High visibility clothing is clothing made of fluorescent material with added reflective tape. The most popular color for high visibility clothing is fluorescent yellow and fluorescent orange. What is the reason?
The sun's ultraviolet rays react with the fluorescent colors to make them appear to "glow" increasing daytime visibility. The effect is stronger in poor light conditions such as in fog or towards dusk.
At night, light from sources such as car headlights bounces off the reflective areas to make the reflective tape glow and increase night time visibility.
Some reflective materials are retro-reflective which means the light bounces back to its source. If a car shines it’s headlights on retro-reflective material, most of the light is reflected back to the driver.
Other reflective materials are made using glass bead technology which scatters the light, some of it getting reflected back to the driver. High visibility products can include either type.
The main thing to remember is: fluorescent for daytime visibility, reflective for night.