My rule is that it's always ok to drive 1-9 miles per hour over the speed limit (average about 5 or so) depending on where you are.  On the expressway, make it 10-15 depending on the circumstances. You know, if you're in the middle of some no-name place with no cars, why not 15 over?

But yes, I have no patience for tail-gaters, can't stand them.  I also have a problem with people who use their horns constantly. 

A horn should be used sparingly, like a strong spice.