The sign doesn't necessarily mean that the dogs are dnagerous. Maybe they want people to think they are. My dogs aren't dangerous, but the mobile home park that I live in requires people to put up a "beware of dog" sign if they have dogs over 30lbs (which all of my dogs are). So I had to put up a sign.
As far as the barking, if the dog's not lunging at you or anything, I would just ignore it. Hope this helps.