drivers may not reenter the freeway they are departing (if, for example, they realize that they have taken the wrong exit) within a SPUI. Three-phase traffic signals are required. Other interchange types designed for efficiency, such as the six-ramp partial cloverleaf and the diverging diamond, require just two signal phases. History The first SPUI opened on February 25, 1974, along U.S. Route 19 (State Road 55), which goes over State Road 60 east of Clearwater, Florida. It was designed by Wallace Hawkes, Director of Transportation Engineering at J. E. Greiner Company (later URS Corporation), who has been called the "granddaddy of the urban interchange". This design has recently been altered to include frontage roads in each direction. Several SPUIs, built in the 1970s and later, are located on German autobahns, like the A40, A42, A44, A46, A57, A59, and A113 in Berlin. There is also a SPUI on the Frankenschnellweg, the urban part of the A73, and Maximilianstraße, in Nuremberg (at ). Smaller versions of the SPUI can also be found on non-autobahn roads in German cities, with right-turning traffic under signal control, located in Heilbronn, Karlsruhe, Sindelfingen, Stuttgart, and Wiesbaden. SPUIs are also found in Australia, Hong Kong, Indonesia, and Singapore. Variants Inverted SPUI A rarely built variant of the SPUI is the inverted SPUI, in which the carriageways of the free-flowing road are separated, with left on- and offramps running between the carriageways and coming to a single at-grade intersection with the cross street. This can be built less expensively than a standard SPUI by allowing for shorter, simpler bridges at the interchange. However, this inverts the usual convention of placing carriageway on- and offramps on the right side of the carriageway, instead placing them on the left side, usually considered the passing lane. Notable examples Interstate 290 in Oak Park, Illinois, near Chicago, at South Harlem Avenue () and at South Austin Boulevard (). Greensboro, North Carolina, connecting West Wendover Avenue to South Holden Road (). Interstate 244 in Tulsa, Oklahoma, at Sheridan Road () and at Memorial Drive (). U.S. Route 422 in Reading, Pennsylvania, at U.S. Route 222 Business (). State Route 18 in St. George, Utah, at Red Hills Parkway. Continuous green T (or seagull) hybrid There is also a variant of the inverted SPUI, which can be used when a side road ends at, rather than crossing, a free-flowing road. In this variant,