Did you know that stop signs used to be yellow back in the 1920s, even though red had already become known as the color for "stop" in traffic lights? The convention in science wasn't yet established, and there were two major problems with using red for stop signs. In this blog post, we'll explore the history and science behind the color choices for stop signs and how an ingenious invention called retroreflection revolutionized their visibility.
The Challenges with Red:
Red posed two main challenges for stop signs: the lack of a durable red pigment and its poor visibility at night. In the early 20th century, there wasn't a way to make a red pigment that wouldn't fade over time. Additionally, red is difficult to see in low light conditions. Our eyes have three types of cones that detect color, including red, green, and blue cones. However, at night, when there's not enough light to activate these cones, our eyes rely on rods, which are more sensitive to low light. Interestingly, rods don't detect red wavelengths, meaning red is the hardest color to see in low light.
The Importance of Visibility:
Not only is red less visible at night, but even in daylight, our eyes are most sensitive to a yellow-greenish color that falls between the red and green cone sensitivities. This color, peaking at around 550 nanometers, offers optimal visibility. While this yellowish hue would be ideal for stop signs, it lacked consistency with the convention of using red to signify "stop."
To overcome these challenges, an ingenious invention called retroreflection came into play. The concept of retroreflection involves using specialized materials that reflect light back toward its source. 3M, a company renowned for innovation, developed retroreflective materials that revolutionized the visibility of road signs.
The Science Behind Retroreflection:
Retroreflection works by utilizing glass beads or cube retroreflectors. When light enters glass, it bends due to its different refractive index. By embedding glass beads or cube retroreflectors in a material, such as a sign, the light that hits them reflects back toward the source at the same angle. This ensures that the light from car headlights or other light sources is directed back to the driver, making the sign highly visible even in low light conditions.
3M's retroreflective materials are used in various applications, including signs, road markings, and even clothing. These materials contain numerous retroreflectors per square centimeter, which bounce the light back toward the source. For sign production, the retroreflective material is applied to a metal backing or used as a sticker. The signs are either made by cutting the retroreflective sheeting to size and applying it or using digital printing with water-based inks to create the desired designs.
Overcoming Rain Visibility Issues:
To tackle the problem of reduced visibility in rainy conditions, 3M developed retroreflective elements with different refractive indices. These elements ensure that even when water is on top of the retroreflective surface, the light is still reflected back toward the driver. By using a combination of elements with varying refractive indices, road signs can maintain their visibility in both dry and wet conditions.
The transition from yellow stop signs to the familiar red ones we see today was not only influenced by the convention of using red in traffic lights but also driven by the need for improved visibility. The invention of retroreflection by 3M revolutionized the visibility of road signs, making them highly visible both day and night. Through the clever use of specialized materials and principles of optics, retroreflective signs have made our roads safer for everyone around.