A journey seems like a meaningful unit of work here. Sometimes the start or end of a journey is more dangerous. The decision to even start journey would look at

A journey seems like a meaningful unit of work here. Sometimes the start or end of a journey is more dangerous. The decision to even start journey would look at the per journey risk perhaps. Think of space launches, as an extreme example. Risk per distance or per hour, if you consider orbiting for a while, could be low but risk per journey quite high.