Cloverleaf

A full cloverleaf has a loop in each of its four corners for the left turns and outer ramps for the right turns, and needs only one bridge.^1,2 In the plain form the loops leave and join the highway itself, one close behind the other, so drivers coming off one loop and drivers heading for the next weave across each other on the highway.^1,3 With collector-distributor roads, or C-D roads, a separate roadway beside the highway leaves it at one exit before the bridge and rejoins it at one entrance, and the ramps on that side leave and join this roadway instead.^2,4 The weaving moves off the through lanes onto the slower C-D road.² Manuals call the form a full cloverleaf with collector-distributor roads or roadways; California's manual calls it Type F-4.^4,5,6

In 1916 Arthur Hale of Rowlandsville, Maryland, was granted a US patent for a street crossing with four spiral ramps and four level ways around a bridge, on which no stream of traffic had to cross another at grade.⁷ The first cloverleaf in the United States was built in 1928 at Woodbridge, New Jersey, where Route 1&9 meets Route 35.^8,9 Canada's first was completed in 1937 at Port Credit, on the QEW between Toronto and Niagara Falls.⁹ After 1956, as the Interstates were built, cloverleafs were relatively common both between freeways and at suburban crossings, with two exits on each approach and four weaving sections.⁹

A 1969 study found that full cloverleafs without C-D roads had a higher crash rate than any other standard type of interchange it studied.¹⁰ A 1973 study found that most state design engineers accepted a 1966 analysis that put the capacity of a weaving section at 1,000 vehicles an hour without a C-D road and 1,500 with one. Most of them agreed that a cloverleaf should not be built without C-D roads.¹⁰ Yet in a survey of ten states for the Federal Highway Administration, published in 1975, all but Texas still built cloverleafs without them. Texas required a C-D road on every freeway through a cloverleaf, and California had altered most of its cloverleafs that had none.¹⁰ In Germany, a 1977 study says the cloverleafs built on the autobahns so far were cheap to build but short of capacity for heavy turning traffic. Comparing cloverleaf designs, it found that laying the weaving lanes directly beside the through lanes, without a C-D road, would cut the cost by about 20 % for the same capacity.¹¹

Texas now says cloverleafs were popular in the early years of freeway building but are usually no longer preferred between freeways, and that one that is built should have C-D roads.¹² California finds the four-quadrant cloverleaf not desirable at a local road, for its cost and its short weaving sections, and wants C-D roads in every one. Between freeways, the cloverleaf with C-D roads, its Type F-4, is ordinarily the most economical interchange that provides every turn; it suits places where the turning traffic is low enough for the short weaving sections.⁶ South Carolina seldom considers a cloverleaf without C-D roads a viable option, and where two fully access-controlled highways meet, a full cloverleaf with C-D roads is the least it builds.⁴ German guidelines have the same roadway, the Verteilerfahrbahn: in place of a weaving section on the main carriageway, it keeps the through traffic clear of both weaving streams.^13,14