If your usual route has a dog that regularly goes after you, consider bringing one of your water bottles filled with vinegar. In most cases a quick shot of vinegar to the face will stop the dog in it's tracks without hurting anybody (dog included). Just make sure you remember which bottle is which or you might be in for an unpleasant surprise.
Alternatively if you don't have a regular dog, your normal water bottle might help deter the dog, if not your best bet is to get away quickly, if you can't out run the dog, dismount and keep the bike between you and the dog. Yell at the dog as loudly and forcefully as you can to get home. Many dogs will respond to this, but if nothing else the owner will probably hear and come get their dog back in line.
A helmet nod goes out to the One-armed Ironman for the tip.
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