roundabout

　

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1. 遠回りの,回り道の・(言葉などが)遠回しの,間接の・= merry-go-round・traffic circle

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/05/08 01:04:27」(JST)

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This article is about the road junction. For other uses, see Roundabout (disambiguation).

See also: Traffic circle

Movement within a roundabout in a country where traffic drives on the left. Note the clockwise circulation.

Movement within a roundabout in a country where traffic drives on the right. Note the anticlockwise circulation.

A roundabout is a type of circular intersection or junction in which road traffic flows almost continuously in one direction around a central island. The modern form was standardised in the United Kingdom.

So-called "modern" roundabouts require entering traffic to yield to traffic already in the circle and optimally observe various design rules to increase safety. Variations on the basic concept include integration with tram and/or train lines, two-way flow, higher speeds and many others.

Traffic exiting the roundabout comes from one direction, rather than 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 queueing 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 on average spend less time idling at roundabouts than at signalled intersections, using a roundabout potentially leads to less pollution. Also, when entering vehicles only need to yield, 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. Additionally, slow moving traffic in roundabouts makes less noise than traffic that must stop and start, speed up and brake. The single greatest benefit of roundabouts is that they eliminate perpendicular/T-bone crashes.^{[citation needed]}

Roundabout in the centre of Colombo, Sri Lanka

Modern roundabouts are commonplace throughout the world, in particularly Australia, Belgium, China, Costa Rica, Cyprus, Denmark, France, Germany, Hungary, Iceland, Indonesia, 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).

History

Thomas Circle in Washington, D.C., 1922

Circular junctions existed before roundabouts, including the Circus, in the English city of Bath a 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.. 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. Its centre originally was intended partly as a traffic island for pedestrians. In the early twentieth century, numerous traffic circles were constructed in the United States, particularly in the northeast. Examples include a circle in Atherton, California.

Small roundabout in Barzio, Italy

Roundabout in Braşov, Romania

Widespread use of the modern roundabout began when the UK's Transport Research Laboratory engineers re-engineered circular intersections during the 1960s. Frank Blackmore led the development of the offside priority rule and subsequently invented the mini-roundabout to overcome capacity and safety limitations. The design became mandatory in Britain for all new roundabouts in November 1966. This yield requirement has been the law in New York state since the 1920s.

In the United States modern roundabouts emerged in the 1990s. 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 the subject of mockery such as featured in the film European Vacation and the television series The Simpsons. By 2011, however, some 3,000 roundabouts had been established, with that number growing steadily. The first modern roundabout in the United States was constructed in Summerlin, Nevada in 1990. This roundabout occasioned 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.

In the United States, municipalities introducing new roundabouts often are met with some degree of public resistance, just as in the United Kingdom in the 1960s. Surveys show that negative public opinion reverses as drivers gain experience with them. 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%.

A pedestrian group in Kinston, North Carolina in 2007 proposed roundabouts in place of traffic lights at major intersections.

Modern roundabout

Road sign examples

Germany (right-hand traffic); in the UK a similar sign, with the arrows reversed, is used at mini roundabouts.

The US and Canada (right-hand traffic); a similar sign is used in Ireland (with directions reversed).

UK (left-hand traffic)

Australia (left-hand traffic)

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 yield the right of way.

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 rotaries are frequently designed to try to increase speeds, and thus have roads that enter the traffic circle tangentially.

Because of the requirement for low speeds, 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 roundabouts, traffic speeds must be reduced via tricks such as curving the approaches.

Terminology

The word roundabout dates from early twentieth century England.

In U.S. dictionaries the terms "roundabout", "traffic circle" and "rotary" are synonyms.

The U.S. Department of Transportation adopted the term "modern roundabout" to distinguish those that require entering drivers to yield to others. This article follows that convention and refers to other types as traffic circles or rotaries. 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.

New York state terms its modern roundabouts traffic circles. In Massachusetts older circular intersections are called "rotaries" and the state enforces^{[citation needed]} that restriction.

The term "traffic circle" is not used in the United Kingdom, where most circular junctions meet the technical criteria for modern roundabouts.

Operation and design

The fundamental principle of modern roundabouts is that entering drivers yield to traffic within the roundabout without the use of traffic signals. Traffic circles typically require circling drivers to yield to entering traffic. Generally, exiting directly from the 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 the inner lane of a traffic circle is usually not permitted without first merging to the circle's outside lane.

Vehicles circulate around the central island in one direction at speeds of 15–25 miles per hour (24–40 km/h). Direction is determined by whether traffic drives on the right- or left-hand side of the road. In left-hand traffic countries the circulation is clockwise; in others, it is anticlockwise.

Multi-lane roundabouts are typically less than 250 feet (76 m) in diameter, Traffic circles and roundabout interchanges may be considerably larger. Roundabouts are roughly the same size as signalled intersections with the same capacity.

Islands

Modern roundabouts feature a central island and pedestrian islands at each entry/exit.

Central

The central island may be surrounded by an apron that is high enough to discourage drivers from crossing over it, but low enough to allow unusually wide/long vehicles to navigate the intersection. The island may provide a visual barrier across the intersection to the drivers entering it. The barrier assists entering drivers into focusing only on the traffic approaching them in the path of the circle. Otherwise, vehicles anywhere in the circle cause entering drivers to stop outside the roundabout, waiting for the vehicles (even on the opposite side) to pass by them. This interferes with traffic flow, unnecessarily reducing the number of circling cars. The barrier may be a landscaped mound, a raised wall, a tree or tall shrubs. Flagpoles may be planted 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 (USA); Bend’s Roundabout sculptures were honored by Americans for the Artsas 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.

Pedestrian

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.

Pedestrian crossing

Crosswalks at each entry/exit may be located at least one full car length outside the circle. The extra space allows an exiting vehicle to stop for a pedestrian without obstructing through traffic. Each crosswalk 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 crosswalk may become diagonal, to direct the gaze of those crossing into exiting traffic.

Bicycles

The Hovenring bicycle roundabout in the Netherlands is an innovative design, completely separating bicycles from vehicular traffic.

Physically separated bikeways best protect cyclists. Less optimally, terminating cycle lanes well before roundabout entrances require 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 use the crosswalks.

Cycle lanes on St. John's roundabout in Newbury, Berkshire, England.

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 crosswalk. 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 bicyclist 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 bicyclists 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 St. John's roundabout in Newbury, Berkshire, England and at Museum Road, Portsmouth, England. The cycle lanes at Museum Road were replaced by a narrowed carriageway to encourage lane sharing, while the St. John's roundabout retains perimeter cycle lanes.

Pavement markings invite bicyclists to enter sidewalk on approach to roundabout in Mesa, Arizona. Bicyclists are still permitted to use the roundabout like any other vehicle.

The roundabout at the Brown Road/202 interchange adopts a U.S.-recommended design. On-street pavement markings direct bicyclists to enter the sidewalk at the end of the bike lane. Bicyclists who choose to travel on the wide sidewalk, cross roundabout arms perpendicularly, well outside the circle. A pedestrian island allows pedestrians and bicyclists to cross one lane at a time.

Cyclists can choose to ride on the sidewalk on far right, or in main lanes of this roundabout in Mesa, Arizona.

Capacity and delays

Traffic approaching Chiverton Cross roundabout in Cornwall, UK

The capacity of a roundabout varies, based on the number of entry and circulating lanes, and on entry angle and lane width. 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 signalized 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 crossing roadways are relatively even, a roundabout can reduce delays, because half of the time a full stop would be required. Dedicated left turn signals 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 modeling

Major research on roundabout capacity has been carried out in multiple countries. 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.

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 gap-acceptance theory rather than 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) 2010 edition and the TRB -FHWA Roundabout Informational Guide (NCHRP Report 672). The HCM 2010 model is based on gap-acceptance theory.

Safety

Small modern roundabout in the United States, where vehicles are driven on the right

Roundabout in the United States with separated side lanes. Vehicles entering the roundabout give way to vehicles in the roundabout. As of March 2009 the road at upper left carries 10,400 vehicles per weekday.

A typical trunk road roundabout in the UK at Carland Cross on the A30 in Cornwall

Statistically, modern roundabouts are safer for drivers and pedestrians than both traffic circles and traditional intersections. Roundabouts are safer than both traffic circles and junctions—experiencing 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). Some larger roundabouts take foot and bicycle traffic through underpasses or alternate routes. Clearwater Beach, Florida's multi-lane roundabout has reduced its previously high bicycle death rate to zero since its construction.^{[citation needed]}

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.

An analysis of the New Zealand national crash database for the period 1996–2000 shows that 26% of cyclist's 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 (50%+ 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. Designs that have marked perimeter cycle lanes were found to be even less safe, suggesting that in roundabouts, cyclists should occupy a vehicle lane rather than a special lane. The researchers advised that drivers be forbidden from overtaking cyclists (as well as other vehicles) while in the circle.^{[citation needed]}

Visually-impaired pedestrians

Poorly designed walkways increase risks for the visually impaired because it is more difficult (than at a signalized intersection) to audibly sense whether there is a sufficient gap in traffic. With a signal traffic comes to a stop and an audible sound pattern can indicate that it is time to cross.

This issue has led to a conflict in the United States between the visually-impaired and civil engineering communities. One alternative is to provide manually operated pedestrian crossing signals at each entry. This increases construction and operation costs and disrupts vehicle traffic to some extent. Signalisation would also increase delays for most pedestrians during light traffic, since pedestrians would need to wait for a signal to change to legally cross.

Signalized 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 add traffic lights to regulate flow.

Some roundabouts have a divider, or subsidiary deflection island, by means of which is provided a segregated left (or right) turn lane 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 priorities, or when buildings occupy the central island.

Smaller, small or midi- roundabouts

As the overall or external size of a roundabout (in the UK referred to as the Inscribd Circle Diameter - ICD) is reduced, so the maximum practicable (and prescribed) diameter for the central island also reduce, 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 off from general use by demarcation lines and differentiated from the outer annulus of carriageway by a combination of 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 being 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 therfore known as an over-run area (in UK useage), 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 28m diameter ICD with a 4m 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 Transport Research Laboratory, turned his attention to the possibility of a roundabout that could be built at sites lacking room for a conventional roundabout.

A mini-roundabout in the United Kingdom, where a painted white circle is used for the centre. The arrows show the direction of traffic flow.

Mini-roundabouts can be a painted circle or a low dome but must be fully traversable by vehicles. The idea is to allow motorists to drive over them absent other traffic, but the practice is dangerous if other cars are present. 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 and Mount Royal and Rosedale in Calgary, Canada. Kemptville, Ontario, has the record for the most roundabouts in Ontario, with a record of three on one highway.

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 28m ICD (external notional diameter).

Raindrop roundabouts

Main article: Dogbone interchange

A hybrid raindrop roundabout in Zagreb, Croatia. These two roundabouts are more akin to a "magic roundabout" since the left turns need not drive through both of them.

Raindrop roundabout at an Interstate interchange in North Carolina.

These roundabouts do not form a complete circle and have a raindrop or teardrop shape. They appear 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

The basic shape of the Dutch turbo roundabout

Turbo roundabout road sign in the Netherlands

In the United Kingdom, Netherlands, Poland, Slovenia, Czech, Hungary, Spain and Belgium, a relatively new type of roundabout is emerging. It provides a forced spiralling flow of traffic, 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 capacity. It is often the case that a turbo roundabout is marked out such that a U-turn by means of the roundabout is not possible for drivers approaching on certain arms.

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 consents from central authorities. 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 wasn't until 1997 that the UK's national highway authorities published guidance (DMRB TA-78/97) that in effect endorsed use of spiral markings in certain circumstances.

Several variations of the turbo roundabout exist. The basic shape is designed for the intersection of a major road crossing a road with much less traffic.

Turbo roundabouts in continental Europe were initially built with raised lane separators. Newer implementations follow UK practice with only lane markings increase efficiency (regarding safety, speed, and capacity) by reducing the safety risk and enabling maintenance vehicles such as snow ploughs.

According to simulations, 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 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. 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.

Motorways

Motorway junction with roundabout in the Czech Republic

See also: Roundabout interchange

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 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.

An 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 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, a roundabout connects two motorways, Autostradan from Lund, and the Inner ring road. It is signposted as a motorway. 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. The junction between the A200 and the A9 uses a 3-level stacked roundabout. Near Eindhoven (the Leenderheide junction), the junction for the A2 uses a roundabout. An overpass was built for the A67 from Antwerp to Germany.

Rotary interchanges 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.

Access-controlled roundabouts

A major signal-controlled roundabout interchange in central Bristol, England. Vehicles drive on the left, and vehicles in the roundabout are stopped by traffic lights to allow other vehicles to enter, while an underpass permits straight-through traffic to bypass the roundabout entirely.

Some bridges on Beijing's Second Ring Road are controlled by traffic lights. While it may appear to defy the logic of roundabouts, it works well to control traffic flow on 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/ring junctions

Line drawing of the roundabout "Magic Roundabout" at Hemel Hempstead illustrating the concept and the reverse (anticlockwise) flow of the inner lane

"Magic roundabouts" direct traffic in both directions around the central island. They are officially known as "ring junctions". The first magic roundabout was constructed in 1972, designed by Frank Blackmore, inventor of the mini-roundabout.

The name derives from the popular children's television series, The Magic Roundabout, and is considered "magic" because traffic flows in both clockwise and anticlockwise directions. This is achieved by surrounding the main island with one smaller roundabout per entry/exit street. This pattern directs traffic in the usual clockwise manner around each mini-roundabout. Exiting the mini-roundabouts, traffic may proceed around the central island either clockwise (via the outer loop), or anticlockwise (the inner loop). The arrangement offers multiple paths between feeder roads. Drivers typically choose the shorter, most fluid route. Although the safety record is good, many drivers find this system intimidating, and some drivers go to great lengths to avoid them.

The town of Swindon in Wiltshire, became home to the first Magic Roundabout. This roundabout joins five roads and consists of a two-way road around the central island with five mini-roundabouts meeting the incoming roads.

Similar systems are found in the Moor End roundabout in Hemel Hempstead (Hertfordshire), which has six intersections; in High Wycombe (Buckinghamshire), the Denham Roundabout in Denham (Buckinghamshire), the Greenstead Roundabout in Colchester (Essex), "The Egg" in Tamworth (Staffordshire) and the Hatton Cross Roundabout in London.

Churchbridge Junction in Staffordshire is a magic gyratory. This type of junction is similar to a magic roundabout, except that the constituent roundabouts are connected by longer lengths of roadway.

Trams

Tram roundabouts merge roundabouts for individual vehicles with tram lines. Large areas are needed for tram roundabouts that include a junction between tram lines. Such systems often contain tram stops.

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, with drivers required to give way to vehicles approaching from their right and to trams approaching from right-angles.

The Haymarket roundabout between Royal Parade and Elizabeth Street contains a tram-stop, pedestrian crossings, three entering tram lines, traffic signals to stop vehicular traffic at each crossing point when a tram is due, service roads and a pedestrian crossing.

Brussels tram roundabouts employ multiple configurations. At the Barrière de St-Gilles/Bareel St-Gillis, tram tracks form a circle in the carriageway, while Verboekhoven and Altitude Cent/Hoogte Honderd have reserved tram tracks inside the roundabout. At Place Stéphanie/Stefaniaplein, they go straight through the 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 grade-separated and is signal-controlled with secondary lanes (separate from the main roundabout) for those making left turns. The junction, the busiest in Ireland, had tram lines added to it with the opening of the Luas system in 2004. The tracks pass across one carriageway of the N7, and across the southern M50 sliproads. Trams pass every five minutes at rush hour. The roundabout was replaced with a grade-separated free flowing junction.

Gothenburg, Sweden has a tram roundabout and tram stop at Korsvägen (the Crossroad). It carries heavy car traffic 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 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.

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, stopping 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. Entering as well as leaving trams give way to vehicles. In the years 1932–1942 trams circulated much like vehicles.

In Kiev, Ukraine an interchange of two "fast tram" lines is below a roundabout.

Oslo, Norway also has many roundabouts with tram tracks passing through; for example at Bislett, Frogner plass, Sinsen, Solli plass, Carl Berners plass and Storo.

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, Lines 4 and 8 pass through the middle of the roundabout at Platz der Deutschen Einheit. The tram stops are in the middle of the roundabout. Roundabout traffic is controlled by traffic lights. Pedestrian access is via subway and street level crossings at the lights.

In Bremen, Germany, tram lines 8 and 6 pass through the middle of the roundabout "Am Stern" east of the main railway station. They enter from the west and exit in a northeastern direction thus making a slight bend within the roundabout. Both stations are situated on the north-eastern edge of the roundabout. Traffic is controlled by two-colour traffic lights inside the roundabout.

Railways

In Jensen Beach, Florida, the main line of the Florida East Coast Railway running north-south bisects the two-lane roundabout at the junction of Jensen Beach Boulevard running east – west. It hosts three other roads and the service entrance to a large shopping plaza. Boom barriers line the railway crossings. The landscaped central island bisected by the tracks was originally curbed, but 18-wheelers had trouble negotiating the roundabout, so the curbs were replaced with painted concrete strips. The roundabout was built in the early 2000s and improved traffic flow, although long freight trains often cause delays.

Two roundabouts in the Melbourne metropolitan area, Highett, Victoria and 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 located next to a shopping centre and pedestrian zone. The flow of traffic and pedestrians is governed by 14 barriers, 22 traffic lights and 8 loudspeakers. The barriers close three times daily for 7 minutes to allow trains to pass.

In New Zealand's South Island, two roundabouts join major roads where a railway cuts through. One is at the intersection between State Highway 1 (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, with additional boom barriers at the Blenheim roundabout.

Hamburger roundabout/throughabout/cut-through

Throughabout

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. The United Kingdom has 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. 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. In Perth, Western Australia, one is found at the intersection of The Strand, Morley and Alexander Drives. 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.

Bicycle-pedestrian roundabouts

The same features that make roundabouts attractive for roadway junctions led to their use at junctions of multi-use trails. The University of California, Davis and Stanford University, as well as the Cape Cod and Old Colony rail trails have bicycle-pedestrian roundabouts.

Roundabouts are used on off-road bicycle trails in Florida, Colorado and Alaska.

An elevated roundabout is located in Eindhoven, serving pedestrian and bicycle traffic only, above the main conventional roadway intersection. It is known as the Hovenring.

Traffic circles

Main article: Traffic circle

Traffic circles are typically larger, operate at higher speeds and often give priority to entering traffic. They may control entering traffic by stop signs or traffic lights. Many older traffic circles allow entry at higher speeds without deflection, or require a stop and a 90-degree turn to enter.

Many traffic circles have been converted to modern roundabouts, including the former Kingstontraffic 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.

See also

• Level of service
• Roundabout Appreciation Society
• Roundabout dog

Notes

Sources

• U.S. Department of Transportation (2013). "Roundabouts: an Informational Guide". 

External links

• City of Carmel, Indiana, USA, Roundabouts page (showing raindrop roundabouts or dogbone interchanges)
• 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
• PBWORKS
• [3]

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