Cloverleaf interchange

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-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 is traffic weaving.

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, cloverleaf interchanges existed long before the Interstate system. They were originally created for busier interchanges that the original diamond interchange 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. This not only made them a viable option for interchanges between freeways (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 expanded rapidly. One problem is that, frequently, large trucks exceeding the area speed limit 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 cloverleaf configurations with full four leaves and full outside slip ramps are extremely rare, though one exists in Toms River, New Jersey. 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 a jughandle or parclo intersection.

The first cloverleaf interchange patented in the US was by Arthur Hale, a civil engineer in Maryland, 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 (IL 19) in Chicago, Illinois, but a diamond interchange was built instead.

The first cloverleaf interchange built in the United States was the Woodbridge Cloverleaf at intersection of the Lincoln Highway (Route 25) and Amboy—now St. Georges—Avenue (Route 4) (now U.S. 1/9 and Route 35) in Woodbridge Township, New Jersey. It opened in 1929, although it has since been replaced with a partial cloverleaf interchange. Before the cloverleaf was replaced in the late 2000s, it was judged eligible for inclusion in the National Register of Historic Places. Because of this, a commemorative film was made of the cloverleaf. The original cloverleaf interchange design was adapted by the Rudolph and Delano building firm from Philadelphia, from a photo Delano saw on a magazine cover about a highway in Buenos Aires, Argentina. The original inventor is unknown.

The first cloverleaf west of the Mississippi River opened on August 20, 1931, at Watson Road and Lindbergh Boulevard near St. Louis, Missouri, as part of an upgrade of U.S. 66.