The organization analyzed traffic fatality data from 1993, when no state had a speed limit higher than 65, to 2017 when states have speed limits as high as 80 or 85 mph. After accounting for other contributing factors to the fatality rate, IIHS found that if the speed limits hadn't changed from 1993, there would have been 36,760 fewer traffic fatalities. And in 2017 alone, having had lower speed limits could have saved 1,934 people. Breaking down the statistics farther, the IIHS found that every 5-mph increase in speed limits led to an 8.5% increase in fatalities on interstates and freeways, and a 2.8% increase in fatalities on other roads.
The IIHS also highlighted historical research that supports the idea that higher speeds lead to more deaths. In the study, the organization notes that a National Research Council Committee found that 3,000 to 5,000 traffic deaths were prevented in 1974 as a result of the national 55-mph speed limit replacing state limits around 60 to 70 mph. That council also found that in 1983, the speed limit was responsible for preventing 2,000 to 4,000 deaths. Additionally, when states were allowed to raise limits up to 65 in 1987, NHTSA found that there were 2,000 more fatalities from 1987 to 1990.
The statistics certainly suggest that speed limits should be lowered. But that might be a hard sell to the American people. The IIHS paper notes that the national speed limit was raised, and eventually removed because Americans were fed up with driving so slow. And now that we're starting to drive even faster, it would probably be pretty hard to get people to give that up.