Here’s how the issue often surfaces in discovery, in the clipped cadence of transcripted Q-and-A:

Q: Did you hit any part of your body upon impact with the interior of the vehicle?
A: Yes. I hit my chest on the steering wheel.

Q: Were you wearing your seatbelt at the time of the accident?
A: Yes.

Q: So why didn’t your seatbelt restrain you?
A: [Stunned silence] …  I don’t know.

Q: Was the seatbelt broken?
A: [More silence]…No.

Q: So are you sure you were wearing your seatbelt?
A: Maybe, I’m not sure now.

Apart from the obvious credibility problem the plaintiff has created for themselves and their counsel, that pause sets the stage for the seatbelt defence. It crystallizes the defence position that if a properly worn or operable seatbelt could have prevented contact with the steering wheel or windshield, then the plaintiff’s injuries may have been amplified by non-use or misuse of the restraint.

The Statutory Framework

Matters that we mediate routinely feature a seatbelt argument, yet it’s still treated as an afterthought. It shouldn’t be.

The defence, when proven, operates as contributory negligence and can trim a plaintiff’s tort recovery by a meaningful percentage. In Ontario, the statutory duty to wear a seatbelt arises under the Highway Traffic Act R.S.O. 1990, c. H.8, s.106^[1]> with technical requirements in Regulation 613 R.R.O. 1990, Reg. 613^[2]. That duty frames the conduct standard, but the courtroom question is narrower. Did non-use (or misuse) of the belt make the injuries worse, and if so, by how much should damages be reduced?

The leading Ontario authority is Snushall v. Fulsang, 2005 CanLII 34561 (ON CA)^[3], where the Court of Appeal rejected a granular, injury-by-injury “strict causation” subtraction in favour of a principled percentage reduction for contributory negligence. The court endorsed the familiar 5%–25% range for seatbelt non-use and set an effective upper cap of 25%, reserving the top end for cases where wearing the belt would have prevented substantially all of the injuries. On the record before it, the court cut the jury’s 35% assessment down to 5%.

Practically, that range—proved with medical/biomechanical evidence linking non-use to enhanced injury—can drive the negotiation math at mediation.

Burden of Proof and Evidence

Critically, the onus rests with the defendant. They must prove both the fact of non-use (or misuse) and the causal connection between that lapse and the severity of injuries. In practice, that usually means adducing persuasive medical/biomechanical evidence that a belt would have prevented particular injuries or materially reduced their severity. Without that evidentiary linkage, courts tend toward the low end of the range (as Snushall itself illustrates). That allocation of proof matters in mediation. Absent credible defence evidence on mechanism and injury amplification, a plaintiff has principled footing to resist anything beyond a single-digit reduction.

The Negligence Act Framework

The seatbelt defence sits within Ontario’s Negligence Act R.S.O. 1990, c. N.1^[4] framework. Contributory negligence doesn’t bar recovery. It apportions it. Fault is compared and damages are reduced according to the plaintiff’s share of fault, applied as a percentage to the tort award. In seatbelt cases, because the defendant’s negligence caused the accident, the plaintiff’s “fault” is their failure to take reasonable self-protection measures. The reduction reflects blameworthiness and injury amplification, not a re-allocation of accident causation. This is why a global percentage reduction—rather than dissecting each injury—has become the accepted approach after Snushall.

Child Passengers

Passenger age and supervision add another dimension. In Galaske v. O’Donnell, 1994 CanLII 128 (SCC)^[5], the Supreme Court of Canada held that drivers owe a duty to take reasonable steps to ensure child passengers wear seatbelts. For mediations involving injured minors, that duty can shift the liability discussion. Rather than a child’s “non-use” diluting the claim, the driver’s failure to ensure proper restraint can increase the driver’s responsibility. That reality often neutralizes a seatbelt deduction argument at the bargaining table where the plaintiff is a child.

Translating Doctrine to Dollars

Translating doctrine to dollars at mediation is straightforward but consequential. Suppose the parties converge on a $600,000 “no-fault-allocation” valuation for the tort claim. A 5% Snushall deduction moves $30,000; 10% moves $60,000; 25% moves $150,000. Because contributory negligence applies across the tort award (subject to the usual heads and set-off rules), even modest percentages reshape brackets, authority, and settlement optics. This is why early, credible expert input on injury biomechanics—belt geometry, kinematics, airbag deployment, contact surfaces—often pays for itself in avoided over- or under-discounting.

Practical Guidance for Counsel

For counsel, the practical takeaways are clean.

Defence: if you intend to press the seatbelt defence above a token percentage, marshal specific proof that a properly worn belt would likely have prevented particular injuries (or the worst of them). Tie mechanism to medical outcome, not just to general safety.

Plaintiff: hold the defence to its proof—where the evidentiary chain is thin, the principled landing is in the low single digits. Where the passenger was a minor, remember the driver’s supervisory duty from Galaske. Those evidentiary realities, more than rhetoric, dictate where the percentage lands within the Snushall range at mediation.

Conclusion

Bottom line for mediations: the seatbelt defence is a valuation lever, not a sideshow. Ontario law anchors the discussion in a stable percentage framework with a 25% ceiling, places the evidentiary burden on the defence to prove injury amplification, and treats minors differently through the driver’s supervisory duty. Knowing where your facts sit on that spectrum—and pricing it early—keeps offers disciplined and prevents a small percentage from quietly eroding a good result.