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{{Short description|A type of freeway interchange}}
{{Use mdy dates|date=April 2018}}
{{more citations needed|date=November 2023}}
A '''directional interchange''', colloquially known as a '''stack interchange''', is a type of [[Interchange (road)|grade-separated junction]] between two [[controlled-access highway]]s that allows for free-flowing movement to and from all directions of traffic. These interchanges eliminate the problems of [[Grade separation#Weaving|weaving]], have the highest vehicle capacity, and vehicles travel shorter distances when compared to different types of interchanges.

The first directional interchange built in the world was the [[Four Level Interchange]] which opened to [[Los Angeles]] traffic in 1949.

==Definition==
A directional interchange is a [[Interchange (road)|grade separated junction]] between two roads where all turns that require crossing over or under the opposite road's lanes of travel to complete the turn utilize ramps that make a direct or semi-direct connection. The difference between direct and semi-direct connections is how much the motorist deviates from the intended direction of travel while on the ramp. Direct ramps are shorter and can handle higher traveling speeds than semi-direct.<ref>{{cite book |title=A Policy on Geometric Design of Highways and Streets |date=2018 |publisher=[[American Association of State Highway and Transportation Officials]] |location=[[Washington, DC]] |isbn=978-1-56051-676-7 |pages=10–64 |edition=7th |ref=AASHTO Geometric}}</ref>

==Four-level stack==
[[File:Stackinterchange.svg|thumb|right|upright=1.2|Four-level stack]]
The four-level stack (or simply ''four-stack'') has one major freeway crossing another freeway with a viaduct, with connector flyover ramps crossing on two further levels. This type of interchange does not usually permit [[U-turn]]s. The four-level stack creates two "inverse" [[dual-carriageway]]s—the turn ramps crossing the middle section have traffic driving on the opposite side of oncoming traffic to usual (see diagram for clarity).

===United States===
[[File:Four Level Interchange.jpg|thumb|left|The [[Four Level Interchange]] of [[Arroyo Seco Parkway]] and Highway 101, looking northeast, in [[Los Angeles]], California. It was the first stack interchange in the world.]]
[[File:Dolphin and Palmetto Expressway Interchange.jpg|thumb|left|Highway Interchange between [[Dolphin Expressway]] and [[Palmetto Expressway]] ([[Dolphin–Palmetto Interchange]]) in [[Miami metropolitan area|Greater Miami]], Florida, United States]]
The first stack interchange was the [[Four Level Interchange]] (renamed the Bill Keene Memorial Interchange), built in [[Los Angeles]], California, and completed in 1949, at the junction of [[U.S. Route 101 in California|US Route 101]] (US 101) and [[California State Route 110|State Route 110]] (SR 110).<ref>[http://cache.search.yahoo-ht2.akadns.net/search/cache?ei=UTF-8&p=Four+Level+interchange&fr=slv8-msgr&u=members.cox.net/mkpl/interchange/interchange.html&w=four+level+leveling+levels+interchange&d=IAEcZZzfQ9Jj&icp=1&.intl=us Four Level interchange-Los Angeles-Orange County Frwys]{{dead link|date=May 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Since then, the [[California Department of Transportation]] (Caltrans) has built eight more four-level stacks throughout the state of [[California]], notably the [[Judge Harry Pregerson Interchange]], as well as a larger number of three-level and four-level stack–[[Cloverleaf interchange|cloverleaf]] hybrids (where the least-used left-turning ramp is built as a cloverleaf-like 270-degree loop). The stack interchange between I-10 and I-405 is a three-level stack, since the semi-directional ramps are spaced out far enough so they do not need to cross each other at a single point as in a conventional four-level stack.

The first four-level stack interchange in Texas was built in [[Fort Worth, Texas|Fort Worth]] at the intersection of [[Interstate 35W (Texas)|I-35W]] and [[Interstate 30|I-30]] (originally I-20) near downtown. This interchange, finished in 1958, was known as "The Pretzel" or the "Mixmaster" by locals. The original contract cost was $1,220,000.<ref name=FW>[http://www.texasfreeway.com/dallas/photos/i35w/i35w.shtml Interstate 35W, Fort Worth]</ref> Improvements to the old Mixmaster over the past 60 years include an upgrade to a Texas-style five-level stack exchange (see below).

[[File:Farmington stack interchange from I-84 westbound, March 2023.JPG|thumb|right|Partially used stack interchange over I‑84 in Connecticut]]

One of the first four-level stack interchanges in the northeastern United States was constructed in the late 1960s over [[Interstate 84 in Connecticut|I-84]] in [[Farmington, Connecticut]], for the controversial [[Interstate 291 (Connecticut)|I-291]] beltway around the city of [[Hartford, Connecticut|Hartford]]. Most of the I‑291 beltway was later cancelled, and the sprawling stack lay dormant for almost 25 years. In 1992 the extension of [[Connecticut Route 9]] to I-84 used the I‑291 right-of-way and some sections of the abandoned interchange. Several ramps still remain unused, including abandoned roadbed for I-291 both north and south of the complex.

Four-level stacks are used for the interchanges between:
* [[The Stack]] in [[Phoenix, Arizona]]
* [[SuperRedTan Interchange]] in [[Mesa, Arizona]]
* [[Mini Stack]] in [[Phoenix, Arizona]]
* [[Interstate 5 in California|I-5]] and [[California State Route 4|SR 4]] in [[Stockton, California]];
* [[Interstate 980 in California|I-980]], [[Interstate 580 in California|I-580]] and [[California State Route 24|SR 24]] in [[Oakland, California]];
* [[Interstate 71|I-71]]/[[Interstate 75 in Kentucky|I-75]] and [[Interstate 275 (Ohio–Indiana–Kentucky)|I-275]] in [[Erlanger, Kentucky]] ([[Cincinnati metropolitan area]]);
* [[Interstate 71|I-71]] and [[Interstate 90 in Ohio|I-90]]/[[Interstate 490 (Ohio)|I-490]] in [[Cleveland|Cleveland, Ohio]];
* [[Interstate 77 in Ohio|I-77]] and [[Interstate 480 (Ohio)|I-480]] in [[Independence, Ohio]] (just outside [[Cleveland]]);
* [[Interstate 77 in Ohio|I-77]] and [[Interstate 490 (Ohio)|I-490]] in [[Cleveland|Cleveland, Ohio]];
* [[Interstate 77 in North Carolina|I-77]] and [[Interstate 485|I-485]] in [[Charlotte, North Carolina]];
* [[Interstate 65 in Tennessee|I-65]] and [[Interstate 440 (Tennessee)|I-440]] in [[Nashville, Tennessee]];
* [[Interstate 20 in Alabama|I-20]] and [[Interstate 459|I-459]] near [[Birmingham, Alabama]];
* [[Interstate 90 in Washington|I-90]] and [[Interstate 405 (Washington)|I-405]] in [[Bellevue, Washington]];
* [[Interstate 110 (Louisiana)|I‑110]] and [[U.S. Route 61 in Louisiana|US 61]]/[[U.S. Route 190 in Louisiana|US 190]] in [[Baton Rouge, Louisiana]];
* [[Interstate 90 in Ohio|I-75]] and [[U.S. Route 35|US 35]] in [[Dayton, Ohio]];
* [[Interstate 75 in Michigan|I-75]] and [[Interstate 696|I-696]] near [[Detroit|Detroit, Michigan]];
* [[Interstate 69 in Michigan|I-69]] and [[Interstate 475 (Michigan)|I-475]] in [[Flint, Michigan]];
* [[Interstate 70 in Missouri|I‑70]]/[[Interstate 270 (Missouri–Illinois)|I‑270]] and I‑270/[[Interstate 64 in Missouri|I‑64]] [[St. Louis|St. Louis, Missouri]];
* The [[Marquette Interchange]] between [[Interstate 794|I-794]], [[Interstate 94 in Wisconsin|I-94]], and [[Interstate 43|I-43]] in [[Milwaukee|Milwaukee, Wisconsin]]; and
* The [[Zoo Interchange]] between [[Interstate 894|I-894]], I-94, [[Interstate 41|I-41]], and [[U.S. Route 41 in Wisconsin|US 41]]/[[U.S. Route 45 in Wisconsin|US 45]] [[Milwaukee|Milwaukee, Wisconsin]].

Another well-known stack interchange lies west of [[Baltimore|Baltimore, Maryland]], serving as the junction between [[Interstate 695 (Maryland)|I-695]] and [[Interstate 70 in Maryland|I-70]]. It was originally built for a planned extension of I‑70 into the city. Due to strong opposition, I‑70 ends at a [[park and ride]] {{convert|3|mi|km|spell=in}} east. As a result, the road east of I‑695 sees little traffic compared to the high volumes to and from the west.

Another four-level stack interchange in the Baltimore area is located at the northeastern junction between I-695 and [[Interstate 95 in Maryland|I-95]]. The stack was built as part of a massive I-95 reconstruction project that includes [[high-occupancy toll lane]]s (HOT lanes), designed to relieve congestion between Baltimore and its northeastern suburbs.

The [[Springfield Interchange]], south of [[Washington, D.C.]], was rebuilt into a four-level stack to accommodate [[Interstate 95 in Virginia|I-95]]'s transition from the [[Capital Beltway]] to its own alignment further south into Virginia. This was necessitated by the inadequacy of the original configuration that was caused by the rerouting of I-95 onto the Beltway after its cancellation within Washington and points north.

In [[Lone Tree, Colorado]], there is a four-level stack serving [[Interstate 25 in Colorado|I-25]], the eastern end of [[Colorado State Highway 470|C-470]] and the southern end of [[E-470]]. In [[Thornton, Colorado]], there is another stack serving I-25 and E-470 at its northern end as it continues west as the [[Northwest Parkway]].

===Canada===

The initial design of [[Ontario Highway 407|Highway 407]] had several four-level stack interchanges planned at junctions with existing [[400-series highways]], but only one example was built: the interchange at [[Ontario Highway 400|Highway 400]] in [[Vaughan, Ontario]], which is also the only true four-level stack in Canada. Highway 407's other proposed four-level stacks at [[Ontario Highway 410|Highway 410]] and [[Ontario Highway 404|Highway 404]] were reduced to three-level [[Interchange (road)#Combination interchange|cloverstack interchanges]], with loop ramps being built instead of a fourth level of semi-directional ramps. Similarly, the interchange with [[Ontario Highway 427|Highway 427]] has four levels but only two semi-directional flyover ramps that cross each other connecting to Highway 427 south of that junction. Two loop ramps link Highway 407 with Highway 427 north of that junction.

===Europe===
In Belgium, on the [[Brussels Ring]] there are {{citation needed span|two|date=December 2020}} four-level stack interchanges: The [[:fr:Échangeur de Grand-Bigard|Grand-Bigard]] and [[:fr:Échangeur de Machelen|Machelen]] interchange (only partly in use).

In Germany, there is one, the [[Wetzlarer Kreuz]].

In Greece, there is also {{citation needed span|one|date=December 2020}} four stack interchange near [[Metamorfosi]], which connects the [[A1 motorway (Greece)|A1]] and [[A6 motorway (Greece)|A6]] (Attiki Odos) motorways.

In the Netherlands there is currently {{citation needed span|one|date=December 2020}} four-level stack interchange: the Prins Clausplein near [[The Hague]]. It forms the junction of the A4 and A12.

[[File:M25-M23 motorway interchange, Surrey, England.jpg|thumb|right|[[M23 motorway|M23]] and [[M25 motorway|M25]] interchange, UK]]
In the United Kingdom there are {{citation needed span|three|date=December 2020}} four-level stacks: at the junction of the [[M4 motorway|M4]] and [[M25 motorway|M25]] near [[Heathrow Airport]] in [[London]] (the [[Thorney Interchange]]), at the junction of the [[M23 motorway|M23]] and M25 to the south of London (the Merstham Interchange), and at the junction of the M4 and [[M5 motorway|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 (a north–south road at this junction) is offset to the east by approximately {{convert|60|m|feet}}. The junction of the [[A19 road|A19]] and [[A66 road|A66]] in [[Teesside]] uses a three-level variant, with a 270-degree loop allowing southbound A19 traffic to exit to the westbound A66.

===Asia===
In Qatar’s capital and largest city, [[Doha]], The Umm Beshr Interchange is a traditional 4-level stack interchange between the Qatar Express Highway, heading north in the direction of Downtown Doha and G Ring Road, which provides access to [[Doha International Airport|Doha]] and [[Hamad International Airport]]. It is currently the tallest interchange in [[Qatar]].<ref>{{cite web|url=http://www.westlinkm7.com.au/cmsAdmin/uploads/Fact_Sheet_Light_Horse_Interchange.pdf|title=Fact Sheet - Light Horse Interchange|publisher=Westlink Motorway Limited|date=May 2006}}</ref>

===Southern Hemisphere===
[[File:Light Horse Interchange (aerial view).jpg|thumb|The [[Light Horse Interchange]] in Sydney is the largest in the southern hemisphere.]]
The [[Light Horse Interchange]] at the junction of the [[M4 Western Motorway|M4]] and [[Westlink M7|M7]] is a four-level stack interchange in [[Sydney, Australia|Sydney]], New South Wales, Australia. Opened in late 2005, it is the largest in the Southern Hemisphere.<ref>{{cite web|url=http://www.westlinkm7.com.au/cmsAdmin/uploads/Fact_Sheet_Light_Horse_Interchange.pdf|title=Fact Sheet - Light Horse Interchange|publisher=Westlink Motorway Limited|date=May 2006}}</ref>

The [[EB Cloete Interchange]] just outside [[Durban, South Africa]], is another four-level stack interchange. The [[N3 road (South Africa)|N3]] is the busiest highway in South Africa and a very busy truck route. Because [[Johannesburg]] is not located near a body of water, most of the city's exports travel through the [[Port of Durban]]. The [[N2 road (South Africa)|N2]] connects [[Cape Town]] with Durban and serves the South African cities of [[Port Elizabeth]], [[East London, Eastern Cape|East London]] and [[George, Western Cape|George]] and the towns of [[Grahamstown]], [[Port Shepstone]], [[Richards Bay]] and the [[iSimangaliso Wetland Park]]. Two busy roads intersect at the junction. A four-level stack interchange was chosen to serve the high volumes of traffic.

The [[Mount Edgecombe Interchange]] is another four-level stack interchange just outside [[Durban, South Africa]], and is the intersection between the [[N2 (South Africa)|N2]] (to [[Durban]] and [[KwaDukuza]]) and the [[M41 (Durban)|M41]] (to [[Mount Edgecombe]] and [[UMhlanga, KwaZulu-Natal|uMhlanga]]). The interchange which was previously a simple diamond interchange was upgraded to a four-level/four-stack interchange, with the upgraded interchange opened in October 2018. A four-level stack interchange was chosen to serve the increasing volumes of traffic in the uMhlanga/Mount Edgecombe area.
{{clear}}

==Five-level stack==

===Texas-style stack===
In [[Texas]], many stacks contain five levels. They usually have the same configuration as four-level stacks, but [[frontage road]]s add a fifth level. The frontage roads usually intersect with [[traffic light]]s and are similar to a grid of nearby [[one-way street]]s. A common setup is for one mainline to go below grade and another to go above grade. The intersection of the frontage roads is typically at grade or close to it. Two pairs of left-turn connectors are built above these.

The [[Dallas–Fort Worth metroplex]] has several five-level stacks, most notably the [[High Five Interchange]] between [[U.S. Route 75 in Texas|US 75]] and [[Interstate 635 (Texas)|I-635]]; completed in 2005 and currently the tallest interchange in the world.<ref>{{Cite web|url =http://uk.chryso.com/upload/t_documents/Fichier_L1/48827/High%20Five%20Interchange.pdf|title =High Five Interchange, Dallas - Texas|publisher =Chryso|url-status =dead|archive-url =https://web.archive.org/web/20120426085950/http://uk.chryso.com/upload/t_documents/Fichier_L1/48827/High%20Five%20Interchange.pdf|archive-date =April 26, 2012|df =mdy-all}}</ref> Others can be found at the interchanges between [[Texas State Highway 121|State Highway 121]] (SH 121) and the [[Dallas North Tollway]], SH 121 and [[Interstate 35E (Texas)|I-35E]]/[[U.S. Route 77 in Texas|US 77]], [[Interstate 30|I-30]] and [[Interstate 35W (Texas)|I-35W]], I-30 and [[President George Bush Turnpike]] and others which are technically five levels but do not fit under a Texas-style stack configuration (i.e. the extra level being located away from the central stack or existing in only one direction).

The [[Greater Houston|Houston]] area has seven five-level stack interchanges along [[Sam Houston Tollway|Beltway 8]]: at [[Interstate 10 in Texas|I-10]] east and west of downtown, [[Interstate 69 in Texas|I-69]] northeast and southwest of downtown, [[Interstate 45|I-45]] north and south of downtown, and [[U.S. Route 290|US 290]] in the beltway's northwest quadrant.<ref>[http://www.texasfreeway.com/Houston/photos/bw8/bw8.shtml TexasFreeway > Houston > Photo Gallery > Beltway 8 Photos]</ref> The newly reconstructed interchange of [[Interstate 610 (Texas)|I-610]] and I-69, 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>{{google maps |url = 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 |title = Interstate 610 at U.S. 59 in Houston, Texas |access-date = November 19, 2006 }}</ref>

Though not a Texas-style stack in the above sense, an unusual stack is nonetheless found in Houston that features more than four levels of traffic but whose fifth level exists in only one direction. In 2011, the previously four-level stack interchange between I-610 and I-10 on the city's east side gained a new (though long-planned)<ref>{{cite web |title = TexasFreeway > Houston > Future Freeway > Future section of US 90, the Northeast/Crosby Freeway |url = http://www.texasfreeway.com/Houston/new_freeway/90/90.shtml |work = TexasFreeway.com |access-date = October 16, 2011 }}</ref> level of complexity with the opening of four ramps connecting the new [[U.S. Route 90 in Texas|US 90]] (Crosby Freeway) to the east, featuring direct movements for the new freeway to and from the southeast quadrant of I-610, to westbound I-10, and from eastbound I-10. It is the latter ramp which gives the interchange the fifth level, as US 90 to I-10 westbound merges onto I-10 before crossing I-610. (None of the frontage roads for these highways cross the interchange itself, and thus do not factor into the complexity of the stack.)<ref>{{google maps |link = no |title=East Fwy at North Loop |url = https://maps.google.com/maps?ll=30.345,-95.083611&spn=0.1,0.1&t=m&q=30.345,-95.083611 |access-date = October 16, 2011 }}</ref>

More than 40 bridges make up the five-level stack interchange known as the [[Big I]] between [[Interstate 40 in New Mexico|I-40]] and [[Interstate 25|I-25]] in [[Albuquerque, New Mexico]].

China is also home to many Texas-style stack interchanges. For example the Nanjing's Yingtian Street Elevated has one each where it intersects the Inner Ring Road twice.

===Other five-level stacks===
[[File:La city hwys.jpg|thumb|right|The [[Judge Harry Pregerson Interchange]] in Los Angeles, California, United States]]
Sometimes a fifth level is added for [[High-occupancy vehicle lane|HOV]] connectors. An example of this exists in [[Los Angeles, California]], at the [[Judge Harry Pregerson Interchange]]. The connector from HOV southbound 110 to HOV westbound 105 can be at the same level as the connector from mixed eastbound 105 to mixed northbound 110, but the connector from HOV southbound 110 to HOV eastbound 105 needs to be higher level, since it crosses over the former connector.

Another case is where connection to nearby arterials suggests that another level may be useful, thus making the interchange more complicated but easier to use. In the Atlanta area, a side ramp forms the fifth level of the [[Tom Moreland Interchange]], colloquially known as ''[[Spaghetti Junction]]'', found in [[DeKalb County, Georgia]].

==Six-level stack==
[[File:Shanghai, China (Unsplash).jpg|thumb|Yan'an East Road Interchange, a six-level urban stack interchange in [[Puxi]], Shanghai, China (Nanbei Elevated Road and Yan'an Elevated Road)]]

There is a six-level stack on the Yan'an East Road Interchange ({{zh|s=延安东路立交}}) in Puxi, Shanghai, with no dedicated HOV/bus/truck lanes. It is a six-level stack because it is formed by two elevated highways, [[North–South Elevated Road (Shanghai)|Nanbei Elevated Road]] and [[Yan'an Elevated Road]] with service roads and a [[footbridge]] underneath. The centrally located interchange has a central pillar known as the Nine-Dragon Pillar ({{zh|labels=no|s=九龙柱}}). The story is that after several construction accidents, a monk suggested the nine-dragon be welcomed with a bas relief sculpture depicting the dragon.{{citation needed|date=January 2017}}

An unusual six-level stack is located at the junction between [[Interstate 35E (Texas)|Interstate 35E]] and [[Interstate 635 (Texas)|I-635]] in [[Dallas|Dallas, Texas]], and does not contain any service or frontage roads. The interchange features two levels of highway with the top three levels consisting of direct connection ramps and HOV connectors. A single ramp leading from I-635 westbound to I-35E southbound weaves underneath the I-635 eastbound bridge, making the interchange six levels.<ref>{{Cite web|title = Phase 2 Entrance and Exit Points {{!}} LBJ TEXpress Lanes|url = http://www.lbjtexpress.com/using-lbj-texpress-lanes/lbj-texpress-lanes-overview/phase-2-entrance-and-exit-points|website = www.lbjtexpress.com|access-date = January 11, 2016}}</ref>

The interchange between [[Interstate 35E (Texas)|I-35E]] and the [[Sam Rayburn Tollway]] in [[Lewisville, Texas]], although similar in design to five-level stacks elsewhere in Texas, also qualifies as a six-level stack, since the ramp connecting the eastbound Sam Rayburn Tollway with northbound I-35E goes over the fifth-level ramps connecting I-35E in both directions with the Sam Rayburn Tollway. The ramp connecting the westbound Sam Rayburn Tollway with southbound I-35E is on the fourth level of the interchange, going under the fifth-level ramps connecting both directions of I-35E with the Sam Rayburn Tollway.<ref>{{Cite web|title = Renderings {{!}} The 35Express Project|url = http://www.35express.org/renderings.php|website = www.35express.org|access-date = October 20, 2017}}</ref>

[[File:Shanghai Interchange 2011-10-11 - 6658.jpg|thumb|Yan'an East Road Interchange, seen from a pedestrian's perspective]]
{{Clear}}

==See also==
{{comparison_of_four_legged_interchanges.svg}}
*[[List of road interchanges in the United States]]

==References==
{{reflist}}

==External links==
{{commons category|Stack interchanges}}
*[http://local.google.com/?ie=UTF8&ll=31.226275,121.464425&spn=0.00344,0.008256&t=h&z=18 Satellite image of Nanbei Elevated Road and Yanan Elevated Road six-level interchange] in [[Shanghai]], China.
*[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 five-level interchange] in [[Los Angeles]].
*[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 four-level interchange] near [[Baltimore]] Maryland.

{{Road junction types}}

{{DEFAULTSORT:Stack Interchange}}
[[Category:Road interchanges]]

Stack interchange - Revision history

imported>SomePacifisticGuy: Minor edit