I'm not sure why looking at the incidents per trip would be preferable over looking at the incidents per mile. If I think about comparing two activities when it comes to safety, I feel like I'd want to look at the number of incidents per second of performing either activity.
EDIT: here's a thought experiment. You're taking a trip to the grocery store. You have two routes in mind, the direct one, which takes 5 minutes, and the detour via the North Pole, which will take weeks. One of these trips is safer than the other.
EDIT: here's a thought experiment. You're taking a trip to the grocery store. You have two routes in mind, the direct one, which takes 5 minutes, and the detour via the North Pole, which will take weeks. One of these trips is safer than the other.