Also, I do not agree with chuboy and the whole bystander argument. There is no way that I would ever be able to leave a run over little toddler laying there on the road. I know for sure that I don't have that in me. Obviously I can't say I know how I would react, but I certainly would have done something about it. Those cruel people walked past like as if nothing had happened...
No, chuboy has it quite correct. Many people are unable to act (or act correctly) in these situations.
In all of my Boy Scout emergency training- CPR, treating serious wounds, etc- the first thing you always do is command a bystander to call 911. You have to point to the person and address them directly. You might scoff at my experience, but the reasons are based off the bystander effect- if you don't act in the situation and order someone to call 911 while you treat the wound (or whatever), people will just watch or in this day and age whip out their iphones and record the event.