A stack interchange (also known as a spaghetti road or butterfly junction) is a free-flowing, grade-separated junction between two roads. It is referred to as a "directional interchange" in AASHTO's Policy on Geometric Design of Highways and Streets.
In countries where one drives on the right, left turns are handled by semi-directional flyovers or under ramps. Vehicles first exit the main carriageway to the right, then complete the turn via a ramp which crosses both highways, eventually merging with the traffic turning right from the opposite side 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 interchanging highways. In countries where one drives on the left, the appearance of the junction is topologically identical, but traffic flows are reversed.
Stacks eliminate the problems of weaving and have the highest vehicle capacity among different types of four-way interchanges. However, they require considerable and expensive construction work for their flyover ramps. Stack interchanges are also widely considered to be an eyesore among residents of homes near existing or proposed interchanges, leading to considerable NIMBY opposition.
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).
North America 
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 and State Route 110. Since then, the California Department of Transportation, or Caltrans, has built eight more four-level stacks throughout the state of California, as well as a larger number of three-level stack/cloverleaf hybrids (where the least-used, left-turning ramp is built as a cloverleaf-like 270-degree loop).
One of the first four-level stack interchanges in the northeastern United States was constructed in the late 1960s over Interstate 84 in Farmington, Connecticut, for the controversial Interstate 291 beltway around the city of Hartford. Most of the I‑291 beltway was later cancelled, and the sprawling stack lay dormant for almost twenty-five years. In 1992 the extension of Connecticut Route 9 to Interstate 84 utilized the I‑291 right-of-way and some sections of the abandoned interchange. Several ramps still remain unused, including abandoned roadbed for Interstate 291 both north and south of the complex.
Four-level stacks are used for the interchanges between Interstate 90 and Interstate 405 in Bellevue, Washington, I‑110 and US 61/US 190 in Louisiana, and Interstate 75 and Interstate 696 near Detroit, Michigan. In St. Louis, Missouri, I‑70/I‑270, and I‑270/I‑64 are all stack interchanges.
Another well-known stack interchange lies west of Baltimore, Maryland, serving as the junction between Interstate 695 and Interstate 70. It was originally built for a planned extension of I‑70 into the city, but because of heavy opposition, I‑70 ends at a park and ride three miles east of here. As a result, the road east of I‑695 sees very little traffic, compared to the high volumes to and from the west.
Though it planned to build many four-level stack interchanges, Canada 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, with loop ramps in lieu of a fourth level of ramps. The interchange between Highway 401, 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). The junction of the A19 and A66 in Teesside uses a three-level variant, with a 270-degree loop allowing southbound A19 traffic to exit to the westbound A66.
Southern Hemisphere 
The EB Cloete Interchange just outside of Durban, South Africa, is another example of a four-level stack interchange. The N3 is the busiest highway in South Africa and is a very busy truck route. Since Johannesburg is not located near a body of water, most of the city's exports travel through the Port of Durban. The N2 connects Cape Town with Durban, and it serves the South African cities of Port Elizabeth, Plettenberg Bay, Margate, 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.
Five-level stack 
Texas-style stack 
Stacks are often five levels in Texas. They have the same configuration as four-level stacks, with frontage roads adding a fifth level. The frontage roads usually intersect with traffic lights, and are similar to a grid of nearby one-way streets. A common setup is for one mainline to go below grade and another mainline to go above grade. The intersection of the frontage roads is typically at‑grade (or close to it). Then two pairs of "left turn" connectors are built above these.
The Houston area has 7 five-level stack interchanges along Beltway 8: at Interstate 10 east and west of downtown, U.S. Highway 59 northeast and southwest of downtown, Interstate 45 north and south of downtown, and U.S. Highway 290 in the beltway's northwest quadrant. 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. 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. Similarly, more than 40 bridges make up the five-level stack interchange known as the Big I between Interstates 40 and 25 in Albuquerque, New Mexico.
Though not a "Texas-style stack" in the above sense, an unusual stack is nonetheless found in Houston which features more than four levels of traffic, but whose fifth level exists in only one direction. In 2011, the previously 4-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 new 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 West 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 in the complexity of the stack.)
Other five-level stacks 
Sometimes a fifth level is added for HOV connectors. An example of this exists in Los Angeles, California at the Judge Harry Pregerson Interchange. The connector from HOV SB 110 to HOV WB 105 can be at the same level as the connector from mixed EB 105 to mixed NB 110, but the connector from HOV SB 110 to HOV EB 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 found in DeKalb County, Georgia.
Six-level stack 
- A Policy on Geometric Design of Highways and Streets. Washington, D.C.: AASHTO. 2011. p. 10-53. ISBN 978-1-56051-508-1.
- Four Level Stack - cbrd.co.uk
- Four Level interchange-Los Angeles-Orange County Frwys
- TexasFreeway > Houston > Photo Gallery > Beltway 8 Photos
- Interstate 610 at U.S. 59 in Houston, Texas. Google Maps. Last accessed November 19, 2006.
- "TexasFreeway > Houston > Future Freeway > Future section of US 90, the Northeast/Crosby Freeway". TexasFreeway.com. Retrieved 16 October 2011.
- "East Fwy at North Loop". Google Maps. Retrieved 16 October 2011.
- Satellite image of Nanbei Elevated Road and Yanan Elevated Road 6-level interchange in Shanghai, China.
- Satellite image of Interstate 105 and Interstate 110 5-level interchange in Los Angeles, CA.
- Satellite image of Interstate 95 and Interstate 695 4-level interchange near Baltimore, MD.