Traffic lights are usually red, yellow, and green because these colors are easily recognizable by drivers and pedestrians. Red signals to stop, yellow warns of the impending red light, and green permits traffic to proceed.
From the age of the first railway lines in the 19th century, English engineers were already using colored lights as signals. The red, a color long associated with danger, was immediately chosen to tell drivers: "Stop! It’s risky!" Initially, white was even used to signify that all was clear, but several confusions due to visibility (or stray white light) quickly led to a switch to green, which was clearer and more distinct at night. Yellow appeared a little later as an intermediate color to clearly warn: "caution, get ready, it's about to change!" It was ultimately in the United States, at the beginning of the 20th century, that this trio of colors was standardized on the roads after proving its usefulness on the rails.
Red is universally perceived as the symbol of danger or urgency. Its ability to immediately attract attention makes it the ideal candidate for the stop message: impossible or nearly impossible to ignore, as this color instinctively provokes alertness and caution.
In contrast, green conveys safety and permission. It reassures, it naturally calms, indicating that the way is clear and that one can proceed without stress.
Yellow, on the other hand, clearly evokes warning. An ambivalent color, it signals an imminent change. Neither entirely reassuring nor completely alarming, it naturally encourages one to slow down and pay attention to what is about to follow.
Our eyes capture certain colors much better than others, especially from a distance or in poor weather. Red, for example, stands out really well in fog or light rain because its wavelength is long and it passes through the atmosphere more easily. It's a color that immediately grabs our attention — perfect for saying "stop! DANGER." Conversely, green is calming, clearly visible in normal conditions but a bit less striking when the weather is bad or dark. As for yellow, it is ideal for alerting us quickly because it is a bright hue that the human eye spots quickly and easily, even in lower light conditions. The choice of these colors also ensures that even those who suffer from certain vision disorders like color blindness usually recognize the differences well, especially between the bright light of the top signal (red) and that of the bottom signal (green).
The use of red, yellow, and green as standard traffic light colors primarily stems from a desire for clear and universally understood uniformity around the world. The Vienna Convention on Road Signs and Signals in 1968 established these colors, ensuring that international drivers do not get lost when changing countries. Thanks to this international harmonization, red always means stop, yellow prepares or warns, and green indicates the right to proceed. This consistency avoids dangerous confusion, simplifies driving, and enhances safety for everyone.
LED bulbs (light-emitting diodes) have seriously changed the game in recent years. With their brighter luminosity, they make lights more visible, even in bright sunlight or bad weather. Another advantage is that they allow colors to be more vivid and accurate, avoiding uncertain shades. Since they consume less energy and last longer than traditional bulbs, they also save money while reducing unexpected outages. As a result, red, yellow, and green stand out better and remain reliable longer, thus reinforcing their universal use. Some recent research is even experimenting with smart lighting capable of adjusting the brightness of colors according to weather conditions or ambient light.
Some countries use horizontal traffic lights rather than vertical ones, for example in Canada, to better cope with harsh weather conditions, particularly in areas exposed to strong winds or snowstorms.
The average duration of a yellow light is adjusted according to the recommended speed limits on a given road, in order to provide drivers with enough time to safely stop before the red light.
The colors red, yellow, and green were chosen particularly for their ease of distinction in cases of low visibility or for individuals with color perception anomalies (color blindness).
The first modern electric traffic lights were installed in 1914 in Cleveland, United States. Before that, police officers manually regulated traffic using gestures and whistles.
The duration of each traffic light color largely depends on the location, traffic conditions, and local regulations. However, in general, the green light lasts between 30 and 90 seconds, the yellow light lasts around 3 to 5 seconds, and the red light usually varies according to the specific traffic regulation needs of the intersection in question.
Here is the translation: "Lights ensure better visibility from both far and near, during the day and at night, and can be easily distinguished by drivers in all conditions. Unlike sounds, they do not cause any confusion in noisy environments or for individuals with hearing impairments. Symbols alone may lack clarity or be difficult to spot quickly."
Red means that the vehicle must stop immediately. Yellow (or orange) indicates that the light will soon turn red and signals to drivers to slow down or stop if they can do so safely. Finally, green clearly indicates that the vehicle may proceed freely.
Generally, countries around the world uniformly use red, yellow, and green to ensure international consistency. However, there are a few rare exceptions for certain specific signals (for example, blue is sometimes used in Japan for green shades), but these remain very marginal and maintain a visual resemblance to traditional green.
The red light is positioned at the top to be immediately visible, even if other lights are blocked or poorly seen by certain vehicles. This standardized position allows for quick intuitive recognition, which is essential for ensuring road safety.
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