A roundabout is a type of circular intersection or junction in which road traffic is slowed and flows almost continuously in one direction around a central island to several exits onto the various intersecting roads. In a modern roundabout, entering traffic must always yield to traffic already in the circle, and roundabouts have additional restrictions on the junction layout to give high safety. Elimination of the opportunity for the most deadly crashes at intersections (T-bone or perpendicular crashes) is the greatest benefit of the roundabout design. Pedestrians are routed away from the intersection, to separate crosswalks located at least one full car length outside of the intersection, where they have a refuge between lanes of traffic. Consequently pedestrians only have to cope with traffic that is coming from one direction at a time, and which is travelling slowly enough for visual engagement with drivers, encouraging deference towards the pedestrians. Roundabouts, in their modern form, were standardised in the U.K., improving on the experience of traffic circles built in the U.S. following WWII. Modern roundabouts are now common in many countries.
The word roundabout dates from the early twentieth century. In the U.S., the term traffic circle is used where entering traffic is either controlled by stop signs, traffic signals, or is not formally controlled at all and speed of vehicles may remain unchanged. The term roundabout is reserved for circular flow intersections where entering traffic must yield to traffic already in the circle, so that traffic is calmed and flow through the intersection is maximized along with the highest safety for pedestrians and cyclists. Not all modern roundabouts are strictly circular, some being elongated to include several former intersections, but the traffic always flows in a loop.
Numerous circular junctions existed before the advent of roundabouts, including the Bath Circus world heritage site completed in 1768, the 1907 Place de l'Étoile around the Arc de Triomphe in Paris, the 1904 Columbus Circle in Manhattan, and several circles within Washington, D.C., however, the operating and entry characteristics of these circles differs considerably from modern roundabouts. The first British circular junction was built in Letchworth Garden City in 1909. Contrary to modern roundabouts, its centre originally was intended partly as a traffic island for pedestrians. In the early twentieth century, numerous traffic circle junctions were constructed in the United States, particularly in the northeast states. There are many instances of traffic circles in the U.S. that predate the modern roundabout, such as the ones that can be found in Atherton, California.
Although numerous circular junctions existed before the advent of modern roundabouts, the widespread use of the modern roundabout began when Transport Research Laboratory engineers re-engineered circular intersections during the 1960s and Frank Blackmore led the development of the offside priority rule and subsequently also invented the mini roundabout  to overcome its limitations of capacity and for safety issues. The rule was adopted as mandatory in Britain for all new roundabouts in November 1966. Unlike traffic circles, traffic approaching roundabouts is normally required to give priority to circulating and exiting traffic (this yield requirement has, however, been the law in the U.S. state of New York since the 1920s) and to eliminate much of the driver confusion associated with traffic junctions and waiting queues associated with junctions that have traffic lights. Roughly the same size as signalled intersections with the same or sometimes a higher capacity, they separate incoming and outgoing traffic, sometimes with pedestrian islands, to encourage slower and safer speeds (see traffic calming).
Initially, they remained much less common in the United States, where limited use began in the 1990s, and they faced some opposition from a population mostly unaccustomed to them. American confusion at how to enter and especially how to exit a roundabout was long the subject of comedic mockery such as featured in the film European Vacation and the television series The Simpsons. By 2011, however, there were about 3,000 U.S. roundabouts, with that number growing steadily. The first modern roundabout in the United States was constructed in Summerlin, Nevada in 1990. This roundabout occasioned a significant amount of "dismay" from residents, and a local news program said about it, "Even police agree, they (roundabouts) can be confusing at times."
As of the beginning of the twenty-first century, roundabouts were in widespread use in Europe. For instance, in 2010 France had more than 30,000 roundabouts.
Modern roundabout 
A "modern roundabout" is a type of circular junction that was developed by the United Kingdom Transport Research Laboratory in the mid-twentieth century, in which road traffic must travel in one direction around a central island and priority is given to the circulating flow. Signs usually direct traffic entering the circle to slow down and to yield the right of way. The centre of a modern roundabout provides a visual barrier across the intersection to the drivers entering it. This functions to assist the drivers into focusing only on the traffic coming toward them in the path of the circle (right in clockwise flow of traffic, and left in anti-clockwise flow). This significantly reduces the conflicts of concern encountered at conventional intersections. When drivers can see across a roundabout, if there are other vehicles in any other part of the device, drivers tend to stop outside the roundabout rather than merging, waiting for the vehicles (even on the opposite side) to come around to pass them. This interferes with the flow of traffic through the intersection, where many vehicles should be able to circulate in the roundabout at the same time. The high barrier may be a landscaped mound, a raised wall, or thickly planted very tall shrubs. Flag poles at the top of a landscaped mound in a roundabout are popular items. Some communities use the area for monuments, the display of large public art, and in a few, for a fountain, but caution must be exercised for these in case the feature might attract pedestrians into the intersection, which is very dangerous. Any lighting should focus carefully on the centre, but not shine out from the feature in the centre of the circle into the eyes of oncoming drivers. Including light-coloured and variegated plants and a good proportion of white flowered plants among those chosen for landscaping makes the mound visible at a greater distance for drivers, especially for those roundabouts without lighting.
These junctions are called "modern roundabouts" in order to emphasise the distinction from older circular junction types which had different design characteristics and rules of operation. Older designs, called "traffic circles" or "rotaries", are typically larger, operate at higher speeds, and often give priority to entering traffic. In some cases, the term "traffic circle" has been used to describe roundabouts in North America by those who do not recognize the distinction. Originally a British term, "roundabout" now is often used in North America by officials and engineers, but until recently it remained rare in general U.S. usage, although commonplace elsewhere.
Statistically, roundabouts are safer for drivers and pedestrians than both traffic circles and traditional intersections. Because low speeds are required for traffic entering roundabouts they usually are not used on controlled-access highways, but may be used on lower grades of highway such as limited-access roads. When such roads are redesigned to take advantage of roundabout principles, steps are taken to reduce the speed of traffic, such as curving the approaches. Sometimes the flow through the roundabout exceeds the level anticipated.
Modern roundabouts are commonplace throughout the world, in particularly Australia, Belgium, China, Costa Rica, Cyprus, Denmark, France, Germany, Hungary, Iceland, Ireland, Israel, Luxembourg, Malaysia, Morocco, the Netherlands, New Zealand, Poland, Portugal, Qatar, Spain, Trinidad and Tobago, the United Arab Emirates, and the United Kingdom. Half of the world's roundabouts are in France (more than 30,000 as of 2008).
Differences from traditional traffic circles 
Following research by the Transport Research Laboratory, which found that circular junctions with certain geometric characteristics and traffic control schemes tended to be safer than those without them, the United Kingdom introduced the rule in 1966 that entering traffic must always give way to traffic already on the roundabout.
Although the term roundabout sometimes is used incorrectly for a traffic circle even in the United States, their traffic engineers now make a clear distinction between them, noting that in a roundabout entering traffic must always yield to traffic already in the circle, whereas in a traffic circle entering traffic is controlled by stop signs, or is not formally controlled, although some states are exceptions, notably New York, which follows the "yield" rule although naming them Traffic Circles. In their use, circular junctions that have the following characteristics are roundabouts and those that do not are considered traffic circles:
- Roundabouts require entering drivers to give way to all traffic within the roundabout, regardless of lane position, while traffic circles typically allow traffic to enter alongside traffic circulating in an inner lane without consequence.
- Generally, exiting directly from the inner lane of a multi-lane roundabout is permitted, and such exiting traffic has the right-of-way over entering traffic. By contrast, exiting from the inner lane of a traffic circle is usually not permitted without first executing a lane change to the outside of the circle. In a state where older circular intersections called "rotaries" are commonplace, Massachusetts has a dwindling number of exceptions.
- Deflection on entry is used to maintain low speed operation in roundabouts. Drivers must manoeuvre (are deflected) around the splitter islands and the central island, at speeds of 15–25 miles per hour (24–40 km/h). Many older traffic circle junctions allow entry at higher speeds due to the lack of deflection, or require a stop and a 90-degree turn to enter, creating a large difference in speed between entering and circulating traffic which can make it difficult for entering drivers to find suitable gaps in heavy traffic.
- Pedestrians are prohibited from entering the intersection and central island of roundabouts and the crosswalk for pedestrians and some cyclists is withdrawn from the junction by at least the length of one vehicle.
- All vehicles circulate around the central island of a roundabout in the same direction, which is determined by whether traffic drives to the right or the left. In left-hand traffic countries the circulation is clockwise; in those that drive to the right, it is anticlockwise.
- Modern multi-lane roundabouts are typically less than 250 feet (75 metres) in diameter, although signalised roundabouts and roundabout interchanges may be considerably larger.
A large number of traffic circles have been converted to other types of junctions. Several have been converted to roundabouts, now meeting modern roundabout design standards, including the former Kingston traffic circle in New York and several in New Jersey. Others have been converted to a series of signalised intersections, such as the Drum Hill Rotary in Chelmsford, Massachusetts, which is now six lanes wide, with traffic lights on all four corners. The formerly oval-shaped rotary is now smaller and more circularly shaped.
In the U.S., many people use the terms "roundabout", "traffic circle", and "rotary" interchangeably, and they are defined as synonyms in dictionaries. This is the reason for the distinction made when engineers use the term, "modern roundabout". Many old traffic circles remain in the northeastern US. Since many of the older junction forms have unfavourable safety records, transportation professionals are careful to use "roundabout" when referring to the newer designs and "traffic circle" or "rotary" when referring to ones that do not meet the criteria listed above.
The term "traffic circle" is not used in the United Kingdom, where most circular junctions meet the U.S. technical criteria for roundabouts. The U.K. does, however, have roundabout variants such as mini-roundabouts and magic-roundabouts — see below for the distinctions from the type of junction generally referred to here as a roundabout. The more precise term, modern roundabout is used often to differentiate more carefully.
Roundabouts are safer than both traffic circles and junctions—having 40% fewer vehicle collisions, 80% 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). Roundabouts also reduce points of conflict between pedestrians and motor vehicles and are therefore considered to be safer for them, additionally, most feature a safety island refuge where pedestrians may pause mid-crossing. Large roundabouts, especially ones with faster traffic, are unpopular with some cyclists. This problem is sometimes addressed at larger roundabouts by taking foot and bicycle traffic through a series of underpasses or alternate routes. Clearwater Beach, Florida which has a multi-lane roundabout, has seen its previously high bicycle death rate plummet to zero since the construction of its roundabout in downtown.
At traditional junctions with stop signs or traffic lights, the most serious accidents are right-angle, left-turn, or head-on collisions that can be severe because vehicles may be moving fast and collide at high angles of impact. Roundabouts virtually eliminate those types of crashes because vehicles all travel in the same direction and most crashes are glancing blows at low angles of impact.
While roundabouts can reduce crashes overall compared to other junction types, crashes involving cyclists may not experience similar reductions in some designs. An analysis  of the New Zealand national crash database  for the period 1996–2000 shows that cyclists were involved in 26% of their reported injury crashes 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 improve the safety of cyclists within roundabouts. These strategies are typically employed on modern roundabouts constructed in the United States.
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 (generally just over 50% of all cyclist/roundabout crashes in New Zealand fall into this category). The next most common crash type they discovered involves motorists leaving the roundabout, colliding with cyclists who are continuing farther around the perimeter of the roundabout. Designs that have marked perimeter cycle lanes are found by their research data to be even less safe than those without them, suggesting that in roundabouts, cyclists should "take the lane", operating as a vehicle rather than tracking around the perimeter. The remedy these researchers advised to cure this was applying the rule prohibiting overtaking and passing on the circular roadway to motor vehicles overtaking cyclists.
If the adjacent footpaths are not properly designed, there are increased risks for persons with visual impairments. This is because it is more difficult (than at a signalized intersection) to detect with hearing whether there is a gap in traffic adequate to cross. During the all-red interval at a signal, traffic comes to a stop, and blind pedestrians can tell by listening which direction gets the green light. Since there is often moving traffic at a roundabout, the sounds of non-conflicting traffic or of a vehicle stopped to yield to the pedestrian might mask gaps.
This issue has led to a conflict in the United States between the visually-impaired and civil engineering communities; some in the visually-impaired community[who?] have taken the position that roundabouts (rather than signal-controlled crossings) are acceptable only if there are pedestrian crossings with signalised control at each road connecting to a roundabout. Engineers point out that since vehicle speeds are slower, crossing gaps are more plentiful, drivers are more apt to give way, and pedestrian crashes are less severe than if the same driver had run a red light. The blind community considers this to be a civil rights issue, however, not an engineering issue. While pedestrian crossings with traffic lights installed in roundabouts exist (see below), signalisation is normally used on large-diameter roundabout interchanges rather than small-diameter modern roundabouts. Signalisation would also substantially increase the cost of roundabout construction and maintenance (essentially, both types of junction being built at every junction). Furthermore, equipping a roundabout with traffic-halting lights would decrease its throughput considerably, thereby artificially reducing or even eliminating a significant advantage of the design over traditional signal-equipped junctions. Signalisation would also increase delays for most pedestrians during light traffic, since pedestrians would need to wait for the signal to change to legally cross.
Capacity and delays 
The capacity of a roundabout varies, based on the number of entry and circulating lanes, and also on more subtle geometrical elements, including entry angle and lane width. Also, as with other types of junctions, the operational performance of a roundabout depends heavily on the flow volumes from various approaches. A single-lane roundabout may be expected to handle approximately 20,000 to 26,000 vehicles per day, while a two-lane roundabout may be expected to handle 40,000 to 50,000 vehicles per day.
Under many traffic conditions, an unsignalised roundabout may operate with less delay to users than traffic signal control or all-way stop control. Unlike all-way stop intersections, a roundabout does not require a complete stop by all entering vehicles, which reduces both individual delay and delays resulting from vehicle queues. A roundabout may also operate much more efficiently than a signalised junction because drivers are able to proceed when traffic is clear without the delay incurred while waiting for the traffic signal to change. These advantages also reduce air pollution from many idling vehicles waiting for traffic lights to change.
Roundabouts can increase delays in locations where traffic would otherwise not be required to stop, however, for example, at the junction of a high-volume and a low-volume road, traffic on the busier road would normally not have to stop if the junction were signalised, because the traffic signals would provide a green signal to the busier road the majority of the time. When the volumes on the approach roadways are relatively balanced, a roundabout can reduce delay because each approach would otherwise encounter a red signal greater than half of the time if the junction were signalised.
Roundabouts can also reduce delays for pedestrians when compared to traffic signals, because pedestrians are able to cross during any safe gap rather than waiting for the traffic signal to provide the right-of-way to the pedestrian. During peak hours of congestion when large gaps are infrequent, the very slow speed of traffic entering and leaving the roundabout can compensate for the smaller gaps and facilitate pedestrian crossings.
Major research on the capacity of roundabouts has been carried out in several countries including the United Kingdom, Australia, United States, Germany, and France. This research has led to the development of several software packages to help professionals with calculating capacity, delay, and queues at roundabouts. The software packages available in the UK, Australia and USA include ARCADY, Rodel, Highway Capacity Software, and Sidra Intersection.
In the U.K., the Transport Research Laboratory (TRL) developed a mathematical model throughout the 1960s and 1970s to facilitate traffic engineers with the necessary knowledge to engineer and design roundabouts. This was delivered in the form of the ARCADY software. The U.K. TRL roundabout capacity model is also the basis of the Rodel software.
Research on Australian roundabouts was conducted in the 1980s at the Australian Road Research Board (ARRB). The analytical capacity and performance models developed as a result of this research differ from the U.K. TRL model significantly, and are based on gap-acceptance theory rather than the effect of roundabout geometric parameters. The UK based approach is derived from empirical models based on roundabout geometric parameters and drivers observed behaviour with regard to lane choice.
Research on U.S. roundabouts sponsored by the Transportation Research Board (TRB) and Federal Highways Administration (FHWA) culminated in a roundabout capacity model being included in the Highway Capacity Manual (HCM) 2010 edition. and the TRB - FHWA Roundabout Informational Guide (NCHRP Report 672) The HCM 2010 model is now a widely accepted lane-based alternative capacity model, based on gap-acceptance theory.
Roundabouts and cyclists 
Cycle facilities at roundabouts 
Research has shown that even in large circular junctions that lack modern roundabout design features, a high rate of bicycle-motor vehicle crashes occurs when bicyclists are riding around the outside. Design guidance for modern roundabouts recommends terminating cycle lanes well before the entrances, so bicyclists merge into the stream of motor traffic.
A 1992 study  from the German Transport Ministry's research institute has cast particular light on this issue. The study found that the risk to bicyclists is high in all such intersections, but it is much higher when the junction has a marked bicycle lane or sidepath around its outside (see "Marked perimeter cycle lanes" below). The results of this study concerning circular junctions are summarised on the web (in German, but partially translated below). A report about accidents at four-arm roundabouts was published by the U.K. Transport and Road Research Laboratory (TRRL) (now TRL) in 1984.
The Dutch Institute for Road Safety Research, however, noted that "roundabouts with separate bicycle tracks have a much lower number of casualties per roundabout than roundabouts with bicycle lanes, van Minnen (1995)". van Minnen advised for separated bicycle tracks, but against bicycle lanes for roundabouts.
Collisions typically occur when a motorist is entering or leaving the circular intersection. A motorist entering the circular intersection must give way to traffic in it, but such traffic will generally keep away from the outside of the circular intersection (as with a vehicle in the photograph) if passing an entrance. A bicyclist close to the edge of the roadway is not in the usual position where an entering motorist expects to look for circulating traffic.
When exiting the circular intersection, a motorist must look ahead to steer, and to avoid colliding with another vehicle ahead or with pedestrians on a footpath. As the circular intersection curves away from the exit, the path of a vehicle exiting it is relatively straight, and so the exiting motorist may often not need to slow substantially. If it is necessary to give way to a bicyclist riding around the outside, however, the exiting motorist must look toward the rear, to the outside of the intersection. With many vehicles, such as vans, the driver's view in this direction is obstructed. The task burden of the motorist is therefore substantially increased if bicyclists ride around the outside. The resulting conflicts, and more frequent requirements for motorists to slow or stop, also reduce the efficiency of traffic flow which is one of the major advantages of the circular junctions. Cycle lanes around the outside of circular junctions, therefore, are falling out of favour in some locales.
Marked perimeter cycle lanes 
An early attempt to deal with the cycle lane problem was to mark preferential lanes for cyclists. With cycle lanes, bicyclists do not merge into the flow of motor traffic in the roundabout, but rather, they travel around the outside, relieving them of the requirement to merge. The coloured road surface and edge lines of the cycle lanes indicate that exiting motorists are required to give way to bicyclists at all locations where their paths may cross. As noted previously, this design has been found to be hazardous to cyclists and has fallen out of favour.
This form of roundabout originally was installed at St. John's roundabout in Newbury, Berkshire, England and at Museum Road, Portsmouth, England. The St. John's roundabout in Newbury is still marked with perimeter cycle lanes. The cycle lanes on the roundabout at Museum Road, Portsmouth have been removed and, instead, the carriageway has been narrowed to encourage drivers and cyclists to circulate together.
Modern design guidance 
The special features of modern roundabouts, including splitter or diverter islands and a limited diameter of the circular path, decrease the speed of motor traffic and so reduce the risk of collisions for motorists as well as cyclists below that of conventional junctions. Design guidance  for roundabouts in the United States recommends terminating cycle lanes well before the entrances, so cyclists merge into the stream of motor traffic. Cyclists who lack the confidence to do this may use the footpaths as pedestrians and push their bicycles on the pedestrian crosswalks. U.S. design guidance also recommends placing the footpath crossings far enough from the roundabout so that at least one exiting vehicle can wait without blocking the circular path. A roundabout with two lanes should place the footpath crossing two car lengths from the junction.
The Dutch CROW (1998) guidelines, following the 'Sustainable Safety' approach, take a much different tack to dealing with cyclists and roundabouts. Instead of merging the cyclists with motor traffic, in providing predictable behaviour to road users, the guidelines recommend cyclist be given separate bicycle tracks on roundabouts, and be given priority on urban roundabouts, but not on rural roundabouts.
The roundabout at the Brown Road/202 interchange includes the U.S.-recommended design. On-street pavement markings indicate bicyclists may enter the sidewalk at the end of the bike lane. Bicyclists who choose to travel on the wide sidewalk, cross roundabout arms perpendicularly some distance removed from the circle. Each crossing crosses two lanes, one in each direction. A median refuge is provided to allow pedestrians and bicyclists to cross one lane at a time, therefore only having to concern themselves with traffic coming from one direction at a time.
The reason for placing crossings away from the circle is to give motorists more space and time to yield to cyclists and pedestrians who cross in them, and to increase the likelihood that crosswalk users and motorists will see each other.
Bicycle-pedestrian roundabouts 
The same features that make roundabouts an attractive option for roadway junctions have led to their use at junctions of multi-use trails. The University of California, Davis and Stanford University, as well as the Cape Cod and Old Colony rail trails have bicycle-pedestrian roundabouts. Roundabouts are used on off-road bicycle trails in Florida, Colorado, and Alaska.
Advantages of roundabouts 
- Roundabouts are safer than signal controlled junctions, with crashes usually occurring at a slower speed and at a slight angle instead of right-angle or rear-end collisions at junctions.
- Roundabouts allow U-turns within the normal flow of traffic, which often are not possible at other forms of junction.
- In general, roundabouts substantially reduce congestion and delays as vehicles are not required to perform a complete stop.
- Compared to intersections, Roundabout operate more efficiently and thus reduce delays and congestion as they are not at the whim of an artificially induced delay by traffic signals. Efficiency is gained by a direct response from the driver to the traffic conditions without any restrictions set by traffic signals - i.e. drivers may proceed when traffic is clear without the delay incurred by a traffic signal.
- The mound of land inside a roundabout may be used for a monument, civic art, or landscaping, thus contributing to the urban environment while at the same time solving a traffic problem. Features that might encourage pedestrians to cross the circulating roadway in order to approach whatever is in the circle should be avoided completely, however, as pedestrians should not be in the circle at any time.
Public opinion 
In the United States, municipalities introducing new roundabouts often are met with some degree of public resistance before trying them, just as in the United Kingdom in the 1960s. Drivers may be unfamiliar with roundabouts initially; however, surveys show that negative public opinion reverses as drivers gain experience with roundabouts. A 1998 survey of municipalities that built roundabouts found public opinion prior to construction as 68% opposed; afterwards it was 73% in favour. A 2007 survey of citizens found public support ranging from 22% to 44% prior to construction, and several years after construction was 57% to 87%.
Special types of circular intersections 
Large roundabouts such as those used at motorway junctions typically have two to six lanes around the central hub, and may have traffic lights regulating flow. They can not function as efficiently as a modern roundabout.
Some roundabouts have a divider between traffic turning from one road onto an adjacent one, and traffic within the roundabout, enabling those making such turns to bypass the roundabout entirely. Some call these slip lanes.
Gyratory system 
The term "gyratory" (for example, Hanger Lane gyratory) is sometimes used in the United Kingdom when a circular intersection is large and has non-standard lane markings or priorities, or when there are buildings on the central island; in fact, they are more like traffic circles.
After developing the offside priority rule, Frank Blackmore, of the U.K. Transport Research Laboratory, turned his attention to the possibility of a new type of roundabout that could be built at sites with inadequate space to build a conventional roundabout.
Mini-roundabouts exist at these smaller junctions to avoid the use of signals, stop signs, or the necessity to give way in favour of one road of traffic. Mini-roundabouts can be a painted circle or a low dome but must be fully traversable. Painted roundabouts and low domes can easily be driven over by most vehicles, which many motorists will do when there is no other traffic, but the practice is dangerous if other cars are present and once the practice is established it may be difficult to discourage. Mini-roundabouts work in the same way as larger roundabouts in terms of right of way, but can give different performance with regard to driver behaviour. Mini-roundabouts are sometimes grouped in pairs (a double mini-roundabout) or in "chains", making navigation of otherwise awkward junctions easier. In some countries there are different road signs used to distinguish mini-roundabouts from larger ones.
Mini-roundabouts are common in the U.K., Ireland, and Hong Kong (particularly on Hong Kong Island), as well as Irapuato in Mexico and Mount Royal and Rosedale in Calgary, Canada. Kemptville, Ontario, a small town located in Canada, has the record for the most roundabouts in Ontario, with a record of three on one highway.
A slightly larger version of a mini-roundabout, sometimes called a "small or midi-roundabout", is designed with a raised centre surrounded by a sloped "overrun area" of a different colour from the roadway and up to a metre wide surface, called a "truck apron" or a "mountable apron". The truck apron design discourages small vehicles from taking a shortcut over it while at the same time allowing the mini-roundabout to more easily accommodate the turning radius required for larger vehicles (such as a truck which may have to navigate the roundabout).
In the U.K. the maximum permissible diameter of the central painted circle of a mini-roundabout is 4m. Whilst it may be physically possible, it is illegal for vehicles which are able to circulate around the central circle of the roundabout to go over the painted circle, or around the wrong way. Vehicles should treat the painted circle as if it were a solid island and proceed around it. (In practice, many motorists ignore these rules, especially when traffic is light.) Some local authorities have installed double white lines around the painted circle to indicate this, but these are not permitted under U.K. traffic signs regulations without authorisation from the Secretary of State for Transport. The centre island also must be able to be overrun by larger vehicles.
Raindrop roundabouts 
These roundabouts do not form a complete circle and have a "raindrop" shape. (They are also sometimes referred to as "teardrop roundabouts".) They are appearing at U.S. Interstate interchanges to provide a free-flowing left turn to the on-ramps and eliminating the need for turn signals and lanes. Since the entry and exit slip roads are one-way, a complete circle is unnecessary. This means that drivers entering the roundabout from the bridge do not need to give way, and that prevents queuing on narrow, two-lane bridges. These roundabouts have been used at dumbbell roundabout junctions, replacing traffic signals that are inefficient without a turning lane. Several junctions along Interstate 70 near Avon, Colorado use teardrop roundabouts.
Turbo roundabouts 
In the Netherlands and Belgium, a relatively new type of roundabout is being built increasingly often. It provides a forced spiralling flow of traffic, thus requiring motorists to choose their direction before entering the roundabout. By eliminating many conflicting paths and choices on the roundabout itself, traffic safety is increased, as well as speed, and as a result, capacity. A turbo roundabout does not allow travelling a full circle.
Several variations of the turbo roundabout exist. The basic turbo roundabout shape is designed for the intersection of a major road crossing a road with much less traffic.
Turbo roundabouts were originally built with raised lane separators. Newer implementations with only lane markings exist, for efficiency of the design (regarding safety, speed, and capacity) by reducing the safety risk to large and small vehicles and enabling maintenance such as snow ploughs. Similar roundabouts, with spiralling lane markings, have been used for many years in the U.K. e.g. the A176/A127 (eastbound) at Basildon, Essex.
According to micro-simulation, a two-lane roundabout with three right turns should offer 12-20% greater traffic flow than a conventional, three-lane roundabout of the same size. The reason offered by authors Ir. Isaak Yperman and Professor Ir. Ben Immers is that there is less weaving in a turbo, making 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), it is often expected that this design is safer, as well. 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 or develop a state-level manual subject to the approval of the federal government.
Roundabouts on 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 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 U.K. 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.
An additional use of roundabouts for 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 use a standard roundabout interchange — although several such junctions have a greater volume of traffic than the capacity such roundabouts can accommodate. In Northern Ireland, the junction between the M1 and M12 (Craigavon connector motorway) is via a standard roundabout with a raised centre, 3 onslips and 3 offslips, and 2 lanes.
In the city of Malmö, Sweden, there is a roundabout connecting two motorways, Autostradan from Lund, and the Inner ring road. It is signposted as a motorway throughout this roundabout. Today these two motorways are considered local, but before the year 2000 they were part of the European roads E6, E20 and E22.
In the Netherlands, A6 motorway and A7 motorway cross near Joure using a roundabout. For the junction between the A200 and the A9 a 3-level stacked roundabout is used. Near Eindhoven (the Leenderheide junction), the junction for the A2 is done with a roundabout. An overpass is built for the A67 from Antwerp to Germany.
Roundabout interchanges are sometimes confused with rotary interchanges, which operate with traffic circles rather than roundabouts. Rotary interchanges are common in New England, particularly in the state of Massachusetts, but a European example of a rotary interchange may be found in Hinwil, Switzerland.
Controlled roundabouts 
Some bridges on Beijing's Second Ring Road are controlled by traffic lights. While it may appear to defy the roundabout system at first, it works well to control the flow of traffic on the bridges, which are two viaducts creating a roundabout suspended over the ring road.
Signal controlled roundabouts are common in Great Britain and Ireland, where they have been introduced in an attempt to alleviate traffic problems at over-capacity roundabout junctions or to prevent some flows of traffic dominating others (around the M50 in Dublin for example). The Cherry Street roundabout in Kowloon, Hong Kong is also one such example.
"Magic" roundabouts or ring junctions 
The town of Swindon in Wiltshire, England, is known for its "Magic Roundabout". This roundabout is at a junction of five roads and consists of a two-way road around the central island with five mini-roundabouts where it meets the incoming roads. Traffic may proceed around the main roundabout either clockwise via the outer lanes, or anticlockwise using the inner lanes next to the central island. At each mini-roundabout the usual clockwise flow applies.
Similar systems are found in various places in England, most famously the Moor End roundabout in Hemel Hempstead (Hertfordshire), which has six intersections; but also in High Wycombe (Buckinghamshire), the Denham Roundabout in Denham (Buckinghamshire), the Greenstead Roundabout in Colchester (Essex), the Sadlers Farm Roundabout in Benfleet (Essex) which is the junction between the A130, the A13 and the B1464 (although this is currently being converted to a signal-controlled roundabout, with work expected to be completed by early 2012,) "The Egg" in Tamworth (Staffordshire) and the Hatton Cross Roundabout in London. Magic roundabouts are also known as "Ring Junctions"
Roundabouts with trams 
Tram roundabouts are found in many countries.
In inner Melbourne, particularly in the inner suburban area of South Melbourne, where the tram network is extensive, tram tracks always pass through the central island of these roundabouts, with drivers required to give way, not only to vehicles coming toward them from their right, but also to trams coming toward them from right-angles.
Having trams pass through small roundabouts is not a problem; through larger roundabouts it can be difficult, particularly when there is a junction between tram lines as well. In these cases, the roundabouts are very large, and often have tram stops in the middle. The Haymarket roundabout between Royal Parade and Elizabeth Street is the most well-known junction of this nature, containing 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.
In Brussels, Belgium, trams treat roundabouts in various ways: at the Barrière de St-Gilles/Bareel St-Gillis, the tram tracks form a circle in the carriageway, while Verboekhoven and Altitude Cent/Hoogte Honderd have reserved tram tracks on the inside of the roundabout. At Place Stéphanie/Stefaniaplein, they go straight through the middle, 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/M50 junction is particularly infamous. It is a grade-separated motorway junction, 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 at a frequency of every five minutes at rush hour. The roundabout has been replaced with grade separated free flowing junction.
In Gothenburg, Sweden the roundabout and tram stop at Korsvägen (the Crossroad) is of this type, and is rather infamous in the city. It is heavily trafficked by cars, and about one tram or bus per minute passes in several directions. This is further complicated by separated rights-of-way for trams and buses and the fact that it is also one of the busiest interchanges in the city. Another one is located at Mariaplan in the inner suburb of Majorna. The trams makes a right turn, giving the roundabout an odd design. Since traffic isn't heavy, this normally doesn't create any problems.
In Warsaw, trams typically cross straight through roundabouts, and have junctions in the middle of them. In Wrocław, Poland, trams pass through the Powstańców Śląskich Roundabout, having a stop in the roundabout (north-headed track).
In Vítězné náměstí (Victory Square) in Prague, Czech Republic, a tramway crosses the carriage way of the roundabout at three places since 1942. Entering as well as leaving trams have to give way to all cars. In years 1932–1942 trams went around much like cars.
In Wolverhampton, England, the Midland Metro tram passes through the centre of a roundabout on approach to its terminus at St. Georges. This also happens in New Addington on the Tramlink 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 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.
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, the tram line serving Lines 4 and 8 passes through the middle of the roundabout at Platz der Deutschen Einheit. The tram stops are in the middle of the roundabout. To allow trams through, traffic is controlled by lights on the roundabout. Pedestrian access is via subway and street level crossings at the traffic lights for the trams.
In Bremen, Germany, tram lines 8 and 6 pass through the middle of the roundabout "Am Stern" east of the main railway station. They enter from the west and exit in a slightly north eastern 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.
Roundabouts with 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 and three other roads and the service entrance to a large shopping plaza. Boom barriers are in place at the railway crossings. The landscaped centre island bisected by the tracks was originally curbed, but 18-wheelers had trouble negotiating the roundabout, so the curbs were replaced with painted drive over concrete strips. A proposed fountain in the centre island has never been built. The roundabout was built in the early 2000s and has improved traffic flow considerably although delays are still caused by the many long freight trains coming through.
Two roundabouts in the Melbourne metropolitan area, Highett, Victoria  and Brighton, have heavy rail crossing the roundabout and through the inner circle. Boom barriers protect the rail 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, which is 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 are closed three times a day for 7 minutes to allow trains to pass.
In New Zealand's South Island, there are two roundabouts between major roads where the railway cuts through the roundabout. One is at the intersection between 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. Here the 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, where the Hokitika Branch separates State Highway 6 southbound from SH 6 northbound and SH 73. Both roundabouts are controlled by flashing red lights, but there are additional boom barriers at the Blenheim roundabout.
Hamburger roundabout/throughabout/cut-through roundabout 
These resemble a typical roundabout but are 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. In the United Kingdom there are examples on the A580 East Lancashire Road in St Helens, on Haydock Island in Merseyside (which also features the M6 passing overhead), and on the Astley/Boothstown border near to Manchester. More examples are the A6003 at Kettering, the 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, Hull, Nottingham and Reading, as well as on the N2/M50 intersection in Dublin, Ireland. Throughabouts are very common in Spain.
A more advanced and safer version of a hamburger roundabout is a roundabout interchange, separating the straight roadway and using underpasses or overpasses to cross the roundabout itself.
See also 
- The New Shorter Oxford English Dictionary, Volume 2, Clarendon Press, Oxford (1993), page 2632
- U.S. Department of Transportation: Roundabouts: an Informational Guide para 1.5
- BBC News (2 November 2004). "Roundabout Magic". Retrieved 13 May 2007.
- Letchworth Garden City Heritage Foundation. "Sign of the Times". Archived from the original on 16 October 2006. Retrieved 14 December 2006.
- "Frank Blackmore: traffic engineer and inventor of the mini-roundabout". The Times (London). 14 June 2008. Retrieved 15 June 2008.
- Keh, Andreh (November 18, 2010). "European Import Has Cars Spinning. Heads, Too.". New York Times. Retrieved 19 November 2010.
- Tom Geoghegan (2011-07-01). "Is the British roundabout conquering the US?". BBC website. Retrieved 2011-07-01.
- Dayna Roselli, "Roundabouts", on lasvegasnow.com, April 18th, 2007
- 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."), Cambridge ("roundabout: UK (US traffic circle)), Concise Oxford (rotary: N. Amer. a traffic roundabout.)
- Shashi S. Nambisan, Venu Parimi. "A Comparative Evaluation of the Safety Performance of Roundabouts and Traditional Intersection Controls". Institute of Transportation Engineers.
- 2008 National Roundabout Conference, B. Guichet's presentation
- fhwa.dot.gov U.S. Department of Transportation: Roundabouts: an Informational Guide para 1.5
- Kittleson & Associates. "Kansas Roundabout Guide: A Supplement to FHWA's Roundabouts: An Informational Guide". Kansas Department of Transportation. Retrieved 26 August 2009.
- Capuzzo, Jill P. (25 November 2007). "A Shift, but for Some Drivers, a Vicious Circle". The New York Times. Retrieved 27 March 2010.
- Richtmeyer, Richard (6 January 2008). "Safer Roundabouts Sprouting Up All Over New York, Nation". Associated Press. Retrieved 10 January 2008.
- Wilke, A. and Koorey, G. (2001). How Safe are Roundabouts for Cyclists? In TranSafe Issue 5, April 2001. Wellington, NZ. PDF
- "Crash analysis system". Retrieved 29 November 2007.
- Campbell, D., Jurisich, I., Dunn, R. 2006. Improved multi-lane roundabout designs for cyclists. Land Transport New Zealand Research Report 287. 140 pp. PDF
- Pedestrian Access to Modern Roundabouts: Design and Operational Issues for Pedestrians who are Blind retrieved 6/26/2010 access-board.gov
- Bill Baranowski, P.E., Pedestrian Crosswalk Signals at Roundabouts: Where are they Applicable? teachamerica.com
- Modern Roundabouts, an Informational Guide, tfhrc.gov/safety
- Taylor, M.P., Barton, E.V., Bliss, J. and O'Brien, A.P. (1993). "Effectiveness Audit of ARRB Intersection Capacity Research". Research Report ARR 242. ARRB Transport Research Ltd, Vermont South, Australia.
- TRB (2010). "Highway Capacity Manual 2010". Transportation Research Board, National Research Council, Washington, DC, USA.
- TRB. "Roundabouts: An Informational Guide". NCHRP Report 672. Transportation Research Board, National Research Council, Washington, DC, USA, in cooperation with US Department of Transportation, Federal Highway Administration.
- US Manual on Uniform Traffic Control Devices, mutcd.fhwa.dot.gov
- R. Schnüll, J. Lange, I. Fabian, M. Kölle, F. Schütte, D. Alrutz, H.W. Fechtel, J. Stellmacher-Hein, T. Brückner, H. Meyhöfer: Sicherung von Radfahrern an städtischen Knotenpunkten [Safeguarding bicyclists in Urban Intersections], Bericht der Bundesanstalt für Straßenwesen zum Forschungsprojekt 8952, 1992
- bernd.sluka.de Scroll to the section labeled "Kreisverkehr". A translation of the text reads: Graphic from Sicherung von Radfahrern an städtischen Knotenpunkten [Safeguarding bicyclists in Urban Intersections], (BASt, 1992). Accident numbers in large circular junctions with different bicycle facilities show: 1. Why there should be no pathways or bike lanes at these junctions; 2. Even when bicyclists use the roadway, their risk is relatively high at these junctions.
- Maycock, G., and Hall, R. D. (1984). "Accidents at 4-Arm Roundabouts." TRRL1120, Transport and Road Research Laboratory (TRRL), Crowthorne, England.
- SWOV Factsheet: Roundabouts
- Arizona Department of Transportation roundabout information page
- Shaw, Jeffrey and Moler, Steve, Bicyclist- and Pedestrian-Only Roundabouts, Public Roads magazine, January/February 2009, tfhrc.gov
- Retting, Richard A., Sergey Y. Kyrychenko, and Anne T. McCartt. "Long-Term Trends in Public Opinion Following Construction of Roundabouts." Journal of the Transportation Research Board 2019 (2007): 219-24.
- Anderson, David (July 27, 2007.). "Pedestrian group shares ideas". The Kinston Free Press. Retrieved November 08, 2012. – via HighBeam Research (subscription required)
- "History of Roundabouts". Retrieved 29 November 2007.
- "The Highway Code – Roundabouts". Retrieved 14 May 2009. Section 188, referring to Road Traffic Act 1988, Section 36, and Traffic Signs Regulations & General Directions 2002, Regulations 10(1) & 16(1)
- Google Maps view of a teardrop roundabout
- Isaak Yperman, Ben Immers (2003). "Capacity of a turbo-roundabout determined by micro-simulation" (pdf). Katholieke Universiteit Leuven, Department of Civil Engineering - Transportation Planning and Highway Engineering. Archived from kuleuven.be the original on 2003.
- mutcd.fhwa.dot.gov "Manual on Uniform Traffic Control Devices for Streets and Highways - Part 3: Markings" (pdf). U.S. Department of Transportation - Federal Highway Administration. 2003.
- Vítězné náměstí, Prague trams fun web
- Google Inc. Google Maps – Roundabout (Map). Cartography by Google, Inc. http://maps.google.co.uk/maps?hl=en&q=27.24395,+-80.22742&safe=active&um=1&ie=UTF-8&hq=&hnear=0x88dee7066d9fab49:0xd4730553b50a5cbf,%2B27%C2%B0+14'+38.63%22,+-80%C2%B0+13'+38.60%22&gl=uk&ei=aIUNTszzGMqphAe-5NXfDQ&sa=X&oi=geocode_result&ct=image&resnum=1&ved=0CBkQ8gEwAA. Retrieved 15 January 20122.
- Jensen Beach Roundabouts Charrette
- Google Inc. Google Maps – Blenheim Roundabout (Map). Cartography by Google, Inc. http://maps.google.com.au/maps?q=-41.51345+173.95987&hl=en&sll=-41.513261,173.960298&sspn=0.004362,0.010568&vpsrc=0&t=h&z=16. Retrieved 15 January 2012.
- Google Inc. Google Maps – Kumara Junction Roundabout (Map). Cartography by Google, Inc. http://maps.google.com.au/maps?q=-42.58515%09171.12974&hl=en&ll=-42.585247,171.129779&spn=0.002145,0.005284&sll=-42.585184,171.129549&sspn=0.004289,0.010568&vpsrc=6&t=h&z=18. Retrieved 1 5 January 2012.
- maps.google.co.uk, Mytongate on the A63 in Hull
- Video of Highway Roundabout in Canada
- TRL, The UK's Transport research Laboratory
- Modern Roundabouts - Geocoded National Database
- Mini-roundabouts - Getting them Right
- Turbo Roundabout Simulation
- Roundabout Benefits from the Washington State Department of Transportation
- Highway Roundabouts from the Ministry of Transportation of Ontario
- Roundabouts Now