A total stranger in NYC decided to help us out when I was pushing my husband's wheelchair across the street towards the ubiquitous clump of ramp-standers on the sidewalk. They were ignoring my polite "Excuse me" but when he bellowed "WATCH YER BACK!" they parted like the sea before Moses. It wasn't just his volume or accent, suddenly it was about THEM and their safety! I've used it ever since, and it never fails. I make no visible attempt to stop or slow (while of course being alert for helpless old people or small children) and use plenty of volume and urgency because, after all, I'm warning them of danger.

And I do the accent. It's part of the magic.