the cause of speed-related accidents is speed variance. There are many situations where the flow of traffic is well in excess of the posted limits. The question here is, who's causing the variance -- the law-observing driver or the majority of speeders?
A friend had a solution: Take down all the speed limit signs, measure the speed of traffic, and post the average as the limit. Exceptions would only be where hazards might not be obvious.
Back in the 1970s when the speed limits dropped to the old "double-nickel" I heard a state official being interviewed on the radio. He said in the year since the speed limit had dropped to 55 there had not been a single fatality in that state where both vehicles were traveling at or below the speed limit and all occupants were wearing their seat belts. Lower speed limits save gas and save lives. Think how much less expensive cars would be without air bags! And how much technology could be eliminated if we all drove 55! STM, the biggest problems with our Compasses seem to be related to the manufacturer trying to cope with CAFE standards and safety.