While bicycles don't have horns, I do have some decent lungs, and every once and a while I'll yell *HEY* loudly at a car driver to get their attention before they do something stupid. It works often enough to make it worthwhile, and even when it doesn't, it makes me feel better.
On Saturday, a van driver totally didn't see me, started to pull out, and I yelled as loudly as I could. In this case, however, it didn't have the desired effect on the driver, as I was sitting in the passenger seat of our 328. It did, however, have an effect on my wife, who was able to take appropriate evasive action.