A cloverleaf interchange is a two-level interchange in which left turns are handled by loop ramps. To go left (in right-hand traffic), vehicles first pass either over or under the other road, then turn right onto a one-way three-fourths loop ramp (270°) and merge onto the intersecting road.

Cloverleaves were named this way because they resemble the leaves of a four-leaf clover.

In the USA, cloverleaf interchanges have existed long before the interstate system. They were originally created for busier interchanges that the original diamond system could not handle. They were used for over 40 years as the US interstate system expanded rapidly. Large transport trucks that exceeded the usual 25 MPH (40 km/h) speed limit too frequently rolled over the side. ("Overturned big-rig on the ramp to..." was and remains a common traffic report on radio.) For this reason, cloverleaf interchanges have become a common point of traffic congestion at busy junctions.

The first cloverleaves

The cloverleaf was patented in the United States by Arthur Hale , a civil engineer in Maryland, on February 29, 1916.[1]

Several cloverleaves were built in the late 1920s, and it is unclear which was firat.

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 Chicago, Illinois, but it is not clear if this was built.

The Woodbridge Cloverleaf was built at Route 25 and Route 4 (now US 1/US 9 and Route 35) in Woodbridge, New Jersey. It opened in 1929, and is currently planned for replacement with a 5-ramp partial cloverleaf interchange (parclo). This was designed by the Rudolph and Delano building firm from Philadelphia, and was modeled after a plan from Buenos Aires, Argentina.

The cloverleaf was patented in Switzerland on October 15, 1928. The first cloverleaf in Europe opened on November 21, 1936 (but was partly closed until November 5, 1938) at Schkeuditzer Kreuz near Leipzig, Germany. This is now the interchange between the and A 14 , and has a single flyover from the westbound A 14 to the southbound A 9.

Problems

A point of conflict is the merging of exiting and entering traffic in the same lane, known as weaving. Most road authorities have since been implementing new interchange designs with more straight exit ramps that do not result in weaving. These interchanges include the parclo and SPUI interchanges when connecting to an arterial road, and the stack and cloverstack interchanges when connecting to another freeway.

A compromise is to add a collector/distributor road next to the freeway; this doesn't eliminate weaving but moves it off the main lanes of the freeway.

A few cloverleaf interchanges in California have been rebuilt to eliminate weaving on the freeway while keeping all eight loop ramps, by adding bridges; a partial example is pictured at right.

External links

• An aerial photo of the Woodbridge Cloverleaf