Nothing

Let's say that the whole 'everything is determined' thing is correct, the only thing I can do in this matter is sit back in watch it live. Trying to prevent something that is set to happen regardless of any outside force trying to stop it is a lost cause. It'd be the same is prolonging a date with someone your friend's thought it was a 'nice' idea to hook up with. [desperate chicks are very persistent]. Sure you'll delay it, but only for so long. If one event gets disrupted, it will try to 'resume' itself in the next possible instance.

Now what if the whole 'change one thing, change the future' theory is correct?
Well then the answer becomes more of a display of human morals than anything else really. Think about it, The very decision of watching or stopping the accident hangs purely on the result of the accident. Will the accident actually start a new study of how cars are constructed for safety? Would letting the accident happen save someone else from dying in case the person who died in the accident was actually some killer HEADING over to said victims house to kill them? How about the possible benefits of preventing the accident?
The only way to honestly answer this question with this theory would be to know the outcome of the outcome from the accident.