Traffic lights, also known as traffic signals, traffic lamps, traffic semaphore, signal lights, stop lights, robots (in South Africa), and (in technical parlance) traffic control signals, are signalling devices positioned at road intersections, pedestrian crossings, and other locations to control flows of traffic.
The world's first, manually operated gas-lit traffic signal was short lived. Installed in London in December 1868, it exploded less than a month later, injuring or killing its policeman operator. Traffic control started to seem necessary in the late 1890s and Earnest Sirrine from Chicago patented the first automated traffic control system in 1910. It used the words "STOP" and "PROCEED", although neither word lit up.
On the 5th of November 1927 Britain’s first automatic traffic lights were launched at a busy junction in Wolverhampton, a large industrial town at the heart of the Black Country. They now appear at almost every major junction throughout the UK and are thought to both alleviate and cause traffic delays.
Traffic lights alternate the right of way accorded to users by displaying lights of a standard color (red, amber (yellow), and green) following a universal color code. In the typical sequence of color phases:
- The green light allows traffic to proceed in the direction denoted, if it is safe to do so and there is room on the other side of the intersection.
- The amber (yellow) light warns that the signal is about to change to red. In a number of countries – among them the United Kingdom – a phase during which red and yellow are displayed together indicates that the signal is about to change to green. Actions required by drivers on a yellow light vary, with some jurisdictions requiring drivers to stop if it is safe to do so, and others allowing drivers to go through the intersection if safe to do so.
- A flashing amber indication is a warning signal. In the United Kingdom, a flashing amber light is used only at pelican crossings, in place of the combined red–amber signal, and indicates that drivers may pass if no pedestrians are on the crossing.
- The red signal prohibits any traffic from proceeding.
- A flashing red indication is treated as a stop sign.
In some countries traffic signals, will go into a flashing mode if the controller detects a problem, such as a program that tries to display green lights to conflicting traffic. The signal may display flashing yellow to the main road and flashing red to the side road, or flashing red in all directions. Flashing operation can also be used during times of day when traffic is light, such as late at night.
Before traffic lights traffic police controlled the flow of traffic, a well-documented example being that on London Bridge in 1722. Three men were given the task of directing traffic coming in and out of either London or Southwark. Each officer would help direct traffic coming out of Southwark into London and he made sure all traffic stayed on the west end of the bridge. A second officer would direct traffic on the east end of the bridge to control the flow of people leaving London and going into Southwark.
The European approach to a signalized crossing is to use dual or, more rarely, a triple aspect with a blackened out lens of a pictogram pedestrian. For cyclists, the same approach is used, with the lens blackened out for a bicycle frame. It is not uncommon to see lenses with both symbols on them. Most European countries use orange instead of yellow for the middle light.
The light sequence is:
- Green: safe to cross.
- Yellow or orange: continue to cross only if unable to stop safely.
- Flashing yellow or orange: cross with caution (often used when lights are out of order or shut down).
- Red: do not cross.
In Germany, the Czech Republic and some other Central European countries, a combination of red and orange lights is used just before the switch back to green. It allows drivers to stop their engines during the red light.
- Green: safe to cross.
- Orange: continue to cross only if unable to stop safely.
- Flashing orange: cross with caution, obey signage (used when lights are out of order or shut down).
- Red: do not cross.
- Red and orange: do not cross, prepare for green.
The light is blackened out with a pedestrian pictogram.
Ampelmännchen pedestrian traffic signals have come to be a nostalgic sign for the former German Democratic Republic.