Good question. But look up the Emergency Highway Energy Conservation Act:
The Emergency Highway Energy Conservation Act, signed on January 2, 1974, established a national 55 miles per hour speed limit to reduce gasoline consumption. It was extended indefinitely on January 4, 1975 (U.S. Dept. of Transportation, 1979c). It also provided that Federal-aid highway funds could be used for ridesharing demonstration programs.
Yes, this occured under the Carter regime. Reagan later eliminated this act and allowed states to set their own max speed limits. However, on many state and county roadways, the Max speed limit was not changed (left at 55) due to safety issues (e.g., traffic volume, narrowness of the roadway, number of highway access points, etc). IMHO, it’s time to revisit the issue of max speed limits in order to increase fuel conservation - that would be one approach to the current oil crisis. Other approaches would be to allow drilling of our own oil reserves, increasing the number of nuclear power plants, incentives for new technology that increases fuel economy for vehicles, etc. Until we get away from middle eastern oil, the problem will only get worse.