A traffic light is a collection of two or more coloured lights found at some junctions and pedestrian crossings which indicates whether it is safe and/or legal to continue across the path of other road users. In the United Kingdom, traffic lights are widely used both on major roads and in built-up areas. Their numbers have increased exponentially since they were first invented in 1868.

History

The first traffic lights actually had their roots in the railway signals used at the time, where two gas lamps, one red and one green, would be alternately hidden by a semaphore arm depending on whether the arm was in a horizontal position or at a 30° angle. The first lights were installed outside the Houses of Parliament in London on 10 December, 1868 to control the increasing number of vehicles there. However, according to some sources, they later exploded and injured the policeman operating them.

The first electric lights were developed in the USA in the early 20th Century. Various people lay claim to the invention of the modern traffic light. These include:

• Lester Wire, a Salt Lake City policeman who set up the first red-green electric traffic lights in 1912.

• James Hoge, from Cleveland, who in 1914 designed some red-green electric lights with a buzzer which sounded when the lights changed.

• William Potts from Detroit, who designed the first three-colour electric traffic lights in 1920.

• John Harriss, a Police Commissioner from New York who developed the first interconnected three-colour electric traffic lights in 1922.

• Garrett Morgan from Cleveland, who in 1923 designed a cross-shaped signalling device which is often mistakenly referred to as the first traffic light.

Once the USA had finished reinventing the traffic light, it was adopted in the UK. The first automatic lights were installed in Princes Square in Wolverhampton. Nowadays, traffic lights are often operated by complex computer software designed to optimise traffic flow.

The Basics of British Traffic Lights

The most basic traffic light consists of three bulbs with different coloured lenses, which from top to bottom are red, amber and green. In the UK, the lights commonly use a sequence of four phases:

1. Red — this indicates that traffic must stop behind the line. It is compulsory for all road users to do so¹. Some traffic lights even have cameras to catch drivers breaking this law.

2. Red and Amber — this combination of bulbs indicates that the lights are about to change to green, and gives drivers time to release their handbrake² and prepare to drive off as soon as they are allowed to do so. This phase was first introduced in 1958.

3. Green — this indicates that traffic may pass through the junction, provided that it is safe to do so and the way is clear. Some junctions are marked with a hash of yellow lines forming a box, which indicates that drivers must not stop on the box unless they are turning right and their exit is clear.

4. Amber — this warns traffic that it should stop unless it is unsafe to do so. In the UK it is legal to pass through an amber light, as the phase exists to warn drivers not yet at the junction that they will have to stop.

Traffic lights at junctions will always follow this pattern, with conflicting flows of traffic being forced to take turns. Often the green bulb is replaced with two or more green arrows or filter lights, which indicate that traffic turning left or right may go, while a red light remains to instruct oncoming traffic to wait. It is now quite common for vehicles turning right to have to wait for a separate filter light, even if the way is clear. Despite being relatively simple, filter arrows are often 'mistaken' for an instruction to go by drivers who want to turn a different way to that shown. Problems are also known to arise from motorists watching the other lights at junctions and anticipating their own movement, and so shades are used to hide the lights from both drivers and from the sun, which would reduce their visibility.

It is interesting to note that the UK is one of only a few countries not to have a 'left on red'³ rule, where cars are allowed to pass through a red light if it is safe to turn left; in the UK, red lights and filter lights must always be obeyed.

A recent improvement in traffic light technology has come with the development of red, amber and green light-emitting diodes (LEDs). Arrays of these tiny bulbs can be used to replace the existing light bulbs in traffic lights and are clearer and more energy-efficient. It is estimated that replacing all the traffic light bulbs in the UK with LEDs would save enough energy to power the city of Norwich.

Pedestrian Crossings

Many junctions also have pedestrian crossings built into them, where red and green signals in the shape of a walking (green) or standing (red) figure indicate to pedestrians whether it is safe to cross. There is also a blank phase where both signals are unlit, indicating that it is still safe to continue crossing but there is not enough time for the average 90-year-old to make it in time if they start now. These crossings often have associated push-buttons for use by pedestrians, but their only apparent action is to display the word WAIT in large, friendly letters. Some of these boxes do, however, have a small knob underneath which revolves when it is safe to cross, which can be useful for the visually impaired. It is important to note that in the UK, although it is not illegal to jaywalk⁴, doing so violates the Highway Code⁵ and those responsible are liable for any resulting accident. Those using pedestrian crossings on side roads have right of way over vehicles once they have begun to cross.

A different sequence to the one mentioned above is used at pelican crossings, where the crossing is not associated with a junction, but is designed purely to allow pedestrians to cross busy roads. The push buttons at these crossings actually stop the traffic after a short delay, and the green figure is often accompanied by a beeping sound. The red and amber phase is replaced by a flashing one, indicating that drivers may continue if there are no pedestrians on the crossing; at the same time the beeping stops and a flashing green figure indicates to pedestrians still waiting to step out onto the crossing that they should wait for the next green man signal to give them right of way. Pedestrians already on the crossing should simply continue to the other side as normal.

Similar crossings are provided for cyclists (toucan crossings) and for horse riders (pegasus crossings). These crossings sometimes feature red and green cycles or horses. Another development on the theme of the pelican crossing is the puffin crossing, where a sensor detects if there are pedestrians on the crossing, making the flashing phase used on pelican crossings obsolete. These crossings do, however, cause confusion, as the red and green men are sited above the push button and not on the opposite side of the road. There are some crossings that do not involve any coloured light sequences. The zebra crossing features a pair of flashing amber Belisha Beacons, while badger crossings do not have any lights at all.

Other Uses

Although their main purpose is to control traffic at junctions and to allow pedestrians to cross safely, traffic lights are used in a variety of situations, including:

• Traffic control at roadworks, where a pair of three-bulb traffic lights have replaced the manual STOP/GO signs⁶.

• Lights at level crossings and drawbridges, where a single steady amber light precedes a pair of flashing red lights indicating that traffic must stop. These are also used to allow emergency services vehicles out of depots on busy roads, and to allow animals to be herded across main roads.

• Lane control on motorways, where white arrows instruct drivers to change lane or leave the motorway, while red crosses indicate closed lanes.

• Lane control on busy roads where the middle lane is used by rush-hour traffic heading one way in the morning and the other in the afternoon. Here, green arrows indicate open lanes and red crosses indicate closed ones.

• As a colour-based system of rating something completely unconnected with driving, where red usually means 'bad' or 'unavailable' and green means 'good' or 'in plentiful supply'. Applications can range from rating the severity of an emergency to use at 'traffic light parties', where the colours give an indication of one's availability to the proposition of a relationship⁷.

• At the cheesy discos of the 1970s, where actual traffic lights were used as disco lights, mostly ignoring the standard sequences.

• In traffic-light jelly.