@juice - people passing using cruise control
The "0.5 mph faster than the car he's passing" driver is a big annoyance to be sure, but what is worse at least here in the US are trucks passing trucks. More and more trucks have GPS devices that limit their top speed. Obviously they aren't all calibrated identically, so you can get two trucks that both think they're doing 70 mph but one is slightly faster. It can take 3-4 minutes for one to pass the other, especially if hills are involved which may result in the passer temporarily losing ground if his truck is more heavily loaded and can't maintain speed up a hill.
I just recently completed a 1200 mile round trip and due to this anytime I was approaching two semis traveling close together I increased my speed by at least 5 mph in an attempt to reach them before the one in the back had the opportunity to pull out and begin passing - especially if there is more than one truck in a group! Even worse, if I'm passing in the left lane behind other vehicles I have to stay less than a truck length (about 20 meters give or take) behind the vehicle in front or the truck will take the opportunity to jump in front of me. My response (which I'm not alone in) to this problem is probably not increasing the overall safety of the roads, though I'm sure those who implemented the GPS speed control devices in the trucks feel it is increasing safety.
Can't wait for self-driving vehicles so kids born in 2015 can tell us old fogies how they can't believe people used to manually drive cars, just like kids born in 2000 can't believe we used to only be able to contact our friends when they were at home.