I live in Wisconsin. I tend to warn "on your left" anytime I am passing people and only in the case of either extremely small children or pokemon go players for whom this is their first excursion in the outdoors, most trailgoers here know what it means, and squeeze right a little more (especially if it is two people walking, or people with dogs).

If there is a small child, or dog with unaware owner, or pokemongo player on the trail, I will slow down and be vocal (give dog owner time to control the dog).

The other thing I do is if I'm riding with anyone, sometimes a group, or just one other person... I will say "ON YOUR LEFT"... and then "there are two of us" (or four of us... or whatever) as I pass by the person, usually holding out my hand with the number of bikers in my group.