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Traffic Signals

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Traffic Signals
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Traffic signals direct the flow of traffic, normally at road junctions and pedestrian crossings, but also at railway level crossings, airfields, opening bridges, emergency vehicle stations and tunnels. Their chief purpose is to minimise congestion and improve road safety, and they have a key advantage over other traffic-engineering solutions, such as grade separation, in being comparatively cheap to install and requiring little additional landtake. Traffic signals have been commonplace in the UK and Ireland since the first modern-style automatic system was introduced in Princes Square, Wolverhampton, in November 1927.


Early Three Light Systems

Modern traffic signals evolved from the coloured-light indications which accompanied railway semaphore signalling, introduced in the 19th century. Red was used to indicate "stop" as it was already widely known to represent danger.

There was a trial of three aspect colour light traffic systems in 1926 in Piccadilly, London. This comprised of 8 signals regulating traffic along Piccadilly from east and west between Piccadilly Circus and the Ritz Hotel, and from the north by Berkeley Street, Dover Street, Albemarle Street and Old Bond Street, and from the south by Arlington Street and St James’s Street. It was provided by the Westinghouse Brake and Saxby Signals Co. Ltd. of Chippenham and authorised in February 1926. The lights were red for “halt”, green for “proceed” and yellow as a warning and “be prepared for a change”.

They were reported by the Daily News London of 4 August 1926 as having started “yesterday”. They were controlled by a policeman in a glass fronted grey cabin at the corner of St James’ Street, Piccadilly by pressing a series of buttons. The switchboard had a plan of the street and junctions with the traffic signals shown and it operated on the same system as the Metropolitan Railway. It had resulted in speeding up traffic, although traffic volumes were lower in August.

A report in the Exeter and Plymouth Gazette on 12 August 1926 stated that some alterations were needed. The cabin was to be replaced by a tower in Piccadilly so that the operator could see the traffic. A better method of showing the policeman what lights were on was also needed. The police needed to look through a small hole at the back of the lights to see which light was on. In sunshine it was difficult seeing any light at all. It was mentioned that the lights were only being operated in daytime since at night there was insufficient traffic to warrant their use. The trial was extended in April 1927 for a further six months.

Automated Traffic Lights

A set of vintage SGE signals on Euston Road, London, in the 1960s

Meanwhile, the first automatic traffic light trials in the UK took place in Wolverhampton, starting on 5 November 1927 in Princes Square. These earliest lights were a three-light system mounted on wires high above the junction, and had a backup police officer stationed underneath the lights during the initial trials. These lights were then replaced with a permanent set from October 1928, still in the centre of the junction but more conventionally situated on a pole. These signals lasted in place until 1968.

Following the successful experiments in Wolverhampton, further installations were trialled across the country, with an early set being recorded in Leeds on 17 February 1928, again being a three-light system. A further trial of automatic signals at some busy junctions in London was agreed in November 1928 with just red and green lights, changing every 30 seconds. Other towns and cities rapidly adopted traffic signals before they became widespread across the United Kingdom.

Traffic signals were regarded as something of a necessary evil during the early to mid 20th century, but by the 1960s, the need to install more signals at pedestrian crossings was becoming more urgent. The first attempt at this was the Panda Crossing, introduced by then transport minister Ernest Marples in 1962, but this suffered criticism for being over-complicated and confusing to users. Consequently, a new design, the now familiar Pelican Crossing, appeared in 1969.

Toucan Crossings followed in the 1990s, for pedestrians and cyclists, so called because "two can cross".

In the 2000s, a new type of pedestrian controlled signal, the Puffin Crossing started to be installed in favour of Pelican Crossings. The principal difference between the two is that the control and the signal is on the same unit facing the user. Also includes on-crossing detectors to reduce unnecessary delays.

In May 2015, Queens Circus Roundabout in Battersea was reconfigured to include a fully signalised cycle route with special cycle aspect signal heads mounted at cyclists eye level. These heads have subsequently been rolled out on some of London's cycle superhighways, where examples can be seen on Farringdon Street.

Display sequence


In the United Kingdom, Guernsey, the Isle of Man and Gibraltar – as well as in Austria, Belarus, Bosnia and Herzegovina, Bulgaria, Croatia, Czechia, Denmark, Estonia, Finland, Germany, Hungary, Iceland, Latvia, Liechtenstein, Lithuania, Malta, Moldova, Montenegro, North Macedonia, Norway, Poland, most of Russia, Serbia, Slovakia, Slovenia, Sweden, Switzerland, and Ukraine – a standard four-period signal sequence is used consisting of the consecutive display of green (proceed if it is safe to do so), amber (stop, unless it is not safe to do so), red (stop), and red-and-amber lights (stop, but prepare to proceed). In Jersey and the Republic of Ireland – as well as other European countries such as France and the Netherlands – the red-and-amber phase is omitted, the lights changing straight from red back to green.

Additionally, in Austria, Belarus, Estonia, Latvia, Lithuania, Montenegro, Russia, Turkey, Ukraine, and parts of Serbia, the green light will flash for 3 seconds before changing to an amber light.

The standard sequence became so well known following its introduction that by the end of the 1920s, when light-controlled pedestrian crossings were being installed in Manchester, the phases were not documented as it was assumed the public would understand them. The Metropolitan Police, in endorsing the design, pointed out that the simple colour-light scheme could be understood without any knowledge of English, and would therefore be of benefit to foreign drivers.


A traffic signal installation comprises various pieces of equipment to make it appear and function as intended. Many of these parts are common for permanent, temporary and portable traffic signals, though different combinations of equipment and technologies may be used that are specific to each type.

Traffic Signal Controller

Main Article: Traffic Signal Controller

The Traffic Signal Controller is the component responsible for operating the traffic signals themselves. In its most basic form, the controller is a simple piece of electrical circuitry that operates the signals in a predetermined sequence at set times. Modern controllers are more akin to computers, and allow for complex detection systems to be utilised. There is typically one controller for each set of traffic signals, though it is possible for a controller to operate several separate sets of signals when they are in close proximity, for example at a roundabout or at two closely-spaced junctions.

Signal Head

Main Article: Traffic Signals - Signal Heads

The signal head is the name given to the unit containing the lights, or aspects, at a set of traffic signals. These may also be referred to as lanterns. Basic signal heads include red, amber and green aspects for vehicular traffic, and red and green figures for pedestrian traffic. The aspects may include symbols such as arrows or pedal cycles, and they may also include signage to convey a certain restriction on the movement of traffic.

Push Button Unit (PBU)

Main Article: Push Button Units
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An old PBU, of a type commonly found in the UK
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The 'Push Button Unit', sometimes also called a Pedestrian Demand Unit (PDU), is located on the signal pole or an auxiliary pole and has the purpose of allowing a crossing user to request the pedestrian or pedal cycle phase at traffic signals. Although historically many fully automated crossings omitted these (e.g. where a pedestrian phase is timed in with existing traffic phases), they are now considered a requirement where pedestrian facilities are included at a set of signals.

The PBU is typically constructed of a housing which includes a push button and a light to indicate that the request has been received by the traffic signal controller. It may also include instructions on how to use the crossing, which may be presented in a simple text format or include diagrams to aid understanding. In the UK, the light indicating that a request has been received is generally either a red rectangle, circle or ring, or alternatively is the word 'WAIT' which illuminates in white or amber.

PBUs regularly house additional pedestrian aids such as audible bleepers and rotating tactile cones for crossing users who are hard of sight. Particularly spurred on by the response to COVID-19, hands-free facilities have also become common in some areas, with the user typically registering a request by waving their hand below the PBU, or in some cases, by using an app.

Nearside Indicators

Born out of the advent of the Puffin Crossing, nearside indicators are units similar in appearance to a PBU, but instead housing the red and green pedestrian, cycle or equestrian aspects. This locates the signals on the same side of the road as the crossing user waiting to cross, and intentionally prevents them from being viewed by crossing users already on the carriageway. They are typically mounted above the PBU, or in some cases are mounted within the same unit as the push button.

Nearside indicators display either red and green pedestrian symbols, red and green pedestrian and cycle symbols, or red and green horse rider and horse symbols.

Early nearside indicators used halogen lamps that were in-keeping with the signal heads of the day, however they were quick to move to LED illumination technology. Nearside indicators were one of the first wide-spread adoptions of LED technology within the UK traffic signal market.


Main Article: Detection

Traffic signal controllers typically use a variety of detectors to influence their decision making on how or when to change the signals. Inductive loop detection is one of the most common forms of detector, where a loop of wire is placed within the carriageway that detects the presence of vehicles. Other common forms of detection include magnetometer, microwave, infra-red and video.


Traffic signal equipment is typically mounted on poles at the roadside. In the UK and Ireland, these poles are typically four metres in length and made of steel, either galvanised or coated with PVC in either grey or black. Aluminium poles are becoming increasingly common. Passively safe poles for use on high speed roads are typically manufactured from aluminium or glass fibre composite material.

In addition to standard poles, short poles that are typically two metres in length may be used to mount pedestrian equipment. Tall poles are often also used to raise signal heads or detection equipment above the usual height, and these typically come in six or eight metre heights, though other heights may also be used.

Mast Arms and Gantries may also be used to suspend signal heads above the carriageway. These are not generally considered a standard feature within traffic signal design in the UK or Ireland, however they have increased in numbers particularly where carriageways have been widened significantly or junctions have become complex.

In West Yorkshire, UK, there are examples of traffic signals suspended above the carriageway using tensioned steel wire rope, something not typically seen in the UK or Ireland, but common in parts of Continental Europe and North America.

Ducting System

To support the electrical infrastructure associated with traffic signals, it is normal for an installation to include a network of underground pipes called ducts and access chambers, similar to manholes, which allow for the routing and inspection of cables between the traffic signal controller and other equipment mounted to poles or other structures.

Many older traffic signal installations did not make use of ducting at all and cables were simply laid and buried in the ground. Cables installed in this way are more prone to damage, and finding faults and working on cables becomes difficult, often involving excavation which is considered undesirable. As a result, it is now considered standard practice to install a ducting network around a traffic signals installation.

Modern ducting equipment is typically manufactured from plastics, however older techniques included clay pipes, brick chambers and iron chamber covers, more akin to drainage systems.

It is now also commonplace to see the use of retention sockets. These metal devices connect directly to the duct network and allow a pole or structure to be installed and secured without it being permanently buried in the ground. Initially developed for use in London where traffic signal equipment must be regularly removed for events, retention sockets are generally considered to have other useful benefits in situations where equipment must be removed and reinstated in a timely manner, such as following a collision or to allow the passage of oversized loads, which would have previously involved excavation of poles buried footways. They are also generally considered to assist in the construction of new traffic signal installations as the retention sockets can be installed with the rest of the ducting system, allowing the poles to follow later. As a result of these benefits, retention sockets are now considered a standard in most areas.

Portable Traffic Signals

Portable variants of traffic signals may be used where temporary traffic management measures are required, such as where roadworks are occurring.

Main Article: Portable Traffic Signals (PTS)


Main Article: Traffic Signals - Signal Heads

SGE were one of the most significant manufacturers of traffic signals from the 1930s onwards. Their design could generally be identified by the text "STOP" on the red signal, and completely separate housing for each lamp. Another major manufacturer at the time was ATE/ATM, who also began installing signals in the 1930s.

Traffic signals took on a more modern appearance with the introduction of the Mellor head from 1970, and this rapidly replaced the SGE design, including a complete retrofit on pelican crossings in 1973. A key advantage of the Mellor design was that it allowed easy installation of fixed-phase timing, whereas previous installations tended to be vehicle activated. Mellor heads were commonplace until the 2000s, when newer signal heads with modular designs started to replace them.


Traffic Signals
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