Editing Traffic light (section)

== Design ==

=== Bulbs ===
Conventional traffic signal lighting, still common in some areas, uses a standard light bulb. The light then bounces off a mirrored glass or polished aluminium reflector bowl, and out through a polycarbonate plastic or glass signal lens. In some signals, these lenses were cut to include a specific refracting pattern. Traditionally, incandescent and halogen bulbs were used. Because of the low efficiency of light output and a single point of failure (filament burnout), some traffic authorities are choosing to retrofit traffic signals with [[Light-emitting diode|LED]] arrays that consume less power, have increased light output, and last significantly longer.{{Citation needed|date=July 2020}} 

In the event of an individual LED failure, the aspect will still operate albeit with a reduced light output. The light pattern of an LED array can be comparable to the pattern of an incandescent or halogen bulb fitted with a [[prismatic lens]].

The low energy consumption of LED lights can pose a driving risk in some areas during winter. Unlike incandescent and halogen bulbs, which generally get hot enough to melt away any snow that may settle on individual lights, LED displays&nbsp;– using only a fraction of the energy&nbsp;– remain too cool for this to happen.<ref>{{cite web |url=http://www.ctv.ca/CTVNews/Canada/20100110/LED_Snow_100110/ |title=LED traffic lights could pose winter driving risk |publisher=CTV |date=5 October 2011 |access-date=5 October 2011}}{{dead link|date=July 2021|bot=medic}}{{cbignore|bot=medic}}</ref><ref>{{cite web|url=https://abcnews.go.com/GMA/ConsumerNews/led-traffic-lights-unusual-potentially-deadly-winter-problem/story?id=9506449|title=LED Traffic Lights Unusual, Potentially Deadly Winter Problem |author=Elizabeth Leamy |author2=Vanessa Weber |date=4 January 2010 |work=ABC News}}</ref> As a response to the safety concerns, a heating element on the lens was developed.<ref>{{cite web |author=Marmarelli, Beth |title=Engineering Team Develops Device to Aid LED Traffic Signals in Inclement Weather & Places Overall in Campus's Senior Design Competition |publisher=University of Michigan |date=22 June 2011 |url=http://sustainability.umich.edu/news/engineering-team-develops-device-aid-led-traffic-signals-inclement-weather-places-overall-campu |access-date=22 June 2011 |archive-date=13 April 2012 |archive-url=https://web.archive.org/web/20120413011756/http://sustainability.umich.edu/news/engineering-team-develops-device-aid-led-traffic-signals-inclement-weather-places-overall-campu |url-status=dead }}</ref><ref>{{cite web |url=http://www.faqs.org/patents/app/20120255942 |publisher=United States Patent Office |title=Patent application title: Traffic Light Heater |work=Class name: Heating devices combined with diverse-type art device electrical devices, 20120255942 |author=Hankscraft Inc. |date=11 October 2012 | access-date=11 October 2012}}</ref>

=== Programmable visibility signals ===
{{Multiple image
| align             = right
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| image1            = First and Mill 3Ms img14.jpg{{!}}3M traffic signals in Shelton, Washington, as seen off-axis from the intended viewing area. These signals appear to be "off" or invisible to adjacent lanes of traffic during the daytime. Only a faint glow can be seen when viewed at night.
| image2            = First and Mill 3Ms img07.jpg{{!}}3M traffic signals in Shelton, Washington, as seen from the signal's intended viewing area. Special light-diffusing optics and a coloured Fresnel lens create the indication
| footer            = Traffic signals installed in [[Shelton, Washington]], seen off-axis from the intended viewing area (''top'') and from the signal's intended viewing area (''bottom'').{{pb}}From off-axis, these signals appear to be "off" or invisible to adjacent lanes of traffic during the daytime. Only a faint glow can be seen when viewed at night.
}}

Signals such as the 3M High Visibility Signal utilize light-diffusing optics and a [[Fresnel lens]] to create the signal indication. The light from a 150 W PAR46 sealed-beam lamp in these "programmable visibility" signals passes through a set of two glass lenses at the back of the signal. The first lens, a frosted glass diffusing lens, diffuses the light into a uniform ball of light around five inches in diameter. The light then passes through a nearly identical lens known as an optical limiter (3M's definition of the lens itself), also known as a "programming lens", also five inches in diameter.{{Citation needed|date=July 2020}}

Using a special aluminium foil-based adhesive tape, these signals are "masked" or programmed by the programming lens so that only certain lanes of traffic will view the indication. At the front of these programmable visibility signals is a 12" Fresnel lens, each lens tinted to meet United States [[Institute of Transportation Engineers]] (ITE) chromaticity and luminance standards. The Fresnel lens collimates the light output created by the lamp and creates a uniform display of light for the lane in which it is intended.

In addition to being positioned and mounted for desired visibility for their respective traffic, some traffic lights are also aimed, louvered, or shaded to minimize misinterpretation from other lanes. For example, a Fresnel lens on an adjacent through-lane signal may be aimed to prevent left-turning traffic from anticipating its own green arrow. Intelight Inc. manufactures a programmable traffic signal that uses a software-controlled LED array and electronics to steer the light beam toward the desired approach.<ref>{{cite web|url=http://intelight-its.com/product/esb-signals/item/34-intelight-esb-traffic-signal-head.html|title=Intelight ESB Traffic Signal Head|work=intelight-its.com|access-date=21 June 2013|archive-url=https://web.archive.org/web/20140808220251/http://intelight-its.com/product/esb-signals/item/34-intelight-esb-traffic-signal-head.html|archive-date=8 August 2014|url-status=dead}}</ref> 

The signal is programmed unlike the 3M and McCain models. It requires a connection to a laptop or smartphone with the manufacturer's software installed. Connections can be made directly with a direct-serial interface kit, or wirelessly with a radio kit over WIFI to the signal. In addition to aiming, Fresnel lenses, and louvers, visors and back panels are also useful in areas where sunlight would diminish the contrast and visibility of a signal face. Typical applications for these signals were skewed intersections, specific multi-lane control, left-turn pocket signals, or other areas where complex traffic situations existed.

[[File:Colors of a Stoplight- Animated- Lewiston.gif|thumb|right|An animated GIF shows a traffic light in 3 reverted phases: red, yellow, then green]]

=== Size ===
In the United States, traffic lights are currently designed with lights approximately {{convert|12|in|-1}} in diameter. Previously the standard had been {{convert|8|in|-1}}; however, those are slowly being phased out in favour of the larger and more visible 12&nbsp;inch lights. Variations used have also included a hybrid design, which had one or more 12&nbsp;inch lights along with one or more lights of {{convert|8|in|-1}} on the same light.

In the United Kingdom, 12-inch lights were implemented only with Mellor Design Signal heads designed by [[David Mellor (designer)|David Mellor]]. These were designed for symbolic optics to compensate for the light loss caused by the symbol. Following a study sponsored by the UK Highways Agency and completed by Aston University, Birmingham, UK, an enhanced optical design was introduced in the mid-1990s. 

Criticism{{by whom|date=February 2024}} of sunlight ''washout'' (cannot see the illuminated signal due to sunlight falling on it), and ''sun-phantom'' (signal appearing to be illuminated even when not due to sunlight reflecting from the parabolic mirror at low sun angles), led to the design of a signal that used lenslets to focus light from a traditional incandescent bulb through apertures in a matt black front mask. This cured both problems in an easily manufactured solution. This design proved successful and was taken into production by a number of traffic signal manufacturers through the engineering designs of Dr. Mark Aston, working firstly at the SIRA Ltd in Kent, and latterly as an independent optical designer.{{Citation needed|date=December 2008}}

The manufacturers took a licence for the generic design from the Highways Agency, with Dr. Aston engineering a unique solution for each manufacturer. Producing both bulb and LED versions of the signal aspects, these signals are still the most common type of traffic light on UK roads. With the invention of anti-phantom, highly visible Aston lenses, lights of {{convert|8|in|mm|-1}} could be designed to give the same output as plain lenses, so a larger surface area was unnecessary. Consequently, lights of {{convert|12|in|-1}} are no longer approved for use in the UK and all lights installed on new installations have to be {{convert|200|mm|0|abbr=on}} in accordance with TSRGD (Traffic Signs Regulations and General Directions). Exemptions are made for temporary or replacement signals.{{Citation needed|date=December 2008}}

=== Mounting and placement ===
<gallery mode="nolines" widths="160">
File:Part time traffic lights on the Pialligo Avenue.jpg|Part time pole/pedestal-mounted traffic lights in [[Canberra]], Australia
File:Traffic lights and vehicular transportation in Ekiti State. 05.jpg|Mast-arm traffic lights in [[Ekiti State]], Nigeria
File:Taiyuan Road shops and pedestrian overpass 20100622.jpg|Horizontal traffic lights mounted on a footbridge in [[Taipei]], Taiwan
File:Dummy Light.jpg|A dummy light in [[Canajoharie (village), New York|Canajoharie]], New York. It was removed in 2021.<ref name="RECORDER">[https://www.recordernews.com/news/local-news/203892 '''''The Recorder''''', ''Village of Canajoharie to permanently move historic dummy light out of Wagner Square'', Shenandoah Briere, August 10, 2022] {{Webarchive|url=https://web.archive.org/web/20230710165750/https://www.recordernews.com/news/local-news/203892 |date=10 July 2023 }}, Retrieved Jul. 10, 2023.</ref>
</gallery>

The MUTCD identifies five types of traffic light mounts. On pedestals, signal heads are mounted on a single pole. This is the normal installation method for the UK.<ref name=":9">{{Cite book |last=Buckholz |first=Jeffrey W. |url=https://www.cedengineering.com/userfiles/Traffic%20Signal%20Supports%20-R1.pdf |title=Traffic Signal Supports, Indications and Signing |publisher=CED Engineering |location=Woodcliff Lake, NJ}}</ref><ref name=":1" /> On mast arms, signal heads are mounted on a rigid arm over the road protuding from the pole. On strained poles, signals are suspended over a roadway on a wire, attached to poles at opposite kerbs. This is the most common installation method in the United States. Unipoles are similar to strain poles, but a single structure over the road, rather than two poles linked with wire. Signals can be attached to existing structures such as an overpass.<ref name=":9" /> Dummy lights are traffic signs located in the centre of a junction, which operate on a fixed cycle. These have generally been decommissioned due to safety concerns. A number remain due to historic value.<ref>{{Cite web |title=A Dummy Forever! |url=https://www.crotonfriendsofhistory.org/dummy-forever |access-date=18 December 2021 |website=Croton Friends of History |language=en-US}}</ref>

Signals can either be placed nearside – between the stop line and the kerbline of the intersecting road – or farside – on the opposite side of the junction. In European countries, signals are often placed on the nearside.<ref name=":10">{{Cite web |date=30 April 2021 |title=Near Side Signals: Thinking Outside the Pedestrian Box |url=https://streets.mn/2021/04/30/thinking-outside-the-pedestrian-box/ |access-date=4 February 2023 |website=Streets.mn |language=en-US}}</ref> In the UK, at least two signal heads are required, known as the primary and secondary heads, one of which is normally nearside and the other of which could be nearside or farside.<ref name=":1" /> In the US, signals are normally located farside, though in some states, nearside signals are also used. Nearside signals can be beneficial to road safety, as drivers have more time to see a red light and are less likely to encroach on pedestrian crossings.<ref name=":10" />