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Acloverleaf interchange is a two-levelinterchange in which all turns are handled byslip 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-quarter 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 istraffic weaving.
Cloverleaf interchanges, viewed from overhead or on maps, resemble theleaves of afour-leaf clover or less often a 3-leaf clover.In theUnited States, cloverleaf interchanges existed long before theInterstate system. They were originally created for busier interchanges that the originaldiamond interchange system could not handle. Their chief advantage was that they were free-flowing and did not require the use of such devices astraffic signals. This not only made them a viable option for interchanges betweenfreeways (where such devices are typically not an option), but they could also be used for very busyarterials where signals could present congestion problems.
They are common in the United States and have been used for over 40 years as theInterstate Highway System expanded rapidly. One problem is that, frequently, largetrucks exceeding the areaspeed limit roll over.[1] 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 cloverleaf configurations with full four leaves and full outside slip ramps are extremely rare, though one exists inToms River, New Jersey.[2] Any other intersection with merely one, two, or three leaf ramps with outer ramps would not be designated a "cloverleaf" and simply be referred to as ajughandle or parclo intersection.
The first cloverleaf interchangepatented in the US was byArthur Hale, acivil engineer inMaryland, on February 29, 1916.[3][4]
A modified cloverleaf, with the adjacent ramps joined into a single two-way road, was planned in 1927 for the interchange betweenLake Shore Drive (US 41) and Irving Park Road (IL 19) inChicago, Illinois, but adiamond interchange was built instead.
The first cloverleaf interchange built in the United States was the Woodbridge Cloverleaf[5] at intersection of theLincoln Highway (Route 25) andAmboy—now St. Georges—Avenue (Route 4) (nowU.S. 1/9 andRoute 35) inWoodbridge Township, New Jersey.[6][7][8] It opened in 1929,[9] although it has since been replaced with apartial cloverleaf interchange.[10] Before the cloverleaf was replaced in the late 2000s, it was judged eligible for inclusion in theNational Register of Historic Places. Because of this, a commemorative film[5] was made of the cloverleaf.[11] The original cloverleaf interchange design was adapted by the Rudolph and Delano building firm fromPhiladelphia, from a photo Delano saw on a magazine cover about a highway inBuenos Aires,Argentina. The original inventor is unknown.
The first cloverleaf west of theMississippi River opened on August 20, 1931, atWatson Road andLindbergh Boulevard nearSt. Louis, Missouri, as part of an upgrade ofU.S. 66.[12]
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 inEurope inSwitzerland on October 15, 1928. The first cloverleaf inEurope opened in October 1935 atSlussen in centralStockholm,Sweden, followed in 1936 bySchkeuditzer Kreuz nearLeipzig,Germany. This is now the interchange between theA 9 andA 14, and has a singleflyover from the westbound A 14 to the southbound A 9.Kamener Kreuz was the first in continental Europe to open fully in 1937, atA 1 andA 2 nearDortmund,Germany.
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 asweaving. 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 thediamond,parclo andsingle-point urban interchanges (SPUI) when connecting to anarterial road in nonfree-flowing traffic on the crossroad and thestack orclover 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. InNorfolk, Virginia, the interchange betweenUS 13 andUS 58 was originally a cloverleaf—it has since been converted to aSPUI. Also, many cloverleaf interchanges onCalifornia freeways, such as U.S. 101, are being converted toparclos. InHampton, Virginia, a cloverleaf interchange betweenInterstate 64 andMercury Boulevard has been partially unwound into a partial stack interchange. During 2008 and 2009, four cloverleaf interchanges alongI-64/US 40 inSt. Louis,Missouri, were replaced withSPUIs as part of a major highway-renovation project to upgrade the highway to Interstate standards. The original cloverleaf interchange inDelmont, Pennsylvania between Routes 22 and 66 was also converted to aSPUI.
A compromise is to add acollector/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 theState Highway 23/Interstate 43 interchange inSheboygan, 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 inCalifornia 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 addingtraffic lights 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 inLakewood, Washington, at the interchange betweenInterstate 5 andWashington State Route 512, where a visibleramp stub shows that one of the four leaves was removed, thus eliminating weave on I-5.[13] In the future, the traffic signal will be replaced by a two-lane flyover, completing the freeway-to-freeway interchange once again.[citation needed] Cloverleaf interchanges also tend to occupy much more land than any other kind of interchange.
Numerous cloverleaf intersections in theLower 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 theUnited Kingdom, because of these performance problems. There were originally three, one inRedditch and two inLivingston. 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 toA14 westbound traffic weaving withM11 traffic.[14] This interchange wasredesigned in 2019 to remove one of the loops. InIreland, partial cloverleaf set-ups exist at the interchanges of the main roads out ofDublin and the M50, allowing free-flow movements in all directions. TheRed Cow Interchange is an example.
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 theHighway 62 andHighway 401 interchange inBelleville, Ontario, theHighway 4 andHighway 401 inLondon, Ontario, as well as theLawrence Avenue andDon Valley Parkway interchange in Toronto. TheDon Mills Road andDon Valley Parkway and theHighway 27 andDixon Road are also other examples; however, one quadrant of each has a loop missing.
