NEWSFLASH: If a dog attacks a rider there are serious and potentially fatal consequences for the rider!
I've spoken to bikers who ended up getting hit by a vehicle because a dog ran into them while riding!
It is the responsibility of the dog owner to prevent the dog from going on a public road and attacking bike riders. Its that simple.

I have used pepper spray once on a German Shepperd and it worked. But wind and speed may make this response problematic.
There is also the "spray it with Gatorade" solution which I was going to use yesterday when a giant black dog started running after me while on mile 23 of the 31 mile route of Bikefest in Hillsborough, NC....But my bottle top was not screwed on properly I realized as the dog was getting closer

I was about to stop and hit it with my helmet --last resort-- when I decided to yell at the top of my lungs "STAY" "STAY"
And it worked
I am going to buy some pepper spray as soon as I can. I am also going to report to the local PD any "HomeOwner" who leaves their dogs to roam around the street unattended. And I encourage any and all other riders to do so....