Interesting that the reply mentions road signs and emergency vehicle retroreflective markings... Road signs are (normally) made of a composite of plastic core with thin aluminium 'skin' each side - of course not retroreflective in the slightest, then the sign face is made from retroreflective self-adhesive plastic (often 3M film). I guess the argument could be made that the 'sign' is made from the 3M film, with the composite board just a support, but the same could be said about a number plate...
And then emergency vehicle markings are the same 3M film applied to a steel body shell...
And then emergency vehicle markings are the same 3M film applied to a steel body shell...