Correct. As a long time roadie, I can tell you that you will never be able to use it before you're down. You just have to accept the disparity of power.
There are no high noon style confrontations; the people who would use a car or truck to harass a cyclist are the same types that will light the tail of a puppy on fire--they do it b/c they can get away with it. They aren't interested in letting you have a chance.
Dogs? If you're harassed by a dog (more accurately, by a poor dog-owner), and you know whose house owns the dog, call the sheriff. Do that repeatedly and the issue should take care of itself. I had problems with a dog owner who would stand and watch as his dogs repeatedly took off after me and my wife as we toodled by. I told him one day that if his dogs bit us on a public road that I would own his house. That ended that.
Backwoods is different. I have a friend who takes a light .357 ("made to be carried much and fired little") when he goes overnight camping using his mtn bike.