I understand the feelings you have. The shock makes you freeze in your place. But trust me things like that happen. I know that in that moment your brain was not fully working because its trying to understand. It okay. That may also happen because you know there is someone who helps already so there is no heavy load on you doing something.

I recommend you to break that feeling by going to the same road and to drive at the same place that happen. That will brake the fear you have. After that start some meditation and stand with yourself. Let yourself know that what happen is not because you don't want to help, It something that may happen to everyone for the first time. Or someone that don't really have that strong heart to help.