He said he tested it a few times (probably made people jump off before). Even in the beginning we can see that Nagare didn't die immediately from the jump, in fact just his legs were fucked up, so if they had rushed him to they hospital they could've saved his life but not his legs and I think thats what Okaya was going for, but then the truck came
That's literally the problem, why wouldn't he consider the possibility of vehicles passing through that path, like that's obviously a freaking road for cars to move on?? If someone falls down there from above and still survives, there is still the risk of getting run over...It's ridiculous how he actually thought his stupid f*cked up plan would work in the first place and it shows you that he doesn't understand the value of a life at all.

Are you serious?? How can you not expect someone to die after you make them jump off from a fucking cliff?? Wtf is his thought process??