page_title: Stack interchange
section_title: United States
hierarchical_section_title: Stack interchange / Four-level stack / United States
caption_attribution_description: English: Looking to the south before landing at Miami International Airport, the Palmetto Expressway (SR 826) ducks under the Dolphin Expressway (SR 836) near Miami Lakes, Florida.
context_page_description: A stack interchange is a particular, free-flowing type of design for interchanges, meaning grade-separated road junctions. It is referred to as a directional interchange in the American Association of State Highway and Transportation Officials 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 road to the right, then complete the turn via a ramp that crosses both highways, eventually merging with the traffic turning right from the opposite side of the interchange. A stack interchange therefore has two pairs of left-turning ramps, which may 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, as well as having a larger footprint.
context_section_description: 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 US Route 101 (US 101) and State Route 110 (SR 110). 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 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 at the intersection of I-35W and 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. Improvements to the old Mixmaster over the past 60 years include an upgrade to a Texas-style five-level stack exchange (see below).
  One of the first four-level stack interchanges in the northeastern United States was constructed in the late 1960s over I-84 in Farmington, Connecticut, for the controversial I-291 beltway around the city of 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:
  I-5 and SR 4 in Stockton, California;
  I-71/I-75 and I-275 in Erlanger, Kentucky (Cincinnati metropolitan area);
  I-71 and I-90/I-490 in Cleveland, Ohio;
  I-77 and I-480 in Independence, Ohio (just outside Cleveland);
  I-77 and I-490 in Cleveland, Ohio;
  I-77 and I-485 in Charlotte, North Carolina;
  I-65 and I-440 in Nashville, Tennessee;
  I-90 and I-405 in Bellevue, Washington;
  I‑110 and US 61/US 190 in Louisiana;
  I-75 and US 35 in Dayton, Ohio;
  I-75 and I-696 near Detroit, Michigan;
  I-69 and I-475 in Flint, Michigan;
  I‑70/I‑270 and I‑270/I‑64 St. Louis, Missouri;
  The Marquette Interchange between I-794, I-94, and I-43 in Milwaukee, Wisconsin; and
  The Zoo Interchange between I-894, I-94, I-41, and US 41/US 45 Milwaukee, Wisconsin.
  Another well-known stack interchange lies west of Baltimore, Maryland, serving as the junction between I-695 and I-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 (4.8 km) east. As a result, the road east of I‑695 sees very 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 I-95. The stack was built as part of a massive I-95 reconstruction project that includes high-occupancy toll lanes (HOT lanes), designed to relieve congestion between Baltimore and its northeastern suburbs.
  In Lone Tree, Colorado, there is a four-level stack serving I-25, the eastern end of 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.
caption: Highway Interchange between Dolphin Expressway and Palmetto Expressway (Dolphin–Palmetto Interchange) in Greater Miami, Florida, United States
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