This is intriguing. The chart does not list the average speed of an automobile, and that is what would be the most obvious correlation. 1920- 1930 the country would still have a very large population of Model T Fords. Although they could be turned into hotrods, left factory standard their top speed was around 40-45 mph. In 1932 Ford introduced the V-8, and with more horsepower speeds of 60 mph and greater became quite easy. Clyde Barrow would love the Ford V-8 , so useful in quickly arriving at a robbery scene and then getting away. Safety equipment, like seat-belts, would not come in until much later in the 1960's. A wreck at 65 mph would tend to be worse than a wreck at 40 mph.

During the Arab Oil Embargo of 1973-74 the Interstate Highway speed limit was dropped from 75 to 55 mph to save fuel. When it was noticed that highway deaths also dropped, the 55 mph limit was , for a time , kept. You can see that small drop in deaths on the chart, right around '74-'75