A Single Point Urban Interchange (SPUI, pronounced spoo-ee), 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. 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.
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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