You will never know how fast you can really go on your bike until you are chased by an angry Rottweiler who thinks he is defending his territory. From personal experience I can tell that having a dog chase you is the highest form of motivation.
I always carry Halt! Dog Repellent with me on night rides, off-road trails, and when I am checking out a new route for the first time. I started carrying Halt! several years ago after two small dogs refused to allow me to get past them on a bike trail. It finally took a well-placed stone the size of a softball for the mutts to back off and let me pass.
Halt! contains capsaicin (chili-peppers) and shoots out a pressurized spray that has an effective range of about 10 feet. This product has been used by the U.S. Postal Service for 35 years and works well.
A couple of years ago I was on a bike trail when a dog came out of nowhere and was very intent on sinking his teeth into my right ankle. I pulled out the Halt! and just as I was about to spray it the dog apparently recognized the can and ran away. I can only imagine that somewhere along the way that dog had a mailman introduce him to Halt! and he decided not to try it again.
On off-road night rides I run into a lot of varmints (both four-legged and two-legged) and I always feel safer knowing I have a can of Halt! in my jersey. I purchased Halt! off of Amazon.com for under $4.50 a can.