My wife's uncle taught a AAA defensive driving class for the elderly. The AAA approved course at that time (this was in the mid-90's) taught them that the safest speed is the average speed of traffic, regardless of the posted speed limit. If they could not drive comfortably at that speed, then they should stay in the right lane. If they were driving much slower than traffic and could not comfortably drive any faster, they should avoid the highways altogether.