If there are lots of other people around, I wouldn't mind a guy approaching me (except at a library where you have to remain quiet). If I'm alone at a bus-stop, late at night, don't even come close to me! It's okay to approach me once we get on the bus, though, if you're friendly.
(I do have a boyfriend already, but I'm happy to talk.)
(I do have a boyfriend already, but I'm happy to talk.)