Speeding drivers

When I drive on the local highways, even at the speed limit my car is often the slowest, often trucks pass me. Yet, I also see plenty of marked police cars around, checking speeds and sometimes pulling people over. I wonder if either the penalties are light or everybody knows how much excess speed is enough to actually get pulled over. Perhaps it depends on jurisdiction where the state highway patrol have one policy but, say, Oak Ridge has another. I know that my car's speedometer is accurate, though it's the first car since my police car where that's been the case; I wonder if Kias in general tend to be accurate.