A suspect there's also an aspect than longer journeys are safer to take into account. Merging onto a high speed road is a time where accidents are more likely, but you might only do it once between cities. Cruising at high speed on a good road0 is relatively safe.

Drive in a city and you may be merging 5-6 times as you move from road to road.

I'm just saying that assuming a linear relationship to distance is probably incorrect as well.