My Broken Speedometer

Is it my speedometer, or do some people just drive painfully slow?
I think it's acceptable to drive five miles per hour, maybe even nine, faster than the posted speed limit. I am pretty sure police don't pull drivers over for speeding unless a)they're going 11 or more miles per hour faster than the speed limit or b)they have some other secret reason to pull them over, like they think the driver is smuggling prohibited chinchillas across state lines.
So what drives me totally batty is when a driver keeps his car in the fast lane, going exactly the speed limit, when he could easily pull over to the slow lane to let me pass. Or worse, when a driver sits in the fast lane, going exactly the speed limit, right next to someone in the slow lane who also is going exactly the speed limit. And inevitably, one of the drivers speeds up just enough to make me think maybe I can pass him, really quickly. But then he slows down again, right when I'm about to make my move. Then there's times when I REALLY want to pass someone, and it's like he knows, and he slows down on purpose, right when he gets lined up with someone else. How about the guy who is going so slow for 10 minutes until you get to a stop light and then he drives like he's trying to outrun a tornado, and you can't keep up with him to save your life, or, Heaven forbid, to pass him.
My favorite is the guy who's driving along, looking off to the right the whole time, which slows him down to unbearable speeds. Or the guy who's gesturing to his passenger the entire time and therefore can't drive any faster than five miles per hour under the speed limit.
Sometimes I think my speedometer is really broken. That's when I'm giving humanity the benefit of the doubt.