Question about the truck accident which left Melissa Moody disfigured.

First of all, I loved this film. It put such a smile on my face and came at the right time. It was delightful and I was particularly impressed by Melissa and the way she was able to transcend her awful accident. She seemed so...well, happy!

Anyway I as so engaged with Melissa and her story of recovery that I ignored the niggling questions around the actual accident until the next day. I just don't get (and this is with all due respect) how the accident happened. Melissa said her hand was caught in the driver's side door which would make her completely visible to the driver (her sister in law). I just don't get why then, the sister in law would not be aware of Melissa running alongside, frantically trying to get her hand out and why she didn't stop the car, then and there...Then of course Melissa was actually dragged under the truck and she still didn't stop? I just don't get it.

Anyone has any answers?