
The Randstad (; "Rim" or "Edge" City) is a roughly crescent-shaped conurbation in the central-western Netherlands, consisting primarily of the four largest Dutch cities (Amsterdam, Rotterdam, The Hague, and Utrecht); their suburbs, and many tow ...
; however, when it appeared the single-lane roundabouts had an even higher capacity than signalised intersections, they were constructed en masse in the western Netherlands as well.
* In 1983 France adopted the yield-at-entry rule on national routes; since then the country's roundabouts have proliferated.
* In 1985 Norway put up yield signs at the entries to all its roundabouts. After this, safety and traffic flow rapidly improved, and Norwegian roundabouts increased from 15 in 1980, to 350 in 1990, to 500 in 1992.
* In 1987 Switzerland introduced the yield-at-entry rule; since then its roundabouts increased from 19 in 1980 to 220 in early 1992, while 500 more were being considered.
* In the late 1980s the Netherlands saw significant growth with about 400 roundabouts constructed in just 6 years. Construction accelerated in the 1990s, and by 2001, there were an estimated 1,500 to 1,800 roundabouts in the Netherlands, more than half of which were located within built-up areas.
* In 1990 US constructed its first roundabout.
* In 1991, France was building 1,000 roundabouts every year.
* As modern roundabouts rose in popularity in the 1980s, the old traffic circles fell out of favour and many were converted into modern roundabouts or other types of intersections.
* In 1999 Canada built its first modern roundabout.
* As of the beginning of the 21st century, roundabouts were in widespread use in Europe. For instance:
** In 2010 France had more than 30,000 roundabouts.
** There were around 25,000 in the United Kingdom in 2015.
File:Aangekocht in 1991 van United Photos de Boer bv. - Negatiefnummer 32989 k 6 a. - Gepubliceerd in het Haarlems Dagblad van 15.09.1990.JPG, Roundabout, Haarlem
Haarlem (; predecessor of ''Harlem'' in English) is a city and municipality in the Netherlands. It is the capital of the province of North Holland. Haarlem is situated at the northern edge of the Randstad, one of the most populated metropoli ...
, Netherlands, 1990. Cyclists may also be users of a roundabout.
File:Pacionfi.JPG, Small roundabout in Barzio
Barzio (Valsassina, Valsassinese lmo, Bàrs) is a ''comune'' (municipality) in the Province of Lecco in the Italy, Italian region Lombardy, located in the Valsassina about northeast of Milan and about northeast of Lecco.
Twin towns
* Magland, F ...
, Italy
File:Straßwalchen kreisverkehr 2.jpg, Roundabout in Straßwalchen
Straßwalchen (Central Bavarian: ''Strosswoicher'') is a market town in the district of Salzburg-Umgebung (''Flachgau'') in the state of Salzburg in Austria.
Geography
The municipal area stretches along the northeastern border of Salzburg with the ...
, Austria
File:Colombo Galle Face Roundabout.JPG, Roundabout in the centre of Colombo
Colombo ( ; si, කොළඹ, translit=Koḷam̆ba, ; ta, கொழும்பு, translit=Koḻumpu, ) is the executive and judicial capital and largest city of Sri Lanka by population. According to the Brookings Institution, Colombo me ...
, Sri Lanka
File:Зеница 20180730 175900.jpg, Double-lane Raindrop Fountain Roundabout in Zenica
Zenica ( ; ; ) is a city in Bosnia and Herzegovina and an administrative and economic center of the Federation of Bosnia and Herzegovina's Zenica-Doboj Canton. It is located in the Bosna (river), Bosna river valley, about north of Sarajevo. Th ...
, Bosnia and Herzegovina, where roundabouts replaced all traffic lights since 2011.
File:Rydsrondellen.JPG, Roundabout signs in Linköping
Linköping () is a city in southern Sweden, with around 105,000 inhabitants as of 2021. It is the seat of Linköping Municipality and the capital of Östergötland County. Linköping is also the episcopal see of the Diocese of Linköping (Church ...
, Sweden
File:British Direction Sign 1.svg, A roundabout sign example used in the UK
North American introduction since 1990s
In the United States modern roundabouts emerged in the 1990s after years of planning and educational campaigning by Frank Blackmore and Leif Ourston, who sought to bring the by then well-established increased safety and traffic flow in other countries to America. The very first was constructed in Summerlin, Nevada
Summerlin is a master-planned community in the Las Vegas Valley of Southern Nevada. It lies at the edge of the Spring Mountains and Red Rock Canyon to the west; it is partly within the official city limits of Las Vegas and partly within uninco ...
in the summer of 1990. This roundabout occasioned dismay from residents, and a local news program said about it, "Even police agree, oundaboutscan be confusing at times." Between 1990 and 1995, numerous modern roundabounts were built in California, Colorado, Florida, Maryland, Nevada, and Vermont.
Municipalities introducing new roundabouts often were met with some degree of public resistance, just as in the United Kingdom in the 1960s.
* American confusion at how to enter and especially how to exit a roundabout was the subject of mockery such as featured in the film ''European Vacation
''National Lampoon's European Vacation'' is a 1985 American comedy film directed by Amy Heckerling and written by Robert Klane. The second film in National Lampoon's ''Vacation'' film series, it stars Chevy Chase, Beverly D'Angelo, Dana Hill, ...
'' (1985).
* A 1998 survey of municipalities found public opinion 68% opposed prior to construction, changing thereafter to 73% in favour.
* A 2007 survey found public support ranging from 22% to 44% prior to construction, and several years after construction was 57% to 87%.
* By 2011, however, some 3,000 roundabouts had been established, with that number growing steadily.
In the mid-2010s, about 3% of the then circa 4,000 U.S. modern roundabouts were located in Carmel, Indiana
Carmel is a suburban city in Indiana immediately north of Indianapolis. With a population of 100,777, the city spans across Clay Township in Hamilton County, Indiana, and is bordered by the White River to the east; the Hamilton-Boone county ...
, whose mayor James Brainard
James Brainard is the mayor of Carmel, Indiana, a principal city in the Indianapolis metropolitan area. Brainard, who first took office January 1, 1996, is currently serving his seventh consecutive four-year term, most recently reelected in 2019 ...
had been actively promoting their construction; because of increased safety, injuries caused by car accidents in the city dropped by 80% after 1996. there were about 4,800 modern roundabouts in the United States. As an example, Washington state
Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...
contained about 120 roundabouts , all having been built since 1997, with more planned.
The first Canadian modern roundabout was built in 1999. They became increasingly popular amongst traffic planners and civil engineers in the 15 years thereafter due to their success in Europe. By 2014 there were about 400 roundabouts in Canada at the time (most in Quebec, Alberta, British Columbia and Ontario), or one per 90,000 inhabitants (compared to one per 84,000 inhabitants in the United States that year).
Modern roundabout
A "''modern'' roundabout" is a type of looping junction
Junction may refer to:
Arts and entertainment
* ''Junction'' (film), a 2012 American film
* Jjunction, a 2002 Indian film
* Junction (album), a 1976 album by Andrew Cyrille
* Junction (EP), by Basement Jaxx, 2002
* Junction (manga), or ''Hot ...
in which road traffic travels in one direction around a central island and priority is given to the circulating flow. Signs usually direct traffic entering the circle to slow and to give way
In road transport, a yield or give way sign indicates that merging drivers must prepare to stop if necessary to let a driver on another approach proceed. A driver who stops or slows down to let another vehicle through has yielded the right of ...
to traffic already on it.[
][
]
Because low speeds are required for traffic entering roundabouts, they are physically designed to slow traffic entering the junction to improve safety, so that the roads typically approach the junction ''radially''; whereas older-style traffic circles may be designed to try to increase speeds, and have roads that enter the circle ''tangentially''.
Roundabouts are normally not used on controlled-access highways
A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms i ...
because of the low speed requirement, but may be used on lower grades of highway such as limited-access road
A limited-access road, known by various terms worldwide, including limited-access highway, dual-carriageway, expressway, limited access freeway, and partial controlled access highway, is a highway or arterial road for high-speed traffic which ...
s. When such roads are redesigned to incorporate roundabouts, traffic speeds must be reduced via tricks such as curving the approaches.
Many traffic circles have been converted to modern roundabouts, including the former Kingston traffic circle in New York and several in New Jersey. Others have been converted to signalised intersections, such as the Drum Hill Rotary in Chelmsford, Massachusetts
Chelmsford () is a town in Massachusetts that was established in 1655. It is located northwest of Boston. The Chelmsford militia played a role in the American Revolution at the Battle of Lexington and Concord and the Battle of Bunker Hill. ...
, which is now six lanes wide and controlled by four separate intersections.
Terminology
The word ''roundabout'' dates from early 20th-century use in the United Kingdom.
In U.S. dictionaries the terms ''roundabout'', ''traffic circle'', ''road circle'' and ''rotary'' are synonyms.[American Heritage](_blank)
("roundabout: ''Chiefly British'' A traffic circle.")
Random House
("roundabout: ''Chiefly Brit.'' See ''traffic circle''.")
Merriam-Webster
"roundabout, noun: ''British'': rotary 2"; "rotary 2: a road junction formed around a central circle about which traffic moves in one direction only—called also ''circle'', ''traffic circle'
Macmillan
"roundabout: a circular area where three or more roads meet that you have to drive around in one direction in order to get onto another road. The American word is ''traffic circle'' or ''rotary''."
("roundabout: UK (US ''traffic circle''))
Concise Oxford
(rotary: N. Amer. a traffic roundabout.) However, several experts such as Leif Ourston have stressed the need to distinguish between the characteristics of the modern roundabout and the nonconforming traffic circle:
;Modern roundabout:
*Entering traffic yields to circulating traffic
*Entering traffic aims at the centre of the central island and is deflected slowly around it
*Upstream roadway often flares at entry, adding lanes
;Nonconforming traffic circle:
*Entering traffic cuts off circulating traffic
*Entering traffic aims to one side of the central island (right side for right-hand traffic) and proceeds straight ahead at speed
*Lanes are not added at entry
The U.S. Department of Transportation
The United States Department of Transportation (USDOT or DOT) is one of the executive departments of the U.S. federal government. It is headed by the secretary of transportation, who reports directly to the President of the United States and ...
adopted the term ''modern roundabout'' to distinguish those that require entering drivers to give way to others. Many old traffic circles remain in the northeastern US
The Northeastern United States, also referred to as the Northeast, the East Coast, or the American Northeast, is a geographic region of the United States. It is located on the Atlantic coast of North America, with Canada to its north, the Southe ...
. Some modern roundabouts are elongated to encompass additional streets, but traffic always flows in a loop.
Rotary
In the United States, traffic engineers typically use the term ''rotary'' for large-scale circular junctions between expressways
Expressway may refer to:
*Controlled-access highway, the highest-grade type of highway with access ramps, lane markings, etc., for high-speed traffic.
*Limited-access road, a lower grade of highway or arterial road.
*Expressway, the fictional slide ...
or controlled-access highway
A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms i ...
s. Rotaries of this type typically feature high speeds inside the circle and on the approaches.
In the United States' New England
New England is a region comprising six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York to the west and by the Canadian provinces ...
region, however, "rotary" is the general term for all roundabouts, including those with modern designs. State laws in these states mandate that traffic already driving in the rotary always has the right of way. For instance, in , "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
Rhode Island (, like ''road'') is a U.S. state, state in the New England region of the Northeastern United States. It is the List of U.S. states by area, smallest U.S. state by area and the List of states and territories of the United States ...
entering vehicles "Yield to vehicles in the roundabout."
Terms used abroad
In the dialect used in the Scottish city of Dundee
Dundee (; sco, Dundee; gd, Dùn Dè or ) is Scotland's fourth-largest city and the 51st-most-populous built-up area in the United Kingdom. The mid-year population estimate for 2016 was , giving Dundee a population density of 2,478/km2 or ...
, ''circle'' is used to refer to roundabouts.
In the Channel Islands
The Channel Islands ( nrf, Îles d'la Manche; french: îles Anglo-Normandes or ''îles de la Manche'') are an archipelago in the English Channel, off the French coast of Normandy. They include two Crown Dependencies: the Bailiwick of Jersey, ...
a third type of roundabout, known as "Filter in Turn", exists. Here approaching drivers neither give way to traffic on the roundabout, as normal, nor have priority over it, but take it in turns to enter from each. Almost all of Jersey
Jersey ( , ; nrf, Jèrri, label=Jèrriais ), officially the Bailiwick of Jersey (french: Bailliage de Jersey, links=no; Jèrriais: ), is an island country and self-governing Crown Dependencies, Crown Dependency near the coast of north-west F ...
's roundabouts are of this type.
In the Philippines
The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no),
* bik, Republika kan Filipinas
* ceb, Republika sa Pilipinas
* cbk, República de Filipinas
* hil, Republ ...
, the term ''rotunda'' or ''rotonda'' is used in referring to roundabouts.
Operation and design
The fundamental principle of modern roundabouts is that entering drivers give way to traffic within the roundabout without the need for traffic signals. Conversely, older traffic circles typically require circling drivers to give way to entering traffic. Roundabouts may also have an interior lane. Generally, exiting directly from an inner lane of a multi-lane roundabout is permitted, given that the intersecting road has as many lanes as the roundabout. By contrast, exiting from an inner lane of an older traffic circle is usually not permitted and traffic must first move into the outside lane.
Vehicles circulate around the central island in one direction at speeds of . In left-hand traffic
Left-hand traffic (LHT) and right-hand traffic (RHT) are the practices, in bidirectional traffic, of keeping to the left side or to the right side of the road, respectively. They are fundamental to traffic flow, and are sometimes referred to ...
countries they circulate clockwise
Two-dimensional rotation can occur in two possible directions. Clockwise motion (abbreviated CW) proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back up to the top. The opposite ...
(looking from above); in right-hand traffic, anticlockwise.
Multi-lane roundabouts are typically less than in diameter; older traffic circles and roundabout interchange
A roundabout interchange is a type of interchange between a controlled access highway, such as a motorway or freeway, and a minor road. The slip roads to and from the motorway carriageways converge at a single roundabout, which is grade-separate ...
s may be considerably larger. Roundabouts are roughly the same size as signalled intersections of the same capacity.
Design criteria include:
* Right-of-way: Whether entering or circling vehicles have the right of way. The New Jersey
New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...
Driver's Manual recommends that, in the absence of flow control signs, traffic yields based on "the circle's historically established traffic flow pattern", and there are no set rules. In New England
New England is a region comprising six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York to the west and by the Canadian provinces ...
, Washington, D.C.
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
and New York State
New York, officially the State of New York, is a state in the Northeastern United States. It is often called New York State to distinguish it from its largest city, New York City. With a total area of , New York is the 27th-largest U.S. stat ...
, entering traffic yields, as is the norm in virtually all countries outside of the U.S.
* Angle of entry: Angles range from glancing (tangent
In geometry, the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve. More ...
ial) that allow full-speed entry to 90 degree angles (perpendicular
In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the ''perpendicular symbol'', ⟂. It can ...
). Deflection is required to avoid vehicles entering at excessive speeds.
* Traffic speed: High entry speeds (over ) 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.
Islands
Modern roundabouts feature a central island and sometimes pedestrian islands at each entry or exit often for decoration.
Denmark has begun widespread adoption of particular high islands, or if not possible, obstacles such as hedges or a ring of trees in larger examples. This is done to further increase the safety benefits of roundabouts, as the obstacles have been found to discomfort drivers more so than the roundabout itself compared to conventional intersections, thus initiating further observation and care taking of the driver. In Denmark, it was found to decrease accidents in roundabouts by -27% to -84% depending on height and type. In studies, heights of 0-0.9, 1-1.9 and 2+ metres were evaluated. It was found that for all heights, especially accidents leading to human injuries were reduced the most, by -47% to -84% for the aforementioned heights. The level of irritation to drivers is not to be understated, as it is the crucial point of the design: to force drivers to pay attention to the sides of the driving direction. This leads to drivers complaining about these designs, as Denmark in most regards embraces designing road infrastructure, such that the wanted driving behaviour leads to comfort i.e., lane width corresponding to speed limit and obstacles encouraging slowdown near points of safety concern such as schools. Such is the controversy for drivers that seasoned drivers teachers complain about this discomfort a decade after its safety is proven and adoption widespread.
Central
The central island may be surrounded by a truck apron
Truck aprons are found at roundabouts and slip lane
A slip lane, also known as a filter lane in the United Kingdom, is a road at a junction that allows motorists to change roads without actually entering an intersection.
Some intersections tha ...
that is high enough to discourage drivers from crossing over it, but low enough to allow wide or long vehicles to navigate the roundabout. The island may provide a visual barrier, to alert approaching drivers to the presence of the roundabout, and to encourage drivers to focus on the traffic in the path of the circle. A visual barrier significantly reduces the accident rate. Otherwise, vehicles anywhere in or near the circle can cause those entering to stop and wait for them to pass, even if they are opposite, which unnecessarily reduces traffic flow. The barrier may be a landscaped mound, a raised wall, a tree or tall shrubs. Road signage or flagpoles may be erected at the top of a landscaped mound.
Some communities use the island for monuments, the display of large public art or for a fountain. Pedestrians may be prohibited from crossing the circling lane(s). Access to the central island requires an underpass or overpass for safety.
= Art installations
=
Roundabouts have attracted art installations around the world:
* Bend, Oregon
Oregon () is a U.S. state, state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington (state), Washington, while the Snake River delineates much of it ...
(United States); Bend's Roundabout sculptures were honoured by Americans for the Arts
Americans for the Arts is a nonprofit organization whose primary focus is advancing the arts in the United States. With offices in Washington, D.C., and New York City, with more than 50 years of service. Americans for the Arts is dedicated to rep ...
as among the 37 most innovative approaches to Public Art in the country.
* Fuerteventura
Fuerteventura () is one of the Canary Islands, in the Atlantic Ocean, part of the North Africa region, and politically part of Spain. It is located away from the northwestern coast of Africa. The island was declared a biosphere reserve by UNES ...
, Canary Islands
The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Morocc ...
(Spain); Local government displayed sculptures at several roundabouts.
* Many countries in Europe (France first, but also Germany, Austria, Italy, Spain, and others) show the widespread use of roundabouts as art installations.
* An inventory of roundabouts in France, made by Marc Lescuyer, listed 3,328 roundabouts with artistic decor early in 2010.
* The Minerva Roundabout in Guadalajara
Guadalajara ( , ) is a metropolis in western Mexico and the capital of the list of states of Mexico, state of Jalisco. According to the 2020 census, the city has a population of 1,385,629 people, making it the 7th largest city by population in Me ...
, Mexico is one of the city's most famous monuments. It features the goddess Minerva
Minerva (; ett, Menrva) is the Roman goddess of wisdom, justice, law, victory, and the sponsor of arts, trade, and strategy. Minerva is not a patron of violence such as Mars, but of strategic war. From the second century BC onward, the Roma ...
standing on a pedestal, surrounded by a large fountain, with an inscription saying "Justice, wisdom and strength guard this loyal city".
* Several famous monuments in Europe, such as the Gate of Alcalá in Madrid
Madrid ( , ) is the capital and most populous city of Spain. The city has almost 3.4 million inhabitants and a metropolitan area population of approximately 6.7 million. It is the second-largest city in the European Union (EU), and ...
, Spain or the Arc du Triomphe
The Arc de Triomphe de l'Étoile (, , ; ) is one of the most famous monuments in Paris, France, standing at the western end of the Champs-Élysées at the centre of Place Charles de Gaulle, formerly named Place de l'Étoile—the ''étoile' ...
in Paris
Paris () is the capital and most populous city of France, with an estimated population of 2,165,423 residents in 2019 in an area of more than 105 km² (41 sq mi), making it the 30th most densely populated city in the world in 2020. S ...
, France, have been isolated from street traffic by means of a roundabout.
* The Garces Memorial Circle
Garces Memorial Traffic Circle, informally known as Garces Circle or just ''The Circle'', is a traffic circle in Bakersfield, California. The traffic circle is located at the intersection of Chester Avenue, Golden State Avenue ( State Route 204) a ...
in Bakersfield, California
Bakersfield is a city in Kern County, California, United States. It is the county seat and largest city of Kern County. The city covers about near the southern end of the San Joaquin Valley and the Central Valley region. Bakersfield's populat ...
contains a statue to Father Francisco Garces
Francisco is the Spanish and Portuguese form of the masculine given name ''Franciscus''.
Nicknames
In Spanish, people with the name Francisco are sometimes nicknamed "Paco". San Francisco de Asís was known as ''Pater Comunitatis'' (father of ...
.
Pedestrian
For larger roundabouts, pedestrian islands at each entry/exit encourage drivers to slow and prepare to enter the circle. They also provide a refuge where pedestrians may pause mid-crossing.
Vehicles or bicycles entering or exiting the roundabout must yield to all traffic including pedestrians.
Pedestrian crossing
Pedestrian crossing
A pedestrian crossing (or crosswalk in American English) is a place designated for pedestrians to cross a road, street or avenue. The term "pedestrian crossing" is also used in the Vienna and Geneva Conventions, both of which pertain to road sig ...
s at each entry/exit may be located at least one full car length outside the circle. The extra space allows pedestrians to cross behind vehicles waiting to enter the circle, and to allow exiting vehicles to stop for pedestrians without obstruction. Each pedestrian crossing may traverse a pedestrian island for protection that also forces drivers to slow and begin to change direction, encouraging slower, safer speeds. On the island, the pedestrian crossing may become diagonal, to direct the gaze of those crossing into exiting traffic.
Bicycles
Physically separated bikeways best protect cyclists. Less optimally, terminating cycle lanes well before roundabout entrances requires cyclists to merge into the stream of motor traffic, but keeps cyclists in full view of drivers, at some cost in motor vehicle speed. Cyclists may also be permitted to use pedestrian crossings.
Traditional cycle lanes increase vehicle–bicycle collisions. When exiting, a motorist must look ahead to avoid colliding with another vehicle or with pedestrians on a pedestrian crossing. As the intersection curves away from the exit, the path of an exiting vehicle is relatively straight, and so the motorist may often not slow substantially. To give way to a cyclist on the outside requires the exiting motorist to look toward the rear, to the perimeter. Other vehicles can obstruct the driver's view in this direction, complicating the motorist's task. The more frequent requirements for motorists to slow or stop reduce traffic flow. A 1992 study found that the risk to cyclists is high in all such intersections, but much higher when the junction has a marked bicycle lane or sidepath around its perimeter. Cycle lanes were installed at Museum Road, Portsmouth
Portsmouth ( ) is a port and city in the ceremonial county of Hampshire in southern England. The city of Portsmouth has been a unitary authority since 1 April 1997 and is administered by Portsmouth City Council.
Portsmouth is the most dens ...
, but were replaced by a narrowed carriageway
A carriageway (British English) or roadway (North American English) consists of a width of road on which a vehicle is not restricted by any physical barriers or separation to move laterally. A carriageway generally consists of a number of tra ...
to encourage lane sharing.
The roundabout at the Brown Road and Loop 202 interchange in Mesa, Arizona
Mesa ( ) is a city in Maricopa County, Arizona, Maricopa County, in the U.S. state of Arizona. It is the most populous city in the East Valley (Phoenix metropolitan area), East Valley section of the Phoenix Metropolitan Area. It is bordered by ...
, adopts a U.S.-recommended design. On-street pavement markings direct cyclists to enter the sidewalk at the end of the bike lane. Cyclists who choose to travel on the wide sidewalk, cross roundabout arms perpendicularly, well outside the circle. A pedestrian island allows pedestrians and cyclists to cross one lane at a time.
Fietsotonde Hovenring Eindhoven.jpg, The Hovenring
The Hovenring is a suspended cycle path roundabout in the province of North Brabant in the Netherlands. It is situated between the localities of Eindhoven, Veldhoven, and Meerhoven, which accounts for its name, and is the first of its kind in t ...
bicycle roundabout in the Netherlands is an innovative design, completely separating bicycles from vehicular traffic.
Okay to enter sidewalk.jpg, alt=Pavement markings indicate sidewalk riding is legal., Pavement markings invite cyclists to enter sidewalk on approach to roundabout in Mesa, Arizona
Mesa ( ) is a city in Maricopa County, Arizona, Maricopa County, in the U.S. state of Arizona. It is the most populous city in the East Valley (Phoenix metropolitan area), East Valley section of the Phoenix Metropolitan Area. It is bordered by ...
. Cyclists are still permitted to use the roundabout like any other vehicle.
Bicyclist in roundabout.jpg, alt=Cyclist rides through the main lane of roundabout, Cyclists can choose to ride on the sidewalk on far right, or in the main lanes of this roundabout in Mesa, Arizona.
Capacity and delays
The capacity of a roundabout varies based on entry angle, lane width, and the number of entry and circulating lanes. As with other types of junctions, operational performance depends heavily on the flow volumes from various approaches. A single-lane roundabout can handle approximately 20,000–26,000 vehicles per day, while a two-lane design supports 40,000 to 50,000.
Under many traffic conditions, a roundabout operates with less delay than signalised or all-way stop
An all-way stop – also known as a four-way stop (or three-way stop etc. as appropriate) – is a traffic management system which requires vehicles on all the approaches to a road intersection to stop at the intersection before proceeding throu ...
approaches. Roundabouts do not stop all entering vehicles, reducing both individual and queuing delays. Throughput further improves because drivers proceed when traffic is clear without waiting for a signal to change.
Roundabouts can increase delays in locations where traffic would otherwise often not be required to stop. For example, at the junction of a high-volume and a low-volume road, traffic on the busier road would stop only when cross traffic was present, otherwise not having to slow for the roundabout. When the volumes on the roadways are relatively equal, a roundabout can reduce delays, because half of the time a full stop would be required. Dedicated left turn signals (in countries where traffic drives on the right) further reduce throughput.
Roundabouts can reduce delays for pedestrians compared to traffic signals, because pedestrians are able to cross during any safe gap rather than waiting for a signal. During peak flows when large gaps are infrequent, the slower speed of traffic entering and exiting can still allow crossing, despite the smaller gaps.
Studies of roundabouts that replaced stop signs and/or traffic signals found that vehicle delays were reduced 13–89 percent and the proportion of vehicles that stopped was reduced 14–56 percent. Delays on major approaches increased as vehicles slowed to enter the roundabouts.
Roundabouts have been found to reduce carbon monoxide
Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
emissions by 15–45 percent, nitrous oxide
Nitrous oxide (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
emissions by 21–44 percent, carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
emissions by 23–37 percent and hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
emissions by 0–42 percent. Fuel consumption was reduced by an estimated 23–34 percent.[
]
Capacity modelling
Many countries have researched roundabout capacity. The software can help calculate capacity, delay and queues. Packages include ARCADY
Junctions is a software package by Transport Research Laboratory. It incorporates the previously separate programs ARCADY, PICADY and OSCADY. The latest version, Junctions 10, was launched Wednesday 3 February 2021.
ARCADY
ARCADY (Assessment o ...
, Rodel, Highway Capacity Software and Sidra Intersection
Sidra Intersection (styled SIDRA, previously called Sidra and aaSidra) is a software package used for intersection (junction) and network capacity, level of service and performance analysis, and signalised intersection and network timing calculat ...
. ARCADY and Rodel are based on the Transport Research Laboratory mathematical model. The TRL approach is derived from empirical models based on geometric parameters and observed driver behaviour with regard to lane choice. Sidra Intersection software includes roundabout capacity models developed in Australia and the US.
Research on Australian roundabouts was conducted in the 1980s at the Australian Road Research Board (ARRB). Its analytical capacity and performance models differ from the TRL model significantly, following a lane-based gap-acceptance theory including geometric parameters.
Research on U.S. roundabouts sponsored by the Transportation Research Board
The Transportation Research Board (TRB) is a division of the National Academy of Sciences, Engineering, and Medicine, formerly the National Research Council of the United States, which serves as an independent adviser to the President of the Unite ...
(TRB) and Federal Highway Administration
The Federal Highway Administration (FHWA) is a division of the United States Department of Transportation that specializes in highway transportation. The agency's major activities are grouped into two programs, the Federal-aid Highway Program a ...
(FHWA) culminated in a capacity model that was included in the ''Highway Capacity Manual
The Highway Capacity Manual (HCM) is a publication of the Transportation Research Board (TRB) of the National Academies of Sciences, Engineering, and Medicine in the United States. It contains concepts, guidelines, and computational procedures for ...
'' (HCM) Edition 6 and the ''TRB-FHWA Roundabout Informational Guide'' ( NCHRP Report 672). The HCM Edition 6 model is based on lane-based gap-acceptance theory. A recent NCHRP survey of US state transport agencies found that Sidra Intersection
Sidra Intersection (styled SIDRA, previously called Sidra and aaSidra) is a software package used for intersection (junction) and network capacity, level of service and performance analysis, and signalised intersection and network timing calculat ...
is the most widely used software tool in the US for roundabout analysis.
Safety
Statistically, modern roundabouts are safer for drivers and pedestrians than both older-style traffic circles and traditional intersections. Compared with these other forms of intersections, modern roundabouts experience 39% fewer vehicle collisions, 76% fewer injuries and 90% fewer serious injuries and fatalities (according to a study of a sampling of roundabouts in the United States, when compared with the junctions they replaced). At junctions with stop signs or traffic lights, the most serious accidents are right-angle, left-turn or head-on collisions where vehicles move fast and collide at high impact angles, e.g. head-on. Roundabouts virtually eliminate those types of crashes. Instead, most crashes are glancing blows at low angles of impact. Further, a study based on satellite imagery of all intersections in Australia observed consistently low speeds on roundabouts compared to other intersection types, contributing to reduced injury severity in case of a crash.
Some larger roundabouts take foot and bicycle traffic through underpass
A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube constr ...
es or alternate routes. However, an analysis of the New Zealand national crash database for the period 1996–2000 shows that 26% of cyclists reported injury crashes happened at roundabouts, compared to 6% at traffic signals and 13% at priority controlled junctions. The New Zealand researchers propose that low vehicle speeds, circulatory lane markings and mountable centre aprons for trucks can reduce the problem. The most common roundabout crash type for cyclists, according to the New Zealand study, involves a motor vehicle entering the roundabout and colliding with a cyclist who already is travelling around the roundabout (more than half of cyclist/roundabout crashes in New Zealand fall into this category). The next most common crash type involves motorists leaving the roundabout colliding with cyclists who are continuing farther around the perimeter.
Vision-impaired pedestrians
Poorly designed walkways increase risks for the vision-impaired, because it is more difficult than at a signalised intersection to audibly detect whether there is a sufficient gap in traffic to cross safely. At a signalised intersection, traffic comes to a stop, and an audible sound can be generated to indicate that it is time to cross.
This issue has led to a conflict in the United States between the vision-impaired and civil engineering
Civil engineering is a professional engineering discipline that deals with the design, construction, and maintenance of the physical and naturally built environment, including public works such as roads, bridges, canals, dams, airports, sewage ...
communities. One solution is to provide manually-operated pedestrian crossing signals at each entry. This increases construction and operation costs, and requires some way to disrupt traffic long enough for the pedestrian to cross (such as a HAWK beacon) that defeats the purpose of the roundabout. Signalisation also increases delays for most pedestrians during periods of light traffic, since pedestrians need to wait for a signal to change before (legally) crossing.
Signalised pedestrian crossings are normally used on large-diameter roundabout interchanges rather than small-diameter modern roundabouts.
Types of circular intersections
Large roundabouts, such as those used at motorway junctions, typically have two to six lanes and may include traffic lights to regulate flow.
Some roundabouts have a divider or subsidiary deflection island, by means of which is provided a "free flow" segregated left- (or right-) turn lane (for the UK see Design Manual for Roads and Bridges TD 51/03) between traffic moving between two adjacent roads, and traffic within the roundabout, enabling drivers to bypass the roundabout.
Gyratory system
The term "gyratory" (for example, Hanger Lane gyratory) is sometimes used in the United Kingdom for a large circular intersection with non-standard lane markings or priority arrangements, or where there are significant lengths of carriageway between the entry arms, or when buildings occupy the central island.
In the 21st century several gyratory systems in London have been removed, including Tottenham Hale and Elephant and Castle.
Smaller, small and mini- roundabouts
As the overall or external size of a roundabout (in the UK referred to as the Inscribed Circle Diameter – ICD) is reduced, so the maximum practicable (and prescribed) diameter for the central island is also reduced, whilst the width of the circulatory carriageway increases (due to the greater width of vehicle swept path at smaller turning radii). In most cases, this results in it being too easy – certainly when traffic is light relative to capacity – for drivers to traverse the roundabout at relatively high speed, with scant regard for road markings or the potential dangers to self or conflicts with other road users. To mitigate this risk, a proportion of the circulatory carriageway – an annulus around the central island – is segregated from general use by demarcation lines and differentiated from the outer annulus of the carriageway by a combination of a slightly raised surface, adverse crossfall, contrasting colours and textures and demarcating lines. The effect of this is to discourage drivers from taking a more direct path through the roundabout, their line of least resistance is more tightly curved (and therefore slower) but more bearable. The inner annulus provides for the trailing axles of longer or articulated vehicles to sweep across the inner annulus, which is therefore known as an over-run area (in UK usage), truck apron, or mountable apron.
The smaller the roundabout, the more such mitigation measures are likely to be abused – the less effective they will be. In the UK the minimum size for roundabouts with raised islands is 28 metre diameter ICD with a 4-metre diameter island. This threshold being driven primarily by vehicle geometry – which is globally relatively consistent – rather than driver behaviour, it is adopted in other jurisdictions too. Below this minimum size, the mini-roundabout prevails.
Mini-roundabouts
After developing the offside priority rule, Frank Blackmore
Frank Blackmore Order of the British Empire, OBE Distinguished Flying Cross (UK), DFC (16 February 1916 – 5 June 2008) was a British airman and traffic engineering (transportation), traffic engineer. He led the development of the offside prio ...
, of the UK's Transport Research Laboratory
TRL Limited, trading as TRL (formerly Transport Research Laboratory) is an independent private company offering a transport consultancy and research service to the public and private sector. Originally established in 1933 by the UK Government a ...
, turned his attention to the possibility of a roundabout that could be built at sites lacking room for a conventional roundabout.
Mini-roundabouts can incorporate a painted circle or a low dome but must be fully traversable by vehicles. Motorists can drive over them when there is no other traffic, but it is dangerous to do so otherwise. Once the practice is established it may be difficult to discourage. Mini-roundabouts use the same right-of-way rules as standard roundabouts but produce different driver behaviour. Mini-roundabouts are sometimes grouped in pairs (a double mini-roundabout) or in "chains", simplifying navigation of otherwise awkward junctions. In some countries road signs distinguish mini-roundabouts from larger ones.
Mini-roundabouts are common in the UK, Ireland and Hong Kong (particularly on Hong Kong Island), as well as Irapuato, Guanajuato, Irapuato in Mexico.
In the UK and also in other jurisdictions that have adopted mini-roundabouts, to drive across the central disc or dome when it is practicable to avoid it is an offence. Vehicles are required to treat the painted circle as if it were a solid island and drive around it. Some local authorities paint double white lines around the circle to indicate this, but these require permission from the Secretary of State (United Kingdom), Secretary of State for Transport. The central dome also must be able to be overrun by larger vehicles.
In the UK – and also in other highway jurisdictions – the maximum size for a mini roundabout is 28 metre (30 yard) ICD (inscribed circle diameter).
Raindrop roundabouts
These roundabouts do not form a complete circle and have a "wiktionary:raindrop, raindrop" or "wiktionary:teardrop, teardrop" shape. They have also been used at bowtie (road), bowtie intersections, replacing traffic signals that are inefficient without a turning lane. In addition to their use at intersections, raindrop roundabouts are also used in dogbone interchanges (described #Motorways, below).
Balcony roundabout
A balcony roundabout is just an elevated roundabout. They are constructed in such a way that vulnerable road users can cross underneath the roundabout. Footpaths and cycle paths along the different roads connect to the square under the roundabout. Vulnerable road users do not interfere with motorised traffic on the roundabout, reducing the risk of collision.
Turbo roundabouts
In the Netherlands, Belgium, Bosnia and Herzegovina, the United Kingdom, Finland, Spain, Poland, Hungary, Slovenia, the Czech Republic, North Macedonia and Croatia a relatively new type of two-lane roundabout designs is emerging, called "turbo roundabouts". These designs require motorists to choose their direction before entering the roundabout, thereby eliminating many conflicting paths and choices on the roundabout itself so that traffic safety is increased, as well as speed and capacity. These designs, seen from above, typically result in a spiralling flow of traffic, giving them the collective name of turbo roundabouts. As a minor drawback turbo roundabouts are often marked out such that a U-turn by means of the roundabout is not possible for drivers approaching on certain arms.
Several variations of turbo roundabouts exist. They are frequently designed for the intersection of a major road crossing a road with less traffic.
An early application of the principle was a six-arm and therefore relatively large (and fast) non-circular roundabout at Stairfoot, Barnsley, South Yorkshire, which was given spiral marking about 1984. At that time the method was considered experimental and needed special consent from central authorities. The turbo roundabout was formally developed in 1996 in the Netherlands by Lambertus Fortuijn, a researcher from the Delft University of Technology. Similar roundabouts, with spiralling lane markings, have been used for many years in the UK e.g. the A176/A127 (eastbound) at Basildon, Essex (). However, it was not until 1997 that the UK's national highway authorities published guidance (DMRB TA-78/97) that in effect endorsed the use of spiral markings in certain circumstances.
Turbo roundabouts can be built with raised lane separators (common in the Netherlands) or with lane markings only. The use of raised lane separators prevents road users from weaving (thereby reducing conflicts) but can make manoeuvring more difficult for large vehicles.
According to simulations, a two-lane roundabout with three exits should offer 12–20% greater traffic flow than a conventional, three-lane roundabout of the same size. The reason is reduced weaving that makes entering and exiting more predictable. Because there are only ten points of conflict (compared with 8 for a conventional single lane roundabout, or between 32 and 64 with traffic signal control), this design is often safer as well. Research and experiments show that traffic accidents are reduced by 72% on turbo roundabouts compared to multi-lane roundabouts, which have 12 points of conflict. Research at Windesheim University of Applied Sciences, Windesheim University also shows that turbo roundabouts reduce accidents including casualties by some 75% when compared to regular intersections, and by 61% compared to single-lane roundabouts. The same research made it very clear that it is safer for cyclists not to have priority over motor vehicles on the roundabout, than to have it.[ At least 70 have been built in the Netherlands, while many turbos (or similar, lane splitting designs) can be found in southeast Asia. Multi-lane roundabouts in the United States of America are typically required to be striped with spiral markings, as most states follow the federal Manual on Uniform Traffic Control Devices.
]
Sub-sea roundabouts
A new development is the roundabout below the seabed, in locations where multiple undersea traffic tunnels join. The first such roundabout is in the Eysturoy Tunnel (Eysturoyartunnilin), opened in December 2020 in the Faroe Islands. It connects the main island Streymoy with two locations on the island Eysturoy that are separated by a long fjord, Skálafjørður. So, three roads meet at this roundabout. Total length of the system is 11.24 km (6.98 mi). It was the largest ever infrastructure project in the Faroe Islands, estimated to have cost around a billion DKK.
Motorways
Roundabouts are generally not appropriate for placement on motorway or freeway mainlines because the purpose of such facilities is to provide for uninterrupted traffic flow. However, roundabouts are often used for the junction between the slip roads (called ''ramps'' in North America) and the intersecting road. A single roundabout, grade separation, grade separated from the mainlines, may be used to create a roundabout interchange
A roundabout interchange is a type of interchange between a controlled access highway, such as a motorway or freeway, and a minor road. The slip roads to and from the motorway carriageways converge at a single roundabout, which is grade-separate ...
. This type of junction is common in the UK and Ireland.
Alternatively, separate roundabouts also may be used at the slip road intersections of a diamond interchange to create what often is referred to as a "dumbbell interchange", which is increasingly common in both Europe and North America due to its reduced need for wide or multiple bridges. A variation of the dumbbell interchange, often called a "dogbone interchange", occurs when the roundabouts do not form a complete circle but are instead raindrop roundabouts (described #Raindrop roundabouts, above). This configuration reduces conflicts between vehicles entering the raindrop roundabouts from the ramps, reducing queueing and delays, compared with the dumbbell interchange.
Additional use of roundabouts for high-speed junctions is the roundabout interchange#Three-level stacked roundabout, 3-level stacked roundabout—this is a roundabout interchange
A roundabout interchange is a type of interchange between a controlled access highway, such as a motorway or freeway, and a minor road. The slip roads to and from the motorway carriageways converge at a single roundabout, which is grade-separate ...
where ''both'' of the roadway mainlines are grade separated. In the United Kingdom, the M25 motorway, M25/A3 road, A3, M8 motorway (Great Britain), M8/M73 motorway, M73 and A1 road (Great Britain), A1(M)/M18 motorway (Great Britain), M18 interchanges are examples of this type. These junctions, however, have less capacity than a full free-flow interchange. A similar design to this is the three-level diamond interchange.
Most junctions on Dublin's M50 motorway C-road were built using a standard roundabout interchange. The traffic volume of several of these junctions increased to a level higher than the capacity such roundabouts can accommodate, and in turn, have been converted into partially or fully free-flowing interchanges. One example is the Red Cow interchange. In Northern Ireland, the junction between the M1 motorway (Northern Ireland), M1 and M12 motorway, M12 (Craigavon connector motorway) is via a standard roundabout with a raised centre, three onslips and three offslips, and two lanes.
In the city of Malmö, Sweden, a roundabout connects two motorways, E22 from Lund, and the Inre Ringvägen, Inner ring road.
In the Netherlands, A6 motorway (Netherlands), A6 motorway and A7 motorway (Netherlands), A7 motorway used to cross near Joure using a roundabout until October 2017, when the junction was turned into a full Interchange (road)#Directional T interchange (Y-interchange), Y-interchange. The junction between the A200 motorway (Netherlands), A200 and the A9 motorway (Netherlands), A9 uses a 3-level stacked roundabout. Near Eindhoven (the Leenderheide junction), the junction for the A2 motorway (Netherlands), A2 uses a roundabout. An overpass was built for the A67 motorway (Netherlands), A67 from Antwerp to Germany.
Near Liège, Belgium, the Cheratte interchange between the A3 motorway (Belgium), A3/E40 and A25 road (Belgium), A25/E25 functions partially as a roundabout, with through traffic allowed to continue without entering the junction and traffic changing between motorways required to use the roundabout.
Rotary interchanges operate with traffic circles rather than roundabouts. Rotary interchanges are common in New England
New England is a region comprising six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York to the west and by the Canadian provinces ...
, particularly in the state of , but a European example of a rotary interchange may be found in Hinwil, Switzerland
). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...
.
Signalised roundabouts
A signalised roundabout is one where one or more entry is controlled by traffic signals, rather than by assumed priority. For each signalised entry there will also be a signalised stopline immediately upstream on the circulatory section. The signals prevent blocking on the roundabout, and balance and improve traffic capacity.
Examples include the M50 in Dublin, the Cherry Street roundabout in Kowloon, Hong Kong, Sheriffhall Roundabout in Edinburgh, Scotland, Newton Circus in Singapore, and many of the roundabouts along the Paseo de la Reforma in Mexico City.
An evolution of the signalised roundabout has been proposed recently. It is based in avoiding stops by eliminating conflict points in roundabouts. This proposed new paradigm (SYROPS) forms platoons of vehicles (e.g. 2 x 3 cars) that arrive at the roundabout with speed identical to the average circulation speed in the roundabout and within the time interval (visualised as a rotating priority sector) assigned to his entrance, avoiding all the conflicts of passage and with it the stops and accelerations required in standard and in signalled roundabouts. Signalling signage is with lights for human drivers and optionally wireless for connected and autonomous vehicles.
"Magic" roundabouts/ring junctions
"Magic roundabouts" direct traffic in both directions around the central island. They are officially known as "ring junctions". The Magic Roundabout (Swindon), first magic roundabout was constructed in 1972 in Swindon, Wiltshire, United Kingdom, designed by Frank Blackmore, inventor of the mini-roundabout. The roundabout joins five roads and consists of a two-way road around the central island, with five mini-roundabouts meeting the incoming roads.
The name derives from the popular children's television series, ''The Magic Roundabout'', and is considered "magic" because traffic flows in both clockwise and anticlockwise directions. This is achieved by surrounding the main island with one smaller roundabout per entry/exit street. This pattern directs traffic in the usual clockwise (in Left- and right-hand traffic, LHT installations) or counter-clockwise (in Left- and right-hand traffic, RHT installations) manner around each mini-roundabout. Exiting the mini-roundabouts, traffic may proceed around the central island either in the usual direction (via the outer loop), or in the inverse direction (the inner loop). The arrangement offers multiple paths between feeder roads. Drivers typically choose the shorter, most fluid route. Although the safety record is good, many drivers find this system intimidating, and some drivers go to great lengths to avoid them.
Similar systems are found in the Magic Roundabout (Hemel Hempstead), Moor End roundabout in Hemel Hempstead (Hertfordshire), which has six intersections; in Magic Roundabout (High Wycombe), High Wycombe (Buckinghamshire), the Denham Roundabout in Denham, Buckinghamshire, Denham (Buckinghamshire), the Magic Roundabout (Colchester), Greenstead Roundabout in Colchester (Essex), "The Egg (roundabout), The Egg" in Tamworth, Staffordshire#The Egg Roundabout, Tamworth (Staffordshire) and the Hatton Cross Roundabout in London.
Churchbridge Junction in Staffordshire is a magic gyratory. This type of junction is similar to a magic roundabout, except that the constituent roundabouts are connected by longer lengths of roadway.
Magicroundabout hemel.svg, Line drawing of the "Magic Roundabout" at Hemel Hempstead illustrating the concept and the reverse (anticlockwise) flow of the inner lane
Magic Roundabout Schild db.jpg, The Magic Roundabout (Swindon), Swindon Magic Roundabout
Dutch-style roundabouts for bicycles and pedestrians
Dutch roundabouts are designed to give cyclists priority over motorists. Cyclists have an outer ring on the roundabout, with cycle crossings over each of the four approach roads in a contrasting red surface. There are also zebra crossings over each approach road for pedestrians.
Reduced lane widths on the roundabout and at exit and entry points are designed to encourage drivers to slow down.
Trams
Tram roundabouts, which are found in many countries, merge roundabouts for individual vehicles with tram lines. Large areas are needed for tram roundabouts that include a junction between tram lines. Such systems often contain tram stops. Tramways usually cross at the centre of the roundabout. This requires traffic lights or special signalling granting the trams priority. However, there are also roundabouts where trams and vehicles use the same lane. Some roundabouts have a tram stop on the island.
*In France, tram roundabouts commonly have radii between 14 and 22 metres, although some have radii outside this range.
*In some cities, the tramway bisects the roundabout. The French considers that the mix of priorities makes these confusing and difficult to understand: a traditional modern roundabout gives the priority to the central ring, while tram roundabouts give priority to the central ring but higher priority still to the tramway. This generates many collisions of cars and trams, between 7 and 10 for each tram roundabout in France between 2006 and 2015 (between 0.37 and 1.01 per year).
*In inner Melbourne, particularly in the inner suburban area of South Melbourne, where the Trams in Melbourne, tram network is extensive, tram tracks always pass through the central island, with drivers required to give way to vehicles approaching from their right and to trams approaching from right angles. The Haymarket roundabout between Royal Parade, Melbourne, Royal Parade and Elizabeth Street, Melbourne, Elizabeth Street contains a tram-stop, pedestrian crossings, three entering tram lines, traffic signals to stop vehicular traffic at each crossing point when a tram is due, service roads and a pedestrian crossing.
*Brussels tram roundabouts employ multiple configurations. At the Barrière de Saint-Gilles, Belgium, St-Gilles (Dutch: ), tram tracks form a circle in the carriageway, while Verboekhoven and Altitude Cent (''Hoogte Honderd'') have reserved tram tracks inside the roundabout. At Place Stéphanie (''Stefaniaplein''), they go straight through the centre, with a slip track up the Chaussée de Charleroi (''Charleroisesteenweg''), while at Montgomery they tunnel underneath.
*In Dublin, Ireland, the Red Cow interchange at the N7 road (Ireland), N7/M50 junction is grade-separated and is signal-controlled with secondary lanes (separate from the main roundabout) for those making left turns. The junction, the busiest in Ireland, had tram lines added to it with the opening of the Luas system in 2004. The tracks pass across one carriageway of the N7, and across the southern M50 sliproads. Trams pass every five minutes at rush hour. The roundabout was replaced with a grade-separated free flowing junction.
*Gothenburg, Sweden has a tram roundabout and Gothenburg tram, tram stop at Korsvägen (the Crossroad). It carries heavy road traffic and about one tram or bus per minute passes in each direction. This is further complicated by separated rights-of-way for trams and buses and the fact that it is one of the busiest interchanges in the city. Another is located at Mariaplan in the inner suburb of Majorna. The trams make a right turn, giving the roundabout an odd design.
*In Warsaw, trams typically cross straight through roundabouts and have junctions in the centre of them. In Wrocław, Poland, trams pass through the Powstańców Śląskich Roundabout, stopping in the roundabout (north-headed track).
*The Silesian Interurbans, Silesian tram network in Poland has two tram roundabouts. In the city centre of Katowice, the tram line passes through the centre of the Ziętka Roundabout in a north–south alignment, with a tram stop in the centre of the circle. In Będzin, unusually, the tram junction itself forms a circular roundabout, with trams going around the circle; there are tram stops immediately outside the roundabout on each branch.
*In Vítězné náměstí (Victory Square) in Prague, Czech Republic, a tramway crosses the carriage way of the roundabout at three places. Entering as well as leaving trams give way to vehicles. In the years 1932–42 trams circulated much like vehicles.
*In Kyiv, Ukraine an interchange of two "Trams in Kyiv, fast tram" lines is below a roundabout.
*Oslo, Norway also has many roundabouts with Oslo Tramway, tram tracks passing through; for example at Bislett, Frogner plass, Sinsen, Solli plass, Carl Berners plass and Storo.
*In Wolverhampton, England, the West Midlands Metro tram passes through the centre of a roundabout on the approach to its terminus at Wolverhampton St George's tram stop, St Georges. This also happens in New Addington in Croydon on the Tramlink north of King Henry's Drive tram stop on Old Lodge Lane at the junction to King Henry's Drive.
*In Sheffield, England the Sheffield Supertram systems crosses two major roundabouts. At the Brook Hill roundabout near University of Sheffield, Sheffield University, the tramway passes underneath the roundabout in a subway, while at Park Square in the city centre it travels above the roundabout on bridges and viaducts with a junction in the central island.
*A roundabout in southern Zagreb, Croatia features tram tracks passing through, curving at a 90° angle, as well as a full tram mini-roundabout inside the middle road island. In Croatia, where tram tracks enter the road without traffic lights, trams have the highest priority and other non-emergency vehicles are required to yield.
*In Salt Lake City, Utah a light rail line on the south side of the University of Utah crosses a roundabout where Guardsman Way meets South Campus Drive. Like virtually all rail crossings in the United States, both crossings in the circle are equipped with boom barriers.
*In Kassel, Germany, Lines 4 and 8 pass through the centre of the roundabout at Platz der Deutschen Einheit. The tram stops are in the centre of the roundabout. Roundabout traffic is controlled by traffic lights. Pedestrian access is via subway and street-level crossings at the lights.
*In Bremen, Germany, tram lines 8 and 6 pass through the centre of the roundabout "Am Stern" east of the main railway station. They enter from the west and exit in a northeastern direction, thus making a slight bend within the roundabout. Both stations are situated on the north-eastern edge of the roundabout. Traffic is controlled by two-colour traffic lights inside the roundabout.
Railways
In Jensen Beach, Florida, the main line of the Florida East Coast Railway running north–south bisects the two-lane roundabout at the junction of Jensen Beach Boulevard running east–west. It hosts three other roads and the service entrance to a large shopping plaza. Boom barriers line the railway crossings. The landscaped central island bisected by the tracks was originally curb (road), curbed/kerbed, but semi-trailer truck, 18-wheelers had trouble negotiating the roundabout, so the curbs were replaced with painted concrete strips. The roundabout was built in the early 2000s and improved traffic flow, although long freight trains often cause delays.
Two roundabouts in the Melbourne metropolitan area, Highett, Victoria and Hampton, Victoria, Hampton, have passenger rail terminology#Heavy rail, heavy rail crossing the roundabout and through the inner circle. Boom barriers protect the railway from oncoming traffic at the appropriate points in the roundabout.
At the Driescher Kreisel in Bergisch Gladbach, Germany, a railway serving a nearby paper factory crosses a roundabout located next to a shopping centre and pedestrian zone. The flow of traffic and pedestrians is governed by 14 barriers, 22 traffic lights and 8 loudspeakers. The barriers close three times daily for 7 minutes to allow trains to pass.
In New Zealand's South Island, two roundabouts join major roads where a railway cuts through. One is at the intersection between State Highway 1 (New Zealand), State Highway 1 (as Sinclair Street and Main Street from the east) and Main Street (from the west), Park Terrace and Redwood Street in the city of Blenheim, New Zealand, Blenheim. Here the Main North Line, New Zealand, Main North Line bisects the roundabout and separates Park Terrace and Main Street eastbound from the rest of the roundabout. The other roundabout is located at Kumara Junction on the West Coast, New Zealand, West Coast, where the Hokitika Branch separates southbound from SH 6 northbound and New Zealand State Highway 73, SH 73. Both roundabouts are controlled by flashing red lights, with additional boom barriers at the Blenheim roundabout.
Through roundabout
Also known as a hamburger roundabout, these junctions are traffic light, signalised and have a straight-through section of carriageway for one of the major routes. The ''hamburger'' name derives from the fact that the plan view resembles the cross-section through a hamburger. The United Kingdom has examples on the A580 road, A580 East Lancashire Road in St Helens, Merseyside, St Helens, on Haydock, Haydock Island in Merseyside (which also features the M6 passing overhead), and on the Astley/Boothstown border. More examples are the A6003 road, A6003 at Kettering, the A538 road, A538 near Manchester Airport, the "Showcase" junction on A329 at Winnersh, Berkshire and the A63/A1079 Mytongate junction in Hull. Examples also exist in Bracknell, Kingston upon Hull, Hull, Bramcote in Nottinghamshire and Reading, Berkshire, Reading, as well as on the N2 road (Ireland), N2/M50 motorway (Ireland), M50 intersection in Dublin, Ireland. In Perth, Western Australia, one is found at the intersection of Alexander Drive, Morley Drive and The Strand. Throughabouts are very common in Spain, where they are called ''raquetas'' (Spanish for racket (sports equipment), racket") or ''glorieta/rotonda partida'' ("split roundabout").
Throughabout.svg, Throughabout
Throughabout Rotonde Verkeersbord 3.jpg, Throughabout road sign in the Netherlands
E37 AlexanderDr-MorleyDr sign.jpg, Throughabout road sign in Australia
Only bicycle-pedestrian roundabouts
The same features that make roundabouts attractive for roadway junctions led to their use at junctions of multi-use trails.
The University of California, Davis and Stanford University, as well as the Cape Cod Rail Trail, Cape Cod and Old Colony Rail Trail, Old Colony rail trails have bicycle-pedestrian roundabouts. A roundabout along the Clear Creek Trail (Indiana), Clear Creek Trail in Bloomington, Indiana, connects the main trail to its spur.
Roundabouts are used on off-road bicycle trails in Florida, Colorado, Alaska, and Wisconsin.
An elevated roundabout is located in Eindhoven, serving pedestrian and bicycle traffic only, above the main conventional roadway intersection. It is known as the Hovenring
The Hovenring is a suspended cycle path roundabout in the province of North Brabant in the Netherlands. It is situated between the localities of Eindhoven, Veldhoven, and Meerhoven, which accounts for its name, and is the first of its kind in t ...
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See also
* Complete streets
* Direction of traffic
* History of road transport
* History of street lighting in the United States
* History of roads in Ireland
* Level of service (transportation), Level of service
* Leif Ourston, an early proponent of roundabouts in the United States
* Roundabout Appreciation Society
* Roundabout dog
* Traffic congestion
Notes
References
Sources
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External links
City of Carmel, Indiana, USA, Roundabouts page (showing ''raindrop roundabouts'' or ''dogbone interchanges'')
Car Free America Roundabout Safety and Design Guide
Video of Highway Roundabout in Canada
TRL, The UK's Transport research Laboratory
Modern Roundabouts
– Geocoded National Database
Turbo Roundabout Simulation
from the Washington State Department of Transportation
Highway Roundabouts
from the Ministry of Transportation of Ontario
Roundabouts Now
Modern Roundabout Practice in the United States
from Transportation Research Board
Proceedings from the Transportation Research Board Standing Committee on Roundabouts (ANB75)
{{Authority control
Roundabouts and traffic circles,
Road junction types
French inventions