driving direction

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 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 US) 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 US, 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 it and rejoins in the opposite direction.

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.

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).

Types of interchanges

Between two freeways

• A cloverleaf is a two-level 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 would be a left 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, Germany, and Netherlands, only three such junctions are found in the short-of-land United Kingdom, none on the motorway network. 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. Cloverleaves are often seen in urban highways within cities that grow out instead of growing up.

• A stack is an 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.

An alternative to the stack which requires fewer levels while retaining semi-directional ramps throughout 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 freeway-to-surface street ramp in this style would be the Interstate 696/Mound Road interchange in Warren, Michigan. [1]

A variation appropriate when the freeways meet at an angle is to have cloverleafs for the sharp left turns (less than 270° turning) and adjacent semi-directional ramps for the gentle left turns, one inside and one outside the junction. This design is compact and has no serious drawbacks.

A junction with two adjacent cloverleafs is sub-optimal and will be the result of a previous junction improvement subject to land restriction.

• A trumpet is an interchange traditionally used where one freeway terminates at another freeway; it involves at least one loop ramp (for traffic leaving the terminating freeway) whose overpass is shared by traffic connecting to the terminating freeway. These junctions are also useful for toll roads as they concentrate all entering and leaving traffic in a single stretch of road, where toll booths can be installed. A double trumpet interchange exists in Vermont where Route 7 and Route 279 meet just north of Bennington, VT.
• 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.
• Hybrids, variations and rare types

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 (sometimes signal-controlled) 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.

See also

• Grade separation
• Junction (traffic)
• Intersection (road)
• Ghost ramp

External links

• Kurumi.com US interchanges directory
• Chris Marshall's UK interchanges directory
• Detailed history of interchanges with diagrams, in German