What's to plan? Go for a drive to "clear your head" until things are clear enough to close your eyes and veer into traffic.
We know she's held herself hostage several times, which Chris took seriously enough to alter his behavior and not do whatever he wants, which is a big deal for him. Obviously she's entertained the option of suicide for a large portion of her life, and it's not like she "planned" by writing notes, taking care of the animals, making peace with anyone left in her life.
The car crash thread has whispers that
this was the crash, 85 miles from her house. Maybe that wasn't her, or maybe she was just driving mindlessly until she got to her final destination.