Lately I've been calling 'rider up!" when I approach someone from behind on a MUT. I have found this much more effective than 'on your left'. Saying 'on your left' seems to confuse too many people. 'Rider up' lets them know I'm there, then they decide how to move, and I can react accordingly. So far I've never seen that moment of hesitation we all know. Most of the time I pass on the left anyway. Give it a try & see if it works for you.....