I also use "passing on your left" when overtaking someone. I ride a MUP daily and try to get there when there aren't too many people using it. During the week, the MUP is not very crowded but on weekends, it's pretty much packed with runners, walkers, cyclists and rollerbladers. It's also when many families go with their children. I have noticed that using "on your left" doesn't register with many of the family riders, but putting the word "passing" into the phrase gets their attention and they almost always check to make sure their children are out of the way. What's unfortunate, is that many of the riders (roadies, mostly) at this MUP seem to think that the $2.00 parking fee comes complete with ESP because they don't say or do anything to let you know they are passing or that they are even there. Several people have been injured because of this but it doesn't change the cyclist attitude about letting others know their intent to pass.