Asingle-point urban interchange (SPUI,/ˈspuːi/SPOO-ee or/ˈspjuːi/SPEW-ee), also called asingle-point interchange (SPI)[1] orsingle-point diamond interchange (SPDI), is a type ofhighway interchange. The design was created in order to help move large volumes of traffic through limited amounts of space efficiently.
A SPUI is similar in form to adiamond 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.[2]
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 oftraffic signals. Due to the space efficiency of SPUIs relative to the volume of traffic they can handle, the interchange design is used extensively in the reconstruction of existing freeways as well as constructing new freeways, particularly in dense urban environments.[3][neutrality isdisputed]
Sometimes a SPUI will allow traffic to proceed straight through from the offramp to the onramp; this usually happens when the ramps connect withfrontage 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 SPUIs are improved operation efficiency and safety as well as reduced right-of-way requirements compared to other interchange forms.[4] Left-turning traffic from both directions of the intersecting roadways is 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).
SPUIs also allow for wider turns, easing movement for large vehicles, such as trucks and RVs. Furthermore, a SPUI takes up considerably less space than a fullcloverleaf interchange, allowing construction to take place on a limited amount of property and minimizing state use ofeminent domain.[5]
Finally, SPUIs are reportedly safer than other space-efficient interchange forms, such as (standard) diamond interchanges. Research suggests that, although there may not be a significant difference between the two types of interchanges in terms of total collisions, the injury and fatality rates are notably lower for SPUIs than diamond interchanges.[6]
The major disadvantage of SPUIs over other types of road junctions is the increased cost due to the need for a longer or wider bridge. A freeway-under SPUI (as in the upper diagram) requires a wider bridge over the free-flowing road to make room for the compressed on- and offramps. However, this disadvantage poses less of a problem in cases where the arterial, or nonfreeway road, already requires a very wide bridge. The intersection of97 Street, having seven throughlanes, withYellowhead Trail inEdmonton, Alberta, Canada, though a diamond interchange in concept, required such a wide bridge that traffic-signal phasing allows this intersection to behave as a SPUI. A freeway-over SPUI (as in the lower photo) requires a longer bridge of the free-flowing road to cross the wider area required for the SPUI intersection below.[citation needed]
Because vehicles must be able to cross the pavement in six different ways, a SPUI generally has a very large area of uncontrolled pavement in the middle of the intersection. This can be unsafe particularly if drivers are unfamiliar with the interchange type. Drivers making a left turn may become confused as oncoming turning traffic passes them on therighthand side.[citation needed]
Due to the large intersection area, the traffic lights need a longer yellow and red phase to clear the intersection, 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.[4] In general, SPUI designs should not be used where bicycle traffic is expected unless fairly substantial changes to the design or special accommodations are provided.[7]
Pedestrians are usually not able to get through the intersection with one green light. It can take up to four cycles to walk through the entire length of a SPUI.[7]
Finally, SPUIs can be somewhat difficult to clear of snow. The large area in which lanes cross may have to be shut down to allow efficient and thorough cleaning lest a snowplow leave piles of snow, interfering with traffic and visibility in the middle of the uncontrolled pavement. Additionally, if the wide area of uncontrolled pavement is on a bridge, as in the diagram, the snow cannot be pushed to the sides of the bridge as it may pose a hazard to the road underneath. This problem can be exacerbated by the comparatively large bridge width required by the SPUI.
Given that a SPUI allows only left and right turns, 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 thesix-ramp partial cloverleaf and thediverging diamond, require just two signal phases.
The first SPUI opened on February 25, 1974,[citation needed] alongU.S. Route 19 (State Road 55), which goes overState Road 60 east ofClearwater, Florida. It was designed by Wallace Hawkes, Director of Transportation Engineering atJ. E. Greiner Company (laterURS Corporation), who has been called the "granddaddy of the urban interchange".[citation needed] This design has since been altered to includefrontage roads in each direction.[8]
Several SPUIs, built in the 1970s and later, are located on German autobahns, like theA40,A42,A44,A46,A57,A59, andA113 inBerlin. There is also a SPUI on theFrankenschnellweg, the urban part of theA73, and Maximilianstraße, inNuremberg (at49°27′06″N11°02′17″E / 49.451708°N 11.038102°E /49.451708; 11.038102). Smaller versions of the SPUI can also be found on non-autobahn roads in German cities, with right-turning traffic under signal control, located inHeilbronn,Karlsruhe,Sindelfingen,Stuttgart, andWiesbaden.[citation needed]
SPUIs are also found in Australia, Canada, Hong Kong, Indonesia, and Singapore.[citation needed]
A rarely built variant of the SPUI is theinverted SPUI, in which thecarriageways of the free-flowing road are separated, with left on- and offramps running between the carriageways and coming to a singleat-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 thepassing lane.
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, one carriageway of the free-flowing road is grade-separated from the side road, as with a typical inverted SPUI, while the other carriageway of the free-flowing road is at-grade with but still separated from the side road, as found in acontinuous green T- (or seagull) intersection.
There is one such interchange inPultney Township, Belmont County, Ohio, betweenState Route 7 (SR 7) andInterstate 470 (I-470) at40°02′52″N80°44′01″W / 40.047657°N 80.733542°W /40.047657; -80.733542. The ramps leading to and from I-470 run west of, but are not directly connected to, SR 7; these ramps form the "side road" at this interchange. The southbound carriageway of SR 7 bridges over the interchange, while its northbound carriageway remains at-grade. SR 7's left on- and offramps run between its carriageways, meeting the I-470 ramps at an at-grade intersection. The I-470 ramps proceed to atrumpet interchange with I-470. I-470 itself bridges over both carriageways of SR 7 a short distance north of the I-470/SR 7 ramps' intersection. This design was likely chosen not for cost savings but because the northbound carriageway of SR 7 is bordered by railroad tracks, commercial properties, and theOhio River and because I-470 bridges the river just east of SR 7—a more conventional interchange design was likely more difficult to achieve.
There is another such interchange inMillvale, Pennsylvania, nearPittsburgh, at the junction betweenPennsylvania Route 28 (PA 28) and the40th Street Bridge. The northbound carriageway of PA 28 passes under the interchange, with lefthand ramps meeting at the bridge (the northbound entrance ramp does not provide direct access to PA 28). The southbound carriageway of PA 28 has a continuous green T-intersection with the bridge.[11]
A similar interchange exists inCoal Grove, Ohio, at the junction ofU.S. Route 52 (US 52) with theBen Williamson Memorial Bridge and theSimeon Willis Memorial Bridge, which cross theOhio River and lead to and fromAshland, Kentucky, respectively. The eastbound carriageway of US 52 passes under the interchange, with lefthand exit and entrance ramps to the bridges. The westbound carriageway of US 52, which narrows to one throughlane, has a continuous green T-intersection with the bridges. The ramps cross each otherat-grade, with atraffic light-controlled intersection.[12]
An interchange, like that in Millvale, Pennsylvania, formerly existed at the junction of SR 7 andSR 822 at the now-demolishedFort Steuben Bridge inSteubenville, Ohio. The northbound carriageway of SR 7 passed under the bridge, with lefthand ramps meeting the bridge approach, while the southbound carriageway had a continuous green T-intersection with the bridge.[13]
An offset SPUI is similar to a diamond interchange, however, by making two of the rampsflyovers or flyunders, all ramps are pushed to one side of the highway, where they meet the surface street at a single intersection.[14] An example of this is the interchange betweenInterstate 225 and Alameda Avenue near theTown Center at Aurora shopping center inAurora, Colorado, United States (at39°42′40″N104°49′33″W / 39.711189°N 104.825807°W /39.711189; -104.825807). Another example is the interchange betweenMinnesota State Highway 36 and Rice Street at the border ofLittle Canada andRoseville, Minnesota.
A three-level SPUI is similar to a standard SPUI but includes an additional grade separation allowing for free-flowing through traffic on the crossroad;[15][16] such through traffic thus bypasses the signalized intersection. The remaining intersection, free of the crossroad's through traffic, uses atwo-phase traffic signal for handling the left turns in pairs, similar to the traffic flow at an inverted SPUI.
Examples include:
