Cloverleaf interchange

A cloverleaf interchange is a two-level interchange in which left turns (reverse directions in left-driving regions) are handled by ramp roads (US: ramps, UK: slip roads). To go left (in right-hand traffic), 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 red lights, even for left and right turns. 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. 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 (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 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.

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Wikipedia