How can a plaintiff prove causation in a motor vehicle accident?

Causation does not mean the sole cause. The plaintiff can prove causation by proving that the defendant’s negligence materially contributed to the plaintiff’s injuries. (See, for example, Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458)

The plaintiff has the ultimate legal burden for establishing causation. However, if the defendant adduces no evidence to the contrary, the court may make an inference of causation. In Snell v. Farrell, 1990 CanLII 70 (SCC), [1990] 2 S.C.R. 311, Sopinka J. explained: ….The legal or ultimate burden remains with the plaintiff, but in the absence of evidence to the contrary adduced by the defendant, an inference of causation may be drawn although positive or scientific proof of causation has not been adduced. If some evidence to the contrary is adduced by the defendant, the trial judge is entitled to take account of Lord Mansfield's famous precept. This is, I believe, what Lord Bridge had in mind in Wilsher when he referred to a "robust and pragmatic approach to the ... facts" (p. 569).