Interchange (road)
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.
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 areas that had such 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).
*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
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.
*
Grade separation*
Junction (traffic)*
Intersection (road)*
Ghost ramp*
Kurumi.com US interchanges directory*
Chris Marshall's UK interchanges directory*
Detailed history of interchanges with diagrams, in German
