Most states subsequently increased the limit on their rural Interstates to 65 miles an hour.
The rate continued to fall even after Congress allowed the speed limit to rise to 65 miles an hour on rural interstates in 1987.
In April 1987, 38 states began raising car speed limits to 65 miles per hour on rural interstates.
The new Federal highway law allows most states to raise the speed limit on rural interstates to 65 miles per hour.
Thirty-eight states now have made the speed limit 65 on rural interstates.
The Senate approved a highway bill allowing states to set a 65-mile limit on rural interstates.
The engineering study had recommended raising the speed limit on rural interstates to 70 mph.
The accident occurred one day before the state raised the speed limit for buses on rural interstates from 55 to 65 m.p.h.
That is up from 6 percent in 1986, before Congress allowed the states to raise speeds on rural interstates to 65 miles an hour from 55.
The maximum speed limit had been set at 55 miles per hour, although in 1987 Congress had allowed the states to raise it on rural interstates.