Exactly, so when I get in that car, I want to know the probability of dying. How do you compute this? By taking the number of crashes per kilometer, and multiplying by the number of kilometers you'll drive for. A trip of 5000km is less safe than a 5km one. That's just common sense. Similarly, when comparing flying versus driving from Amsterdam to Paris, you are, as you say, interested in which method is most likely to get you to your destination alive. So you take the number of incidents per kilometer for planes and cars, and multiply them by the distance between Amsterdam and Paris. The interesting figure really is incidents / kilometer, that's just basic math.