Is it obligatory for me to fast, pay a diyah, or offer a fidyah after an unintentional car accident that resulted in the death of a Christian, especially when there is a witness who saw another person hit the deceased and flee?

If a person's death is a result of your negligence, such as speeding while driving, it is considered involuntary manslaughter (قتل خطأ) which necessitates blood money (Diyya) upon you and your 'Aqila (male relatives on the father's side), in addition to expiation (Kaffara). However, if you adhered to driving rules and were not negligent, then there is no Diyya and no Kaffara.

In a case of negligence where the person dies before being hit by the second party, the Diyya is solely upon you. If both of you participated in the killing and both were negligent, then the Diyya is shared between you, and each of you owes a separate Kaffara according to the majority of scholars. The Kaffara is incumbent upon each killer; it is not divisible and must be paid in full by every participant in the killing. It is also obligatory for killing a non-Muslim.