What is the test for determining whether defendants had a duty of care in the context of a motor vehicle accident?

Having determined that defendants had such a duty of care, the breach of which was a possible cause of plaintiff's injury (a matter which ultimately must be decided by the trier of fact), this case necessarily falls into the category of secondary assumption of risk. In such circumstance, it is for the trier of fact to determine the cause of the injury and to apportion the loss resulting from the injury; in so doing it may consider the relative responsibility of the parties. (Knight v. Jewett, supra, 3 Cal.4th 296, at p. 315, 11 Cal.Rptr.2d 2, 834 P.2d 696.)