Why Are Stoplights Red, Yellow, and Green?

Why Are Stoplights Red, Yellow, and Green?
Stoplights are everywhere. They tell us when to go, pause, or stop. But why are stoplights red, yellow, and green? Let’s find out!

Why Are Stoplights Red, Yellow, and Green?

In the 1830’s, railroads were being constructed at a rapid pace and it became apparent that traffic signals were needed. Engineers began brainstorming and came up with a three color light system: white meant go, green meant caution, and red meant stop. Due to its wavelengths, red can be seen at a farther distance than any other color on the spectrum, which is why it has historically been used to signify danger. Green is said to have been chosen because it is easily distinguished from red.

This system worked fairly well until train conductors started running into issues with the white “go” signal. In one accident, a red lens fell out, exposing the white light behind it. This led to a train running a “stop” signal, and crashing into another train.

After these incidents started piling up it was clear that a change in colors was necessary. They slightly altered the system so that green signified go and yellow meant caution, since yellow is the second most visible color on the spectrum. Interestingly, for a while, yellow was the chosen color for stop signs since red proved difficult to see in low lighting. Once reflective technology came out, however, stop signs were painted red, but yellow can still be found on school buses and warning signs.

The First Lights for Road Traffic

There were many inventions all over the world that took place before the international traffic light system that’s used today. One such invention was created in London 1865 due to concern over horse-drawn traffic interfering with pedestrians. A man named John Peake Knight sought to solve this issue with the use of a semaphore system.

A semaphore is a structure with colored arms that would be lowered or raised by a police officer to control traffic. At nighttime, the semaphore would flash either a red or green light instead of using the arms. This system was implemented in London and worked well for about a month. One day, however, one of the gas lines that supplied the lights began to leak. The policeman operating the device was unaware of the leak and was severely burned after the lamp exploded on him, ending the semaphore system.

In the U.S. during the early 20th century, there was a slightly different system in place. Watchtowers had been constructed all over the country for policeman as they used their whistles to guide the flow of traffic. In 1920, a policeman named William L. Potts sought change and invented the three color light system using red, yellow, and green. Detroit was the first city in history to use these lights to control traffic.

What Came Before the System We Have Now?

Inventors all over the country continued to create different mechanisms for road traffic since the three color light system had not yet made its way to all of the states.  The majority of these designs required someone to push a button or flip a switch to change the color of the light which made them very costly.

In the late 1920’s, automatic signals were invented and allowed the lights to change at different time intervals. This meant, however, that some cars would be stopped even when there were no other cars in sight. To counter this, one inventor came up with a signal that could detect a car honking its horn. A microphone was mounted on a pole at the intersection that would change the light when a car honked. To prevent people from honking continuously, they created a ten second wait interval after each light change. This was obviously quite annoying for anyone within hearing range, and was considered one of the less favorable stoplight inventions.

Having different systems proved to be a major problem for cars traveling through multiple areas. It was possible to encounter several systems in a single drive which caused a lot of frustration. In 1935, the Federal Highway Administration created The Manual on Uniform Traffic Control Devices. This manual set a standard for all traffic signals, road signs, and pavement markings that we see today.

How the System Works Today

Today, these traffic systems sometimes use lasers or rubber tubes, but the most common is the inductive loop method. This method requires grooves to be cut in the roadway at the stop line of traffic lights. It’s a common belief that these grooves sense the weight of a vehicle, but actually there is a loop that detects change in the magnetic field. When a car enters the field the inductance rises and tells system there’s a car present.

Red, yellow, and green have become universal for stop, pause and go, and are meant to be visible in all types of weather. Today, this system helps create order and safety on increasingly busy roads.

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