Parclo interchange
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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] 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.
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[edit] Naming
In Ontario, the specific variation is identified by a letter/number suffix after the name. Other jurisdictions do not have 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.
[edit] Parclo A2 and B2
The Parclo A2 and B2 are commonly used on rural freeways such as the Highway 402 and Highway 416, where the ramps can be added without widening the street overpass/underpass to include deceleration lanes (normally needed on the A4 in order to safely enter the loop ramps that lead to the freeway). The Parclo A2 and B2 ramps are also usually longer and allow for higher speeds than their A4 counterparts because of rural land availability. It is possible to upgrade an A2 to an A4 by adding directional ramps, serving arterial-to-freeway traffic that would otherwise be forced to make a left turn to enter the loop ramps.
[edit] Parclo A4
The Parclo A-4 contains 6 ramps. On each side on the freeway, there is an (often multi-lane) exit ramp, followed by a loop ramp and directional ramp entering the freeway. 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 (which require queues waiting to make such turns). This also makes logical sense since freeways have a higher capacity than arterials and right-turn ramps serve effectively to absorb capacity and reduce traffic on the arterial. Traffic exiting the freeway to the arterial road is also provided with a long, relatively straight exit ramp, preventing most speed-related rollovers. These exit ramps are also frequently multi-laned to accommodate traffic turning left, right, or going straight ahead in some cases. Traffic signals are commonly installed at the end of the exit ramp to regulate the freeway traffic flowing onto the arterial. The Parclo A4 is particularly well-suited to suburban areas with high traffic levels; Ontario freeways built in the Greater Toronto Area since the 1970s such as Highway 403, Highway 404, and Highway 410 uses this design at almost all individual junctions except where land conditions do not permit it. The first freeway constructed in the GTA thirty years later, Highway 407, still retains the Parclo A4 interchanges wherever possible.
[edit] Parclo B4
The Parclo B4 interchange, while superficially appearing to be a mirror image of the Parclo A4, is rarely used; the only examples in Ontario are the Highway 402/Highway 40 interchange in Sarnia, several interchanges on the Conestoga Parkway in Kitchener-Waterloo (at Fisher-Hallman Road, Courtland Avenue, and Bridgeport Road) and the Highway 17/RR 55 interchange near Sudbury, while three exist in British Columbia. In addition, the Solomon Pond Road interchange on I-290 in Northborough, Massachusetts was converted from a B2 interchange to a B4 interchange in the mid-1990s with the construction of the Solomon Pond Mall. The B4 design provides freeway-to-arterial ramps without traffic lights, which is undesirable due to a lower capacity arterial being potentially flooded with freeway traffic. Entering the freeway often requires a left-hand turn lane.
[edit] Other Parclos
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 and Queen Elizabeth Way, both 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 the United States, folded diamonds are frequently used in interchanges with roads that have a railroad line closely paralleling the surface street; entrance/exit ramps are not permitted to have level crossings in modern American practice.
[edit] Implementation
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.
Various forms of parclo interchanges are used in the Philippines' North Luzon Expressway: a recently-modernized tollway. The configuration of parclo interchanges (particularly those of the "folded diamond" type) allows for the consolidation of toll barriers at points where onramps and offramps run alongside each other. A single large barrier can serve each onramp/offramp pair simultaneously, reducing construction and operation expenses.
A "cloverleaf-stack" or "clover-stack" (also known as a partial stack or hybrid parclo/stack) is a three level freeway-to-freeway interchange which mostly resembles the Parclo A4, but adds third-level flyover ramps to eliminate the need for left turns. Though having lower capacity than a four-level stack interchange, clover-stacks can be constructed significantly cheaper and also require less land. Alternatively, clover-stacks are formerly cloverleaf interchanges with two loop ramps (typically diagonal from each other) replaced with flyover ramps.
| Types of road junctions | |
| Interchanges (grade separated) |
Cloverleaf - Diamond - Directional T - Diverging diamond Parclo - Trumpet - SPUI - Stack - Three-level diamond |
| Intersections (at-grade) |
Box junction - Continuous flow - Crossroads - Hook turn - Jughandle - Michigan left Quadrant roadway - Roundabout - Superstreet - 3-way junction - Traffic circle |
