Roundabout

A roundabout is a type of circular intersection or junction in which road 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 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 lights. 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 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: the Circus in the city of Bath, Somerset was completed. This was constructed based on architectural considerations and not for traffic purposes.
* 1780 (ca.) France: the Place de l'Étoile around the Arc de Triomphe in Paris
* 1791 US: Pierre Charles L'Enfant (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) of Indianapolis, Indiana (which gave the city the nickname "The Circle City");
* 1877 France: The French architect Eugène Hénard was designing one-way circular intersections.
* 1879 Netherlands: the Keizer Karelplein in Nijmegen;
* 1899 Germany: Brautwiesenplatz (Bride Meadow('s) Place) in Görlitz;
* 1904 US: Columbus Circle in Manhattan, New York;
* 1905 US: American architect William Phelps Eno 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.
* 1909 United Kingdom: The first British circular junction was built in Letchworth Garden City.

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 plaque on the first traffic circle in the United States, at the intersection of River and Pleasant streets in Yarmouth, Massachusetts
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''. This traffic circle surrounds the ''Arc de Triomphe'' 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 engineers re-engineered and standardised circular intersections during the 1960s. Frank Blackmore 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 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 and Norway adopted the modern roundabout.
* In 1980 Switzerland 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. 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; 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, Netherlands, 1990. Cyclists may also be users of a roundabout.
File:Pacionfi.JPG, Small roundabout in Barzio, Italy
File:Straßwalchen kreisverkehr 2.jpg, Roundabout in Straßwalchen, Austria
File:Colombo Galle Face Roundabout.JPG, Roundabout in the centre of Colombo, Sri Lanka
File:Зеница 20180730 175900.jpg, Double-lane Raindrop Fountain Roundabout in Zenica, Bosnia and Herzegovina, where roundabouts replaced all traffic lights since 2011.
File:Rydsrondellen.JPG, Roundabout signs in Linköping, 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 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'' (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, whose mayor James Brainard 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 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 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 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 because of the low speed requirement, but may be used on lower grades of highway such as limited-access roads. 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, 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
("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 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. 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 or controlled-access highways. Rotaries of this type typically feature high speeds inside the circle and on the approaches.

In the United States' New England 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 Massachusetts, "Any operator of a vehicle entering a rotary intersection shall yield the right-of-way to any vehicle already in the intersection." In Rhode Island entering vehicles "Yield to vehicles in the roundabout."

Terms used abroad

In the dialect used in the Scottish city of Dundee, ''circle'' is used to refer to roundabouts.

In the Channel Islands 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's roundabouts are of this type.

In the Philippines, 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 countries they circulate clockwise (looking from above); in right-hand traffic, anticlockwise.

Multi-lane roundabouts are typically less than in diameter; older traffic circles and roundabout interchanges 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 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, Washington, D.C. and New York State, entering traffic yields, as is the norm in virtually all countries outside of the U.S.
* Angle of entry: Angles range from glancing (tangential) that allow full-speed entry to 90 degree angles (perpendicular). 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 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 (United States); Bend's Roundabout sculptures were honoured by Americans for the Arts as among the 37 most innovative approaches to Public Art in the country.
* Fuerteventura, Canary Islands (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, Mexico is one of the city's most famous monuments. It features the goddess Minerva 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, Spain or the Arc du Triomphe in Paris, France, have been isolated from street traffic by means of a roundabout.
* The Garces Memorial Circle in Bakersfield, California contains a statue to Father Francisco Garces.

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 crossings 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, but were replaced by a narrowed carriageway to encourage lane sharing.

The roundabout at the Brown Road and Loop 202 interchange in Mesa, Arizona, 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 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. 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 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 emissions by 15–45 percent, nitrous oxide emissions by 21–44 percent, carbon dioxide emissions by 23–37 percent and hydrocarbon 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, Rodel, Highway Capacity Software and Sidra Intersection. 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 (TRB) and Federal Highway Administration (FHWA) culminated in a capacity model that was included in the ''Highway Capacity Manual'' (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 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 underpasses 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 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, of the UK's Transport Research Laboratory, 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 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 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 "raindrop" or " teardrop" shape. They have also been used at 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 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 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 separated from the mainlines, may be used to create a roundabout interchange. 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 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 3-level stacked roundabout—this is a roundabout interchange where ''both'' of the roadway mainlines are grade separated. In the United Kingdom, the M25/ A3, M8/ M73 and A1(M)/ 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 and 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 Inner ring road.

In the Netherlands, A6 motorway and A7 motorway used to cross near Joure using a roundabout until October 2017, when the junction was turned into a full Y-interchange. The junction between the A200 and the A9 uses a 3-level stacked roundabout. Near Eindhoven