A cloverleaf interchange is a two-level interchange in which all turns are handled by slip roads. To go left (in right-hand traffic; reverse directions in left-driving regions), vehicles first continue as one road passes over or under the other, then exit right onto a one-way three-fourths loop ramp (270°) and merge onto the intersecting road. The objective of a cloverleaf is to allow two highways to cross without the need for any traffic to be stopped by traffic lights. The limiting factor in the capacity of a cloverleaf interchange is traffic weaving.

Overview

Cloverleaf interchanges, viewed from overhead or on maps, resemble the leaves of a four-leaf clover or less often a 3-leaf clover. In the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

, cloverleaf interchanges existed long before the Interstate system. They were originally created for busier interchanges that the original

diamond interchange A diamond interchange is a common type of road junction, used where a controlled-access highway crosses a minor road. Design The freeway itself is grade-separated from the minor road, one crossing the other over a bridge. Approaching the ...

system could not handle. Their chief advantage was that they were free-flowing and did not require the use of such devices as

traffic signals Traffic lights, traffic signals, or stoplights – known also as robots in South Africa are signalling devices positioned at road intersections, pedestrian crossings, and other locations in order to control flows of traffic. Traffic light ...

. This not only made them a viable option for interchanges between

freeways A controlled-access highway is a type of highway that has been designed for high-speed vehicular traffic, with all traffic flow—ingress and egress—regulated. Common English terms are freeway, motorway and expressway. Other similar terms i ...

(where such devices are typically not an option), but they could also be used for very busy arterials where signals could present congestion problems. They are common in the United States and have been used for over 40 years as the

Interstate Highway System The Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly known as the Interstate Highway System, is a network of controlled-access highways that forms part of the National Highway System in the United States. T ...

expanded rapidly. One problem is that, frequently, large

truck A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame constructi ...

s exceeding the area

speed limit Speed limits on road traffic, as used in most countries, set the legal maximum speed at which vehicles may travel on a given stretch of road. Speed limits are generally indicated on a traffic sign reflecting the maximum permitted speed - expre ...

(i.e., 25 mph; 40 km/h) roll over. Another problem is the merging of traffic ( see below). For these reasons, cloverleaf interchanges have become a common point of traffic congestion at busy junctions.

At-grade At-grade may refer to: *At-grade intersection, a crossing between roads on the same level * Road junction *Level crossing, where a road or path crosses a railway on the same level * Diamond crossing, where two railway tracks cross * At-grade railway ...

cloverleaf configurations with full four leaves and full outside slip ramps are extremely rare, though one exists in

Toms River, New Jersey Toms River is a township in Ocean County, New Jersey, United States. Its mainland portion is also a census-designated place of the same name, which serves as the county seat of Ocean County.jughandle or parclo intersection.

History

Background

The first cloverleaf interchange patented in the US was by Arthur Hale, a

civil engineer A civil engineer is a person who practices civil engineering – the application of planning, designing, constructing, maintaining, and operating infrastructure while protecting the public and environmental health, as well as improving existing ...

Maryland Maryland ( ) is a state in the Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; and Delaware and the Atlantic Ocean t ...

, on February 29, 1916. A modified cloverleaf, with the adjacent ramps joined into a single two-way road, was planned in 1927 for the interchange between Lake Shore Drive ( US 41) and Irving Park Road ( ILL 19) in

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, but a

was built instead.

Beginning

The first cloverleaf interchange built in the United States was the Woodbridge Cloverleaf at intersection of the

Lincoln Highway The Lincoln Highway is the first transcontinental highway in the United States and one of the first highways designed expressly for automobiles. Conceived in 1912 by Indiana entrepreneur Carl G. Fisher, and formally dedicated October 31, 191 ...

( Route 25) and Amboy—now St. Georges—Avenue ( Route 4) (now U.S. 1/9 and Route 35) in

Woodbridge Township, New Jersey Woodbridge Township is a township in Middlesex County, in the U.S. state of New Jersey. The township is both a regional hub for Central New Jersey and a major bedroom suburb of New York City in the New York metropolitan area located within the ...

. It opened in 1929, and has been replaced with a partial cloverleaf interchange. Before the cloverleaf was replaced, it was judged eligible for inclusion in the

National Register of Historic Places The National Register of Historic Places (NRHP) is the United States federal government's official list of districts, sites, buildings, structures and objects deemed worthy of preservation for their historical significance or "great artistic ...

. Because of this, a commerative film was made of the cloverleaf. The original cloverleaf interchange design was adapted by the Rudolph and Delano building firm from

Philadelphia Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Since ...

, from a photo Delano saw on a magazine cover about a highway in

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Argentina Argentina (), officially the Argentine Republic ( es, link=no, República Argentina), is a country in the southern half of South America. Argentina covers an area of , making it the List of South American countries by area, second-largest ...

. The original inventor is unknown. The first cloverleaf west of the

Mississippi River The Mississippi River is the List of longest rivers of the United States (by main stem), second-longest river and chief river of the second-largest Drainage system (geomorphology), drainage system in North America, second only to the Hudson B ...

opened on August 20, 1931, at Watson Road and Lindbergh Boulevard near St. Louis, Missouri, as part of an upgrade of U.S. 66. The first cloverleaf interchange in Canada opened in 1937 at the junction of the Queen Elizabeth Way (QEW) and Provincial Highway 10 in Port Credit, Ontario (now a part of Mississauga, Ontario). As originally built, Highway 10 passed over the QEW. In 1962, the interchange was rebuilt with sub-collector roads along the QEW, and the orientation was also changed so that Highway 10 then passed under the QEW. The interchange was further modified between 2008 and 2010 by removing all but one loop ramp, creating a partial cloverleaf/diamond hybrid. The cloverleaf was patented in

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

Switzerland ). Swiss law does not designate a ''capital'' as such, but the federal parliament and government are installed in Bern, while other federal institutions, such as the federal courts, are in other cities (Bellinzona, Lausanne, Luzern, Neuchâtel ...

on October 15, 1928. The first cloverleaf in

opened in October 1935 at Slussen in central

Stockholm Stockholm () is the capital and largest city of Sweden as well as the largest urban area in Scandinavia. Approximately 980,000 people live in the municipality, with 1.6 million in the urban area, and 2.4 million in the metropo ...

Sweden Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic countries, Nordic c ...

, followed in 1936 by Schkeuditzer Kreuz near

Leipzig Leipzig ( , ; Upper Saxon: ) is the most populous city in the German state of Saxony. Leipzig's population of 605,407 inhabitants (1.1 million in the larger urban zone) as of 2021 places the city as Germany's eighth most populous, as ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...

. This is now the interchange between the A 9 and A 14, and has a single flyover from the westbound A 14 to the southbound A 9.

Kamener Kreuz The Kamener Kreuz was formerly a full cloverleaf interchange near Dortmund in North Rhine-Westphalia Germany where the Autobahnen Bundesautobahn 1, A1 and Bundesautobahn 2, A2 meet. It lies between the towns of Kamen and Bergkamen in the west and ...

was the first in continental Europe to open fully in 1937, at A 1 and A 2 near

Dortmund Dortmund (; Westphalian nds, Düörpm ; la, Tremonia) is the third-largest city in North Rhine-Westphalia after Cologne and Düsseldorf, and the eighth-largest city of Germany, with a population of 588,250 inhabitants as of 2021. It is the la ...

Problems

The primary drawback of the classic design of the cloverleaf is that vehicles merge onto the highway at the end of a loop immediately before other vehicles leave to go around another loop, creating conflict known as

weaving Weaving is a method of textile production in which two distinct sets of yarns or threads are interlaced at right angles to form a fabric or cloth. Other methods are knitting, crocheting, felting, and braiding or plaiting. The longitudinal ...

. Weaving limits the number of lanes of turning traffic. Most road authorities have since been implementing new interchange designs with less-curved exit ramps that do not result in weaving. These interchanges include the

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...

, parclo and

single-point urban interchange A single-point urban interchange (SPUI, or ), also called a single-point interchange (SPI) or single-point diamond interchange (SPDI), is a type of highway interchange. The design was created in order to help move large volumes of traffic thr ...

s (SPUI) when connecting to an arterial road in non free-flowing traffic on the crossroad and the

stack Stack may refer to: Places * Stack Island, an island game reserve in Bass Strait, south-eastern Australia, in Tasmania’s Hunter Island Group * Blue Stack Mountains, in Co. Donegal, Ireland People * Stack (surname) (including a list of people ...

or clover and stack hybrids when connecting to another freeway or to a busy arterial in free-flowing traffic where signals are still not desired. Not only are these ideas true for new interchanges, but they also hold when existing cloverleaf interchanges are upgraded. In

Norfolk, Virginia Norfolk ( ) is an independent city in the Commonwealth of Virginia in the United States. Incorporated in 1705, it had a population of 238,005 at the 2020 census, making it the third-most populous city in Virginia after neighboring Virginia B ...

, the interchange between US 13 and US 58 was originally a cloverleaf—it has since been converted to a

SPUI A single-point urban interchange (SPUI, or ), also called a single-point interchange (SPI) or single-point diamond interchange (SPDI), is a type of highway interchange. The design was created in order to help move large volumes of traffic thro ...

. Also, many cloverleaf interchanges on

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

freeways, such as U.S. 101, are being converted to parclos. In Hampton, Virginia, a cloverleaf interchange between Interstate 64 and

Mercury Boulevard Mercury Boulevard in the cities of Hampton and Newport News in the Peninsula region of southeastern Virginia carries U.S. Route 258 (US 258) approximately south from Fort Monroe at Old Point Comfort on Hampton Roads to the north end of ...

has been partially unwound into a partial stack interchange. During 2008 and 2009, four cloverleaf interchanges along I-64/ US 40 in St. Louis,

Missouri Missouri is a state in the Midwestern region of the United States. Ranking 21st in land area, it is bordered by eight states (tied for the most with Tennessee): Iowa to the north, Illinois, Kentucky and Tennessee to the east, Arkansas t ...

, were replaced with SPUIs as part of a major highway-renovation project to upgrade the highway to Interstate standards. The original cloverleaf interchange in Delmont, Pennsylvania between Routes 22 and 66 was also converted to an

. A compromise is to add a collector/distributor road next to the freeway; this does not eliminate weaving but moves it off the main lanes of the freeway. An example of this is the State Highway 23/ Interstate 43 interchange in Sheboygan, Wisconsin, where the exit/entrance roads on and off Highway 23 are two lanes next to the main I-43 freeway on the north and southbound sides of the road. A few cloverleaf interchanges in

have been rebuilt to eliminate weaving on the freeway while keeping all four loop ramps, by adding bridges, similar to braided ramps. Several cloverleaf interchanges have been eliminated by adding

traffic light Traffic lights, traffic signals, or stoplights – known also as robots in South Africa are signalling devices positioned at road intersections, pedestrian crossings, and other locations in order to control flows of traffic. Traffic light ...

s on the non-freeway route. Sometimes, this is even done at the intersection of two freeways, particularly when one freeway terminates at an interchange with another. An example of this is in

Lakewood, Washington Lakewood is a city in Pierce County, Washington, United States. The population was 63,612 at the 2020 census. History Lakewood was officially incorporated on February 28, 1996. Historical names include Tacoma/Lakewood Center and Lakes Distric ...

, at the interchange between Interstate 5 and Washington State Route 512, where a visible

ramp stub An unused highway is a highway or highway ramp that was partially or fully constructed, but went unused or was later closed. An unused roadway or ramp may often be referred to as an abandoned road, ghost road, highway to nowhere, stub ramp, g ...

shows that one of the four leaves was removed, thus eliminating weave on I-5. In the future, the traffic signal will be replaced by a two-lane flyover, completing the freeway-to-freeway interchange once again. Cloverleaf interchanges also tend to occupy much more land than any other kind of interchange. Numerous cloverleaf intersections in the Lower Mainland of British Columbia, Canada, require the merging of traffic from the clover directly onto the collector/distributor lane. This requires the slowly moving driver coming around the loop ramp to merge with the quickly moving driver exiting on the collector/distributor lane with no opportunity to accelerate to match the flow of the oncoming driver. This speed differential in merging can be as great as 65 km/h (approx. 43 mph). The cloverleaf interchange was not implemented in great numbers in the

United Kingdom The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain, is a country in Europe, off the north-western coast of the continental mainland. It comprises England, Scotland, Wales and ...

, because of these performance problems. There were originally three, one in

Redditch Redditch is a town, and local government district, in north-east Worcestershire, England, approximately south of Birmingham. The district has a population of 85,000 as of 2019. In the 19th century, it became the international centre for the ...

and two in

Livingston Livingston may refer to: Businesses * Livingston Energy Flight, an Italian airline (2003–2010) * Livingston Compagnia Aerea, an Italian airline (2011–2014), also known as Livingston Airline * Livingston International, a North American customs ...

. One of the Livingston examples was remodeled in the mid-2000s as part of a public transport project. The Girton interchange near Cambridge was a "half"-cloverleaf interchange that regularly experienced peak-time congestion due to A14 westbound traffic weaving with M11 traffic. This interchange was redesigned in 2019 to remove one of the loops. In

Ireland Ireland ( ; ga, Éire ; Ulster-Scots: ) is an island in the North Atlantic Ocean, in north-western Europe. It is separated from Great Britain to its east by the North Channel, the Irish Sea, and St George's Channel. Ireland is the s ...

, a partial cloverleaf set-up exists at the interchange of the N4 and the M50 in

Dublin Dublin (; , or ) is the capital and largest city of Ireland. On a bay at the mouth of the River Liffey, it is in the province of Leinster, bordered on the south by the Dublin Mountains, a part of the Wicklow Mountains range. At the 2016 ...

, allowing free-flow movements in all directions. A similar design has been implemented further down the M50 at the

Red Cow interchange The Red Cow interchange is a major road junction in west Dublin, Ireland on the M50, meeting the N7 Naas Road (to Cork and Limerick) at a free-flow grade-separated junction which incorporates a tram line. The N7 route commences at this junction, ...

. Most cloverleaf interchanges have been phased out in Ontario, but some close variants do remain with similar traffic flows. The main difference however is that adjacent on and off ramps are shared together by single bidirectional carriageways. Examples include the Highway 62 and

Highway 401 King's Highway 401, commonly referred to as Highway 401 and also known by its official name as the Macdonald–Cartier Freeway or colloquially referred to as the four-oh-one, is a controlled-access 400-series highway in the Canadian provin ...

interchange in Belleville, Ontario, as well as the Lawrence Avenue and

Don Valley Parkway The Don Valley Parkway (DVP) is a municipal expressway in Toronto, Ontario, Canada, which connects the Gardiner Expressway in downtown Toronto with Highway 401. North of Highway 401, it continues as Highway 404. The parkway ...

interchange in Toronto. The Don Mills Road and

and the Highway 27 and Dixon Road are also other examples; however, one quadrant of each has a loop missing.

References

External links

* Map of the Woodbridge, NJ Cloverleaf * Map of the Toms River, NJ At Grade Cloverleaf
UK-based discussion on cloverleaf interchanges
{{Road junction types Road interchanges Road junction types de:Autobahnkreuz#Kleeblatt pl:Węzeł drogowy#Koniczynka