My fascination with the roundabout – I go through seven in the space of 15-20 minutes on the way to work – has led me to delve into the subject a bit. It is clear to me, when traffic is light, that the roundabout is an enormously efficient way of keeping traffic moving. My brief but intense involvement with them suggests that at a certain point traffic reaches a critical mass and traffic lights become more efficient and far less dangerous. It is clear that Doha is replacing a number of roundabouts with traffic signal controlled intersections. In other cases, they have created and are continuing to construct by-pass lanes for those wishing to simply go right. This allows those vehicles to bypass entering the roundabout and then simply merge with traffic coming off the roundabout. In Wikipedia one reads that a roundabout is a type of road junction at which traffic enters a one-way stream around a central island. In the United States it is technically called a modern roundabout, to emphasize the distinction from the older, larger type of traffic circle.
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The first roundabout was technically square. Constructed in Paris around the Arc de Triomphe in 1901, closely followed by Columbus Circle in New York City in 1904. The first British roundabout was five years later, in Letchworth Garden City in 1909 - originally intended partly as a traffic island for pedestrian. However, the widespread use of roundabouts began when British engineers re-engineered 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 signaled 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). I have not seen one pedestrian island in Doha to date nor can I concur that roundabouts encourage slower speeds and traffic calming.
…… roundabouts do not cope well with the traffic on motorways or similar roads, 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 a few 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".
And here’s the most obvious thing for me: Roundabouts are not suitable for junctions where the exits suffer from traffic congestion. Congestion on one exit commonly blocks a roundabout and spreads to all entering directions. The roundabout of Kwai Tsing Interchange in Hong Kong was replaced by a large box junction with traffic lights after recurring area traffic congestion when numerous container trucks journeyed to Kwai Chung Container Port after a typhoon.
The Wikipedia piece continues with descriptions of various types of roundabouts (gyratory system, mini roundabouts, raindrop or pear shaped roundabouts, controlled roundabouts, roundabouts with trams, hamburger roundabouts and my favorite, whose description is reproduced here, magic roundabouts, the image at the top left of the post is a magic roundabout. ).
"Magic" roundabouts
Swindon's "Magic roundabout": Map in GoogleMaps (click for image).
The town of Swindon in Wiltshire, England is known for its "Magic Roundabout". This roundabout is at an intersection 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 anti-clockwise using the inner lanes next to the central island. At each mini-roundabout the usual clockwise flow applies. Please do click on the image above for the full effect; written description pales beside the image under Google Maps.
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The first roundabout was technically square. Constructed in Paris around the Arc de Triomphe in 1901, closely followed by Columbus Circle in New York City in 1904. The first British roundabout was five years later, in Letchworth Garden City in 1909 - originally intended partly as a traffic island for pedestrian. However, the widespread use of roundabouts began when British engineers re-engineered 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 signaled 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). I have not seen one pedestrian island in Doha to date nor can I concur that roundabouts encourage slower speeds and traffic calming.
…… roundabouts do not cope well with the traffic on motorways or similar roads, 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 a few 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".
And here’s the most obvious thing for me: Roundabouts are not suitable for junctions where the exits suffer from traffic congestion. Congestion on one exit commonly blocks a roundabout and spreads to all entering directions. The roundabout of Kwai Tsing Interchange in Hong Kong was replaced by a large box junction with traffic lights after recurring area traffic congestion when numerous container trucks journeyed to Kwai Chung Container Port after a typhoon.
The Wikipedia piece continues with descriptions of various types of roundabouts (gyratory system, mini roundabouts, raindrop or pear shaped roundabouts, controlled roundabouts, roundabouts with trams, hamburger roundabouts and my favorite, whose description is reproduced here, magic roundabouts, the image at the top left of the post is a magic roundabout. ).
"Magic" roundabouts
Swindon's "Magic roundabout": Map in GoogleMaps (click for image).
The town of Swindon in Wiltshire, England is known for its "Magic Roundabout". This roundabout is at an intersection 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 anti-clockwise using the inner lanes next to the central island. At each mini-roundabout the usual clockwise flow applies. Please do click on the image above for the full effect; written description pales beside the image under Google Maps.
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 one in High Wycombe (Buckinghamshire) and the Denham Roundabout in Denham (Buckinghamshire), the Greenstead Roundabout in Colchester (Essex) and the Sadler's Farm Roundabout in Benfleet (Essex); "The Egg" in Tamworth, (Staffordshire); and the Hatton Cross Roundabout in London. Magic roundabouts are also known as "Ring Junctions".
