I'd be all in favor of "on your left" or other overtaking signals as a standard practice, if I thought they did anything to enhance safety. I don't believe they do. As others have reported, the phrase "on your left" seems to confuse most MUPpets: they grind to a halt, spin around, wobble, do everything but maintain a steady course (if they were already over on the right) or get to the right if they weren't there already. So, when you're dealing with the confused and inattentive, it doesn't seem to help much, and when you're dealing with the clued-in and attentive, it's unnecessary and annoying: "On your left, on your left, on your left, yabbayabbayabbayabbayabba," SHUT UP ALREADY.