It's seems that these days 1/2 or more of the people on the MUP's around here are wearing ear bud's so calling out that I am passing on the left whether they have a dog or not is futile. I do however call out, and if they have to get startled as a pass by them, well, that's their problem (unfortunately if I were to run into somebody or some animal because they weren't paying attention it would become my problem as well). I really think it would be helpful if there were some signs posted on MUP's encouraging people to stay to the right if not passing and to keep in mind that there will be faster moving traffic. Most of the MUP's I ride on are pretty wide so people could stay to the right and still walk side by side no problem, unfortunately many people salmon all over the place completely oblivious to the world around them.