#11 wrote: "On your left" is something that runners and cyclists say, but something the general public isn't always used to hearing. That's why they have trouble comprehending what you saying in real time. I can imagine them first being startled by the voice, then have first thoughts like: Oh, someone is coming up behind me; left? what's on my left? or you want me to go to the left?
This is right. I can't tell you how many times I have said, "On your left" only to have the person step briskly to the left and block me. It is just better to not say anything and blow by them. If they are running, it is not usually an issue. If they are walking, I am gone in two seconds. If they are walking a dog, I try to pass on the side that the leash can't reach out and trip me.
I prefer to do ALL the thinking; "bringing them into the conversation" by saying anything is not useful.
As for "scaring" them, that is unlikely unless they obliviously listening to headphones on a busy running trail/bike path. If that is the case, they don't care about others or their own safety anyway.
p.s. sometimes when I try to pass two people (especially who see me coming), they swap sides of the path so by changing positions, they still manage to block me. Oof. It is chaos out there.