Cloverleaf interchange
From Wikipedia, the free encyclopedia
A cloverleaf interchange is a two-level interchange in which left turns (in countries that drive on the right) are handled by loop roads (US: ramps, UK slip roads). 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.
Contents |
[edit] Overview
Cloverleaf interchanges, viewed from overhead or on maps, resemble the leaves of a four-leaf clover.
In the USA, 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 didn't 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 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.[citation needed] ("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.
[edit] 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 first.
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 Township, 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 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 US 66.[2]
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 A 9 and A 14, and has a single flyover from the westbound A 14 to the southbound A 9. Kamener Kreuz was the first in continental Europe to open fully in 1937, at A 1 and A 2 near Dortmund Germany.
At-grade Cloverleaf configurations with full four leafs and full outside slip ramps are extremely rare, though one is known to exist in Toms River, New Jersey. [3] Any other intersections 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 intersection.
[edit] 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 Single-point urban (SPUI) interchanges when connecting to an arterial road, and the stack interchange when connecting to another freeway or to a busy arterial where signals are still not desired.
Not only is this true for new interchanges but it also holds when existing cloverleaf interchanges are upgraded. In Norfolk, Virginia, the interchange between U.S. 13 and U.S. 58 was originally a cloverleaf--it has since been converted to a SPUI. And many cloverleaf interchanges on California freeways are being converted to parclos. In Hampton, Virginia, a cloverleaf interchange between Interstate 64 and U.S. Route 258 has been partially unwound into a partial stack interchange.
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.
Several cloverleafs have been eliminated by adding traffic 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 in Lakewood, Washington, at the interchange between Interstate 5 and Washington State Route 512. A visible ramp stub shows that one of the four leaves was removed; thus eliminating an unsafe weave on I-5. [4]
Cloverleafs also consume much more land than any other kind of interchange (with the possible exception of stack interchanges).
[edit] See also
- Jughandle, which can resemble the loop ramp of a cloverleaf with no bridge
[edit] External links
- Map of the Woodbridge, NJ Cloverleaf
- Map of the Toms River, NJ At Grade Cloverleaf
| Types of road junctions | |
| Interchanges (grade separated) |
Cloverleaf - Diamond - Directional T - Diverging diamond Parclo - Trumpet - SPUI - Stack - Three-level diamond - Raindrop |
| Intersections (at-grade) |
Box junction - Continuous flow - Hook turn - Jughandle - Michigan left Quadrant roadway - Roundabout - Superstreet - 3-way junction - Traffic circle - Bowtie |
