sfrider 

This is why professional instructors in any area are taught to consistently use positive directions: tell the listener what you DO want them to do. Never tell them not to do what you don't want them to do - at least if you expect an immediate response. It takes longer to parse the double negative and turning it into action is error prone. Mainly because it leaves the recipient to figure out an action that falls outside the negative, and if misheard could be interpreted in the positive. So for instance, never say "don't turn!" - say "continue straight!". Not "don't touch" - but "keep your hands off". If you say "on your left" it could easily be misheard as "left!" meaning you want (in the positive) the person to move left. When someone isn't listening or paying attention, or thinking of something else, it's pretty common to miss the first couple of words if unexpectedly addressed.

This is pretty elementary instructional stuff. There is no basis for "to your left" in any state law, nor is it normally recognized idiomatically among non-cyclists, nor is it particularly good practice. Instead a quick "passing!" works better, or do what the law usually mandates and get a bell.