Interchange (road)
In the field of road transport, an interchange or a grade-separated junction is a road junction that uses grade separations to allow for the movement of traffic between two or more roadways or highways, using a system of interconnecting roadways to permit traffic on at least one of the routes to pass through the junction without interruption from crossing traffic streams. It differs from a standard intersection, where roads cross at grade. Interchanges are almost always used when at least one road is a controlled-access highway or a limited-access highway, though they are sometimes used at junctions between surface streets.
Terminology
Note: The descriptions of interchanges apply to countries where vehicles drive on the right side of the road. For left-side driving, the layout of junctions is mirrored. Both North American and British terminology is included.; Freeway junction, highway interchange, or motorway junction
; System interchange
; Service interchange
; Complete interchange
; Incomplete interchange
; Ramp, or slip road
; Directional ramp
; Semi-directional ramp
; Weaving
History
The concept of the controlled-access highway developed in the 1920s and 1930s in Italy, Germany, the United States, and Canada. Initially, these roads featured at-grade intersections along their length. Interchanges were developed to provide access between these new highways and heavily-travelled surface streets. The Bronx River Parkway and Long Island Motor Parkway were the first roads to feature grade-separations.Maryland engineer Arthur Hale filed a patent for the design of a cloverleaf interchange on May24, 1915, though the conceptual roadwork was not realised until a cloverleaf opened on December15, 1929, in Woodbridge, New Jersey, connecting New Jersey Route 25 and Route 4. It was designed by Philadelphia engineering firm Rudolph and Delano, based on a design seen in an Argentinian magazine.
System interchange
A system interchange connects multiple controlled-access highways, involving no at-grade signalised intersections.Four-legged interchanges
Cloverleaf interchange
A cloverleaf interchange is a four-legged junction where left turns across opposing traffic are handled by non-directional loop ramps. It is named for its appearance from above, which resembles a four-leaf clover. A cloverleaf is the minimum interchange required for a four-legged system interchange. Although they were commonplace until the 1970s, most highway departments and ministries have sought to rebuild them into more efficient and safer designs. The cloverleaf interchange was invented by Maryland engineer Arthur Hale, who filed a patent for its design on May24, 1915. The first one in North America opened on December15, 1929, in Woodbridge, New Jersey, connecting New Jersey Route25 and Route4. It was designed by Philadelphia engineering firm Rudolph and Delano based on a design seen in an Argentinian magazine.The first cloverleaf in Canada opened in 1938 at the junction of Highway 10 and what would become the Queen Elizabeth Way. 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]. Some may be half cloverleaf containing ghost ramps which can be upgraded to full cloverleafs if the road is extended. US 70 and US 17 west of New Bern, North Carolina is an example.
Stack interchange
A stack interchange is a four-way interchange whereby a semi-directional left turn and a directional right turn are both available. Usually, access to both turns is provided simultaneously by a single off-ramp. Assuming right-handed driving, to cross over incoming traffic and go left, vehicles first exit onto an off-ramp from the rightmost lane. After demerging from right-turning traffic, they complete their left turn by crossing both highways on a flyover ramp or underpass. The penultimate step is a merge with the right-turn on-ramp traffic from the opposite quadrant of the interchange. Finally, an on-ramp merges both streams of incoming traffic into the left-bound highway. As there is only one off-ramp and one on-ramp, stacks do not suffer from the problem of weaving, and due to the semi-directional flyover ramps and directional ramps, they are generally safe and efficient at handling high traffic volumes in all directions.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 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.
Stacks are significantly more expensive than other four-way interchanges are due to the design of the four levels; additionally, they may suffer from objections of local residents because of their height and high visual impact. Large stacks with multiple levels may have a complex appearance and are often colloquially described as Mixing Bowls, Mixmasters, or as Spaghetti Bowls or Spaghetti Junctions. However, they consume a significantly smaller area of land compared to a cloverleaf interchange.
Combination interchange
A combination interchange 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 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.File:Autobunnskräiz-RO-A201_cropped.jpg|thumb|center|A turbine interchange between A201 and the Brussels Ring in Brussels
Windmill interchange
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
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
Three-legged interchanges
These interchanges can also be used to make a "linking road" to the destination for a service interchange, or the creation of a new basic road as a service interchange.Trumpet interchange
Trumpet interchanges may be used where one highway terminates at another highway, and are named as such for to their resemblance to trumpets. They are sometimes called jug handles.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.