Stack interchange

Adirectional interchange,colloquially known as astack interchange,is a type ofgrade-separated junctionbetween twocontrolled-access highwaysthat allows for free-flowing movement to and from all directions of traffic. These interchanges eliminate the problems ofweaving,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 theFour Level Interchangewhich opened toLos Angelestraffic in 1949.

Definition

A directional interchange is agrade separated junctionbetween 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.

Four-level stack

Four-level stack

The four-level stack (or simplyfour-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 permitU-turns.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).

United States

TheFour Level InterchangeofArroyo Seco Parkwayand Highway 101, looking northeast, inLos Angeles,California. It was the first stack interchange in the world.

Highway Interchange betweenDolphin ExpresswayandPalmetto Expressway(Dolphin–Palmetto Interchange) inGreater Miami,Florida, United States

The first stack interchange was theFour Level Interchange(renamed the Bill Keene Memorial Interchange), built inLos Angeles,California, and completed in 1949, at the junction ofUS Route 101(US 101) andState Route 110(SR 110).Since then, theCalifornia Department of Transportation(Caltrans) has built eight more four-level stacks throughout the state ofCalifornia,notably theJudge Harry Pregerson Interchange,as well as a larger number of three-level and four-level stack–cloverleafhybrids (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 inFort Worthat the intersection ofI-35WandI-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.Improvements to the old Mixmaster over the past 60 years include an upgrade to a Texas-style five-level stack exchange (see below).

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 overI-84inFarmington, Connecticut,for the controversialI-291beltway around the city ofHartford.Most of the I‑291 beltway was later cancelled, and the sprawling stack lay dormant for almost 25 years. In 1992 the extension ofConnecticut Route 9to 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:

• I-5andSR 4inStockton, California;
• I-980,I-580andSR 24inOakland, California;
• I-71/I-75andI-275inErlanger, Kentucky(Cincinnati metropolitan area);
• I-71andI-90/I-490inCleveland, Ohio;
• I-77andI-480inIndependence, Ohio(just outsideCleveland);
• I-77andI-490inCleveland, Ohio;
• I-77andI-485inCharlotte, North Carolina;
• I-65andI-440inNashville, Tennessee;
• I-20andI-459nearBirmingham, Alabama;
• I-90andI-405inBellevue, Washington;
• I‑110andUS 61/US 190inBaton Rouge, Louisiana;
• I-75andUS 35inDayton, Ohio;
• I-75andI-696nearDetroit, Michigan;
• I-69andI-475inFlint, Michigan;
• I‑70/I‑270and I‑270/I‑64St. Louis, Missouri;
• TheMarquette InterchangebetweenI-794,I-94,andI-43inMilwaukee, Wisconsin;and
• TheZoo InterchangebetweenI-894,I-94,I-41,andUS 41/US 45Milwaukee, Wisconsin.

Another well-known stack interchange lies west ofBaltimore, Maryland,serving as the junction betweenI-695andI-70.It was originally built for a planned extension of I‑70 into the city. Due to strong opposition, I‑70 ends at apark and ridethree miles (4.8 km) 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 andI-95.The stack was built as part of a massive I-95 reconstruction project that includeshigh-occupancy toll lanes(HOT lanes), designed to relieve congestion between Baltimore and its northeastern suburbs.

TheSpringfield Interchange,south ofWashington, D.C.,was rebuilt into a four-level stack to accommodateI-95's transition from theCapital Beltwayto 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.

InLone Tree, Colorado,there is a four-level stack servingI-25,the eastern end ofC-470and the southern end ofE-470.InThornton, Colorado,there is another stack serving I-25 and E-470 at its northern end as it continues west as theNorthwest Parkway.

Canada

The initial design ofHighway 407had several four-level stack interchanges planned at junctions with existing400-series highways,but only one example was built: the interchange atHighway 400inVaughan, Ontario,which is also the only true four-level stack in Canada. Highway 407's other proposed four-level stacks atHighway 410andHighway 404were reduced to three-levelcloverstack interchanges,with loop ramps being built instead of a fourth level of semi-directional ramps. Similarly, the interchange withHighway 427has 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 theBrussels Ringthere aretwofour-level stack interchanges: TheGrand-BigardandMacheleninterchange (only partly in use).

In Germany, there is one, theWetzlarer Kreuz.

In Greece, there is alsoonefour stack interchange nearMetamorfosi,which connects theA1andA6(Attiki Odos) motorways.

In the Netherlands there is currentlyonefour-level stack interchange: the Prins Clausplein nearThe Hague.It forms the junction of the A4 and A12.

M23andM25interchange, UK

In the United Kingdom there arethreefour-level stacks: at the junction of theM4andM25nearHeathrow AirportinLondon(theThorney Interchange), at the junction of theM23and M25 to the south of London (the Merstham Interchange), and at the junction of the M4 andM5nearBristol(theAlmondsbury 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 60 metres (200 ft). The junction of theA19andA66inTeessideuses a three-level variant, with a 270-degree loop allowing southbound A19 traffic to exit to the westbound A66.

Southern Hemisphere

TheLight Horse Interchangein Sydney is the largest in the southern hemisphere.

TheLight Horse Interchangeat the junction of theM4andM7is a four-level stack interchange inSydney,New South Wales, Australia. Opened in late 2005, it is the largest in the Southern Hemisphere.

TheEB Cloete Interchangejust outsideDurban, South Africa,is another four-level stack interchange. TheN3is the busiest highway in South Africa and a very busy truck route. BecauseJohannesburgis not located near a body of water, most of the city's exports travel through thePort of Durban.TheN2connectsCape Townwith Durban and serves the South African cities ofPort Elizabeth,East LondonandGeorgeand the towns ofGrahamstown,Port Shepstone,Richards Bayand theiSimangaliso Wetland Park.Two busy roads intersect at the junction. A four-level stack interchange was chosen to serve the high volumes of traffic.

Five-level stack

Texas-style stack

TheHigh Five InterchangeinDallas,Texas, United States, is a five-level interchange.

InTexas,many stacks contain five levels. They usually have the same configuration as four-level stacks, butfrontage roadsadd a fifth level. The frontage roads usually intersect withtraffic lightsand are similar to a grid of nearbyone-way streets.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.

TheDallas–Fort Worth metroplexhas several five-level stacks, most notably theHigh Five InterchangebetweenUS 75andI-635;completed in 2005 and currently the tallest interchange in the world.Others can be found at the interchanges betweenState Highway 121(SH 121) and theDallas North Tollway,SH 121 andI-35E/US 77,I-30andI-35W,I-30 andPresident George Bush Turnpikeand 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).

TheHoustonarea has seven five-level stack interchanges alongBeltway 8:atI-10east and west of downtown,I-69northeast and southwest of downtown,I-45north and south of downtown, andUS 290in the beltway's northwest quadrant.The newly reconstructed interchange ofI-610and 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.

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)level of complexity with the opening of four ramps connecting the newUS 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.)

More than 40 bridges make up the five-level stack interchange known as theBig IbetweenI-40andI-25inAlbuquerque, 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

TheJudge Harry Pregerson Interchangein Los Angeles, California, United States

Sometimes a fifth level is added forHOVconnectors. An example of this exists inLos Angeles, California,at theJudge 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 theTom Moreland Interchange,colloquially known asSpaghetti Junction,found inDeKalb County, Georgia.

Six-level stack

Yan'an East Road Interchange, a six-level urban stack interchange inPuxi,Shanghai, China (Nanbei Elevated Road and Yan'an Elevated Road)

There is a six-level stack on the Yan'an East Road Interchange (Chinese:Duyên an đông lộ đan xen) in Puxi, Shanghai, with no dedicated HOV/bus/truck lanes. It is six-level stack because it is formed by two elevated highways,Nanbei Elevated RoadandYan'an Elevated Roadwith service roads and afootbridgeunderneath. The centrally located interchange has a central pillar known as the Nine-Dragon Pillar (Cửu Long trụ). The story is that after several construction accidents, a monk suggested the nine-dragon be welcomed with a bas relief sculpture depicting the dragon.

An unusual six-level stack is located at the junction betweenInterstate 35EandI-635inDallas, 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.

The interchange betweenI-35Eand theSam Rayburn TollwayinLewisville, 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.

Yan'an East Road Interchange, seen from a pedestrian's perspective

See also

Comparison of traffic flows for some four-legged complete interchanges(animation)

• List of road interchanges in the United States

External links

• Satellite image of Nanbei Elevated Road and Yanan Elevated Road six-level interchangeinShanghai,China.
• Satellite image of Interstate 105 and Interstate 110 five-level interchangeinLos Angeles.
• Satellite image of Interstate 95 and Interstate 695 four-level interchangenearBaltimoreMaryland.