The number 6,400 resulted not from a study but from a simple projection of data by the National Highway Traffic Safety Administration (NHTSA) on the effects of the congressionally approved change in 1987 from a 55 mph to 65 mph speed limit on rural interstates.
The 1992 report estimated that the 2,336 fatalities on the 29,475 miles of rural freeways then posted at 65 mph were 30 percent higher than might be expected from historic trends. These are raw fatality numbers and do not reflect miles driven, traffic mix, etc. so there is no basis for reaching a true conclusion on the impact of speed or the speed limit. In fact, that report said "it is not possible to prove scientifically that the higher speed limit on rural interstates actually caused an increase in fatalities." Likewise, the report said that the fatality rate in 1990 was the same as the last year the limit was at 55 mph.
"Because we didn't have anything else, we went back to the 30 percent level," said William A. Boehly, NHTSA associate administrator for research and development. "We merely said if you apply the 30 percent to the body count, that will get you a number."
From the NMA