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Inevitable Accident Defence in the Context of Mechanical Failure


In Tam v. Allard, 2022 BCSC 274, our firm successfully represented the defendant driver and owner in the trial at the BC Supreme Court. On appeal, the BC Court of Appeal upheld the trial judge's decision regarding, among other things, the inevitable accident defence in favour of our defendants.


The inevitable accident defence is a legal concept that is used where an individual has caused harm or damage unintentionally as a result of an unforeseeable or unavoidable event. In Tam v. Allard, 2023 BCCA 178, a recent decision by the British Columbia Court of Appeal considered the issue of inevitable accident in the context of a motor vehicle collision and whether expert evidence must be introduced to prove a mechanical failure.

The case stemmed from a 2016 motor vehicle accident where the defendant (respondent in appeal) Allard was driving a “cube” van which rear-ended a stopped vehicle at an intersection, pushing that vehicle into the vehicle ahead which was driven by the plaintiff (appellant in appeal) Tam.

The plaintiff has the onus of proving on balance of probabilities the negligence of a defendant in causing an accident. In the context of a “rear-end” collision, the court may draw an inference of negligence. At trial, the defendants argued they were not negligence because the brakes on the van suddenly failed, and they had no warning of the hazard of the van’s apparently faulty brakes.

In the case of an accident caused by an alleged mechanical failure, the defence of inevitability requires that a defendant prove that (1) there was a mechanical failure, (2) the mechanical failure could not have been prevented by the exercise of reasonable care, and (3) once the failure became apparent, the driver exercised reasonable care required of a driver faced with the emergency situation.

The trial judge ultimately concluded that the defendants were not negligent in their operation and maintenance of the van, and that the driver reacted reasonably given the sudden emergency situation. There was nothing that would have drawn the driver’s attention to any problem with the van’s brakes. The owner had conducted regular visual inspections of the braking system. The brake failure was likely the result of a simultaneous sequential leakage of brake fluid from multiple brake lines. The action was dismissed.

The plaintiffs appealed, arguing among other reasons, that the trial judge erred in concluding the cause of the brake failure without specific expert opinion evidence from the defendants’ expert witness. They argued that it was not open to the trial judge to find a non-tortious explanation of inevitable accident in the circumstances. The Court of Appeal considered whether expert evidence was required to prove mechanical failure, and whether an inability to prevent that failure through reasonable care could be established without proof of the precise cause of the failure.

The Court of Appeal held that there is no rule requiring expert opinion evidence to establish mechanical failure as part of a defence of inevitable accident. Defendants are not require to prove the precise mechanism of mechanical failure. At trial, the defendants had given evidence that the van had been reasonably serviced and maintained and this was sufficient to establish that the sudden brake failure could not have been detected and prevented with reasonable care. In written reasons, the Court of Appeal noted that previous cases held that the loss of steering control in a motor vehicle can be based on the evidence of the vehicle’s driver or passenger(s).

Establishing the cause of a mechanical failure might be relevant if the allegations of failure are speculative or unreasonable based on the available evidence. It could also be relevant in determining whether an accident could have been prevented through the exercise or reasonable care, but it is not require to establish that a mechanical failure actually occurred. The defence can be made out without defendants providing the precise reason for the failure. In this case, the owner had maintained the van and the driver had no reasonable opportunity to avoid the accident when the brakes suddenly failed. That the defendants did not have an expert witness opine on the exact reason why the brakes failed is separate from the fact that they did fail.

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