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initially passed in the 1974 Emergency Highway Energy Conservation Act the law was relaxed in 1987 and finally repealed in 1995 allowing states to choose their speed limits.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.
That's why life in the slow lane is not always the safe route. In the fast lane, I think this is precisely why Montana's old Reasonable and Prudent speed limit was dangerous (and eventually voted out).
After the federal repealing of the national speed limit law in 1995, Montana had 4 years of "Reasonable and Prudent" driving on the interstate. Growing up in Montana and turning 15 in 1997, I was one of the least reasonable and prudent people to drive on Montana Interstate highways. Because the 16-year-old version of me had a different idea than most of what constituted reasonable driving, I often passed people while going 20 mph faster than them (not proud of this). Although it was fun to Tony.16, in retrospect (from Tony.29's perspective), this didn't feel safe.
As a 17-year-old driver, I was happy when they imposed limits on everyone in 1999. Interestingly, one role for speed limits is to solve the coordination game of how fast we should drive. If everyone sees the same number, we (more or less) tend to drive the same speed (5-10-15 mph higher than that number). Except for when the number is absurdly low, but you should read the Tabarrok article for that.