What is the test for determining whether a plaintiff is a sole or superseding cause in a motor vehicle accident?

"[I]t is only an unforeseeable intervening cause, an extraordinary and abnormal occurrence, which rises to the level of an exonerating, superseding cause. [Citations.]" (People v. Armitage (1987) 194 Cal.App.3d 405, 420-421.) "Facts attacking legal causation are only relevant if the defendant's act was not a substantial factor in producing the harm or injurious situation. [Citation.]" (People v. Wattier (1996) 51 Cal.App.4th 948, 953, italics in original.) To constitute a sole or superseding cause, the victim's conduct must have been "so unusual, abnormal, or extraordinary that it could not have been foreseen. [Citation.]" (People v. Schmies (1996) 44 Cal.App.4th 38, 52, italics added.) Absent such conduct, evidence the victim "may have shared responsibility or fault for the accident does nothing to exonerate [a] defendant for his role" and "is not relevant." (Id. at p. 51.)