Actually...

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initially passed in the 1974 Emergency Highway Energy Conservation Act the law was relaxed in 1987 and finally repealed in 1995 allowing states to choose their speed limits.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.

Unfortunately vestiges of the 55mph limit remain, in part because police like the 55mph limit which lets them write tickets at will whenever they need an increase in revenues.

So, Alejandro's response is correct, but all of this seems rather tangential to the question you quote. The reason the speed limit is 55 rather than 56 or 54 is because we have a cultural preference for multiples of 5... which is also why all the other speed limits I see posted are multiples of 5. Seeing a speed limit sign that read "33" or something would cause me to do a potentially life-threatening double-take.