| | Parclo B2 Interchange (folded diamond) | Parclo AB3 Interchange (hybrid) | The parclo interchange (short for partial cloverleaf interchange) is a successor to the cloverleaf interchange. The parclo interchange was developed by the Ontario Ministry of Transportation [1] (http://www.mto.gov.on.ca) as a replacement for the cloverleaf on 400-Series Highways, removing the dangerous weaving patterns and allowing for more acceleration and deceleration space on the freeway. The design has been well received, and has since become one of the most popular freeway-to-arterial interchange designs in North America.
In Ontario, the specific variation is identified by a letter/number suffix after the name. Other jurisdictions don't have any naming conventions, so Ontario's are used in this article. The letter A designates that two ramps each are in the upper left and lower right of the arterial crossing, while B designates the opposite. The number designates how many quadrants of the interchange contain ramps. In left-hand drive countries, the designations are reversed. Common parclo configurations include the Parclo A2, Parclo B2 and Parclo A4.
The Parclo A4 is one of the most popular designs, as all movements from the arterial road to the entrance ramps are made by right-hand turns, providing for a safer entrance to the freeway by eliminating left-hand turns into opposing traffic, and queues waiting to make such turns. Traffic exiting the freeway to the arterial road is also provided with a long, relatively straight exit ramp, preventing most speed-related rollovers. The Parclo A4 is particularly well-suited to suburban areas with high traffic levels. Highway 407 in the Greater Toronto Area exclusively uses this design at all individual junctions.
Parclo designs with only two quadrants are commonly referred to as folded diamonds, due to their similarity with diamond interchanges. Sometimes the ramps in a folded diamond are actually local streets; surface roads upgraded to higher standards often do this to save money on land acquisition. This type of interchange long predates the Parclo; the Merritt Parkway, built in the 1930s, used mainly folded diamonds and cloverleaves.
Depending on traffic and land needs, hybrid designs, such as the Parclo AB and Parclo A3, can be created. Other variants, not describable using Ontario's system, eliminate one or more outside ramps, while leaving the loop ramps in those quadrants.
In California, Caltrans currently has a policy that whenever cloverleaf interchanges between freeways and surface streets are being rebuilt to handle higher traffic loads, they will be turned into parclo interchanges by removing some of the loop ramps (or in rare cases bridges will be added between adjacent loop ramps - see cloverleaf interchange for details). However, as for cloverleaf interchanges between freeways, they are being unwound into partial stack interchanges or made safer with slip ramps as funds permit.
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