My collie is not ferocious, but she is big and has a very loud deep bark. Someone came to the door the other day and I opened the inside door but not the storm door. I just said "I'm sorry I can't open this door because she is not leashed." I was holding her collar and she was barking up a storm. He left in a hurry.
About signs that say Beware of Dog: I was always told that indicates you already know you have a bad dog and if someone gets bitten you can get sued. But instead put signs that say "Guard Dog on Duty" which indicates it's a good dog, but trained to be a guard dog and only doing his job if someone comes on the property.