Claire, if a dog continues to go after cyclist aggressively I will report it to the local animal control office. They will need info including address or location, details of the incident(s), and MOST IMPORTANTLY whether the dog came out into the travelled portion of the roadway in chasing and/or attempting to bite the cyclist.
This extra effort is not only important for your own safety, but has an impact on the potential safety of other riders that pedal past that location. I have reported two dogs. One owner now has his dog behind a fence, while the other dog still chases cyclists. The next time I get chased by that dog I will be making my third compliant. This builds a record of non-compliance and gives animal control grounds for taking more serious action against the owner. Or it could be the good ole boy network in action? I plan to stick with it and go to a supervisor if necessary or my local County Commissioner.
In the meantime practice using HALT, so when you need it you're ready. A key factor with any spray weapon is wind variables. You don't want to fire a burst of HALT at a dog about to bite your ankle, only to have that spray blow back in your own face!!
Oxon Hill Bicycle and Trail ClubP.O. Box 81 Oxon Hill, MD 20750-0081