The suspension oddly works better at higher speeds, so rutted country roads tend to be dealt with fairly well at national speed limits. (autoexpress.co.uk)
Jumbo jets doubled airplane passenger capacity, and the federal government instituted the first national speed limit, 55 mph, to conserve energy during the 1973 oil embargo. (educationworld.com)
But when energy concerns became a priority during World War II, the U.S. government stepped in to create national speed limits. (automobilemag.com)