Utility meter readers are real experts in dealing with aggressive dogs. The meter readers around here carry a repellant spray called Halt, which is made for dogs. My bike shop sells this stuff.

Halt has a belt clip that can be attached to brake or shifter cables right by your hands for easy access. Its cheap protection that really works. It produces effective, but temporary, discomfort to the dog. My daughter's mailman sprayed her feisty terrier recently, and he came home yelping; an hour later, he was back to normal.