What is the ruling of Islamic law concerning accidental killing resulting from a car colliding with a motorcycle driven by an addicted person of unknown identity? Is blood money (diyya) obligatory, and to whom should it be paid?

If the death resulted from an error and transgression on the part of the motorcyclist, then there is no liability on the driver of the car, unless he could have avoided the collision and failed to do so. However, if he could not avoid it, then there is no guarantee (liability) on him, based on the rule: "That which cannot be avoided entails no guarantee," because the car driver did not directly commit or cause the killing. The damage occurred due to the motorcyclist's transgression.