Like Sundy, I MUST use sunglasses to drive because I am photosensitive. (The rest of the time, my regular glasses can generally compensate--the lenses have been treated with silver chloride and darken in sunlight, but this doesn't work in the car.) I don't usually use them as a tool to 'block people out,' though--how does that work? I can see them exactly the same with my sunglasses or without them and I feel the same, too, since I'm always wearing glasses (unless I want to be blind). Eye contact is easily solved by looking at people's mouth when talking (necessary, since I don't always have good listening comprehension), and I don't see how sunglasses prevent those annoying people from coming up and talking to you.
Besides, my sunglasses are very expensive, because I need prescription sunglasses, and I really don't want to run the risk of losing them.
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cogito, ergo sum.
non cogitas, ergo non es.