I've argued for a long time now that, regardless of the posted speed limit, driving at a speed lower than the rate at which traffic is flowing — particularly in an "inside lane" — should be a primary offense (that is, behavior that's probable cause for an officer to pull you over). While you may think you're making traffic "safer" by observing the posted speed limit, you're actually increasing the risk to yourself and everyone around you by causing eddies in the flow of traffic as people merge into normally slower-traveling lanes to pass you on the "wrong" side. Those local instabilities in traffic flow significantly increase the likelihood of an accident because they engender unpredictable (or at least less predictable) behavior in your fellow drivers. Yes, traffic probably would be safer if everyone drove at the posted limit, but so long as a significant enough number of people aren't, behavior that complicates traffic flow increases risk for everyone.

(Then again, some of the worst offenders in that regard tend to be the police, themselves, in my experience. I can't count the number of times I've been caught in some huge knot of traffic pacing a cop who's driving two or three MPH under the posted limit, as if he's daring people to pass him...)

Of course, I've also argued for just as long if not longer that one should have to pass a university-level course in fluid dynamics in order to get a driver's license, and I don't see that happening any time soon, either.