one ding about 50' out.
if no reaction, 2 dings 25' out.
if still no reaction, "coming around on your left", while giving as wide a berth as possible and slowing down.
about 75% of the time i have good/amiable results. the rest of the time i get the finger/stare down/"f'u" from those that refuse to move. last week was the best: single pedestrian on a paved trail with two dogs off leash and headphones so loud i could hear his music.... *sigh*