Partial cloverleaf

A partial cloverleaf, or parclo, has loops in one, two or three of the four corners between the roads, where a full cloverleaf has one in each.¹ The loops remove the main left-turn conflicts, and outer ramps carry the remaining turns.¹ Manuals name the forms by where the loops lie for a driver on the highway: before the cross road in a Parclo A, where they lead onto the highway, and beyond it in a Parclo B, where they lead off.^2,3

In North America, after 1941 and into the mid-1950s, freeways were built mostly by local governments and toll authorities. Partial cloverleafs were among the interchanges of those years; Leisch and Morrall show one from 1950.⁴ By the mid-1950s every basic interchange form had been designed and built, the partial cloverleaf among them.⁴

From the late 1960s into the 1980s, engineers developed more efficient partial cloverleafs for suburban areas, some of them rebuilt from full cloverleafs. Leisch and Morrall picture a Parclo A on Highway 401 in Toronto and a Parclo B on Deerfoot Trail in Calgary, both with more capacity than most diamond forms.⁴ California's Type L-9 has loops that lead onto the highway in addition to the four ramps of a diamond.⁵ In a Parclo A built that way, each direction of the highway has two entrances in a row, the loop and the right-turning ramp, though the two may merge into one first.^2,6 In Ontario, two entrances in a row from a Parclo A are common.⁶

In South Carolina's manual, a loop may be added where a left turn carries 500 vehicles an hour or more, so that it flows continuously.³ California's manual says a parclo with ramps in only two corners should be used only where something rules out diamond ramps in all four, such as the right-of-way, a railway along the cross road or a short weaving distance to the next interchange.⁵ With their loops in opposite corners, the A and B forms avoid a full cloverleaf's weaving.² The loops need a lot of land.⁷ In a Parclo B they leave the highway, where drivers can find them hard to see.⁶