Stack interchange

A stack interchange is a free-flowing junction between two or more roads that allows turning in all directions. A stack interchange has the highest vehicle capacity among different types of interchanges. Also, stack interchanges are most expensive, both in terms of land and construction costs, and can be subject to closure during icy weather.

Four-level stack

The four-level stack (or simply "four-stack") has one major road crossing another on a bridge, with connector roads crossing on two further levels. This type of interchange does not usually permit U-turns. Interestingly, the four-level stack creates two "inverse" dual-carriageways—the turn ramps crossing the middle section have traffic driving on the opposite side of oncoming traffic to usual (see diagram for clarity).

The first stack interchange in the world was the aptly-named Four Level Interchange (renamed the Bill Keene Memorial Interchange), built in Los Angeles, California at the junction of U.S. Route 101 and State Route 110. Since then, Caltrans has built eight more four-level stacks throughout the state of California, as well as a larger number of three-level variants (where the least-used left-turning ramp is built as a cloverleaf-like 270-degree loop to save money).

A four-level stack was constructed over Interstate 84 in Farmington, Connecticut for the controversial Interstate 291 beltway around the city of Hartford. Most of the interstate was later canceled and the sprawling stack lay dormant for almost twenty-five years. In 1992, the extension of Connecticut Route 9 to Interstate 84 utilized sections of the abandoned interchange. Several ramps still remain unused, including abandoned roadbed for Interstate 291 both north and south of the complex.

Though it planned to build many four-level stack interchanges, the Canadian province of Ontario has only one true four-level stack interchange—the interchange between Highway 400 and Highway 407. Planned four-level stacks at Highway 407 and Highway 410, and Highway 407 and Highway 404 were reduced to three-level interchanges. The interchange between Highway 401s, 403, and 410 is almost a full four level stack, with a loop ramp planned to be added in the northeast quadrant.

In the United Kingdom there are three four-level stacks: at the junction of the M4 and M25 near Heathrow Airport in London, at the junction of the M23 and M25 to the south of London, and at the junction of the M4 and M5 near Bristol (the Almondsbury Interchange). The M4/M25 junction is particularly unusual as it also has a railway line bisecting it at its lowest level. The M4/M25 junction is slightly offset so there is no point where all four levels are directly above each other. M25 (north-south road in this junction) is offset to the east by approximately 60 metres (200 feet).

In Argentina there are two four-level stacks: the first one is located at the intersection of Av. General Paz (the innermost beltway of Buenos Aires) with the Autopista Perito Moreno (AU6)/Autopista del Oeste (National Route 7), in the western limit of the city of Buenos Aires); the second one at the intersection of Autopista 25 de Mayo (AU 1) with Autopista 9 de Julio/Avenida 9 de Julio south of Downtown Buenos Aires.

The Lighthorse Interchange at the junction of the M4 and M7, is an example of a four-level stack interchange in Sydney, New South Wales, Australia.

Five-level stack

Five-level stacks may have the same configuration as four-level stacks, except that service roads may constitute the fifth level, often being placed at or below grade. U-turns are often possible on the below-grade service roads.

The Houston, Texas area has 5 five-level stack interchanges along Beltway 8: at Interstate 45 north of downtown, U.S. Highway 290, Interstate 10 west of downtown, U.S. Highway 59 southwest of downtown, and Interstate 45 southeast of downtown. [ [http://www.texasfreeway.com/Houston/photos/bw8/bw8.shtml TexasFreeway > Houston > Photo Gallery > Beltway 8 Photos ] ] The newly-reconstructed interchange of Interstate 610 and U.S. 59, with the new I-610 northbound feeder road built underground and the new I-610 southbound feeder road overpass, is also a five-level stack interchange. [ [http://www.google.com/maps?ie=UTF8&oe=UTF-8&hl=en&q=&z=18&ll=29.728873,-95.460173&spn=0.002227,0.003626&t=k&om=1 Interstate 610 at U.S. 59 in Houston, Texas] . Google Maps. Last accessed November 19, 2006.] In Dallas, the award-winning High Five Interchange was completed in 2005 and features some ramps which are more than 12 stories above the ground.

An alternative configuration of a five-level stack exists for dedicated HOV lane intersections. An example of this configuration exists in Los Angeles, California between Interstate 110 and Interstate 105, where the intersection has a normal four-level configuration plus a fifth level for HOV-only traffic to transition from either direction on I-105 to I-110 northbound and from I-110 southbound to either direction on I-105. This configuration allows these HOVs to avoid the congested general use lanes and ramps, particularly on Interstate 110.

In the Atlanta area, a side ramp forms the fifth level of the Tom Moreland Interchange in DeKalb County, Georgia.

References

External links

* [http://local.google.com/?ie=UTF8&t=k&om=1&ll=33.928388,-118.280354&spn=0.007763,0.014462 Satellite image of Interstate 105 and Interstate 110 5-level interchange] in Los Angeles, CA.

* [http://local.google.com/?ie=UTF8&om=1&z=16&ll=39.350876,-76.496601&spn=0.007417,0.014634&t=k Satellite image of Interstate 95 and Interstate 695 4-level interchange] near Baltimore, MD. This is a rare type of interchange in which the opposing lanes of traffic on both freeways cross over each other, so that vehicles drive through the interchange "on the wrong side of the road" in order to facilitate left exits and entrances. This interchange is not a four-level stack, the overpasses are all two-level, but the design achieves the same "higher-spreed" ramps as a typical four-level with logical "turn left -- exit left" ramps. This interchange is currently being reconstructed as a conventional four-level stack interchange with the addition of two more levels of restricted (express toll lane) ramps, bringing the total number of levels to six. [http://www.i-95expresstolllanes.com/constr_projects/i695_etl.asp]