Has anyone mentioned how far the sound of a whistle will carry in the woods, anyway?
I think the usefulness depends on a few things, like where you are, for instance. Where I go, as an example, it's heavily wooded and generally you have to stay on the trail. So if you need help, it's going to come from someone who is ahead of you and getting further away or from someone behind you who's going to be passing by sooner or later anyway. I'm not saying blowing a whistle will make any difference but that's for when you're out there by yourself.
But if you're out there with a group, large or small, a whistle is probably a good idea unless you've all brought along walkie-talkies. Either way, I've never heard any kind of signal in the woods, whistle, gunshot or siren, unless you count shouting. Some people seem to shout a lot.
At home, it might be more useful, oddly enough. I live in a densely wooded suburban neighborhood. Lots of folks go walking in the woods, with or without a dog. The paths are all fairly near houses. Except for technical rock climbing, there is nearly every kind of terrain within a mile of the back door of my house as there is 75 miles from home on the Appalachian Trail. So, in theory, the same kinds of mishaps could happen. In such an event, a whistle might be even more useful than it would in the woods because of the close proximity of habitations, provided anyone is actually outside, which is problematic. In fact, recently a woman died on such a walk, walking her dog. But she was 73 and she died of a stroke, I believe. Her dog remained with her. She wasn't found for hours.