This is manslaughter of the vehicular variety
Manslaughter is the unlawful killing of a human being without malice.
It is of three kinds:...
(1) ... driving a vehicle in the commission of a lawful act which
might produce death, in an unlawful manner, and with gross negligence.
This is what they call a wobbler, and could be charged as a felony or a misdemeanor. In the latter case, the maximum penalty is a year in county jail and in the former it is six years in state prison. If the cause was ordinary negligence, it is just a misdemeanor. The details of gross negligence are set forth in the jury instruction CALCRIM 592
A person acts with gross negligence when:
- He or she acts in a reckless way that creates a high risk of death or great bodily injury; AND
- A reasonable person would have known that acting in that way would create such a risk. In other words, a person acts with gross
negligence when the way he or she acts is so different from how an
ordinarily careful person would act in the same situation that his or
her act amounts to disregard for human life or indifference to the
consequences of that act.
The "absent-mindedness" defense seems a bit far fetched, but still not everybody who drives inattentively is prosecuted for a felony, or even a misdemeanor. However, it is really hard to imagine not being acutely aware of the fact that people drive on the right side of the road here. Without a more-detailed story, I don't see a basis for deciding what level of negligence a prosecutor is likely to argue for, and what the prosecutor's agenda is: somewhere between no prosecution, and (most likely) misdemeanor manslaughter.