It would depend for each person. In the end I don't see that it matters. Chances are if they really didnt remember the events prior to the accident (due to an actual TBI) chances are you will have other obvious signs of head injury than just amnesia. I doubt you would get, it never happened to me anyways, a pt that can't remember what happened yet you have nothing else that could be obvious of a possible head injury (contusions, facial abrasions, changes in BP, pulse pressure, agitation etc) Then you would also potentially have other signs like damage to the inside of the vehicle, or cracked/broken helped etc...
Maybe its me, maybe a lot of you have seen that happen before, who knows. But from what I had experienced, I wouldnt get one without getting at least one more of the other.