Why does a pure white patch of snow on a white speed limit sign appear pitch black in oncoming headlights?

Question

I know this has something to do with the reflective nature of the speed limit sign, but can someone please give me the direct cause and effect that creates this phenomenon?

Answer 1

Signs designed to be seen at night are coated with special reflective materials consisting of very tiny plastic spheres that reflect light back to the source. While snow is white, meaning it reflects light well, it scatters the reflected light in all directions. This means that much less reflected light reaches you from snow than from the special reflective material so the snow appears much darker than the rest of the sign.

Signs that are not intended for night use, such as school crossing signs, are typically not specially coated. Look for one at night and you will see how dark it is and what a difference the coating can make.

Answer 2

The snow does not form a perfectly flat reflective surface on the sign. The shape of the snow crystals causes the light to reflect off of it in several different directions and not just straight back to your eyes. When little or no light is reflected back to your eyes, everything will appear black. In physics, black is the absence of all colors of light.