The regression-to-mean concept describes which phenomenon?

• A. Crashes are random with no pattern
• B. Crashes always increase after interventions
• C. Crashes depend only on weather
• D. The tendency for crashes to fluctuate year to year but converge to a long-term mean

Answer: (D)

Regression to the mean describes how extreme values tend to be followed by more typical ones, drifting toward the long-term average because random variation produces spikes and dips. In road safety, crashes fluctuate from year to year due to many random factors. When a year is unusually high, the next year often moves back toward the average, and similarly an unusually low year tends to rise toward that average. Over time, these fluctuations converge around a stable long-term mean. This captures why crashes don’t just keep climbing or staying at an extreme, and why the pattern is a pull toward the average rather than a simple, unpatterned change. It’s not that crashes are completely random, not that they always increase after interventions, and not solely determined by weather—the key idea is the tendency to move toward the long-term average after an extreme value.