Kelownagirl, it's interesting. I think the 10km over concept is purely Canadian.
Friends were up from Washington state one time and asked "what do those numbers on the signs mean?" He was trying to point out that he didn't see *anyone* doing the speed limit or under, at least outside of any city, once he got into BC. I do think it's cultural to whatever location you are driving in, so it's a good idea if you travel to find out what the highway speed enforcement is like in the area.
I set the cruise to 10km over and have not been stopped for speeding. Truth be told, in my old hometown, there were several locations in town where I would be passed by the police when I was doing 10km over....and they were not going to an emergency.
All that said, whenever possible, I let DH drive. I get way to stressed out when I'm driving.
Hugs and butterflies,
~T~