If you need to go somewhere with a realistic choice between e.g. driving and flying (which seems quite rare), risk per distance is fine for choosing one or the

If you need to go somewhere with a realistic choice between e.g. driving and flying (which seems quite rare), risk per distance is fine for choosing one or the other via risk per trip. But the implication of it being a realistic choice is that the travel times are comparable, meaning risk per time will also be comparable, and risk per time has the benefit of not producing absurd answers for edge cases like my Mars example. Every new world-shrinking transportation innovation has changed the distribution of things in the world and increased the distance society expects us to travel on various kinds of "required" trips. That's because time and money are more important than distance in deciding whether a trip is worth it.