Roundabout interchange
Encyclopedia
A roundabout interchange is a type of interchange
Interchange (road)
In the field of road transport, an interchange is a road junction that typically uses grade separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream. It differs from a standard intersection, at which...

 between a controlled access highway such as a motorway or freeway, and a minor road, in which the slip roads to and from the motorway carriageways converge at a single roundabout
Roundabout
A roundabout is the name for a road junction in which traffic moves in one direction around a central island. The word dates from the early 20th century. Roundabouts are common in many countries around the world...

, which is grade-separated
Grade separation
Grade separation is the method of aligning a junction of two or more transport axes at different heights so that they will not disrupt the traffic flow on other transit routes when they cross each other. The composition of such transport axes does not have to be uniform; it can consist of a...

 from the motorway lanes with bridges. A roundabout interchange is similar to a rotary interchange, which uses a rotary rather than a roundabout. Roundabouts may also be used in conjunction with other interchange types such as a standard or folded diamond interchange
Diamond interchange
A diamond interchange is a common type of road junction, used where a freeway crosses a minor road. The freeway itself is grade-separated from the minor road, one crossing the other over a bridge...

, though such use should not be confused with a roundabout interchange.

Roundabout interchanges are extremely common in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

, with hundreds on the motorway network alone. However, recent cost-cutting has meant that "dumbbell interchange" are increasingly used instead. These are essentially diamond interchange
Diamond interchange
A diamond interchange is a common type of road junction, used where a freeway crosses a minor road. The freeway itself is grade-separated from the minor road, one crossing the other over a bridge...

s with roundabouts instead of signals or stop signs where the slip roads meet the minor road. They are cheaper than roundabout interchanges as only one bridge is required instead of two. Roundabout interchanges are much less common in North America, but have been built more frequently since 1995, to improve safety, and to reduce traffic delays and bridge widening costs. However, and conversely, many of the older and more dangerous rotary-style overpass interchanges have been signalized to improve throughput and safety, such as the Drum Hill Rotary in the New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...

 state of Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

, where such interchanges are unusually commonplace.

A "divided diamond," where the minor road is separated into four intersections, rather than two, also acts like a roundabout interchange, however is more square in shape, and typically has traffic light control.

Examples

  • M1
    M1 motorway
    The M1 is a north–south motorway in England primarily connecting London to Leeds, where it joins the A1 near Aberford. While the M1 is considered to be the first inter-urban motorway to be completed in the United Kingdom, the first road to be built to motorway standard in the country was the...

     & M69
    M69 motorway
    The M69 is a lightly used dual three lane dual carriageway motorway in Leicestershire and Warwickshire, England connecting Leicester and Coventry. It opened in 1977.-History:...

    , Braunstone
    Braunstone Town
    Braunstone is a civil parish and is the largest parish within the district of Blaby in Leicestershire, England, now known as the Town of Braunstone or more commonly, Braunstone Town. At 2007 the population is around 15,000. There are around 7,500 households including Thorpe Astley.Braunstone is...

    , Leicestershire, is an English style signalized roundabout interchange. 52°36′2"N 1°11′42"W
  • Kinsale Road Roundabout
    Kinsale road roundabout
    The Kinsale Road Roundabout , sometimes nicknamed The Magic Roundabout, is a five-arm signalised roundabout located approximately 3km south of Cork city centre in Ireland at the junction of the N25 South Ring Road and the N27 South City Link/Airport Road.The South Ring Road has been constructed on...

     south of Cork, Ireland
    Cork (city)
    Cork is the second largest city in the Republic of Ireland and the island of Ireland's third most populous city. It is the principal city and administrative centre of County Cork and the largest city in the province of Munster. Cork has a population of 119,418, while the addition of the suburban...

     on the N25 South Ring Road at the N27 Airport Road is a signalized roundabout interchange.51°52′25"N 8°28′12"W
  • New York State Route 201
    New York State Route 201
    New York State Route 201 is a north–south state highway located west of the city of Binghamton in Broome County, New York...

     & Riverside Drive, Johnson City, New York
    Johnson City, New York
    Johnson City is a village in Broome County, New York, United States. The population was 15,535 at the 2000 census. It is part of the Binghamton Metropolitan Statistical Area....

    , has the roundabout to the side of the freeway with underpasses for the on and off ramps. 42°6′25"N 75°57′59"W
  • Denham Roundabout
    Denham Roundabout
    The Denham Roundabout is a large road intersection to the west of London, at junction 1 on the M40 motorway.When Western Avenue was originally built, it ended at a point west of Uxbridge at the intersection of the road from Uxbridge towards Gerrards Cross and High Wycombe with the road from...

     (M40
    M40 motorway
    The M40 motorway is a motorway in the British transport network that forms a major part of the connection between London and Birmingham. Part of this road forms a section of the unsigned European route E05...

     & Oxford Road), Denham, Buckinghamshire
    Denham, Buckinghamshire
    Denham is a village and civil parish in the South Buckinghamshire district of Buckinghamshire, England. It is north west of Uxbridge and north of junction 1 of the M40 motorway. Denham contains the Buckinghamshire Golf Club.-Origin:...

    , connects the motorway ramps to local streets using a large circular roadway containing multiple intersections, several of which are roundabouts. 51°33′40"N 0°29′45"W
  • U.S. Route 9 & New York State Route 2
    New York State Route 2
    New York State Route 2 is a state highway in the Capital District of New York, United States. It extends from an interchange with Interstate 87 and NY 7 in the hamlet of Latham within the town of Colonie to the Massachusetts state line, where it continues to Boston as Route 2....

    , Latham, New York
    Latham, New York
    Latham is a hamlet in Albany County, New York, United States. It is located along U.S. Route 9 in the town of Colonie, a dense suburb north of Albany.-History:...

    , is a roundabout interchange with U-turn lanes for the freeway. 42°44′53"N 73°45′39"W


A roundaboat is used as an interchange between Quebec Autoroute 20
Quebec Autoroute 20
Autoroute 20 is a major Quebec Autoroute, following the Saint Lawrence River through one of the more densely-populated parts of Canada, and is part of the Trans-Canada Highway. At , it is the longest Autoroute in Quebec...

 and Quebec Autoroute 520
Quebec Autoroute 520
Autoroute 520, Autoroute Côte de Liesse, or Côte-de-Liesse Road connects Autoroute 20 and Montréal-Pierre Elliott Trudeau International Airport to Autoroute 40 on Montreal Island.- Interchanges from west to east :...

 in Montreal, Quebec. The roundaboat is known as
the Dorval Circle. Ramps to side streets are added as well. The interchange is
currently being replaced.

Three-level stacked roundabout

The three-level stacked roundabout is a variation on the roundabout interchange in which both roads are grade-separated. It is similar to the three-level diamond interchange except that the small square of that latter interchange is enlarged to a true roundabout.
  • Causeway Boulevard and Airline Highway, Metairie, Louisiana
    Metairie, Louisiana
    Metairie is a census-designated place in Jefferson Parish, Louisiana, United States and is a major part of the New Orleans Metropolitan Area. Metairie is the largest community in Jefferson Parish. It is an unincorporated area that would be larger than most of the state's cities if it were...

    , has the traffic circle between the two roads. 29°58′29"N 90°9′23"W
  • Staples Corner
    Staples Corner
    Staples Corner is a major road junction in London, United Kingdom.It has two linked roundabouts and flyovers, which connect the A406 North Circular Road with the A5 Edgware Road and the start of the M1 motorway...

     (A406
    A406 road
    The A406 or the North Circular Road is a road which crosses North London, UK, linking West and East London. It, together with the South Circular Road, forms a ring road through the inner part of Outer London...

     and A5), Dollis Hill
    Dollis Hill
    Dollis Hill is an area of north-west London. It lies close to Willesden, in the London Borough of Brent. As a result, Dollis Hill is sometimes referred as being part of Willesden, especially by the national press...

    , Greater London, has the roundabout on the ground. 51°34′16"N 0°13′55"W


Three-level stacked roundabouts are quite common in Britain because they use less land than other four-way junctions where both roads are grade separated. However, they have lower capacity for turning movements – some have had direct-linking slip roads added later in an attempt to solve this problem. Examples of such junctions are found at Lofthouse, West Yorkshire
Lofthouse, West Yorkshire
Lofthouse is a village in West Yorkshire, England between the cities of Wakefield and Leeds. It is in the City of Leeds metropolitan borough but with a Wakefield postal address . It is mentioned as Locthuse also Loftose in the 1086 Domesday Book....

 (M1
M1 motorway
The M1 is a north–south motorway in England primarily connecting London to Leeds, where it joins the A1 near Aberford. While the M1 is considered to be the first inter-urban motorway to be completed in the United Kingdom, the first road to be built to motorway standard in the country was the...

/M62
M62 motorway
The M62 motorway is a west–east trans-Pennine motorway in Northern England, connecting the cities of Liverpool and Hull via Manchester and Leeds. The road also forms part of the unsigned Euroroutes E20 and E22...

)53°43′55"N 1°30′47"W and Swanley
Swanley
Swanley is a town and civil parish in the Sevenoaks District of Kent, England. It is located on the south-eastern outskirts of London, north of Sevenoaks town. The town boundaries encompass the settlements of Swanley itself, Hextable and Swanley Village...

, Kent
Kent
Kent is a county in southeast England, and is one of the home counties. It borders East Sussex, Surrey and Greater London and has a defined boundary with Essex in the middle of the Thames Estuary. The ceremonial county boundaries of Kent include the shire county of Kent and the unitary borough of...

 (M20
M20 motorway
The M20 is a motorway in Kent, England. It runs from the M25 motorway to Folkestone, providing a link to the Channel Tunnel and the ports at Dover. It is long...

/M25
M25 motorway
The M25 motorway, or London Orbital, is a orbital motorway that almost encircles Greater London, England, in the United Kingdom. The motorway was first mooted early in the 20th century. A few sections, based on the now abandoned London Ringways plan, were constructed in the early 1970s and it ...

) 51°23′19"N 0°11′38"E.

External links

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