What is the test for establishing causation in a motor vehicle accident?

In Athey v. Leonati et al (1996), 1996 CanLII 183 (SCC), 203 N.R. 36 (S.C.C.), the court states at p. 43: Causation is established where the plaintiff proves to the civil standard on a balance of probabilities that the defendant caused or contributed to the injury. The general, but not conclusive, test for causation is the “but for” test, which requires the plaintiff to show that the injury would not have occurred but for the negligence of the defendant. Where the “but for” test is unworkable, the courts have recognized that causation is established where the defendant’s negligence “materially contributed” to the occurrence of the injury. In some circumstances an inference of causation may be drawn from the evidence without positive scientific proof. The plaintiff need not establish that the defendant’s negligence was the sole cause of the injury. The law does not excuse a defendant from liability merely because other causal factors for which he or she is not responsible also helped produce the harm. It is sufficient if the defendant’s negligence was a cause of the harm.