This is a question about the real world.
In the story that I am writing, one person suffers the destruction of (most of) their brainstem and cerebellum. Other parts of the brain and body remain largely unaffected or receive only minor injuries that can be healed (e.g. a fracture of a bone). The face remains completely unscathed, maybe with the exception of the lower jaw.
I have attempted to research head injuries in traffic incidents, but have only been able to find statistics or generalities, and all examples of head injuries that I found are described as involving either the whole brain (e.g. concussion) or other parts of the head (e.g. injuries to the face when wearing a helmet).
What I probably need is some kind of Phineas Gage accident, where not the whole head is bashed against another vehicle or the ground, but where a slim, sharp object like a spiked fence post or torn vehicle door does some very limited damage.
Ideally the victim of the accident was riding a bike and the perpetrator was driving a car. If the victim is not riding a bike, they need to be travelling individually (e.g. in a car or on foot), not in public transportation (bus or train).
The victim dies from the injury.
The cause of the injury must be a traffic accident (not a bomb damaging cars passing by or a terrorist knifing passengers).