The other day, my DH and I were riding along a country road. There was a man up ahead with his back to us, very slowly walking his big German Shepard on the edge of the road.
Well of course we wanted him to know we were coming so that in case his dog wanted to lunge or chase us, he would be forewarned (you know how dogs love to chase/attack bikes!). And we don't like to startle pedestrians from behind as we pass them.
My DH was ahead of me, and called out "hello" ahead of time. The man turned towards him and asked "Yes?" but made no attempt to shorten the leash or anything... by that time my DH had passed him. I was now approaching him on the road, he seemed to be looking in my direction, but that's when I saw that he was blind, and his dog was a guide dog. I rung my bell, and he said to his dog "It's a bicycle".
Naturally we were in no danger from his well trained dog.