I don't think anyone expects the police to show up fast enough to protect from an imminent dog attack. The point is to report the owner to the police and hopefully that will prevent such a situation in the future.

It does matter where you live. I've never been hiking on public land anywhere in the country -- local, state or national -- where dogs were not required to be on leash. Whether the requirement is enforced is another matter. In any event, people are criminally and civilly liable for their dogs attacking people.

And in my experience, private landowners have been even more strict about leashes because they've had livestock.