Editing Traffic light (section)

== Operation ==
{{Main|Traffic light control and coordination}}
[[File:Traffic Phases and Stages.png|thumb|398x398px|A staging diagram for a typical signalised T-junction]]
A variety of different control systems are used to operate signal cycles smoothly, ranging from simple clockwork mechanisms to sophisticated computerised control systems. Computerised systems are normally [[Actuator|actuated]], i.e. controlled by [[Induction loop|loop detectors]] or other sensors on junction approaches. Area-wide coordination can allow [[green wave]] systems to be set up for vehicles or cycle tracks.<ref name="progr">{{cite web |last=Robinson |first=Larry |title=Traffic Signal Progression |url=http://midimagic.sgc-hosting.com/progreso.htm |access-date=22 May 2014}}</ref> [[Smart traffic light]] systems combine traditional actuation, a wider array of sensors and [[artificial intelligence]] to further improve performance of signal systems.<ref>{{Cite web |date=18 May 2022 |title=Going Nowhere Fast? Smart Traffic Lights Can Help Ease Gridlock |url=https://science.howstuffworks.com/engineering/civil/smart-traffic-lights-news.htm |access-date=4 February 2023 |website=HowStuffWorks |language=en-us}}</ref> A traffic signal junction or crossing is typically controlled by a [[Controller (control theory)|controller]] mounted inside a cabinet nearby.<ref name="ts101">[http://www.dot.state.mn.us/trafficeng/publ/signals101/index.html Traffic Signals 101] {{Webarchive|url=https://web.archive.org/web/20210228141151/http://www.dot.state.mn.us/trafficeng/publ/signals101/index.html |date=28 February 2021 }}, [[Minnesota Department of Transportation]], 2006</ref>

"Phases" (or "signal groups" in Australia and New Zealand) are indications show simultaneously, e.g. multiple green lights which control the same traffic approach. A "movement" is any path through the junction which vehicles or pedestrians are permitted to take, which is "conflicting" if these paths cross one another. A stage (or "phase" in ANZ) is a group of non-conflicting phases which move at the same time. The stages are collectively known as a "cycle".<ref name=":0">{{Cite web |title=Traffic Signal Design Terminology |url=https://www.traffic-signal-design.com/traffic-signal-design-terminology.htm |access-date=2 October 2022 |website=www.traffic-signal-design.com |language=en}}</ref> 

The time between two conflicting green phases is called an "intergreen period", which is set at an appropriate length for the junction to safely clear, especially for turning traffic which may be waiting in the centre of the junction. This often results in an all red stage, when all approaches are shown a red light and no vehicle can proceed. This all red is sometimes extended to allow a [[pedestrian scramble]], where pedestrians can cross the empty junction in any direction all at once.<ref>DfT Traffic Advisory Leaflet 1/06, Part 4 – https://tsrgd.co.uk/pdf/tal/2006/tal-1-06_4.pdf</ref> Some signals have no "all red" phase: the light turns green for cross traffic the instant the other light turns red.{{efn|These are typically older signals. There are many examples in Houston, Texas, of this. Suspended lights constructed so that a single source simultaneously illuminates all four directions always have this characteristic: Red (in two directions) and green (in the two cross directions) with Red-Yellow-Green sequence on two sides and Green-Yellow-Red sequence on the cross sides{{citation needed|date=February 2014}} }}

Many traffic light installations are fitted with vehicle actuation, i.e. detection, to improve the flexibility of traffic systems to respond to varying traffic flows. Detectors come in the form of [[digital sensor]]s fitted to the signal heads or [[induction loop]]s within the road surface. Induction loops are beneficial due to their smaller chance of breakdown, but their simplicity can limit their ability to handle some situations, particularly involving lighter vehicles such as motorcycles or pedal cycles.<ref>{{cite web |date=April 2000 |title=How does a traffic light detect that a car has pulled up and is waiting for the light to change? |url=https://auto.howstuffworks.com/car-driving-safety/safety-regulatory-devices/how-does-a-traffic-light-detect-that-a-car-has-pulled-up-and-is-waiting-for-the-light-to-change.htm |access-date=7 January 2021 |website=Howstuffworks}}</ref> This situation most often occurs at times of day when other traffic is sparse as well as when the small vehicle is coming from a direction that does not have a high volume of traffic.<ref>{{cite web |title=Motorcycles And Stoplights |url=http://motorcycleassistant.com/motorcycles-and-stoplights/ |website=Motorcycle Assistant}}</ref>

=== Timing ===
{{Main|Signal timing}}
[[File:Traffic_light_in_Tehran.jpg|thumb|A traffic light with a timer in [[Tehran]], Iran]]
The timing of the intergreen is usually based on the size of the intersection, which can range from two to five seconds.{{Citation needed|date=February 2023}} Modelling programs include the ability to calculate intergreen times automatically. Intergreen periods are determined by calculating the path distance for every conflict point in the junction, which is the distance travelled to the conflict point by the movement losing right of way minus the distance travelled to the same conflict point by the movement gaining right of way using the possible conflict points (including with pedestrians) and calculating both the time it would take the last vehicle to clear the furthest collision point and the first vehicle from the next stage to arrive at the conflict point. At actuated junctions, integreens can be varied to account for traffic conditions.<ref name=":1" />

Engineers also need to set the amber timings (and red–amber, where appropriate), which is normally standardised by a traffic authority. For example, in the UK, the amber time is fixed nationally at three seconds and the red–amber time at two seconds, which results in a minimum intergreen time of five seconds (plus any all-red time).<ref name=":1" /> The US also uses a minimum of three seconds, but local traffic authorities can make timings longer, especially on wider, suburban roads. This variation has resulted in controversy when municipalities with shorter amber times use red light cameras.<ref>{{Cite web |title=Yellow lights shorter in Chicago |url=https://www.chicagotribune.com/news/ct-xpm-2010-03-22-ct-met-yellow-light-20100322-story.html |access-date=4 February 2023 |website=Chicago Tribune|date=22 March 2010 }}</ref> Where pedestrian signals are used, the timing of the "inivitation to cross" – the period where a steady walk signal shows – and clearance periods – time when the walk signal flashes or no signal is shown – need to be calculated. This is normally set against a design speed, e.g. {{Convert|1.2|m/s|ft/s|abbr=on}}. Similarly, these can be made extendable using sensors, allowing slower-moving pedestrians more time to cross the street.<ref name=":1" />

=== Design guidance ===

National or sub-national highway authorities often issue guidance documents on the specification of traffic signals and design of signalised intersections according to national or local regulations. For example, in the United States the [[Federal Highway Administration]] issues the [[Manual on Uniform Traffic Control Devices]] and the ''Signalized Intersections Information Guide'', which is a synthesis of best practices and treatments to help practitioners make informed decisions.<ref>{{Cite web |title=Signalized Intersections Informational Guide |url=https://safety.fhwa.dot.gov/intersection/signal/fhwasa13027.pdf}}</ref>