What is the test for determining whether there is proximate cause in a motor vehicle accident?

"[T]he question of proximate cause . . . becomes one of law where the facts are uncontroverted and only one deduction or inference may reasonably be drawn therefrom." (Whinery v. Southern Pac. Co. (1970) 6 Cal.App.3d 126, 128.) " 'A mere possibility of . . . causation is not enough; and when the matter remains one of pure speculation or conjecture, or the probabilities are at best evenly balanced, it becomes the duty of the court to direct a verdict for the defendant.' [Citation.]" (Saelzler v. Advanced Group 400 (2001) 25 Cal.4th 763, 775-776; italics omitted.)