In general, people in a public space are to an extent consenting to interact with other humans. If they aren't, we have a system of recognized signals for it: Walking fast, looking downward, listening to music, reading, etc. I don't think you should feel too guilty about imposing a brief few seconds of interaction on people out and about in public.
You know the drill - If it's worth saying, but not worth its own post (even in Discussion), then it goes here.