Editing Single-point urban interchange (section)

==Description==
[[File:Florida SR 4080 at Valencia College Lane.jpg|thumb|upright=0.75|right|A typical freeway-over SPUI. This example, near [[Orlando, Florida|Orlando]], has since been demolished. ]]
[[File:Taylor Street single-point urban interchange on CA-87.jpg|thumb|upright=1.5|[[California State Route 87]] at Taylor Street, a freeway-under SPUI]]

A SPUI 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.<ref>{{cite journal |author= Anonymous |title= Urban interchange moves more traffic in same space; a design new to California improves congested diamond interchanges without taking more high-priced real estate |journal= Highway & Heavy Construction |volume= 132 |issue= 8 |date=July 1989 |pages= 52–53 |issn= 0362-0506}}</ref>

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 light|traffic signal]]s. 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.<ref>{{cite web |author= St. Louis District |publisher= [[Missouri Department of Transportation]] |url= http://www.modot.org/stlouis/links/SinglePointUrbanInterchanges.htm |title= Single Point Urban Interchanges |access-date= July 15, 2012 |archive-date= July 1, 2017 |archive-url= https://web.archive.org/web/20170701201331/http://www.modot.org/stlouis/links/SinglePointUrbanInterchanges.htm |url-status= dead }}</ref>{{POV inline|date=June 2023}}<!-- Some would see this as a bad design due to the ped and bike problems; is this actually popular or just in certain places? -->

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 road]]s. Since most through traffic travels over or under the intersection, the SPUI is still much more efficient than a surface intersection.

===Advantages===
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.<ref name=bonneson>{{cite journal |last= Bonneson |first= James A. |date=January 1993 |title= Bridge Size and Clearance Time of Single Point Urban Interchange |journal= Journal of Transportation Engineering |publisher= American Society of Civil Engineers |volume= 119 |issue=1 |pages=77–93 |doi=10.1061/(ASCE)0733-947X(1993)119:1(77) |issn= 0733-947X}}</ref>  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 full [[cloverleaf interchange]], allowing construction to take place on a limited amount of property and minimizing state use of [[eminent domain]].<ref>{{cite web |author= St. Louis District |publisher= Missouri Department of Transportation |url= http://www.modot.org/stlouis/links/SPUIFrequentlyAskedQuestions.htm |title= Single Point Urban Interchange Frequently Asked Questions |access-date= July 15, 2012 |archive-date= July 23, 2012 |archive-url= https://web.archive.org/web/20120723034644/http://www.modot.org/stlouis/links/SPUIFrequentlyAskedQuestions.htm |url-status= live }}</ref>

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.<ref>{{cite journal |first1= Joe |last1= Bared |first2= Alvin |last2= Powell |first3= Evangelos |last3= Kaisar |first4= Ramanujan |last4= Jagannathan |date=May 2005 |title= Crash Comparison of Single Point and Tight Diamond Interchanges |journal= Journal of Transportation Engineering |publisher= American Society of Civil Engineers |volume= 131 |issue= 5 |pages= 379–381 |doi= 10.1061/(ASCE)0733-947X(2005)131:5(379)  |s2cid= 110625815 |issn= 0733-947X}}</ref>

===Disadvantages===
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 of [[97 Street, Edmonton|97 Street]], having seven throughlanes, with [[Alberta Highway 16|Yellowhead Trail]] in [[Edmonton]], 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.{{Cn|date=October 2022}}

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 the ''righthand'' side.{{Cn|date=October 2022}}

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.<ref name=bonneson/> 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.<ref name="kcbike.info">{{cite web |first1= Mohammad |last1= Qureshi |first2= Navin |last2= Sugathan |first3= Rohit |last3= Lasod |first4= Gary |last4= Spring |title= Design of Single Point Urban Interchanges, Research Investigation 02-015 |publisher= Missouri Department of Transportation Research, Development, and Technology, University of Missouri-Rolla |date= September 2004 |url= https://library.modot.mo.gov/RDT/reports/Ri02015/BrfApr05.pdf |access-date= February 10, 2010 |archive-date= May 30, 2010 |archive-url= https://web.archive.org/web/20100530115607/http://library.modot.mo.gov/RDT/reports/Ri02015/BrfApr05.pdf |url-status= live }}</ref>

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.<ref name="kcbike.info"/>

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.

[[Traffic light#Rules|Three-phase traffic signals]] are required. Other interchange types designed for efficiency, such as the [[partial cloverleaf interchange#A4|six-ramp partial cloverleaf]] and the [[diverging diamond interchange|diverging diamond]], require just two signal phases.