Flashing lights at a slow-moving driver is the generally accepted way of asking them to move over amongst the driving cognoscenti. Getting mad at it would be the same as getting mad at someone in a crowded grocery store aisle saying to you ``Excuse me, could I squeeze though, please?''

Of course, if he's also blowing his horn and weaving about the road, or failing to give you an appropriate amount of time to get over, then get mad all you want.

I just wish that I could avoid passing people on the right, and flashing lights seems like the best signal, seeing as how it's pretty much defined to be so.