Zinc sulfide is used in road signs because when light from cars hit the road signs, it glows. What actually is the reason for the glowing of road signs (actually glowing of $\ce{ZnS}$)?
Answer
In the Wikipedia article Phosphorescent paint,
Phosphorescent paint is commonly called "glow-in-the-dark" paint. It is made from phosphors such as silver-activated zinc sulfide or doped strontium aluminate, and typically glows a pale green to greenish-blue color. The mechanism for producing light is similar to that of fluorescent paint
The process is summarised in the article Fluorescence vs. Phosphorescence, The explanation from the article:
Phosphorescent materials produce light in a similar way as does fluorescence materials. A visible difference between these two types of luminescence, the ability of phosphorescence materials to glow after the excitation energy source is removed.
The website has a diagram exhibiting the process (in a bit of a simplistic, but understandable way - just replace the 'UV' with 'car headlights etc'):
Essentially, photons are emitted from every step back from the excited state to the ground state, unlike fluorescence, where the photon is emitted when the electron goes straight to the ground state.
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