Why didnt she try harder?

I just wonder if they started to write the film that I think could so easily have been made (see my rather overlong post yesterday - "could have been greatly improved") - a film without any definitive supernatural premonition/time slipping but which leaves this ambiguous - but thought this too sophisticated for the market they were aiming for so added some supernatural only moments.

As I point out in that post, you highlight one of many things that is not consistent with her actually believing that he is going to die (because you'd do almost anything to stop that!) but is completely consistent with a film one interpretation (you could also have a supernatural one of course which is why it is ambiguous) where what we are seeing is her grief stricken shattered memories played back in a non linear 'memento' style as she looks back at the days from her hospital bed, and she has misinterpreted her own vague premonition, and through grief or guilt concluded she *could* have done something about it as she (erroneously) has it in her own head that she predicted it in detail, although this only being an artefact of her own shattered recollections.

Instead they firmly went for the supernatural only option ruling out any logic and creating the flaw you point out along with many others (an equal or even bigger hole is why she didn't put some window stickers up to stop that accident! - the effort/reward ratio probably even better than slashing the car tires!).

Probably because the entire message of the movie was that some things are just meant to happen and you cant change fate