I've owned large dogs and medium sized dogs - the mediums were Pits or in the family of Pits (AmStaffs, American Bull Dogs) so people have a certain reaction to them without the dogs ever acting agressive in the first place. I never let my dogs run where there are people around. At the first sign that a person has arrived - the leashes go on. I trust my dogs, but one can never tell, and they may interpret something as an attack that isn't.

You might consider mace - like postal delivery people carry. If someone has their dog off leash and the dog is charging you - to my mind they can hardly complain if you defend yourself in a non-lethal manner. Better that than you getting bit and the dog getting the blue juice.