there was a discussion about this on the Portland subreddit a year or two ago, and some people noticed that people (including themselves) tend to step or veer in the direction that's called out. I know I've done that. one time, though, the person coming up behind me called out, "coming up behind you," or something similar. I thought that was kind of a cool idea, just say you're coming and let the person who needs to move decide which way to go.