Roundabout

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Image:1 3 3.svg A roundabout, rotary, or gyratory circus is a type of road junction (or traffic calming device) at which traffic streams around a central island, after first yielding to the circulating traffic. In the United States it is technically called a "modern roundabout", to emphasize the distinction from the older, larger type of traffic circle. Roundabouts are statistically safer than both traffic circles and traditional intersections, though they do not cope as well with the traffic on motorways or similar fast roads.

1 Difference between roundabouts and traffic circles
2 History and safety
3 Types of roundabout
4 Trivia
5 See also
6 External links

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Difference between roundabouts and traffic circles

Roundabouts are sometimes referred to as traffic circles, but a technical distinction was made between roundabouts and traffic circles in the mid-1960s:

roundabout	traffic circle
Entering vehicles yield	Stop sign, stop signal, or giving priority to entering vehicles
Vehicles in the roundabout have priority over the entering vehicle	Allow weaving areas to resolve conflicted movement
Use deflection to maintain low speed operation	Some large circles provide straight path for higher speed
No parking is allowed	Some large circles permit parking within the circle
Pedestrians are (usually) prohibited from the central island	Some large circles allow pedestrians on central island
All vehicles circulate around the central island	Mini-traffic circles with left-turning vehicles passing to the left¹ of the central island.
(Source for table: Oregon Department of Transportation [1]) 1. For countries that drive on the right-hand side of the road.

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History and safety

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The first modern roundabout was constructed in New York City, United States in 1904. However, the widespread use of roundabouts began when British engineers reengineered the traffic circle in the mid-1960s to overcome its limitations of capacity and for safety issues. Unlike traffic circles, roundabouts operate with yield control to give priority to circulating traffic and eliminate much of the driver confusion associated with traffic circles and driver wait associated with junctions that have traffic lights. Roughly the same size as signalized intersections with the same capacity, roundabouts also are significantly smaller in diameter than traffic circles, separate incoming and outgoing traffic with pedestrian islands and therefore encourage slower and safer speeds (see traffic calming).

Roundabouts are safer than both traffic circles and traditional intersections—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, compared with the intersections they replaced). Roundabouts also significantly reduce points of conflict between pedestrians and motor vehicles and are therefore considered to be safer for them. However, roundabouts, especially large fast moving ones, are unpopular with some cyclists. This problem is sometimes handled on larger roundabouts by taking foot and bicycle traffic through a series of underpasses.

In addition to improved vehicle and pedestrian safety, and in spite of lower speeds, roundabouts dramatically outperform traffic circles in terms of vehicle throughput and, because a roundabout's circular traffic is always moving, they outperform ordinary junctions with traffic signals as well.

However, due to the fact that vehicle traffic must yield instead of stop, there are some safety concerns for bicyclists who cycle alongside the road and especially for persons with visual impairments. Safety concerns for the second group of people is especially important in countries that have legislation protecting the rights of people with disabilities.

This issue has led to a mutually frustrating conflict in the United States between the visually impaired and civil engineering communities; the visually impaired have taken the position that roundabouts are acceptable only if there are pedestrian crossings with lights at each road connecting to a roundabout. Although such crossings would reduce the possibility that a blind pedestrian might be run over by vehicles entering or exiting the roundabout at unsafe speeds, they would also increase the cost of a roundabout and decrease its throughput.

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In addition, roundabouts do not cope well with the traffic on motorways or similar roads, thus leading to long tailbacks when they are encountered by the motorist. Britain's strategic road network has many isolated roundabouts on otherwise almost motorway-like roads (for example, A1/A421) and even on motorways (for example, the A601(M), A627(M), and M271 have roundabouts on the main line). Some of these roundabouts, as well as other busy roundabouts, have had traffic lights added and are termed "signal controlled roundabouts".

Roundabouts are not suitable for junctions with high traffic volume. When traffic congestion occurs in one direction, it would quickly block a roundabout and spread to all other entering directions. The congestion would be further deteriorated by large number of long vehicles like container trucks. The roundabout of Kwai Tsing Interchange in Hong Kong was replaced by a large box junction with traffic lights after recurring area traffic congestions when numerous container trucks journeyed to Kwai Chung Container Port after a typhoon.

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Types of roundabout

Large roundabouts such as those used at motorway intersections typically have two to four lanes around the central hub, and frequently have traffic lights regulating flow during peak hours.

Some roundabouts have a divider between traffic turning left (in right-hand drive countries) and other traffic, enabling those making left turns to bypass the roundabout entirely. Another type of roundabout is the through-about roundabout or "hamburger" junction. This type of roundabout enables straight-through traffic on one road to cross over the central island, whilst all other traffic must drive around the island. As a consequence this junction must always be controlled by traffic lights. Examples of this type exist in Bracknell and Reading, Berkshire, as well as on the N2/M50 intersection in Dublin, Ireland.

The term "gyratory" (for example, the Hanger Lane Gyratory System) is sometimes used in England when a roundabout is large and has non-standard lane markings or priorities; in fact, they are more like traffic circles.

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Mini roundabouts

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Mini-roundabouts exist at smaller intersections to avoid the use of signals, stop signs or the necessity to yield in favour of one road of traffic. Mini-roundabouts can be a painted circle, a low dome, or often are small garden beds. Painted roundabouts and low domes can be driven over; in the UK, some motorists ignore convention at times of low traffic presence and drive directly over such roundabouts as if they do not exist. Mini-roundabouts work in the same way as larger roundabouts in term of right of way. They can often come in "chains", making navigation of otherwise awkward junctions easier. There are usually different road signs used to distinguish mini roundabouts from larger ones.

Mini-roundabouts are also common in Irapuato, Mexico, usually marked with a tiny grassy circle enclosed with sidewalk edges, and in Calgary, Canada's inner-city Mount Royal and Rosedale neighborhoods, where mini-roundabouts recently replaced intersections formerly controlled by stop signs to combat increasing cut-through traffic.

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Roundabouts on motorways

While roundabouts do not usually interrupt motorways in the UK or Ireland, a common type of motorway intersection (suited only for lower volumes of traffic) consists of a grade separated roundabout above the main motorway, accessed via sliproads. Most intersections on Dublin's M50 motorway ring-road use this configuration - although several junctions have a greater volume of traffic than the capacity such roundabouts can accommodate.

An additional improvement is the 3-level stacked roundabout - this is a roundabout interchange where both roads are grade separated. In the United Kingdom, the M25/A3 and A1(M)/M18 interchanges of this type. These junctions however have less capacity than a full free-flow interchange. An equivalent version of this in the United States would be the volleyball interchange.

The A52 motorway in Switzerland links with three sections of road near Hinwil heading toward Hinwil, Forch and Rapperswil. The intersection takes shape in the form of a massive roundabout on the motorway. However, the sign for a roundabout is not used and a speed limit of 80 km/h (50 mph) applies. The size of the roundabout conceals the fact that it is a roundabout at all.

The A7 motorway also has a motorway roundabout (at its terminus in Kreuzlingen), but it is smaller in size.

Massachusetts Route 128, a motorway/freeway in the United States, also has two large at-grade roundabouts (or "rotaries" as they are called in that state) in the town of Gloucester. They are signed as Exits 10 and 11 on the freeway. Roundabouts in Massachusetts follow the same general rule as they do in the UK, with circling traffic getting the right of way.

There is also a roundabout at the southern terminus of Massachusetts Route 3, a motorway linking Cape Cod to Boston; directly opposite is another motorway, the start of the Mid-Cape Highway section of US 6 heading over the Sagamore Bridge. Due to the size of the roundabout, and the fact that it's the shortest way to get between the Cape and Boston, traffic tie-ups and accidents are common, especially in the busy summer months, when drivers from other states are often confused at how to navigate the roundabout. Work started in late 2004 to convert the roundabout to a freeway interchange. Completion is scheduled for 2007.

In the town of Greenfield, Massachusetts, there is a roundabout that connects the exits of Interstate 91 (North-South), Route 2 (East-West) and Route 2A. I-91 continues over the roundabout. Route 2 west heads off the roundabout, Route 2 east joins with I-91 north. Route 2A diverts through downtown Greenfield before reconnecting to Route 2 East. This roundabout is most busy during the snow/ski season when skiiers head into Vermont, three exits up on I-91.

In the city Malmö, Sweden, there is a roundabout connecting two motorways, "Autostradan" from Lund, and the "Inner ring road". It is signposted as a motorway through this roundabout. Today these two motorways are considered local, but before year 2000 they were part of the European roads E6, E20 and E22.

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Controlled roundabouts

Some bridges on Beijing's 2nd 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 themselves are two viaducts creating a roundabout suspended over the ring road itself.

Signal controlled roundabouts are relatively common in Ireland, where they have come about in an attempt to alleviate traffic problems at over-capacity roundabout intersections (around the M50 in Dublin for example).

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"Magic" Roundabouts

The town of Swindon in Wiltshire, England is famous for its "Magic Roundabout". This roundabout is at an intersection of five roadways, comprising one large center roundabout and five smaller (mini) roundabouts. Traffic flow around the larger, inner roundabout is counter-clockwise, whereas traffic flows in the usual clockwise direction around the five mini-roundabouts and the outer loop.

Similar systems are found in various places in England, most famously the Moor End roundabout in Hemel Hempstead in Hertfordshire, which has six intersections, but also High Wycombe and Denham in Buckinghamshire, Colchester and Benfleet in Essex, Tamworth in Staffordshire and Hatton Cross in London. Magic roundabouts are also known as "Ring Junctions".

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Roundabouts with Trams

Most notably found in inner Melbourne, where the tram network is quite extensive. Tram tracks always pass through the central island of one of these roundabouts. They can be particularly daunting prospects for inexperienced drivers, as they must give way to vehicles coming towards them from their right, but also trams coming at them from right-angles.

Trams passing through small roundabouts is not a problem; through larger roundabouts it can be difficult, particularly when there is a tram junction (i.e. the tram may do more than simply pass straight through the intersection). In these cases, the roundabouts are very large, and often have tram stops in the middle. Flemington Junction is the most notorious intersection of this nature, containing a tram-stop, pedestrian crossings, three entering tram lines, traffic lights, service roads and a large flag-pole.

In Toronto, the Spadina Streetcar goes around Spadina Crescent, which can be considered a roundabout. The streetcar tracks go around the roundabout in a circular fashion, with their own lane inside of the car lanes.

In Dublin, Ireland, the Red Cow ("Mad Cow") roundabout at the N7/M50 intersection 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 hand turns. The junction, the busiest in Ireland, had tram lines added at-grade 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 5 minutes at rush hour.

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Trivia

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The Lambeth Bridge Roundabout in London was featured in the film National Lampoon's European Vacation.
The first roundabout in the UK was built in Leicester.
The Brunel Bridge Roundabout in Slough was made famous by its appearance in the opening titles of the Ricky Gervais sitcom The Office.
The Kinsale Road Roundabout in Cork, Ireland is a very large and irregularly shaped, three lane, signal controlled roundabout. The roundabout is situated on the N25 southern ring road dual carriageway, at the intersection with a dual-carriageway to the city centre, a road to Bishopstown, and the airport or Kinsale road. With the huge traffic volumes at the intersection, the dangerous and difficult navigation of the roundabout has given rise to the "Magic Roundabout" nickname. Traffic jams of a kilometer in length are not uncommon, especially during peak periods. This roundabout is scheduled to be replaced with a grade separated junction in late 2006.
The Magic Roundabout at the junction of Tyndall Street and Ocean Way in Cardiff has cubes and other geometric shapes constructed from road signs.
Crawley, West Sussex has a large rectangular roundabout which contains a hotel in the centre. Giving rise to the address, "The Squareabout, Crawley".
Mooroolbark, Melbourne contains a triple-roundabout [2], that is, three consecutive roundabouts, on Hull Rd. They were built to replace a dangerous area where three secondary roads joined Hull Rd at separate places within 200 m. Each roundabout allows drivers to turn onto or off from one of the three secondary roads.
Two roundabouts in Melbourne, Highett [3] and Brighton [4], have heavy rail crossing through the roundabout and through the inner circle. Boom gates protect the rail from oncomming traffic at the appropriate points in the roundabout.
St Kilda Junction, St Kilda, Melbourne, was once an un-controlled seven-street roundabout also containing a tram junction in the middle; it was drastically altered in the 1970s to make it a two-level, light-patrolled intersection.
One of the most famous landmarks in Guadalajara, Mexico is the Minerva Fountain Roundabout, which contains a huge fountain surrounding a statue of the Roman goddess Minerva. It's in the middle of a series of 3 consecutive roundabouts, located in the intersection of 5 avenues: López Mateos, Vallarta, López Cotilla, Agustín Yáñez and Golfo de Cortez. A long tunnel runs under all the three roundabouts.