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Interchange (road)
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In the field of road transport, an interchange is a road junction that typically utilizes grade separation, and one or more ramps, to permit traffic on at least one road to pass through the junction without crossing any other traffic stream. It differs from an intersection, at which roads cross at grade.
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Encyclopedia
In the field of road transport, an interchange is a road junction that typically utilizes grade separation, and one or more ramps, to permit traffic on at least one road to pass through the junction without crossing any other traffic stream. It differs from an 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.
Terminology
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.) or motorway junction (in the UK) 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.
- 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. Right ramps are almost always directional.
- 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.
- A U-turn ramp leaves the road in one driving direction, turns over or under it and rejoins in the opposite direction. The Texas U-turn is a form of the U-turn that allows traffic to pass under or over a freeway, primarily to access businesses on the opposite side.
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, it separates the weaving traffic from the freeway's main lanes, 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 onramp passes over or under an offramp using an overpass structure, which is commonly used on Ontario's Highway 407 and the other 400-series highways in Ontario.
The German Autobahn system splits Autobahn-to-Autobahn interchanges into two types—Autobahnkreuz (AK, translates as "motorway cross"), a crossing of two Autobahns (or something more complicated), and Autobahndreieck (AD, translates as "motorway triangle"), an interchange where one of the Autobahns ends.
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.
Barring u-turns, a complete interchange between two freeways requires eight ramps, while a complete interchange between a freeway and another road (not a freeway) requires at least four ramps. Using u-turns these numbers can be halved to four and two respectively, by making cars that want to turn left pass by the other road first, then make a u-turn and turn right. The use of u-turn ramps is common in less wealthy countries.
The general rule today is that freeways should have at least one mile between interchanges to prevent excessive weaving between entering and leaving traffic. Unfortunately, most older freeway interchanges do not follow this rule (and are congested as a result).
Between two freeways
Four-way
*A cloverleaf is a two-level four-way interchange in which left turns are handled by loop ramps. To go left, vehicles first pass either over or under the other road, then bear right onto a one-way ramp that loops 270 degrees to the right and then merges onto the intersecting road. (In countries where cars drive on the left, this is a right turn.) The major advantage of cloverleafs is that they require only one bridge, which makes such junctions cheap if land is plentiful. A major problem with cloverleafs is weaving (see definition of weaving, above). Cloverleafs also have a considerable land consumption which means that while they are common in the United States, Canada (mainly parclos), Germany, and Netherlands, only four such junctions are found in the land-scarce United Kingdom, with just one on the motorway network (partial cloverleaf exists where the M25 meets the M40 west of London). In Germany the standard design is to separate all turning traffic into a parallel lane so that the extra road space minimizes the problem of weaving. Collector-distributor roads are similar, but are separated from the main freeway lanes by a divider. Collector-distributor roads can be seen at the junction of Interstate 64 and Interstate 295 east of Richmond, Virginia, USA. Cloverleafs are often seen in urban highways within cities that grow out instead of up.
- A stack is a four-way interchange in which left turns are handled by semi-directional flyover ramps. In order to go left, vehicles first turn slightly right (on a 'right-turn' ramp), then go left on a ramp which goes over (or under) both freeways and connects to the 'right-turn' ramp in the opposite quadrant of the interchange. A stack interchange, then, has two pairs of left-turning ramps, which can be "stacked" in various configurations above or below the two through routes. Stacks do not suffer from the problem of weaving associated with cloverleafs, but require massive construction works. A basic stack involves roads on four levels, but in stacks including ramps for direct movement between high occupancy vehicle lanes, even more levels of stack can be needed; the interchange between Interstate 105 and Interstate 110 in Los Angeles County, California, is a five-level stack. This is not only expensive, but also unsightly, leading to considerable NIMBY opposition. Large stacks with multiple levels are sometimes referred to as Mixmasters or Spaghetti Bowls in the United States.
- In the late 1960s, there were parclo designs modified for freeway traffic. The ramps are longer to allow for higher speeds, and the loop ramp radii is larger as well. In a right-hand traffic (RHT) system, the signalized left turn is eliminated by an underpass or overpass. This includes the Highway 401-404 junction, and the former interchange between I-635 and US-75, the latter which has since been replaced by a five-level stack.
- Another alternative to the stack for four-way junctions is the turbine interchange (known as a "whirlpool" in the UK). The turbine/whirlpool requires fewer levels while retaining semi-directional ramps throughout and has the left-turning ramps sweep around the center of the interchange in a spiral pattern. This has slightly less capacity because the ramps turn and change height more quickly. One example of a half-turbine-half-stack interchange is the Interstate 696/Mound Road interchange in Warren, Michigan, USA. Another example is the Interstate 95/U.S.Highway 6/Memorial Blvd. full turbine interchange in Providence, Rhode Island, USA. A clearer example of a turbine interchange (without left-side ramps) is at the junction of Interstate 4 and Interstate 75 east of Tampa, Florida, USA.
Yet another example is the junction of Interstate 787 and US 9/US 20 in Albany, New York. The Highway 401-Allen Road interchange is a hybrid of the turbine and clover-stack, with some ramps dedicated to serving the adjacent Yorkdale Mall rather than freeway to freeway traffic.
Three Way
- A trumpet is an interchange traditionally used where one expressway terminates at another expressway; it involves at least one loop ramp connecting traffic either entering or leaving the terminating expressway with the far lanes of the continuous expressway. These junctions are useful for freeways as well as toll roads, as they concentrate all entering and leaving traffic in a single stretch of road, where toll booths can be installed. Double Trumpet interchanges can usually be found when a toll road meets another toll road or a freeway.
- A Directional T provides for high-speed 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.
- A Y is where one freeway terminates at another freeway with the same general directional alignment. 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. An example of this type of interchange is the El Toro Y.
- Hybrids, variations, and rare types also exist.
Between a freeway and a non-freeway road
- Diamond interchange
- Cloverleaf interchange
- Parclo interchange (partial cloverleaf—sometimes called a folded diamond when only four ramps, in two quadrants, are used)
- Single-point urban interchange
- Roundabout interchange —this uses a single roundabout or traffic circle which spans the freeway with two over/underpasses (either above or below the freeway). These are very common in the UK, Ireland and France, but almost unknown in the rest of Europe and the United States. Such junctions can be improved by adding a flyover for straight-through traffic on the non-freeway, creating a stacked roundabout.
- Dumbbell interchange —this is similar to a diamond except that it uses roundabouts rather than signals or stop signs where the ramps meet the non-freeway road. These are common in Ireland.
- Inner diamond interchange — this type of interchange allows opposing left turns to proceed concurrently where the ramps meet the surface street due to all four ramps meeting at a single intersection either above or below the freeway median, just as if it were an intersection of two surface streets. However, all of the ramps enter and exit the freeway from the innermost passing lanes, creating the potential for confusion to people not familiar with the area. Examples in the U.S. include Interstate 290 in Chicago and Interstate 244 in Tulsa.
- Diverging diamond interchange —similar to a traditional diamond interchange, except that the directional lanes of the non-freeway road cross over each other on either side of the freeway, preventing left-turning movements from crossing traffic, and allowing two-phase traffic signals (which handle higher volume), as no signaled left turns are required. As of April, 2008, at least three are known to exist in the world; the Missouri Department of Transportation is proposing a second in the Kansas City area. Construction is also underway to create such an interchange to replace a diamond interchange at the junction of Interstate 44 and Missouri Highway 13 in Springfield, Missouri.
See also
External links
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