There are people who simply do not have the balls to make their own rules (speed in this case), so they take it upon themselves to hold others back. They live bitter lives, and resist friendly courtesy if it isn't required by law. What is so hard about letting someone make their own choices, and face their own consequences? Seeing someone with more courage than themselves, and facing that reality.
It is easy to be irritated by them, but what they really deserve is our pity.