What is the test for causation in motor vehicle accidents?

The classic test for causation in tort was articulated in Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R.458 at paras. 14-19: 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. . . . The “but for” test is unworkable in some circumstances, so the courts have recognized that causation is established where the defendant’s negligence “materially contributed” to the occurrence of the injury. . . . It is not now necessary, nor has it ever been, for the plaintiff to 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 is not responsible also helped produce the harm.