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A high-capacity interchange in Los Angeles, California, United States.
In the field of road transport, an interchange is a road junction that typically uses grade separation, and one or more ramps, to permit traffic on at least one highway to pass through the junction without directly crossing any other traffic stream. It differs from a standard intersection, at which roads cross at grade. Interchanges are almost always used when at least one of the roads is a limited-access divided highway (expressway or freeway), though they may occasionally be used at junctions between two surface streets.


Note: The descriptions of road junctions are for countries where vehicles drive on the right side of the road. For countries where driving is on the left the layout of the junctions is the same, only left/right is reversed.

  • A freeway junction or highway interchange (in the U.S.marker) or motorway junction (in the UKmarker) is a type of road junction, linking one motorway to another; to other roads; or sometimes to just a motorway service station. In the UK, all junctions on a motorway with other roads are numbered—the first being called "Junction 1", etc. In the U.S., interchanges are either numbered according to cardinal interchange number, or by mileage (typically the latter in most states).
  • A highway ramp (as in exit ramp and entrance ramp) or slip road is a short section of road which allows vehicles to enter or exit a freeway (motorway).
  • A directional ramp always tends toward the desired direction of travel. This means that a ramp that makes a left turn exits from the left side of the roadway (a left exit). Left directional ramps are relatively uncommon as the left lane is usually reserved for high-speed through traffic. Ramps for a right turn are almost always right directional ramps. Where traffic drives on the left, these cases are reversed.

  • A non-directional ramp goes in a direction opposite to the desired direction of travel. Many loop ramps (as in a cloverleaf) are non-directional.
  • A semi-directional ramp exits a road in a direction opposite from the desired direction of travel, but then turns toward the desired direction of travel. Many 'flyover ramps' (as in a stack) are semi-directional.

Weaving is an undesirable situation in which traffic veering right and traffic veering left must cross paths within a limited distance, to merge with traffic on the through lane. In the worst circumstances, a large portion of through traffic must change lanes to stay on the same roadway. Weaving creates both safety and capacity problems. Some interchanges use collector/distributor roads to deal with weaving—while doing so does not eliminate the problem entirely. Collector/distributor roads separate the weaving traffic from the highway's main lanes or carriageway, thus improving traffic flow. Some areas that had such bad junctions have gone through the expensive process of "unweaving the weave" to improve traffic flow. Another way to avoid weaving is to have braided ramps, in which an on-ramp passes over or under an off-ramp using an overpass structure, which is commonly used on the 400-series highways in Ontario, Canada.

The Germanmarker Autobahn system splits Autobahn-to-Autobahn interchanges into two types: a four-way interchange, the Autobahnkreuz (ABK), where 2 motorways cross, and a three-way interchange, the Autobahndreieck (AD) where two motorways merge.

Complete interchanges

A complete interchange has enough ramps to provide access from any direction of any road in the junction to any direction of any other road in the junction. Complete interchanges typically use four to eight dedicated ramps to connect various directions of travel, but can require more depending on the interchange type and the connectivity offered. For example, if a highway interchanged with a highway containing a collector/express system, additional ramps could be used to strictly link the interchanging highway with the collector and express lanes respectively. For highways with high occupancy vehicle/HOV lanes, ramps can be used to service these carriageways directly, thereby increasing the number of ramps used.

Between two highways

Four-way interchanges

Cloverleaf interchange

A typical cloverleaf interchange in Ohio, United States.
A cloverleaf interchange is a typically a two-level, four-way interchange whereby all left turns are handled by loop ramps (right turns if traveling on the left). To go left, vehicles first cross over or under the targeted route, then bear right onto a sharply curved ramp that loops roughly 270 degrees, merging onto the interchanging road from the right, and crossing the route just departed.

The major advantage of cloverleafs is that they require only one bridge, which makes such junctions inexpensive as long as land is plentiful. A major shortcoming of cloverleafs, however, is weaving (see definition above).

Cloverleafs also require considerable land consumption, hence they appear mostly in the United States, Canada (mainly parclos, or partial cloverleaf interchanges), Germany, and the Netherlands. In Germany, the standard design is to separate all turning traffic into a parallel carriageway so that the extra road space can help minimize the problem of weaving. Collector and distributor roads are similar, but are usually separated from the main carriageway by a divider, such as a guard rail or Jersey barrier.

Cloverleafs are more often found along older highways in rural areas and within cities with low population densities.

Stack interchange

A multi-level stack interchange in Shanghai, China.
A stack interchange is a four-way interchange whereby left turns are handled by semi-directional flyover/under ramps. To go left (right in countries with left-hand drive), vehicles first turn slightly right (on a 'right-turn' off-ramp) to exit, then complete the turn via a ramp which crosses both highways, eventually merging with the 'right-turn' on-ramp traffic from the opposite quadrant of the interchange. A stack interchange, then, has two pairs of left-turning ramps, of which can be "stacked" in various configurations above or below the two interchanging highways.

Stacks do not suffer from the problem of weaving, but require massive construction work for their flyovers. A standard stack interchange includes roads on four levels. This is not only expensive, but also creates an eyesore among local residents, leading to considerable NIMBY opposition. Large stacks with multiple levels are often colloquially described as “mixmasters” or “spaghetti Bowls” due to their complex appearance, being compared to boiled spaghetti.

Cloverstack interchange

Three-level cloverstack
Two-level cloverstack
In the late 1960s, partial cloverleaf interchange (parclo) designs modified for freeway traffic emerged, eventually leading to the cloverstack interchange. Its ramps are longer to allow for higher ramp speeds, and loop ramp radii are made larger as well. For countries using right hand-drive, the large loop ramps eliminate the need for a fourth, and sometimes a third level in a typical stack interchange, as only two directions of travel use flyover/under ramps.

Cloverstacks are cheaper to build than stack interchanges and are less of an eyesore for local residents. By using the loop ramps in opposite quadrants, weaving is also eliminated. However, cloverstacks require a lot of land to construct and the loop ramps are not as efficient as flyover/under ramps in terms of traffic flow. The cloverstack design is becoming more and more popular, and is commonly used to upgrade cloverleaf interchanges to increase their capacity and eliminate weaving.

Turbine interchange

Two-level Turbine
Three-level Turbine
Another alternative to the four level stack interchange is the turbine interchange (also known as a whirlpool). The turbine/whirlpool interchange requires fewer levels (usually two or three) while retaining semi-directional ramps throughout, and has its left-turning ramps sweep around the center of the interchange in a spiral pattern in right-hand drive.

Turbine interchanges offer slightly less vehicle capacity because the ramps typically turn more often and change height more quickly. They also require more land to construct than the typical four level stack interchange.

In areas with rolling or mountainous terrain, turbines can take advantage of the natural topography of the area due to the constant change in the height of their ramps and thus can are used in these areas where conditions apply, reducing construction costs compared to turbines built on level ground.

Roundabout interchange

Roundabout interchange
A further alternative found often in the United Kingdommarker is the three-level roundabout interchange. The ramps of the interchanging highways meet at a roundabout or rotary on a separated level above, below, or in the middle of two highways.

Roundabout interchanges are much more economical in use of materials and land than other interchange designs, as the junction does not normally require more than three bridges to be constructed. However, their capacity is limited when compared to other interchanges and can become congested easily with high traffic volumes.

Other/Hybrid interchanges

Hybrid interchange near Rotterdam, Netherlands.
Hybrid interchanges use a mixture of interchange types and are not uncommon. Their construction can consist of multiple interchange designs such as loop ramps, flyovers and roundabouts.

A windmill interchange is similar to a turbine interchange, but it has much sharper turns, reducing its size and capacity. 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. The interchange is named for its similar overhead appearance to the blades of a windmill.

Divided volleyball interchanges create a wide median between the carriageways of the two interchanging highways, using this space for connecting ramps.

Full diamond interchanges are large, multi-level interchanges that use flyover/under ramps to handle both right and left ramps.Image:Knooppunt windmolen.png|WindmillImage:Divergerend windmolenknooppunt.png|Diverging windmillImage:Knooppunt barok.png|Divided volleyballImage:Knooppunt gotisch.png|Full diamond

Three-way interchanges

Trumpet interchange

Trumpet interchanges are traditionally used where one highway terminates at another highway; it involves at least one loop ramp connecting traffic either entering or leaving the terminating expressway with the far lanes of the continuous highway.

These interchanges are useful for highways as well as toll roads, as they concentrate all entering and exiting traffic into a single stretch, where toll booths can be installed. A double trumpet interchange variant can be usually found when a toll road meets another tolled or non-tolled road. These are commonly found on the Pennsylvania Turnpike.

Trumpet interchanges are named as such due to their resemblance to a trumpet. The bell of a trumpet can be seen where the terminating highway begins to interchange with the continuous highway, and the tubing is resembled along the connecting loop ramp.

Directional T interchange

Directional T
Directional-T interchange in Lausanne, Switzerland.
A directional T interchange uses flyover/under ramps in all directions at a three-way interchange. A semi-directional T does the same, but some of the splits and merges are switched to avoid ramps to and from the passing lane. Directional T interchanges are very efficient, but are expensive to build compared to other three-way interchanges. They also require three levels, which can be an eyesore for local residents.

Full Y interchange

Full Y
A full Y interchange is typically used when a three-way interchange is required for two or three highways interchanging in semi-parallel/perpendicular directions, but can be used in ninety degree instances as well. The connecting ramps can spur from either the right or left side of the highway, depending on the direction of travel and the angle.

Full Ys use flyover/under ramps for both connecting and mainline segments, and require a moderate amount of land and a moderate cost as only two levels are typically used. They get their name due to their resemblance of a capital Y letter, depending on the angle the interchange is viewed.

Other/Hybrid interchanges

Hybrid interchanges use a mixture of interchange types and are not uncommon. Their construction can consist of multiple interchange designs such as loop ramps and flyovers.

A half clover is essentially half a cloverleaf interchange, constructed to interchange three ways instead of four. They are rarely used due to the weaving they cause and the land they consume, but can be built in areas where the connecting ramp along the loop of a trumpet interchange isn't feasible due to development or a physical limitation. Half clovers are designed to be easily upgraded to a full cloverleaf if the terminating highway is extended past the continuous highway.

A 3/4 volley interchange is a divided volleyball interchange designed to meet at three points instead of four. Like the half clover, it can easily be upgraded to a full divided volleyball interchange if the terminating highway is extended past the continuous highway.

A T-bone interchange is essentially a compacted directional T interchange with sharper turns and a lower capacity, built when there is insufficient land or funds to build a directional T. The two flyover/under ramps contract together after overpassing the continuous highway then split into two segments, resembling the bone in a T-bone steak, from which it gets its name.Image:Knooppunt half klaverblad.png|Half cloverImage:Knooppunt half barok.png|3/4 volleyImage:As-gruszka.svg|T-bone

Two-way interchanges

A half trumpet interchange is essentially a trumpet interchange without its loop ramp and one of its directional ramps designed to meet the continuous highway in one direction, usually on a ninety degree or semi-perpendicular angle. Should the need arise, it can easily be constructed into a trumpet interchange, making it a three-way interchange.

A partial y interchange is used where one highway terminates at another highway with the same general directional alignment (usually a maximum of sixty degrees). The trunk of the terminating highway merges with the trunk of the continuous highway; vehicles traveling into the interchange may only exit traveling in the same direction. This type of interchange is often used for bypass routes, and is named for the shape the two highways' confluence makes when drawn on a map, creating a lower-case "y".

Hybrids, variations, and rare types also exist for two-way interchanges.Image:Knooppunt onvolledig.png|Half trumpetImage:Saut-de-mouton2.svg|Partial YImage:I-287 I-87.jpg|Partial y interchange in New York, United States.

One-way interchanges

Dual highway setup
A Dual highway setup is used when one highway (usually parallel) joins up with another highway, creating two highways in one direction. These two highways can further join together and form one highway.

A basketweave interchange is commonly found on highways using a collector/express system or long collector/distributor lanes. In a basketweave, one highway is able to interchange with itself, allowing traffic traveling in the same direction to switch between carriageways through the use of flyover/under ramps created between two carriageways without causing weaving. These interchanges usually involve left exits and entry for the outer carriageway (right in left-hand drive) but can be configured to meet on the right.

Between a highway and a non-highway road

Diamond interchange

A diamond interchange is an interchange involving four ramps where they enter and leave the freeway at a small angle and meet the non-freeway at almost right angles. These ramps at the non-freeway can be controlled through stop signs, traffic signals, or turn ramps. Diamond interchanges are inexpensive to build and require little land but are prone to congestion and accidents if there is high traffic.

A dumbbell interchange is similar to the diamond interchange, but uses a pair of roundabouts to join the ramps with the non-highway. This typically increases the efficiency of the interchange when compared to a diamond.

Parclo interchange/folded diamond

A parclo/diamond hybrid in British Columbia, Canada.
A Parclo interchange, also known as a 'partial cloverleaf', is an interchange usually involving four to six ramps, two of which are loop ramps, which connect to the non-highway.

The parclo is a safer modification of the cloverleaf design. Depending on the number of ramps used, they take up a moderate to large amount of land and are typically inexpensive to build. Parclos with more ramps have a greater capacity and efficiently than parclos with fewer ramps. Parclos are sometimes called a folded diamond when only four ramps, in two quadrants, are used. If the loop ramps are constructed opposite or mirrored along the highway, weaving is avoided. Cloverleaf interchanges that involve a non-highway can be changed to parclos without too much reconstruction. Although the interchange's capacity is reduced, weaving is eliminated, increasing the safety and efficiency of the interchange.

Diverging diamond interchange

Diverging diamond
A diverging diamond interchange is similar to a traditional diamond interchange, except that it uses directional lanes for the non-highway to cross over each other on either side of the highway, altering the direction of travel on the over/underpass through the use of traffic lights. This allows all turns to and from the highway to be made without crossing the opposite direction of travel, increasing the capacity when compared to a typical diamond interchange.

The idea for the diverging diamond interchange came from a freeway-to-freeway connection north of Baltimore, Marylandmarker, where I-695 interchanged with I-95. While this interchange has since been replaced with a four-level stack, it was once of a design known as the synchronized split-phasing interchange.

Single-point urban interchange

Single point urban interchange (SPUI)
  • The Single-point urban interchange, often abbreviated to SPUI, is a modification of the diamond interchange and has its ramps meet at one point, usually on the overpass/underpass of the non-highway. This requires only one set of traffic signals, increasing its efficiency and capacity when compared to a diamond.

Other/Hybrid interchanges

Highway/non-highway hybrid interchanges consist of diamond and partial cloverleaf elements. Their construction can consist of multiple interchange designs such as loop ramps, flyovers and roundabouts.

A form of the roundabout interchange can be used to connect a highway with a non-highway. It uses a single roundabout, rotary or traffic circle which spans the highway as a over/underpass. Such junctions can be improved by adding a flyover for straight-through traffic on the non-freeway, creating the Roundabout interchange.

The three level diamond/volleyball interchange is three levels and handles interchanging ramps via four intersections. This interchange can also be used to connect two highways together, but due to the use of intersections it is not free-flowing.

Image:Parclo B2.svg|Four ramp partial cloverleaf (B-2)Image:Parclo A4.svg|Six ramp partial cloverleaf (A-4)Image:Halfklaverbladaansluiting.png|Folded diamond/parcloImage:Volleyball Interchange Diagram.gif|Three level diamond/volleyballImage:Salida autovia a rotonda.svg|Roundabout

In popular culture

A compact stack interchange in SimCity 4.
Interchanges are used widely in popular culture, from movies to video games. Interchanges can be closed to the public so that recording can take place without posing any risk to motorists, whereas other interchanges are constructed explicitly for filming. Interchanges can also be computer generated for movies to create specific conditions which is prominently found in newer movies.

In video games, interchanges have long existed and can be used for racing such as in the Need for Speed series or constructed in city building simulators, such as in SimCity 4.

See also


External links

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