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.
Read more about Stack Interchange: Four-level Stack
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