A system interchange connects multiple controlled-access highways, involving no at-grade signalised intersections. It is named for its appearance from above, which resembles a
four-leaf clover. The first cloverleaf outside of North America opened in
Stockholm on October15, 1935. Nicknamed
Slussen, it was referred to as a "traffic carousel" and was considered a revolutionary design at the time of its construction. A cloverleaf offers uninterrupted connections between two roads but suffers from weaving issues. Along the mainline, a loop ramp introduces traffic prior to a second loop ramp providing access to the crossroad, between which ingress and egress traffic mixes. For this reason, the cloverleaf interchange has fallen out of favour in place of
combination interchanges. A standard stack interchange includes roads on four levels, also known as a four-level stack, including the two perpendicular highways, and one more additional level for each pair of left-turn ramps. These ramps can be stacked (cross) in various configurations above, below, or between the two interchanging highways. This makes them distinct from turbine interchanges, where pairs of left-turn ramps are separated but at the same level. There are some stacks that could be considered 5-level; however, these remain four-way interchanges, since the fifth level actually consists of dedicated ramps for
HOV/bus lanes or frontage roads running through the interchange. The stack interchange between I-10 and I-405 in
Los Angeles is a 3-level stack, since the semi-directional ramps are spaced out far enough, so they do not need to cross each other at a single point as in a conventional four-level stack. is a hybrid of other interchange designs. It uses loop ramps to serve slower or less-occupied
traffic flow, and flyover ramps to serve faster and heavier traffic flows. If local and express ways serving the same directions and each roadway is connected righthand to the interchange, extra ramps are installed. The combination interchange design is commonly used to upgrade cloverleaf interchanges to increase their capacity and eliminate weaving.
Turbine interchange The
turbine interchange is an alternative four-way directional interchange. The turbine interchange requires fewer levels (usually two or three) while retaining directional ramps throughout. It features right-exit, left-turning ramps that sweep around the center of the interchange in a clockwise
spiral. A full turbine interchange features a minimum of 18 overpasses, and requires more land to construct than a four-level stack interchange; however, the bridges are generally short in length. Coupled with reduced maintenance costs, a turbine interchange is a lower cost alternative to a stack. and the
Brussels Ring (R0) in
Brussels Windmill interchange , Netherlands was a windmill before it was renovated in 1997. A windmill interchange is similar to a turbine interchange, but it has much sharper turns, reducing its size and capacity. The interchange is named for its similar overhead appearance to the blades of a
windmill. A variation of the windmill, called the diverging windmill, increases capacity by altering the direction of traffic flow of the interchanging highways, making the connecting ramps much more direct. There also is a
hybrid interchange somewhat like the diverging windmill in which left turn exits merge on the left, but it differs in that the left turn exits use left directional ramps.
Braided interchange in 1998.A
braided or diverging
interchange is a two-level, four-way interchange. An interchange is braided when at least one of the roadways reverses sides. It seeks to make left and right turns equally easy. In a pure braided interchange, each roadway has one right exit, one left exit, one right on-ramp, and one left on-ramp, and both roadways are flipped. The first pure braided interchange was built in
Baltimore at
Interstate 95 at
Interstate 695; however, the interchange was reconfigured in 2008 to a traditional stack interchange. ;Examples •
Interstate 65 and
Interstate 20/
Interstate 59 in
Birmingham, Alabama () •
Interstate 196 and
U.S. Route 131 in
Grand Rapids, Michigan () •
Interstate 77 and
Interstate 85 in
Charlotte, North Carolina () • Eastern Ring Road and Southern Ring Branch Road,
Riyadh ()
Three-level roundabout A three-level roundabout interchange features a grade-separated roundabout which handles traffic exchanging between highways. These interchanges are very common on
toll roads, as they concentrate all entering and exiting traffic into a single stretch of roadway, where toll plazas can be installed once to handle all traffic, especially on
ticket-based tollways. A double-trumpet interchange can be found where a toll road meets another toll road or a free highway. They are also useful when most traffic on the terminating highway is going in the same direction. The turn that is used less often would contain the slower loop ramp. Trumpet interchanges are often used instead of directional or semi-directional T or Y interchanges because they require less bridge construction but still eliminate weaving.
T and Y interchanges A
full Y interchange (also known as a directional T interchange) is typically used when a three-way interchange is required for two or three highways interchanging in semi-parallel/perpendicular directions, but it can also be used in right-angle cases as well. Their connecting ramps can spur from either the right or left side of the highway, depending on the direction of travel and the angle. Directional T interchanges use flyover/underpass ramps for both connecting and mainline segments, and they require a moderate amount of land and moderate costs since only two levels of roadway are typically used. Their name derives from their resemblance to the capital letter T, depending upon the angle from which the interchange is seen and the alignment of the roads that are interchanging. It is sometimes known as the "New England Y", as this design is often seen in the northeastern United States, particularly in Connecticut. This type of interchange features directional ramps (no loops, or weaving right to turn left) and can use multilane ramps in comparatively little space. Some designs have two ramps and the "inside" through road (on the same side as the freeway that ends) crossing each other at a three-level bridge. The directional T interchange is preferred to a trumpet interchange because a trumpet requires a loop ramp by which speeds can be reduced, but flyover ramps can handle much faster speeds. The disadvantage of the directional T is that traffic from the terminating road enters and leaves on the passing lane, so the semi-directional T interchange (see below) is preferred. The interchange of
Highway 416 and
Highway 417 in Ontario, constructed in the early 1990s, is one of the few directional T interchanges, as most transportation departments had switched to the semi-directional T design. As with a directional T interchange, a
semi-directional T interchange uses
flyover (overpass) or
underpass ramps in all directions at a three-way interchange. However, in a semi-directional T, some of the splits and merges are switched to avoid ramps to and from the
passing lane, eliminating the major disadvantage of the directional T. Semi-directional T interchanges are generally safe and efficient, though they do require more land and are costlier than trumpet interchanges. Semi-directional T interchanges are built as two- or three-level junctions, with three-level interchanges typically used in urban or suburban areas where land is more expensive. In a three-level semi-directional T, the two semi-directional ramps from the terminating highway cross the surviving highway at or near a single point, which requires both an overpass and underpass. In a two-level semi-directional T, the two semi-directional ramps from the terminating highway cross each other at a different point than the surviving highway, necessitating longer ramps and often one ramp having two overpasses.
Highway 412 has a three-level semi-directional T at
Highway 407 and a two-level semi-directional T at
Highway 401. Image:Knooppunt driehoek.png|Full Y interchange Image:California SR 85 and SR 87 Interchange.jpg|Complex T interchange of SR 85 and SR 87 in
San Jose, California. File:Knooppunt halve ster.png|Semi-directional T interchange Image:Interchange-colour-img 0526.jpg|Two level semi-directional T interchange in
Orbe, Switzerland. == Service interchange ==