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.
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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.
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.
The German Autobahn system splits Autobahn-to-Autobahn interchanges into two types: a four-way interchange, the Autobahnkreuz (AK), where 2 motorways cross, and a three-way interchange, the Autobahndreieck (AD) where two motorways merge.
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.
A cloverleaf interchange is typically a two-level, four-way interchange where 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), and the consequent low capacity of this design.
Cloverleafs also use a considerable area of land, and are suitable for countries such as the United States and Canada (mainly parclos, or partial cloverleaf interchanges). In Germany, the standard design is to separate all turning traffic into a parallel carriageway to 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.
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 (Not In My Back Yard) 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.
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. 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.
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 driving.
Turbine interchanges offer slightly less vehicle capacity because the ramps typically turn more often and change height quicker. They also require more land to construct than the typical four-level stack interchange.
In areas with rolling or mountainous terrain, turbine interchanges can take advantage of the natural topography of the land due to the constant change in the height of their ramps, and hence these are commonly used in these areas where conditions apply, reducing construction costs compared to turbine interchanges built on level ground.
A further alternative found often in the United Kingdom is called a roundabout interchange. This is a normal roundabout except one (two-level) or both (three-level) mainlines pass under or over the whole thing. The ramps of the interchanging highways meet at a roundabout or rotary on a separated level above, below, or in the middle of the 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.
A variation of this interchange has been proposed by a Ukrainian engineer Viktor Petruk. The claimed advantages of this type of interchange are: low number of conflict points, short car paths on the circle, and relatively small overall size that allows it to be used in dense urban environment. Unfamiliarity of clockwise traffic flows on roundabouts is its most frequently cited drawback.
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.
Trumpet interchanges have been used where one highway terminates at another highway. These involve 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 of roadway, where toll booths can be installed. A double-trumpet interchange version can be found where a toll road meets another toll road or a free highway.
Trumpet interchanges are named as such due to their resemblance to trumpets. The bell of a trumpet can be seen where the terminating highway begins to interchange with the continuous highway, and the resemblance to the tubing is seen along the connecting loop ramps.
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. However, the T-interchange is perferred to a trumpet interchange because a trumpet requires a loop ramp which speeds can be reduced to as much as 25 MPH (US), but flyover ramps can handle much faster speeds.
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 it can also be used in right-angle case 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.
Full Y-interchanges use flyover/flyunder ramps for both connecting and mainline segments, and they require a moderate amount of land and moderate costs since only two levels or roadway are typically used. They get their name due to their resemblance to the capital letter "Y", depending upon the angle from which the interchange is seen. Two examples of this type of interchange can be found in the Province of Ontario: the Highway 403/Highway 6 interchange in Hamilton, and the Highway 417/Highway 416 interchange in Ottawa.
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. One place to find hybrids are near Santa Clarita, California, all along Interstate 5 and California State Route 14.
A half-clover interchange is essentially half a cloverleaf interchange, constructed to connect in just three directions instead of four. These are rarely used due to the traffic weaving that they cause and the large amount of land that they consume, but they can be built in areas where the connecting ramp along the loop of a trumpet interchange is not feasible due to building developments or physical limitations. Half-clovers are designed to be readily upgraded to full cloverleafs if the terminating highway is ever extended past the through highway. A notable example of a half-clover interchange is with M-47 and U.S. Route 10 near Midland, Michigan, since M-47 was the relic of a scrapped plan to extend a freeway further north. Interstate 75 to its east near Bay City, Michigan, a more important destination, made this highway unnecessary.
A 3/4 volleyball 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 fully divided volleyball interchange if the terminating highway is extended beyond the through 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. Its two flyover/flyunder ramps contract together after passing over the through highway, but then they split into two segments, resembling the bone in a T-bone steak.
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.
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.
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.
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.
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, where I-695 has an interchange with Interstate 95. Whereas this interchange has been replaced with a four-level stack, it was once of a design known as the "synchronized split-phasing interchange".[2]
The first diverging diamond interchange in the United States opened on July 7, 2009, in Springfield, Missouri, at the junction of Interstate 44 and Missouri Route 13.[3]
Freeways in the Phoenix Metropolitan area are great examples of the utilization of SPUI interchanges. Some examples of SPUI are along AZ-51 from Downtown Phoenix all the way to Loop 101, another location where SPUI is common.
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.
One 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 high, and it handles interchanging ramps via four intersections. This kind of an interchange can also be used to connect two highways together, but due to the use of intersections, its traffic is not free-flowing.
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