I haven't really figured out the best solution for that situation. One problem with saying "On your left" or whatever ("Passing on your left" is more clear) is that you have to be pretty close for it to work, and if you're going fast and they don't move, you don't have time to dodge. The path I ride on, you can generally go around people if you need to, and that seems the more prudent course of action if you're making any speed.