Stack interchange

A stack interchange (also known as a butterfly junction) is a free-flowing, grade-separated junction between two roads. 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 carriageway 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. Stack interchanges are also widely considered to be an eyesore among residents of homes near existing or proposed interchanges, leading to considerable NIMBY opposition.

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 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).

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 (a north–south road at 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.

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