Jesus that must be a hairy moment. Folding door mirrors at 50 with a HGV coming at you. Personally I would just use the extra time it took to fold the mirrors to brake lmao.
In response to your question I have no idea, never folded them in over 5mph but would be an interesting test. If you haven't got a response by the morning I'll try it on the way to work and let you know.