I don't think you can be certain of that. If you'd been first on the scene and witnessed the accident, you may have responded very differently. As it was part of you was aware that there were others around who were helping the victim (and quite possibly those others have been through a fairly recent tragedy). As you say, you simply don't know. In my experience, it varies wildly according to the circumstance. I've been stupid brave in a couple of instances (both when I saw a woman being abused), but when I was snorkeling on my honeymoon, I thought I was caught in a rip tide — whereupon I panicked and swam like a demon for the beach without so much as looking back to check on my beloved until I'd reached dry ground. And did I feel like a lily liver.