The 55 mph speed limit was a vain attempt by the Federal government to reduce gasoline consumption; initially passed in the 1974 Emergency Highway Energy Conservation Act the law was relaxed in 1987 and finally repealed in 1995 allowing states to choose their speed limits. Highways and cars are safer today than in the 1970s and on many highways speed limits were increased to 65 mph. Higher speed limits are often safer because what is worse than speed is variable speed, some people driving fast and some driving slow. When the speed limit is set too low you get lots of people who safely break the law and a few law-abiders who make the roads more dangerous.
He links to a paper in the American Economic Review by Charles Lave from 1985 which says:
Based on analysis of 1981 and 1982 state cross-section data, I find that there is no statistically discernable relationship between the fatality rate and average speed, though there is a strong relationship to speed variance. When most cares are traveling at about the same speed, whether is is a high speed or a low one, the fatality rate will be low-- presumably because the probability of collision will be low. Variance kills, not speed.