Roundabout
|
Sign for indicating a roundabout |
A
roundabout,
rotary (the term used almost universally in the US state of
Massachusetts), or
gyratory circus is a type of
road junction (or
traffic calming device) at which
traffic streams around a central island, after first yielding (giving way) 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.
Roundabouts are sometimes referred to as
traffic circles, but a technical distinction was made in some jurisdictions between roundabouts and traffic circles in the mid-
1960s:
In the
United Kingdom the term 'traffic circle' is not used, and all types are known as roundabouts (but see
mini-roundabouts and
magic roundabouts below).
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 signalised 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.
|
A major signal controlled roundabouts in Bristol, England. The traffic drives on the left. |
In addition, roundabouts do not cope well with the traffic on motorways or similar roads, thus leading to long queues. 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.
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 (and vice-versa), 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.
Mini roundabouts
 |
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. |
|
An early mini-roundabout in the US, built in 1930 and located at the intersection of River Street & Pleasant Street, South Yarmouth, Massachusetts |
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 neighbourhoods, where mini-roundabouts recently replaced intersections formerly controlled by stop signs to combat increasing cut-through traffic.
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 â€" 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 â€" 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, using a conventional overpass, or "flyover", directly onto the bridge. 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 town of
Concord, Massachusetts,
Route 2 is again split by a roundabout. In addition to several local roads, Route 2 Ease and
Route 2A East from Acton meet and join as Route 2 East. Conversely, Route 2A West splits off from Route 2. As with many at-grade intersections, there is a large tieup of commuter traffic at the rotary during morning and evening
rush hour.
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.
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
Great Britain and
Ireland, where they have been introduced in an attempt to alleviate traffic problems at over-capacity roundabout intersections or to prevent some flows of traffic dominating others (around the
M50 in
Dublin for example).
"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 centre 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 (where it is colloquially known as "the egg") and
Hatton Cross in
London. Magic roundabouts are also known as "Ring Junctions".
Roundabouts with Trams
Most notably found in inner
Melbourne, particulary in the inner suburban area of
South 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 signals to stop vehicular traffic at each crossing point when a
tram is due, service roads,
pedestrian crossings, and a large flag-pole.
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.
In
Gothenburg,
Sweden the roundabout and
tram stop "Korsvägen" (the Crossroad) at the
Trade fair 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 pass.
In
Kiev,
Ukraine an interchange of two "Fast Tram" lines is done below a roundabout (see picture to the right).
Hamburger Roundabout or Throughabout
These resemble a normal 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. An example can be seen at the junction of
Morley Drive and Alexander Drive in Perth, WA, Australia.
*
Elephant and Castle,
London*
The Plain, Oxford*
St George's Circus, London
*
Los Alamitos Traffic Circle,
Long Beach, California |
An IMAX cinema built in a roundabout |
*The
Lambeth Bridge Roundabout in
London was featured in the film
National Lampoon's European Vacation.
*
Leicester claims to have had the first roundabout in the UK, around the
Clock Tower.
*
Letchworth Garden City has a rival claim to having the first roundabout in the country, from 1909.
*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 roundabout at the junction of Tyndall Street and Ocean Way in
Cardiff has
cubes and other
geometric shapes constructed from
road signs. Link to Google Maps view [
1].
*
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 the Melbourne metropolitain area,
Highett [
3] and
Brighton [
4], have
heavy rail crossing the roundabout and through the inner circle.
Boom barriers 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 is in the middle of a series of 3 consecutive roundabouts, located in the intersection of 5 avenues: López Mateos, Vallarta, López Cotilla, AgustÃn Yáñez and Golfo de Cortez. A long tunnel runs under all the three roundabouts.
*In
Birmingham, United Kingdom,
Haden Circus is a roundabout with only half of the central island grassed over, the other half being demarcated by concrete blocks. This originates from when Formula 3 racing was held on the city streets.
*The city of
Griffith in
New South Wales,
Australia contains no traffic lights, only roundabouts. It is the only Australian city with a population of above 20,000 to do so.
*
Cycle facilities at roundabouts*
Roundabout Appreciation Society*
Roundabouts in North America*
The Magic Roundabout of Swindon*
The Hanger Lane Gyratory System, London, England*
The Kinsale Road Roundabout, Cork, Ireland, on Google Maps*
how to use UK roundabouts, video tutorials
