Driving around town, you might have wondered why the colours used in traffic lights are red, yellow and green. The R-Y-G (Red-Yellow-Green) and not any other colours like blue, purple, or white.
You already know that in traffic regulations red means “stop” and green means “go”. Yellow that comes in between red and green simply means get “ready to go” or simply “caution”.
As to why the R-Y-G colours are used in traffic lights, some have posited that it has something to do with wavelengths of the colours. There is a scientific backing to this theory which Naijauto will discuss further down in this article.
Interestingly, the first sets of traffic signals to ever be used were made for trains. However, at that time, they were only red and green and were powered by gas.
Road traffic light was adopted from railroad crossing lights
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1. Red color has the longest wavelength
As it is, the colour red is always used to symbolize danger in many instances. Additionally, red has the longest wavelength in the spectrum of visible light (R-O-Y-G-B-I-V). This means it can easily be seen even from a far distance when compared to other colours on the spectrum.
Red can easily be seen even from a far distance when compared to other colours
Either way, red has always been used as a stop signal way before cars came into existence. Its use as a train stop signal dates as far back as when a mechanical arm lifted or lowered to show whether the rail ahead of the train was clear or not.
2. Green intially meant "caution" but changed to "go"
Just like the red light signal was earlier used on railroads, so also was the green signal. However, the meaning of the green light signal in traffic regulations had changed from what it was initially.
In the earlier railway days, green light meant “caution” while the white light meant “go” but today, green means “go”. On the spectrum of visible light, green is the next colour of longer wavelength after yellow (R-O-Y-G-B-I-V) which we will talk about later in this article.
Green was changed its meaning to "go" after a serious mistake
As to why the original meaning of green light was changed from “caution” to “go,” it was rumoured that a train engineer once mistook stars in a night horizon as the white light which initially meant “go”, resulting in several train collisions.
So, to avoid such mistakes in future, the initial green light which meant “caution” was specified to mean “go” - which was later adopted in road traffic regulations.
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3. Yellow is almost as visible as red
As early as before 1900, all stop signs weren’t red, some were actually yellow. Reason being that it was difficult to spot a red stop sign in an unlit area.
However, with advancement in material science and technology today, it is now possible to produce highly illuminated and very reflective signs. Thus, bringing back red to its natural position as stop signal colour.
Leaving the next very visible colour after red, this is yellow as “caution”. More specifically, yellow is only second to red in terms of visible wavelength or ability to be seen.
Yellow is the next most easy to be seen colour after red hence it is used as "caution"
Just as traffic lights are red, yellow and green, that is the same reason school children crossing and crosswalks are usually yellow and not any other colour.
In summary, we have the set of traffic light colors as today because red is the colour with the longest wavelength on the spectrum of visible light, followed by yellow and green respectively and are easy to see in that order.