Traffic signals represent one of the most effective and universally understood communication systems in the modern world. Every driver and pedestrian immediately understands the meaning of red, yellow, and green, which collectively orchestrate the flow of billions of vehicles daily. The success of this system relies entirely on the instant, unambiguous recognition of each color in all conditions, from bright sun to foggy rain. The color chosen for “go” is a deliberate choice rooted in both historical precedent and the biological mechanics of human sight.
The Standardization Story
Traffic control signals originated in the 19th-century railway industry. Early railway signaling used a system where red indicated danger or stop, green meant caution, and white light signified a clear track or “go.” This system proved dangerous because a broken lens could expose the clear white light behind it, mistakenly signaling a clear track and leading to catastrophic accidents.
Following several such incidents, railroad companies standardized their signals by the 1890s, removing the problematic white light. Green was promoted to mean “all clear” or “go,” and yellow was introduced as the intermediate caution signal. When early electric road traffic lights were installed in the United States in the 1910s, they adopted this standardized railroad color scheme. The Federal Highway Administration later mandated this uniform system in its 1935 Manual on Uniform Traffic Control Devices, establishing red, yellow, and green as the national standard.
The Physics of Visibility
Green was chosen for “go” due to its exceptional visibility to the human eye, particularly in daylight. The human retina contains specialized photoreceptor cells called cones, which are responsible for color perception. Of the three types of cones, the medium-wavelength cones (M-cones) are maximally sensitive to light in the yellow-green region of the visible spectrum.
This peak sensitivity occurs at approximately 550 nanometers (nm), which is near the color of a typical green traffic light. Actual traffic signal green is often shifted toward the blue end of the spectrum (520 to 522 nm) to ensure it is distinct from the yellow light. This high sensitivity means the eye requires less energy to register the color, making the green signal appear brighter and more noticeable. The color choice ensures maximum contrast, allowing drivers to perceive the signal clearly from a greater distance and in adverse conditions.
The Modern Green
The earliest traffic lights utilized incandescent bulbs paired with colored glass lenses to produce the required hues. This method was inefficient because the bulb produced light across the entire visible spectrum, and the glass filter blocked most of it to isolate the desired color. These older bulbs were also power-intensive, typically requiring around 175 watts of electricity.
Modern traffic signals have universally transitioned to Light Emitting Diode (LED) technology, which significantly enhances visibility and efficiency. LEDs are semiconductor devices that emit light at a much narrower, more focused band of wavelengths. This allows for a purer, more precise green color that aligns better with the eye’s peak sensitivity. The shift to LEDs has drastically reduced power consumption, requiring only about 10 to 25 watts, while extending the operating lifespan to over 100,000 hours.