The basic principle of “Retroreflection” – When the light is incident into three mirror surfaces that orthogonal oriented, it reflects three times and then, return to the original direction

Reflex reflectors provide drivers visual guidance and help ensuring the safe and smooth flow of traffic, especially at night. On the top of that, reflex reflectors are energy saving products which function only using reflection of light from other light resources. Thus, in the current automobile society in which there are increased safety and environmental concerns, reflex reflectors have increasing importance

(※further information - https://en.wikipedia.org/wiki/Retroreflector)