I have worked on injured people when my car was behind one which was hit and summersaulted....that said i have been trained in first aid and was the lead first aider for my nursery school at the time. I personally could not stand by and watch someone bleed to death if i am able to help them!
on the other hand if i needed help myself i would tell the person helping me what i needed them to do , but if i was unable to tell them , then i would be happy that they try their best!
if you are not happy to help or don't know how to help the best thing is call emergency services and the operator will talk you through what to do until the medics arrive