I call this feeling "being on auto-pilot." You're functioning and mostly behaving appropriately, but it's as if you just flipped a switch to "auto" while you're out. It's a very, very common response to trauma.
Right now, she's coping not only with the accident itself, but with the way it made her feel about the world. Up until now, she's probably felt like most young people, and thought that bad things will always happen to somebody else. It never really occurred to her that she couild be killed, crippled, the cause of someone else being hurt or anything else of the kind. Now she's realizing that reality is different than the way she pictured it. She feels very vulnerable and like the universe is out of control.
It might last for months, or even a year or more, depending on how disillusioned she is, and whether there's physical problems from the accident. Last time I had something like this happen to me, over a year later I broke down in tears in the cereal aisle of the grocery store. I just felt that nothing, and no one, was safe, and it was awful.
Let her work through at her own pace and in her own way. Don't try to rush her to "snap out of it." If she's become fearful of anything - not just cars, but any situation - don't tell her it's silly or recite statistics. Tell her, in real terms, that yes, the world is a scary, random place, but she DOES have the ability to cope with it. She has the knowledge and skill to make the best of life.
All my best to both of you!