Interchange (road)

In the field of road transport, an interchange (American English) or a grade-separated junction (British English) 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 (freeway or motorway) or a limited-access divided highway (expressway), 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 (NA) and British (UK) terminology is included.

; Freeway junction, highway interchange (NA), or motorway junction (UK)
: A type of road junction linking one controlled-access highway (freeway or motorway) facility to another, to other roads, or to a rest area or motorway service area. Junctions and interchanges are often (but not always) numbered either sequentially, or by distance from one terminus of the route (the "beginning" of the route).
:The American Association of State Highway and Transportation Officials (AASHTO) defines an interchange as "a system of interconnecting roadways in conjunction with one or more grade separations that provides for the movement of traffic between two or more roadways or highways on different levels."
; System interchange
: A junction that connects multiple controlled-access highways.
; Service interchange
: A junction that connects a controlled-access facility to a lower-order facility, such as an arterial or collector road.
: The mainline is the controlled-access highway in a service interchange, while the crossroad is the lower-order facility that often includes at-grade intersections or roundabouts, which may pass over or under the mainline.
; Complete interchange
: A junction where all possible movements between highways can be made from any direction.
; Incomplete interchange
: A junction that is missing at least one movement between highways.
; Ramp (NA), slip road (UK), or link (Ireland)
: A short section of road that allows vehicles to enter or exit a controlled-access highway.
:''Ingressing'' traffic is entering the highway via an on-ramp or entrance ramp, while ''egressing'' traffic is exiting the highway via an off-ramp or exit ramp.
; Directional ramp
: A ramp that curves toward the desired direction of travel; i.e., a ramp that makes a left turn exits from the left side of the roadway (a left exit).
; Semi-directional ramp
: A ramp that exits in a direction opposite from the desired direction of travel, then turns toward the desired direction. Most left turn movements are provided by a semi-directional ramp that exits to the right, rather than exiting from the left.
; Weaving
: An undesirable situation where traffic entering and exiting a highway must cross paths within a limited distance.

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 was the first road 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 (now U.S. Route 1/9 and New Jersey Route 35). 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 absolute 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 (now U.S. Route1/9 and New Jersey Route35). 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 1937 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 offramp. Assuming right-handed driving, in order 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 onramp merges both streams of incoming traffic into the left-bound highway. As there is only one offramp and one onramp (in that respective order), 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 (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 five-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 three-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, 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'' (for a Sunbeam Products brand of electric kitchen mixers), or as ''Spaghetti Bowls'' or ''Spaghetti Junctions'' (being compared to boiled spaghetti). However, they consume a significantly smaller area of land compared to a cloverleaf interchange.

Combination interchange

A combination interchange (sometimes referred to by the portmanteau, ''cloverstack'') 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

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