Traffic control devices like signs and road markings need to be visible both day and night. They rely on brightness and contrast for nighttime visibility. This contrast is achieved by reflecting light back to drivers' eyes, often using special retroreflective materials. These materials return light in a cone towards the light source, ensuring visibility. The amount of light reflected depends on the material's retroreflective properties. Coefficient of retroreflection is a key factor in determining the visibility of traffic control devices.