From the record
Real cases, written from the published record. Predict first; the record and its sources come after.
2004 · Sweden
The fourth power
Göran Nilsson studied what happened to crashes when average speeds on Swedish roads went up or down. He found a simple rule of thumb: fatal crashes change with about the fourth power of the speed change.
Suppose average speeds on a road rise by 10%.
About how many more fatal crashes would you expect?
What happened. 1.1 × 1.1 × 1.1 × 1.1 ≈ 1.46, so nearly 50% more. Small speed changes matter a lot because crash energy grows with the square of speed. In 1997 Sweden's parliament adopted Vision Zero: roads should be designed so that a mistake is not a death sentence.
What we don’t know. Later studies fitted slightly different powers for different roads; the steep shape held.