I'm amazed at how self centered we are sometimes. On you left really means, Oh Please pay attention to me, I'm going soooo fast and you need to let me by. I think On Your Left has more to do with the lack of skills on the part of the person doing the passing than it does with the parties being passed. On your left is dangerous because in a high percentage of cases it causes the person being passed to move to the left. You'd be much better off saying nothing and just passing or if you feel the need to announce your presence the just say Passing. I ride mostly on a MUP. Half the people I pass are wearing headphones and can't hear a thing I say anyways. Sometimes I have to slow to a crawl and just wait for the right time to pass, that's life, we have to share the path/road