Question: Why can't Henry save his mother from being in the car crash? Couldn't he have warned her when they met on the subway?
Sereenie
27th Aug 2009
The Time Traveler's Wife (2009)
Answer: Of course not. Why would she listen to him? He's a total stranger. And if he tells her he's her time-travelling son, she'll think he's a nutjob to boot. It's well-established in the book that he tried everything to save her but could never do so, which made him recognise a well-accepted convention of time-travelling lore: big past events can never be changed. Diana Gabaldon wrote an excellent and extensive essay on time-travelling laws, which is probably still available somewhere on the Internet.
Answer: He could have had Claire distract her long enough to delay her car ride and miss the accident.