Science Fair Project Encyclopedia
- This article is about traffic roundabouts - for the childrens' rides see carousel or roundabout (play).
A roundabout, rotary, or gyratory circus is a type of road junction (or traffic calming device) at which traffic streams circularly 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 sort 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.
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:
History and safety
The first actual 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 deaths (according to a study of a sampling of roundabouts in the United States, compared with the intersections they replaced). Roundabouts also significantly reduce potential 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, and can be dangerous for, 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 circular traffic is always moving, in roundabouts, 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.
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".
Types of roundabout
There are many variations in the design of roundabouts. 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.
Mini roundabouts exist at smaller intersections to avoid the use of signals, stopsigns or the necessity to yield in favour of one road of traffic. Mini-roundabouts can either be a painted circle or dome shaped. Painted roundabouts 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 roundabout 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.
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.
The A52 motorway in Switzerland links with three sections of road heading toward Hinwil , Forch and Rapperswil near Hinwil. 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 applies. The size of the roundabout conceals the fact that it is a roundabout at all.
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). The Red Cow ("Mad Cow") roundabout at the N7 intersection is particularly infamous, being signal-controlled, having secondary lanes for those making left hand turns, and now accommodating a tram line.
The town of Swindon in Wiltshire, England is famous for its "Magic Roundabout" (not to be confused with The Magic Roundabout) which is made up of one large center roundabout and five smaller (mini) roundabouts around the center. Traffic circulates anti-clockwise around the main central roundabout, with the normal rule applying at each mini-roundabout within.
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".
The Kinsale Road Roundabout in Cork, Ireland is nicknamed "The Magic Roundabout" although not of the aforementioned type. It is in fact 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.
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.
Note that junctions with the name "Magic Roundabout" are not necessarily of this type. For example, the Magic Roundabout at the junction of Tyndall Street and Ocean Way in Cardiff is magic in name only - it is a normal 4-way roundabout with its exits at near-right angles.
Roundabouts often have unique designs in the central island. These can range from flower beds, rocks, hills, statues and fountains.
- Portland, Oregon's city government page on the subject
- Reducing traffic frustration: roundabouts in North America
- The Magic Roundabout of Swindon
- The BBC takes a lighthearted look at The Hanger Lane Gyratory System in London (aimed at non-native drivers)
- List of UK Magic Roundabouts
- Roundabouts Canada
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