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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 usually 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 with the traffic on motorways or similar fast roads.
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 left* of the central island. |
(Source for table: Oregon Department of Transportation )
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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 signalized intersections. 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 motorized traffic 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.
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 signalized 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 ride on pedestrian walkways 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.
In addition, roundabouts do not cope 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), M271 and even the M60 have roundabouts on the main line). Some of these roundabouts, as well as other busy roundabouts, have been signalized.
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, England.
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. These work in the same way as larger roundabouts in term of right of way. Mini roundabouts 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.
The A52 motorroad 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 motorroad. However, the sign for a roundabout is not used and a speed limit of 80 km/h applies.
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.
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 is able to circulate in both directions 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 Plough Roundabout in Hemel Hempstead in Hertfordshire, which has five (originally six) intersections, but also High Wycombe and Denham in Buckinghamshire, Colchester and Benfleet in Essex, Tamworth in Staffordshire and Hatton Cross in London
In addition, many British cities have ring roads consisting effectively of a magic roundabout around the city—normally a two way A-Road or Dual carriageway with roundabouts instead of intersections.
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.
Roundabouts often have unique designs in the central island. These can range from flower beds, rocks, hills, statues and fountains.
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