A partial cloverleaf interchange or parclo is a modification of a cloverleaf interchange.
The design has been well received, and has since become one of the most popular freeway-to-arterial interchange designs in North America. It has also been used occasionally in some European countries, such as Germany, Hungary, Italy, the Netherlands, and the United Kingdom.
Comparison with other interchanges
A diamond interchange has four ramps. A cloverleaf interchange has eight ramps, as does a stack interchange. They are fully grade separated, unlike a parclo, and have traffic flow without stops on all ramps and throughways. A parclo generally has either four or six ramps but less commonly has five ramps.
Naming
In Ontario, the specific variation is identified by a letter/number suffix after the name. Ontario's naming conventions are used in this article. The letter A designates that two ramps meet the freeway ahead of the arterial road, while B designates that two ramps meet the freeway beyond the crossing.
The number designates how many quadrants of the interchange contain ramps. In left-hand drive countries, the ramps function the same as in right-hand drive countries, but ramps with the same designation appear visually reversed. Common parclo configurations include the parclo A2, parclo B2 and parclo A4.
The Department of Transportation in the United States refers to the A and B variants without numbers.
Caltrans refers to the A2, B2, and A4 as types L-7, L-8, and L-9 respectively
Configurations
A2 and B2
Parclo A2 and B2 configurations contain four ramps. On each side of the freeway there is a loop ramp and a directional ramp. In parclo A2, the loop ramps serve as on-ramps and the directional ramps serve as off-ramps. In parclo B2 the roles are reversed. Both on- and off-ramps require controlled intersections at the arterial road (sometimes loop and directional ramps from the same side of the freeway will utilize the same intersection).
The parclo A2 and B2 are commonly used on rural freeways such as 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.
A4
The parclo A4 (also called "six-ramp partial cloverleaf") contains six 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 on-ramps are in the same configuration as a cloverleaf interchange, but there is one off-ramp for each freeway direction instead of two, typically requiring a controlled intersection where the off-ramp meets the arterial road.
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.
Some parclos have the right-turning (directional) on-ramp leave the arterial road after the exit ramp intersection. This configuration is often used when another road meets the freeway off-ramp and gives traffic from this road the option of turning right to use the directional on-ramp to enter the freeway.
Traffic exiting the freeway to the arterial road is 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.