What is the test for determining liability for injuries caused by careless driving?

Because there is often not a single cause sufficient by itself to produce the plaintiff’s harm, it is not necessary for the plaintiff to prove that the defendant’s carelessness was the only cause of the injury. It is enough if the defendant’s conduct was part of a combination of causal factors that produced the plaintiff’s injury. In Athey v. Leonati, the plaintiff, whose history of back problems predisposed him to further injury, was injured in two separate motor vehicle collisions, and then, while recovering from those injuries, he suffered a herniated disc. Major J. said, at para. 17: 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. The plaintiff had to prove on a balance of probabilities using the but-for test that the impugned conduct was a contributing cause of the injury (para. 41). The defendants were held liable because, “[a]lthough the accidents played a lesser role than the pre-existing problems, the accidents were nevertheless a necessary ingredient in bringing about the herniation” (para. 43). Since the contribution of the accidents was more than de minimis, it was a material contribution (para. 44).