A "stack interchange" (sometimes called a four-level interchange) is a type of interchange fully connecting two perpendicular limited access roads, or freeway
s. Stack interchanges vary from cloverleaf interchange
s in that stacks cost more to build but take up less surface area. Usually, stack interchanges are also more efficient at transporting cars than cloverleafs.
Simply put, a stack interchange features a direct ramp from each stream of traffic, to each stream of traffic. Stack interchanges do not feature ramp looping or weaving common to cloverleafs. Because of this construction style, stack interchanges form four visually imposing ramps stacked on top of each other, making them look complex and unsightly. From above, a stack interchange would form the shape of a diamond. Stacks should not be confused with diamond interchanges, however.
Because of their high building cost and visually unpleasing appearance, stack interchanges are usually only used in highly urban areas where land development precludes the use of a cloverleaf. In 1952, the United States built the first stack interchange in Los Angeles connecting US 101 with CA 110.