Stack interchange
From Wikipedia, the free encyclopedia
A stack interchange is a free-flowing junction between two or more roads that allows turning in all directions. This is the best type of four-way interchange for the driver and in terms of capacity, but is also the most expensive, both in terms of land and in terms of construction.
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[edit] 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.
The first stack interchange in the world was the aptly-named Four Level Interchange, built in Los Angeles, California at the junction of U.S. Route 101 and California State Route 110.
Though there were plans for more such interchanges, the Canadian province of Ontario has only one four-level stack interchange--the interchange between Highway 400 and Highway 407.
In Great Britain 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 M4/M25 junction is particularly unusual as it also has a railway line bisecting it at its lowest level.
[edit] 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.[1] 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.<ref>Interstate 610 at U.S. 59 in Houston, Texas. Google Maps. Last accessed November 19, 2006.</ref> 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.
In theory, stacks with more than five levels are possible. However, the amount of land required for the highest ramps increases significantly as each stack is added, in order for high speed traffic to maintain velocity during the climb to the top of the highest stack. Stack interchanges are also associated with a strong NIMBY feelings, due to their size and noise, and higher stacks would be visible from a greater radius, increasing the number of potential opponents.[citation needed] High level stacks also impact drivers who suffer from mild to extreme forms of Acrophobia.[citation needed]
[edit] References
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[edit] External links
- 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.
| Types of road junctions | |
| Interchanges (grade separated) | Cloverleaf - Diamond - Directional T - Diverging diamond Parclo - Trumpet - SPUI - Stack - Three-level diamond |
| Intersections (at-grade) | Box junction - Continuous flow - Crossroads - Hook turn - Jughandle - Michigan left Quadrant roadway - Roundabout - Superstreet - 3-way junction - Traffic circle |
