This happened the year I was born (though I was not around yet). On this date in 1974, President Richard Nixon signed the Emergency Highway Energy Conservation Act into law. This meant that the national maximum speed limit became 55. Before that time, it was up to the states. They could set the limits from 40 to 80. Cheap Arab gas had been the norm in this country until the Arab-Israeli conflict of 1973. This law was supposed to help with fuel conservation. It further stated that highway projects would not be approved nor funded within the states that did not comply with this act. It was in 1995, under President Clinton, when this was finally repealed.
For more info:
http://famousdaily.com/history/us-president-nixon-signs-into.html
http://www.history.com/this-day-in-history/nixon-signs-national-speed-limit-into-law
http://www.republicanoperative.com/forums/f13/nixon-signs-national-speed-limit-into-law-43033/