I thought the question here was whether the acquantaince was rude. But I must admit I am stunned at the number of people who are supporting violations of the law.
Most of us aren't so perfect that we see driving a few miles over the speed limit as a particularly egregious sign of moral turpitude.
Yep. If that's the case, then toss me in jail and throw away the key. 
People are not simply saying "hey, not perfect, I admit I speed from time to time". They are basically maintaining that that makes them good drivers because they are keeping up with traffic or that they have superior judgement to the people who set the speed limit in the first place and thus are justified in disregarding it. I think it is that response that Kitty Hawk is pointing out. I have to say that when I do catch myself going over the speed limit I do not find myself saying "My, I am driving particularly safely today". If you have to justify yourself by saying you are less dangerous than a distracted driver that is not the best argument that you are actually safe, just that you are lower down on the "dangerous continuum".
I don't think anyone said that. I mentioned that my driving instructor told me that it was safest to stay with the flow of traffic but specified that I live in a state where people are speed demons. It is probably not safe to be driving more than 10 MPH slower OR faster than most people around you, that's just common sense - think of blizzard conditions where everyone might be going 20-30 MPH on a highway where normally people go 60, for example.
There really isn't a need to get into a debate over how to drive. The question was how, when, and where is it polite to point out potential safety trumps etiquette issues.
