My dad has been bit by loose dogs a few times. Once he took the brunt of an attacking large dog to save our cocker spaniel from harm (she was leashed). He picked her up (an option I don't think you have, given your large dog), but not in time to save his leg from a nasty bite. Rabies shots are not fun.

When facing an irresponsible owner, who is really the root of the problem, there is not much you can do before an incident. It's not too much fun to keep dogs on a leash when they want to run, but at least you wouldn't be contributing to the problem of loose dogs.