Roundabout

This article is about the road junction. For other uses, see Roundabout (disambiguation).

Road sign for a roundabout in Germany (right-hand traffic). In the UK this symbol (with the arrows reversed) is used at mini roundabouts.

File:Circular Intersection sign.svg

Road sign before a roundabout in the U.S. (right-hand traffic).

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Traffic sign to identify a roundabout in the UK (left-hand traffic).

File:Give way sign at roundabout (Australia).svg

Give way sign at a roundabout in Australia (left-hand traffic).

A roundabout is one of several types of circular road junctions or intersections at which traffic is slowed down and enters a one-way stream around a central island. Technically these junctions sometimes are called modern roundabouts, in order to emphasize the distinction from older circular junction types which had different design characteristics and rules of operation. In the United States those older designs commonly are referred to as "rotaries" or "traffic circles".

In countries where people drive on the right, the traffic flow around the central island of a roundabout is anticlockwise (also known as ''counter''clockwise). In countries where people drive on the left, the traffic flow is clockwise.

Statistically, roundabouts are safer for drivers and pedestrians than both traffic circles and traditional intersections. Because low speeds are required for traffic entering roundabouts they are not designed for high-speed motorways (expressways). When such roads are redesigned to take advantage of roundabout principles, steps are taken to reduce the speed of traffic, such as adding additional curves on the approaches.

Modern roundabouts are particularly common in Australia, the United Kingdom, Ireland, and France. Half of the world's roundabouts are in France (over 30,000 as of 2008). The first modern roundabout in the United States was constructed in Summerlin, Nevada in 1990, and roundabouts have since become increasingly common in North America.

Difference from traffic circles and rotaries

Starting in the 1960s, research in the United Kingdom found that circular junctions with certain geometric characteristics and traffic control schemes tended to be safer than those without them and the engineers building them distinguished them with the term roundabout.

In technical use, circular junctions with the following characteristics are roundabouts and those that do not are considered traffic circles or rotaries:

• Roundabouts require entering drivers to give way to all traffic within the roundabout regardless of lane position, while rotaries and traffic circles typically allow traffic to enter alongside traffic circulating in an inner lane without consequence.
• Generally, exiting directly from the inner lane of a multi-lane roundabout is permitted, and such exiting traffic has the right-of-way over entering traffic. By contrast, exiting from the inner lane of a traffic circle or rotary is usually not permitted without first executing a lane change to the outside of the circle.
• Deflection on entry is used to maintain low speed operation in roundabouts. Drivers must maneuver (are deflected) around the splitter islands and the central island, at speeds of 15–25 miles per hour (24–40 kilometres per hour). Many older rotary and traffic circle junctions allow entry at higher speeds due to the lack of deflection, or require a stop and a 90-degree turn to enter, creating a large difference in speed between entering and circulating traffic which can make it difficult for entering drivers to find suitable gaps in heavy traffic.
• Pedestrians are usually prohibited from the central island of roundabouts, and the crosswalk for pedestrians and some cyclists is withdrawn from the junction by at least the length of one vehicle.
• All vehicles circulate around the central island of a roundabout in the same direction, which is determined by whether traffic drives to the right or the left. In left-hand traffic countries the circulation is clockwise; in those that drive to the right, it is anticlockwise.
• Modern multi-lane roundabouts are typically less than 250 feet (75 meters) in diameter, although signalised roundabouts and roundabout interchanges may be considerably larger.

The term "traffic circle" is not used in the United Kingdom, where most circular junctions meet the criteria for roundabouts. The U.K. does, however, have roundabout variants such as mini-roundabouts and magic-roundabouts — see below for the distinctions from the type of junction generally referred to here as a roundabout).

In the U.S., many people incorrectly use the terms "roundabout", "traffic circle", and "rotary" interchangeably. Many old rotaries and traffic circles remain in the Northeastern United States. Since many of the older junction forms have unfavourable safety records, transportation professionals are careful to use "roundabout" when referring to newer designs and "traffic circle" or "rotary" when referring to ones that do not meet the criteria listed above. The more precise term, modern roundabout is used often to differentiate more carefully.

A large number of traffic circles and rotaries have been converted to other types of junctions. Several have been converted to roundabouts, now meeting modern roundabout design standards, including the former Kingston traffic circle in New York and several in New Jersey..

History

Movement within a roundabout in a country where traffic drives on the left. Note the clockwise circulation. There is also a right hand side version of the animation.

Numerous circular junctions existed before the advent of roundabouts, including the Place de l'Etoile around the Arc de Triomphe in Paris, Columbus Circle in New York City, and several circles within Washington, DC. However, the operating and entry characteristics of these circles differed considerably from modern roundabouts. The first British roundabout was built in Letchworth Garden City in 1909 - originally intended partly as a traffic island for pedestrians. In the early 20th century, numerous rotary junctions were constructed in the United States, particularly in the northeast states, which allow entry at relatively high speeds and require entering drivers to weave with exiting and circulating traffic.

However, the widespread use of roundabouts began when British engineers re-engineered circular intersections during the mid-1960s and Frank Blackmore invented the mini roundabout to overcome its limitations of capacity and for safety issues. Unlike traffic circles, traffic approaching roundabouts is normally required to give priority to circulating and exiting traffic and to eliminate much of the driver confusion associated with traffic circles and waiting queues associated with junctions that have traffic lights. Roughly the same size as signalled intersections with the same capacity, roundabouts also are significantly smaller in diameter than most traffic circles and rotaries, separate incoming and outgoing traffic with pedestrian islands to encourage slower and safer speeds (see traffic calming).

Safety

A major signal-controlled roundabout in central Bristol, England. Vehicles drive on the left, and vehicles in the roundabout are stopped by traffic lights to allow other vehicles to enter.

Small modern roundabout in the United States. Vehicles drive on the right.

Roundabout in the United States with separated side lanes. Vehicles entering the roundabout give way to vehicles in the roundabout. As of March 2009 the road at upper left carries 10,400 vehicles per weekday.

Roundabouts are safer than both traffic circles and traditional junctions—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, when compared with the junctions they replaced). Roundabouts also 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 or alternate routes.

At traditional junctions with stop signs or traffic lights, the most serious accidents are right-angle, left-turn, or head-on collisions that can be severe because vehicles may be moving fast and collide at high angles of impact. Roundabouts virtually eliminate those types of crashes because vehicles all travel in the same direction and most crashes are glancing blows at low angles of impact.

While roundabouts can reduce crashes overall compared to other junction types, crashes involving cyclists may not experience similar reductions.. An analysis of the New Zealand national crash database for the period 1996–2000 shows that cyclists were involved in 26% of the reported injury crashes at roundabouts, compared to 6% at traffic signals and 13% at priority controlled junctions. The New Zealand researchers propose that low vehicle speeds, circulatory lane markings, and mountable centre aprons for trucks can improve the safety of cyclists within roundabouts. These strategies are typically employed on modern roundabouts constructed in the United States.

The most common roundabout crash type for cyclists, according to the New Zealand study, involves a motor vehicle entering the roundabout and colliding with a cyclist who already is traveling around the roundabout (generally just over 50% of all cyclist/roundabout crashes fall into this category). The next most common crash type involves motorists leaving the roundabout, colliding with cyclists who are continuing further around the perimeter of the roundabout. Designs that have marked perimeter cycle lanes are found by research data to be even less safe than those without them, suggesting that in roundabouts cyclists should "take the lane", operating as a vehicle rather than riding on the exterior.

If the adjacent footpaths are not properly designed, there are increased risks for persons with visual impairments. This is because, unlike traffic signals, it is hard to hear if there is an adequate gap in traffic to cross. During the all-red interval at a signal, traffic comes to a stop, and blind pedestrians can tell by listening which direction gets the green light. Since there is often moving traffic at a roundabout, the sounds of non-conflicting traffic will mask gaps, or the sound of an idling vehicle whose driver has stopped to give way to the pedestrian.

This issue has led to a conflict in the United States between the visually impaired and civil engineering communities; some in the visually impaired community have taken the position that roundabouts (rather than signal-controlled crossings) are acceptable only if there are pedestrian crossings with signalised control at each road connecting to a roundabout. Engineers point out that since vehicle speeds are slower, crossing gaps are more plentiful, drivers are more apt to give way, and the severity of pedestrian crashes are lower than if the same driver had run a red light. However, the blind community considers this to be a civil rights issue, not an engineering issue. While pedestrian crossings with traffic lights installed in roundabouts are not unheard of (see below), signalisation is normally used on large-diameter roundabout interchanges rather than small-diameter modern roundabouts. Signalisation would also substantially increase the cost of roundabout construction and maintenance (essentially, both types of junction being built at every junction). Furthermore, equipping a roundabout with traffic-halting lights would decrease its throughput considerably, thereby artificially reducing or even eliminating the design's main advantage over traditional signal-equipped junctions. Signalisation would also increase delays for most pedestrians during light traffic, since pedestrians would need to wait for the signal to change to legally cross.

Capacity and Delays

The capacity of a roundabout varies based on the number of entry and circulating lanes, and also on more subtle geometry elements including entry angle and lane width. Also, like other types of junctions, the operational performance of a roundabout depends heavily on the flow volumes from various approaches. A single-lane roundabout can be expected to handle approximately 20,000 to 26,000 vehicles per day, while a two-lane roundabout can be expected to handle 40,000 to 50,000 vehicles per day .

Under many traffic conditions, an unsignalised roundabout can operate with less delay to users than traffic signal control or all-way stop control. Unlike all-way stop intersections, a roundabout does not require a complete stop by all entering vehicles, which reduces both individual delay and delays resulting from vehicle queues. A roundabout can also operate more efficiently than a signalised junction because drivers are able to proceed when traffic is clear without the delay incurred while waiting for the traffic signal to change.

However, roundabouts can increase delays in locations where traffic would otherwise not be required to stop. For example, at the junction of a high-volume and a low-volume road, traffic on the busier road would normally not have to stop if the junction were signalised, because the traffic signals would provide a green signal to the busier road the majority of the time. When the volumes on the approach roadways are relatively balanced, a roundabout can reduce delay because each approach would otherwise encounter a red signal greater than half of the time if the junction were signalised.

Roundabouts can also reduce delays for pedestrians when compared to traffic signals, because pedestrians are able to cross during any safe gap rather than waiting for the traffic signal to provide the right-of-way to the pedestrian.

Several software packages exist to help with calculating capacity and queues at roundabouts. These include ARCADY, RODEL and aaSIDRA.

Types

Large roundabouts such as those used at motorway junctions typically have two to six lanes around the central hub, and may have traffic lights regulating flow.

Some roundabouts have a divider between traffic turning from one road onto an adjacent one, and traffic within the roundabout, enabling those making such 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, while 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, Hull, Nottingham and Reading (all in England), as well as on the N2/M50 intersection in Dublin, Ireland.

Gyratory system

The term "gyratory" (for example, Hanger Lane gyratory) is sometimes used in the United Kingdom 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.

Mini-roundabouts exist at smaller junctions to avoid the use of signals, stop signs or the necessity to give way 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 easily be driven over by most vehicles, which many motorists will do when there is no other traffic, but the practice is dangerous if other cars are present. Mini-roundabouts work in the same way as larger roundabouts in terms 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 Ireland, as well as Irapuato, Mexico, usually marked with a tiny grassy circle enclosed edge paving, and in Calgary, Canada's inner-city Mount Royal and Rosedale neighbourhoods, where mini-roundabouts recently replaced junctions formerly controlled by stop signs to combat increasing cut-through traffic.

A slightly larger version of a mini-roundabout, sometimes called a "small roundabout", is designed with a raised centre surrounded by a sloped "overrun area" of a different colour from the roadway and up to a metre in thickness called a "truck apron" or a "mountable apron". The truck apron's design discourages small vehicles from taking a shortcut over it while at the same time allowing the mini-roundabout to more easily accommodate the turning radius of larger vehicles (such as a truck which may have to navigate the roundabout). These are not well suited for bus routes, as mounting the apron can be somewhat uncomfortable to passengers.

In the UK the maximum diameter permissible of a mini-roundabout is 4m. Whilst it may be physically possible, it is illegal for vehicles like cars, which can turn around the mini-roundabout, to go over the painted island, or around the wrong way- vehicles should treat it like a solid island and proceed around it. (In practice, few motorists obey these rules). Some local authorities have installed double white lines around the island to indicate this, but these are not permissible. The centre island also must be able to be over-run by larger vehicles. If this is not possible, perhaps due to plants, or street furniture it is considered a small roundabout not a mini roundabout and as such must adhere to the stricter roundabout guidelines.

At Hatton Cross roundabout, close to Heathrow Airport, five small ‘mini-roundabouts’ have been constructed where the roads join/leave the main roundabout, requiring traffic to circle the main roundabout in an anti-clockwise direction.

Raindrop Roundabouts

A hybrid raindrop roundabout in Zagreb, Croatia. These two roundabouts are more akin to a magic roundabout since the left turns need not drive through both of them.

Raindrop roundabout at an Interstate interchange in North Carolina.

These roundabouts do not form a complete circle and are in a "raindrop" shape. They are appearing at U.S. Interstate interchanges to provide a free-flowing left turn to the on-ramps and eliminating the need for turn signals and lanes. Since the entry and exit slip roads are one-way, a complete circle is unnecessary. This means that drivers entering the roundabout from the bridge do not need to give way and prevents queuing on narrow, two-lane bridges. These roundabouts have been used at dumbbell roundabout junctions, replacing traffic signals that are inefficient without a turning lane. Several junctions along Interstate 70 near Avon, Colorado use teardrop roundabouts.

Turbo roundabouts

The basic shape of the Dutch turbo roundabout

In the Netherlands, a relatively new type of roundabout is built increasingly often. It provides a forced spiralling flow of traffic, thus requiring motorists to choose their direction before entering the roundabout. By eliminating many conflicting paths and choices on the roundabout itself, traffic safety is increased, as well as speed, and as a result, capacity. A turbo roundabout does not allow traveling a full circle.

Several variations of the turbo roundabout exist. The basic turbo roundabout shape is designed for where a major road crosses a road with less traffic.

Turbo roundabouts are typically built with raised lane separators. Cheaper implementations with only road markings exist, but hurt the efficiency (regarding safety, speed and capacity) of the design by enabling users to cheat the system.

According to micro-simulation, a two-lane roundabout with free right turns should offer 12-20% greater traffic flow than a conventional, three-lane roundabout of the same size. The reason offered by authors Ir. Isaak Yperman and Prof. Ir. Ben Immers is that there is less weaving in a turbo, making entering and exiting more predictable. Because there are only ten points of conflict (compared with 16 for a conventional roundabout, or 64 with a traffic signal), it is expected that these new designs will be safer, as well. At least 15 have been built in the Netherlands, while many turbos (or similar, lane splitting designs) can be found in southeast Asia.

Multi-lane roundabouts in the United States are typically required to be striped with spiral markings, as most states follow the federal Manual on Uniform Traffic Control Devices or develop a state-level manual subject to the approval of the federal government.

Roundabouts on motorways

See also: Roundabout interchange

Roundabouts are generally not appropriate for placement on motorway or freeway mainlines because the purpose of such facilities is to provide for uninterrupted traffic flow. However, roundabouts are often used for the junction between the slip roads and the intersecting road. A single roundabout, grade separated from the mainlines, may be used to create a roundabout interchange. This type of junction is common in the UK and Ireland. Alternatively, separate roundabouts may also be used at the slip road intersections of a diamond interchange to create what is often referred to as a "dumbbell interchange", which is increasingly common in both Europe and North America due to its reduced need for wide or multiple bridges.

An additional use of roundabouts for junctions is the 3-level stacked roundabout — this is a roundabout interchange where both of the roadway mainlines are grade separated. In the United Kingdom, the M25/A3, M8/M73 and A1(M)/M18 interchanges are examples of this type. These junctions, however, have less capacity than a full free-flow interchange. A similar design to this is the three-level diamond interchange.

Most junctions on Dublin's M50 motorway C-road use a standard roundabout interchange — although several such junctions have a greater volume of traffic than the capacity such roundabouts can accommodate. In Northern Ireland the junction between the M1 and M12 (Craigavon connector motorway) is via a standard roundabout with a raised centre, 3 onslips and 3 offslips, and 2 lanes.

In the city of 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.

In the Netherlands, A6 motorway and A7 motorway cross near Joure using a roundabout. For the junction between the A200 and the A9 a 3-level stacked roundabout is used. Near Eindhoven (the Leenderheide junction), the junction for the A2 is done with a roundabout. An overpass is built for the A67 from Antwerp to Germany.

Roundabout interchanges are sometimes confused with rotary interchanges, which operate with rotaries rather than roundabouts. Rotary interchanges are common in New England, particularly in the state of Massachusets, but a European example of a rotary interchange may be found in Hinwil, Switzerland.

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 common in Great Britain and Ireland, where they have been introduced in an attempt to alleviate traffic problems at over-capacity roundabout junctions or to prevent some flows of traffic dominating others (around the M50 in Dublin for example).

"Magic" roundabouts

File:Swindon Magic Roundabout eng.svg

The "Magic roundabout" in Swindon, England _{Map in GoogleMaps} A mirrored version is available here, to illustrate the traffic flow of a potential right-hand traffic version of the roundabout

Sadlers Farm Roundabout

The town of Swindon in Wiltshire, England, is known for its "Magic Roundabout". This roundabout is at a junction 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 anticlockwise using the inner lanes next to the central island. At each mini-roundabout the usual clockwise flow applies.

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 in High Wycombe (Buckinghamshire), the Denham Roundabout in Denham (Buckinghamshire), the Greenstead Roundabout in Colchester (Essex), the Sadler's Farm Roundabout in Benfleet (Essex) which is the junction between the A130, the A13 and the B1464. "The Egg" in Tamworth (Staffordshire) and the Hatton Cross Roundabout in London. Magic roundabouts are also known as "Ring Junctions", while larger ones are sometimes known as "Gyratories".

Roundabouts with trams

Tram roundabouts are most notably found in inner Melbourne, particularly in the inner suburban area of South Melbourne, where the tram network is extensive. Tram tracks always pass through the central island of these roundabouts, with drivers required to give way, not only to vehicles coming towards them from their right, but also to trams coming at them from right-angles.

Having trams pass through small roundabouts is not a problem; through larger roundabouts it can be difficult, particularly when there is a junction between tram lines as well. In these cases, the roundabouts are very large, and often have tram stops in the middle. The Haymarket roundabout between Royal Parade and Elizabeth Street is the most notorious junction 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 and pedestrian crossing.

There are a few larger roundabouts in Brussels, Belgium, where several tram routes converge from different directions, and the tram lines are laid around the roundabout allowing the trams to follow the same path as other traffic.

In Dublin, Ireland, the Red Cow ("Mad Cow") roundabout at the N7/M50 junction 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 turns. The junction, the busiest in Ireland, had tram lines added 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 at Korsvägen (the Crossroad) 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 passes in several directions. This is further complicated by separated rights-of-way for trams and buses and the fact that it is also one of the busiest interchanges in the city. Another one is located at Mariaplan in the inner suburb of Majorna. The trams makes a right turn, giving the roundabout an odd design. Since traffic isn't heavy, this normally doesn't create any problems.

In Wrocław, Poland, trams pass through the Powstańców Śląskich Roundabout, having a stop in the roundabout (north-headed track).

In Vítězné náměstí (Victory Square) in Prague, Czech Republic, a tramway crosses the carriage way of the roundabout at 3 places since 1942. Entering as well as leaving trams have to give way to all cars. In years 1932–1942 trams went around much like cars.

In Kiev, Ukraine an interchange of two "fast tram" lines is done below a roundabout.

Oslo, Norway also has many roundabouts with tram tracks passing through; for example at Bislett, Frogner plass, Sinsen, Solli plass and Storo.

In Wolverhampton, England, the Midland Metro tram passes through the centre of a roundabout on approach to its terminus at St Georges. This also happens in New Addington on the Tramlink on Old Lodge Lane at the junction to King Henry's Drive.

An underpass in Sheffield, England, allows the tram to travel below a major roundabout, avoiding what would otherwise be a serious hindrance to traffic. A second notable tram/roundabout feature on the Sheffield system has three main branches meet and junction together atop a large, earth-worked traffic island with each line being bridged across to the outside of the feature high above the traffic, along with a couple of pre-existing walkway bridges that are now complemented by the pedestrian paths set alongside two of the incident lines.

In Salt Lake City, Utah a light rail line on the south side of the University of Utah crosses a roundabout where Guardsman Way meets South Campus Drive. Like virtually all rail crossings in the United States, both crossings in the circle are equipped with boom barriers.

Roundabouts with railways

In Jensen Beach, Florida, the main line of the Florida East Coast Railway running north-south bisects the two-lane roundabout at the junction of Jensen Beach Boulevard running east-west and three other roads and the service entrance to a large shopping plaza. Boom barriers are in place at the railway crossings. The landscaped centre island bisected by the tracks was originally curbed, but 18-wheelers had trouble negotiating the roundabout, so the curbs were replaced with painted drive over concrete strips. A proposed fountain in the centre island has never been built. The roundabout was built in the early 2000s and has improved traffic flow considerably although there are still delays caused by the many long freight trains coming through. Two roundabouts in the Melbourne metropolitan area, Highett and Brighton, have heavy rail crossing the roundabout and through the inner circle. Boom barriers protect the rail from oncoming traffic at the appropriate points in the roundabout.

Hamburger roundabout/throughabout/cut-through roundabout

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. There are two such examples on the A580 East Lancashire Road in St Helens, England, one at Haydock Island in Merseyside (which also features the M6 passing overhead), the second is on the Astley/Boothstown border near to Manchester. More examples are the A6003 at Kettering and the A538 near Manchester Airport.

A more advanced and safer version of a hamburger roundabout is a roundabout interchange, separating the straight roadway and using underpasses or overpasses to cross the roundabout itself.

Roundabouts and cyclists

Cycle facilities at roundabouts

Research has shown that even in large circular junctions that lack modern roundabout design features, a high rate of bicycle/motor vehicle crashes occurs when bicyclists are riding around the outside. Design guidance for modern roundabouts recommends terminating cycle lanes well before the entrances, so bicyclists merge into the stream of motor traffic.

A 1992 study from the German Transport Ministry's research institute has cast particular light on this issue. The study found that bicyclists' risk is high in all such intersections, but it is much higher when the junction has a marked bicycle lane or sidepath around its outside (see "Marked perimeter cycle lanes" below). The results of this study concerning circular junctions are summarised on the web (in German, but partially translated below). A report about accidents at four-arm roundabouts was published by the UK Transport and Road Research Laboratory (TRRL) (now TRL) in 1984.

Collisions typically occur when a motorist is entering or leaving the circular roadway. A motorist entering the circular roadway must give way to traffic in it, but such traffic will generally keep away from the outside of the circular roadway (as with a vehicle in the photo) if passing an entrance. A bicyclist close to the edge of the roadway is not in the usual position where an entering motorist expects to look for circulating traffic.

When exiting the circular roadway, a motorist must look ahead to steer, and to avoid colliding with another vehicle ahead or with pedestrians on a footpath. As the circular roadway curves away from the exit, the path of a vehicle exiting the circular roadway is relatively straight, and so the exiting motorist may often not need to slow substantially. However, if it is necessary to give way to a bicyclist riding around the outside, the exiting motorist must look toward the rear, to the outside of the intersection. With many vehicles, such as vans, the driver's view in this direction is obstructed. The task burden of the motorist is therefore substantially increased if bicyclists ride around the outside. The resulting conflicts, and more frequent requirements for motorists to slow or stop, also reduce the efficiency of traffic flow which is one of the major advantages of the circular junctions. Cycle lanes around the outside of circular junctions are therefore falling out of favour.

Marked perimeter cycle lanes

Cycle lanes on St John's roundabout in Newbury, Berkshire, England. This example is from a country where traffic keeps to the left, and traffic in the circular roadway of the roundabout travels clockwise.

An early attempt to deal with the problem was to mark preferential lanes for cyclists. With cycle lanes, bicyclists do not merge into the flow of motor traffic in the roundabout, but rather, they travel around the outside, relieving them of the requirement to merge. The coloured road surface and edge lines of the cycle lanes indicate that exiting motorists are required to give way to bicyclists at all locations where their paths may cross. As noted previously, this design has been found to be hazardous to cyclists and has fallen out of favour.

Modern design guidance

The special features of modern roundabouts, including splitter/diverter islands and small diameter of the circular roadway, decrease the speed of motor traffic and so reduce the risk of collisions for motorists as well as cyclists below that of conventional junctions. Design guidance for modern roundabouts recommends terminating cycle lanes well before the entrances, so cyclists merge into the stream of motor traffic. Cyclists who lack the confidence to do this may use the footpaths as pedestrians. Modern design guidance also recommends placing the footpaths far enough from the roundabout so that at least one exiting vehicle can wait without blocking the circular roadway. A roundabout with 2 lanes should place the footpath two car lengths from the junction.

Bicycle/pedestrian roundabouts

The same features that make roundabouts an attractive option for roadway junctions have led to their use at junctions of multi-use trails. The University of California, Davis[3] and Stanford University both have built bicycle-pedestrian roundabouts. Roundabouts are used on off-road bicycle trails in Florida, Colorado and Alaska.

See the United States DOT publication, Roundabouts: An Informational Guide.

Advantages of roundabouts

• Roundabouts are safer than signal controlled junctions, with accidents usually occurring at a slower speed and at a slight angle instead of right-angle or rear end collisions at junctions.

• Most roundabouts are not controlled by traffic lights. This reduces waiting time, compared to crossroads that have traffic lights. The absence of traffic lights can also equate to reduced operating costs for lamp replacement and electricity.

• Roundabouts allow for easy u-turns, including for larger vehicles, which are sometimes impossible or forbidden in normal road junctions.

• Roundabouts (and other circular intersection types) allow for landscaping, monuments, and other aesthetic uses within the central island.

• Reduced waiting time waiting for lights means less fuel wasted while idling & better air quality

• No overhead traffic lights means drivers can focus on the street level for other cars and pedestrians

• Roundabouts still function as designed during electrical power outages

Disadvantages of roundabouts

• Roundabouts cannot give priority to major-road approaches and require all traffic to slow down, and as such may be undesirable where a high-volume road would otherwise not be required to stop.

• A roundabout occupies more space than crossroads at the intersection point. However, traffic signals often require construction of turn lanes for capacity and safety reasons which typically extend a considerable distance from the junction, and therefore traffic signals may occupy more space overall than a roundabout at the same location.

• Unfamiliar and elderly drivers may become confused and use roundabouts improperly in areas where roundabouts are otherwise uncommon, especially in areas where traffic circles or rotaries also exist.

• The central island of a roundabout can be a hindrance to movement of oversize vehicles, such as those carrying large machinery or houses. Highway designers can design for oversized loads, if they are aware that they will be using the route. An example can be seen here.

• Obstruction to departing vehicles, such as a nearby grade level railway crossing, continuous pedestrian flows, or signalised pedestrian crossings can result in blockage of all approaches to the roundabout until the obstruction clears. In this scenario, exiting vehicles queue into the roundabout, thereby obstructing both entering and circulating traffic. In a non-roundabout intersection, particularly those with exclusive turn lanes, traffic flow can typically be maintained in directions parallel to the obstruction.

Examples of roundabouts

• St George's Circus, London
• Van Dyke Ave at 18 1/2 Mile Road, Sterling Heights, Michigan (3-Lane Roundabout)
• Interstate 70 at Avon Road, Avon, Colorado
• Interstate 17 at Happy Valley Road, north of Phoenix, Arizona
• Los Alamitos Traffic Circle, Long Beach, California
• Kinsale road roundabout (Kinsale Rd/N27 at South Ring Road (N25), Cork, Ireland (Signalised roundabout)
• The Plain, Oxford, England (Irregularly shaped signalised roundabout)
• Ancaster Roundabout (Kings Hwy 2 at Hamilton Drive), Hamilton, Ontario, Canada
• Trade Center roundabout, Dubai, United Arab Emirates (UAE)
• Highways E6 at E39 in Klett, Norway
• Dunning Street at Interstate 87 ramps, Malta, New York
• New Seward Highway at Dowling Road, Anchorage, Alaska, USA (A dumbbell junction)
• Udyog Path at Sarovar Path, in Chandigarh, India. Most major grid intersections in Chandigarh are circular, many of which are roundabouts.
• The Australian cities of Griffith in New South Wales, and Albany in Western Australia contain no traffic lights, only roundabouts.
• The town of Milton Keynes, England is famous for having grid roads which have roundabouts at almost every junction.
• Seattle, Washington has well over one hundred roundabouts that have been built during the last two decades, with many more located throughout the Seattle Metropolitan area. In addition, hundreds of mini-roundabouts have long existed in the city's residential neighborhoods.

See also

• Roundabout Appreciation Society
• Roundabout interchange
• Traffic circle
• Rotary
• Give way/Yield Sign
• Junction
• All-way stop
• Roundabout dog

External links

• Modern Roundabouts - Geocoded National Database
• Mini Roundabouts Good Practice Guidance - Department for Transport (United Kingdom)
• DLZ Roundabouts
• Roundabouts in North America
  • Roundabouts: An Informational Guide by the U.S. Federal Highway Administration, FHWA-RD-00-67, June 2000 (or see the entire pdf in one file)
  • Secion 3B.24 from the U.S. Manual on Uniform Traffic Control Devices
  • Roundabouts: Interim Requirements and Guidance by the New York State Department of Transportation, June 20, 2000
• The Magic Roundabout of Swindon
• Multilane Roundabouts an Information Sheet
• The Hanger Lane Gyratory System, London, England
• The Kinsale Road Roundabout, Cork, Ireland, on Google Maps
• Approaching a roundabout
• Mini-roundabouts - Getting them Right
• Washtenaw County Road Commission's Modern Roundabout Information
• Roundabout Information and related CBC Article and Video
• Traffic Congestion in City Streets: one suggestion for relief takes the form of a novel safety island, Popular Science monthly, February 1919, page 69, Scanned by Google Books: http://books.google.com/books?id=7igDAAAAMBAJ&pg=PA69
• Don't Be So Square: Why American drivers should learn to love the roundabout, Slate, July 20, 2009