Here in Chicago, most of the time "on your left" works on the paths. A few cases it doesn't... Foreign tourists, esp Japan. They tend to walk on their "normal" side and when you call out anything behind them, they just crowd closer to your left. Probably due to driving side habits.
The "ooooh, what a big city, golly them buildings is tall" people. They slowly meander all over the path. I've lost track of the number of times I have come up on a group like this who may be more left of center, call out "on your left" and it seems 9 out of 10 times, they will look on their RIGHT to see what is coming. I'm on the brakes and will wait until they clear the left side, in a slightly worthless attempt to show them what side to be on. And I ALWAYS say "thanks" as I pass and don't condescend. Goodwill in small doses.