Why are traffic signs and signal lights around the world predominantly certain specific colors? Has it always been this way? The answer to these question involves both history and science.
Colors of Road Signs
The scientific reason is obvious. It is based on the visibility of the different colors used in traffic signal devices. According to an article in RoadTrafficSigns.com, red can be seen the farthest distance of any color because it has the longest wavelength. In good light, it jumps out at drivers and demands their immediate attention. Its bloody hue puts a driver in mind of danger and produces a subconscious sense of warning. Red traffic signs and lights, in essence, issue a command: Stop. Yield. Do Not Enter.
That said, stop signs in fact used to be yellow with black lettering, because it was difficult to see a red sign in poor lighting before the invention of reflective aluminum for road signs. The stop sign officially became red with white lettering in the United States in 1961. This made its color association consistent with that used for traffic lights and train signals.
Yellow or orange signs are the next easiest to see. These are used to warn drivers of hazards, but they do not issue commands. Road signs in these colors are divided into two types, temporary cautions and permanent ones.
Temporary (orange) caution signs are used to warn travelers of road construction or other temporary changes to the route of travel. Permanent (yellow) cautions include school crossing signs or pedestrian crossing signs to indicate hazards that drivers should always look out for.
Other colors of road signs include green, blue, brown and black and white. These are purely informational, and their colors are soothing, blending in with the shades of nature, so as to not distract drivers.
Green signs typically give suggestions about the route ahead. They include highway exit signs appearing to the side of a highway or lane signs appearing above a highway, which tell drivers which lane to get in if they want to travel to particular destinations.
Blue signs provide drivers information about amenities and services, such as the locations of restaurants or truck stops. Brown signs indicate picnic areas or parks. Black and white signs indicate regulatory information, such as signs announcing speed limits or railroad crossings.
Was visibility always the primary consideration for the colors of road signs and traffic lighting? Sadly, no. In the early days of interstate road construction, highways and roads were often constructed and maintained by automobile clubs, which would design their own signage with no consideration for visibility or consistency with other clubs’ signage at all.
Colors of Traffic Lights
The first traffic light, which had to be manually operated, was put up outside the Parliament buildings in London in 1868.
We got our colors for traffic lights from the red and green lights used for trains. A red train light meant “stop.” Green train lights, however, meant “decrease speed,” while white lights signified “go ahead.”
According to one story, a train engineer confused the white train lights with stars in the night sky and so kept his train running through something it should have stopped for. At this point, green lights supposedly became the signal for a train to proceed, and red lights remained the stop signal. From then on, these were the only colors used for trains. When automobiles were invented, the train signals’ use was repurposed to the new mode of transportation, with yellow lights (called amber in the UK), being used as the signal for drivers to decrease their speed.
One more interesting note–The Japanese language, among others, does not distinguish between blue and green, using the single base word ao for them. They consider their traffic lights to be blue. To maintain a standard system with other countries, they made their traffic lights the bluest shade of green that they could obtain.
There is no single international standard regarding placement of the colors or orientation of the lights, though some groups of countries have agreed to international traffic standard conventions. The United States uses the Manual on Uniform Traffic Control Devices (MUTCD) as its guide. Traffic signals in the southern US are often placed horizontally to reduce wind resistance during hurricanes. People driving on hills must be able to see the red light first, so they can know to start breaking as they crest a hill. Thus, the red light was placed above the other two colors in the US.
In some northern countries, however, as explained by Karen Hill of Super Beefy.com, the red light is placed at the bottom of the signal so that the red light can still be seen even if snow accumulating on the traffic light hoods obscures the colors above it.