The interesting part for me was that the posted speed limit only really changes the behaviour of the slowest drivers - the 10% who "identify the speed limit sig

The interesting part for me was that the posted speed limit only really changes the behaviour of the slowest drivers - the 10% who "identify the speed limit sign and drive at or near that limit". Bringing their speed up to match the rest of us reduces the variance in speed; and minimizing the spread of speeds is a key to safety.