Rear-end collisions look simple until you’re litigating them. Ontario law imposes a common-law reverse onus on the rear driver. But courts still expect a coherent evidentiary story. The Divisional Court’s decision in Kou v. Karmah 2025, ONSC 3815^[1] is a timely reminder of both points.

The facts in Kou were familiar and contested. Around 2:00 p.m. on a clear, dry day on Bloor Street in Mississauga, two westbound vehicles were in the left lane near Queen Frederica Road, an intersection with no westbound stop or signal and no designated left-turn lane. The plaintiff, Ms. Kou, said she was travelling roughly 20 km/h, about half a car length behind, when the defendant Karmah missed his turn and stopped abruptly without signalling. She had no time to brake and struck him from behind. Karmah’s account was very different. He said he signalled left, slowed normally, came to a full stop to wait for a gap in oncoming traffic, and remained stopped 30–60 seconds before impact; his back-seat passenger corroborated that timeline.

At a two-day simplified-procedure trial on liability only, the trial judge split fault 65/35 against Kou and Karmah. On appeal, the Divisional Court allowed the appeal and dismissed the action. The Court held the trial judge had conflated the parties’ competing stories in a way that led to an unwarranted adverse credibility finding against the defendants. More importantly, the Court reiterated the settled rule: in a rear-end collision, the following driver bears the burden to explain how the crash occurred without their negligence. Even accepting the plaintiff’s version—an abrupt stop after a missed turn within the same lane—there was no “unusual” circumstance capable of shifting any liability to the lead driver. Result: Kou was 100% liable, and costs of both trial and appeal were awarded to the defendants.

The reverse onus—what it is, and where it comes from

Ontario courts have said for nearly a century that when one car runs into another from behind, the starting point is that the fault lies with the rear car and it is up to that driver to satisfy the court the collision was not the result of their negligence. The classic articulation appears in the case of Beaumont v. Ruddy 1932 CanLII 147 (ON CA)^[2], and remains good law.

Modern appellate guidance sharpened this in Iannarella v. Corbett 2015 ONCA 110^[3]. The Court of Appeal confirmed that once a rear-end collision is proven, the evidentiary burden shifts to the rear driver to show they were not negligent. The trial judge’s jury instruction must reflect that. The defendant’s “excuse” (whiteout, sudden stop ahead, etc.) does not flip the burden back—the rear driver must still provide a persuasive, non-speculative explanation consistent with due care.

Chernet v. RBC General Insurance Company 2017 ONCA 337^[4] illustrates how unforgiving this can be. The Court of Appeal upheld summary judgment where the rear driver offered no coherent account to rebut the presumption arising from a straightforward rear-end. Bare assertions and conjecture won’t do.

Trial-level cases apply the same framework to “sudden stop” and poor-conditions scenarios. In Ozimkowski v. Raymond 2018 ONSC 5779^[5], aff’d Ozimkowski v. Raymond 2019 ONCA 435^[6] the rear driver couldn’t escape liability by pointing to ice and a sudden stop in the same lane. The expectation remains that you leave enough distance to stop safely for foreseeable manoeuvres in traffic.

Can the reverse onus be overcome? Yes—but only with truly unusual lead-driver conduct or comparable circumstances. Rahimi v. Hatami 2015 ONSC 4266^[7] explains that a rear driver may disprove negligence by showing that the lead vehicle’s conduct materially contributed in a way that a prudent driver could not reasonably anticipate. Classic examples are abrupt, unlawful lane intrusions or departures from the lane immediately before impact. The Court of Appeal’s decision in Martin-Vandenhende v. Myslik 2012 ONCA 53<^[8] is often cited at the edge of the doctrine: liability could shift where the lead vehicle appears to yield off the roadway and then darts back into the lane without warning. But even then, the evidence has to clearly establish that atypical manoeuvre.

Ontario judges have also emphasized that “sudden stop” within the same lane—without more—is not unusual. That’s why Kou<^[9]matters: it rejects an attempt to transform a missed turn and abrupt stop into an exception. Absent a lane departure or other truly erratic conduct by the lead driver, the reverse onus remains intact. See also Ozimkowski v. Raymond 2019 ONCA 435^[10].

For completeness, some trial decisions show how courts deal with cut-off or chain-reaction theories. In Sobh v. RBC General Insurance 2016 ONSC 7382^[11] the court analyzed whether a sudden, unsafe lane intrusion could shift or share fault, underscoring that the defendant still needs hard evidence—not speculation—of an unforeseeable lead-vehicle manoeuvre. The through-line across these cases is simple: unless you can prove an authentic, atypical precipitating event, the rear driver wears it.

What this means when you get to mediation

Mediations in rear-end cases usually don’t turn on fault, because the law—and the caselaw above—gives the defence (for the lead driver) a strong liability posture. If you represent the rear driver, be candid with your client about the uphill climb. To create “real” liability risk for the lead driver, you need admissible, credible evidence of an out-of-the-ordinary event: dash-cam video, a neutral eyewitness who saw a cut-off or a swerve back from the shoulder, or physical evidence (debris, scuff marks, vehicle positioning consistent with a lane incursion). Absent that, you are negotiating damages, not liability.

Plaintiff’s counsel in a rear-end case should come to mediation with medicals and functional evidence ready to carry the day on quantum, because liability arguments framed as “sudden stop” tend to fall flat under Beaumont, Iannarella, Chernet or Ozimkowski. Defence for the lead driver can, and should, press that advantage: insist on a liability concession or a heavy discount unless the plaintiff brings persuasive, contemporaneous evidence of an exception that fits Rahimi or Martin-Vandenhende.

From a mediator’s chair, the path is practical. Test the rear driver’s story against the reverse onus. Can they actually explain, with proof, why due care would not have avoided the impact? If not, steer the session toward damages, credibility, and statutory issues—deductibles and threshold implications—because that is where these files settle. If the plaintiff does have genuine exception evidence (say, clear dash-cam of a shoulder-to-lane dive), you’ve got a liability live-issue. Carve out time to value comparative negligence and talk ranges on split-fault scenarios, but be precise about how far Martin-Vandenhende truly goes.

Bottom line

Kou v. Karmah^[12] does not reinvent the wheel—it re-affirms it. The reverse onus is alive and well in Ontario. The rear driver carries the burden to show non-negligence. “Sudden stop in the same lane” won’t cut it. If you don’t have unusual facts backed by solid evidence, expect mediation to be a numbers exercise, not a liability seminar.