Early on in my dog "career" I was involved in Schutzhund. (Google it and you'll see what it is about.) Part of the sport is protection work. As opposed to what most think, dogs are trained to do protection work off of prey drive as opposed to fear drives. Dogs trained off of fear drive are extremely dangerous dogs. The old "junk yard dog" type of thing.
I've done the protection work. You dress up in padded bib overalls, with a sleeve that has heavy plastic piping (so the dog doesn't break your arm) that is covered with burlap. That's at the advanced levels. At beginner levels, you actually just use a towel.
The only time that I was truly afraid was when I slipped and I was face to face with a dog that I knew was trained to bite. It certainly does get your heart pounding. The dog hit the sleeve and didn't think of anything else.
In the 10 years, or so, that I was involved in the sport, none of the dogs that we trained were ever in a dangerous situation.
That being said.
A woman friend with a highly trained (obedience) Rottweiler. She was out at a beach by herself. A bunch of teenage males came toward her thinking about hassling her. She called her dog over. Grabbed his collar. Told the dog in a VERY LOUD voice, "NO BITE". The teens couldn't get out of there quick enough. I'm sure her dog looked at her thinking what the heck are you talking about.
This is a tip that I've always given to my students.
Most people are extremely frightened of dogs and don't want to hang around long enough to actually see if the dog is protection trained.
Added - UHave2be. I can't find the article. Many years ago, there was a California police officer who was "attacked" by a group of small dogs. They were the size of Pomeranians. Owner got fined. I'm sure that the officer took a LOT of grief for a while.