Single-point urban interchange
A single-point urban interchange (SPUI, /ˈspuːiː/ or /ˈspjuːiː/), also called a single-point interchange (SPI) or single-point diamond interchange (SPDI), is a type of highway interchange. The design was created in order to help move large volumes of traffic through limited amounts of space safely and efficiently.
A single point urban interchange 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 term "single point" refers to the fact that all through traffic on the arterial street, as well as the traffic turning left onto or off the interchange, can be controlled from a single set of traffic signals. Due to the space efficiency of SPUIs relative to the volume of traffic they can handle, the interchange design is being used extensively in the reconstruction of existing freeways as well as constructing new freeways, particularly in dense urban environments.
Sometimes a SPUI will allow traffic to proceed straight through from the off-ramp to the on-ramp; 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 most commonly cited advantages of single point urban interchanges are improved operation efficiency and safety as well as reduced right-of-way requirements compared to other interchange forms. Left turning traffic from both directions of the intersecting roadways are able to turn simultaneously without crossing the path of the opposing left turns. Because traffic passing through the interchange can be controlled by a single signal, vehicles can clear the intersection much more quickly than in a diamond interchange (which requires two sets of traffic signals).
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