Seagull intersection

A seagull intersection[1] or continuous green T-intersection[2] (also known as a turbo-T[3] (in Florida) or High-T intersection (in Nevada and Utah)[4][5]) is a type of three-way road intersection, usually used on high traffic volume roads and dual carriageways. This form of intersection is popular in Australia and New Zealand, and sometimes used in the United States and other countries.

Sketch in right-hand traffic layout

Design
Seagull intersection on Broadway in Everett, Washington, United States (47.9649°N 122.201003°W / 47.9649; -122.201003)

Seagull intersections get their name from the pattern that the two cross-traffic turn lanes make when looking down at them from the air.

In a seagull intersection, one or more lanes of traffic on the arterial road, on the carriageway opposite the intersecting side road, are free flowing, that is, one direction of traffic on the arterial is allowed to travel straight through without stopping. The free-flowing lane(s) are called "continuous green through lane(s)" (CGTL).[6] For the free-flowing through lanes, access into and out of the side road is provided via turn lanes separated from the through lanes in a configuration similar to exit and entrance ramps at an interchange; however, the turn lanes are at the same grade as both carriageways of the arterial and are located on the same side as the oncoming traffic.

Those wishing to turn across traffic into the side road at the intersection drive into the turn lane, which forms one "wing" of the seagull. Here, they meet the opposite carriageway and the side road. Traffic wishing to turn across traffic out of the side road, cross the intersecting carriageway, drive up the other "wing" of the seagull, and merge onto the other carriageway.[7]

For the arterial carriageway adjacent to the side road, access into and out of the side road is ordinarily handled like a conventional T intersection. However, some seagull intersections may have a second, smaller "seagull" formed by two left turning lanes into and out of the side road.

Road sign used in Florida when a continuous green through lane is ahead.[8]

Different methods are used to control traffic where two right-turning movements and the through movement meet. Most intersections use traffic lights, while others use give way (yield) and stop signs, and sometimes roundabouts.

This design type has been proven to provide sustainable benefits when compared the traditional T-intersection design. By reducing delay through the intersection, automobiles use less fuel on average passing through the intersection, and thus emissions are reduced across the intersection. The savings per vehicle may not seem very significant, but when scaled to account for all automobiles passing through the intersection, the total emission savings are significant. Additionally, there are economic and social benefits to reducing delay time and allowing drivers to pass through the intersection quicker.[9] In the Netherlands, this type of intersection occurs by default when a T-junction has a bicycle path on the continuing road, and is not intersected by a roadway for motorized vehicles. The bicycle path may ignore red lights; it is only turning cyclists that must wait for the red light.[10]

History
Note: This section refers to countries driving on the right. For countries that drive on the left, reverse right and left.

An experiment was done in Illinois, United States to allow going straight on red (following rules analogous to those used for turns on red) when approaching a T junction on the main road, with the intersecting road on the left. It was a failure. However, at some T junctions where the main road includes at least two lanes on the side away from the intersecting road, the farthest (rightmost, in areas where traffic drives to the right) lane is given the right of way to proceed straight through the intersection at all times, denoted by a "green arrow" signal if a traffic light is installed at the intersection. In such cases, often that lane is also specially delimited with pavement markings or other lane separation devices, to keep left-turning traffic on the intersecting road from colliding with traffic proceeding through the intersection on the main road. The seagull intersection was developed as a safer variation of this.[3]

References
  1. John Harper, Wal Smart, Michael de Roos: Seagull Intersection Layout. Island Point Road – A Case Study Archived 2013-05-07 at the Wayback Machine 2000 – 2010
  2. Federal Highway Administration: Alternative Intersections/Interchanges: Informational Report (AIIR), Publication Number: FHWA-HRT-09-060, April 2010
  3. Jonathan Reid, P.E. (July 2004). "Unconventional arterial intersection design, management and operations strategies" (PDF) (PDF). Parsons Brinckerhoff. Retrieved September 9, 2012.
  4. Utah DOT: "5400 South". Archived from the original on 2013-12-13. Retrieved 2013-08-30.
  5. Nevada DOT: US93 Lakeshore v4 Handouts
  6. https://www.fhwa.dot.gov/publications/research/safety/16036/16036.pdf
  7. Intersection Decision Guide, Indiana Department of Transportation, Version 1.1 published 14 January 2014, retrieved 25 August 2015, P. 44
  8. Edward S. Jarem: Safety and Operational Characteristics of Continuous Green Through Lanes at Signalized Intersections in Florida, Lake Mary, Florida 2004
  9. Litsas, Stephen (July 31, 2012). "Evaluation of Continuous Green T-Intersections on Isolated Under-Saturated Four-Lane Highways" (PDF). TRB.org. Retrieved September 19, 2013.
  10. Cycling past red lights in the Netherlands, YouTube, published 24 October 2012
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