What is the legal test for determining whether a defendant is liable for a motor vehicle accident?

The legal test for causation is outlined in Athey v. Leonati (1996), 1996 CanLII 183 (SCC), 140 D.L.R. (4th) 235 at 238-9 (S.C.C.). A defendant is liable for any injuries caused or contributed to by his or her negligence. The presence of a non-tortious contributing cause does not reduce the liability. The plaintiff must prove on a balance of probabilities that the defendant caused or contributed to the injury. Causation is established where the defendant’s negligence “materially contributed” to the occurrence of the injury. According to Athey, supra at 239, the plaintiff does not need to prove that the defendant’s negligence was the sole cause of the injury. As long as a defendant is part of the cause of an injury, the defendant is liable, even though his act alone was not enough to create the injury. There is no basis for a reduction of liability because of the existence of other preconditions: defendants remain liable for all injuries caused or contributed to by their negligence.