Science Fair Project Encyclopedia
Single Point Urban Interchange
A Single Point Urban Interchange (SPUI, pronounced spoo-ee, rhymes with phooey), also called a Single Point Interchange (SPI) or Single Point Diamond Interchange (SPDI), is a type of highway interchange. It is similar in form to a diamond interchange but has the advantage of allowing opposing left turns to proceed simultaneously, by compressing the two intersections of a diamond into one single intersection over or under the free-flowing road.
The major disadvantage is that a SPUI costs more due to the need for a longer or wider bridge. A freeway-under SPUI (as in the diagram) requires a wider bridge over the free-flowing road to make room for the compressed on- and off-ramps. A freeway-over SPUI (as in the photo) requires a longer bridge of the free-flowing road to cross the wider area required for the SPUI intersection below. Additionally, due to a bigger intersection area, the traffic lights need a longer yellow or red phase, and even then it may not be long enough for a bicyclist entering on green or yellow to make it across before opposing traffic gets a green. Also, a SPUI generally has a very large area of uncontrolled pavement in the middle of the intersection because vehicles must be able to cross the pavement in six different ways. This can be unsafe particularly if drivers are unfamiliar with the interchange type.
Sometimes a SPUI will allow traffic to proceed straight through from the offramp to the onramp; this usually happens when the ramps connect with frontage roads. Since most through traffic travels over or under the intersection, the SPUI is still much more efficient than a surface intersection.
The first SPUI opened on February 25, 1974 along US 19 (SR 55 ), which goes over SR 60 east of Clearwater, Florida. It was designed by Wallace Hawkes, Director of Transportation Engineering at Greiner, Inc., who has been called the "granddaddy of the urban interchange". The SPUI is currently in the process of being expanded to provide more lanes over the intersection; only two lanes in each direction go over but US 19 has three in each direction on both sides of the interchange.
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