Rear underride crashes are among the most lethal events in highway transportation. A passenger vehicle traveling at highway speed strikes the back of a commercial trailer, and the trailer's frame overrides the car's crumple zone, cutting into the passenger compartment at neck height. Federal regulations address this risk through rear underride guard standards, but the regulatory floor is not the end of the liability analysis. The gap between what the regulation requires and what engineering can provide is where the defect case lives.
What FMVSS 224 Requires and Where It Falls Short
Federal Motor Vehicle Safety Standard 224 sets a minimum performance requirement for rear impact guards on trailers and semitrailers over 10,000 pounds GVWR. The standard specifies that the guard must withstand specified test forces at defined test points, essentially requiring that the guard prevent underride in a center-impact crash at relatively modest speeds.
The standard has two widely recognized limitations. First, it tests only center-impact scenarios. A passenger vehicle striking the corner or offset portion of the trailer rear may not engage the guard at all, and FMVSS 224 does not address that configuration. Corner and offset impacts are common in real-world crashes, particularly when a vehicle drifts, changes lanes, or swerves before striking the trailer. Second, the standard was established decades ago, and the force requirements in the regulation are lower than what modern testing shows is achievable with readily available guard designs.
When a guard meets FMVSS 224 but fails in an offset or corner-impact real-world crash, the defense will argue regulatory compliance as a shield. Courts have generally allowed design defect cases to proceed even when a product meets applicable federal standards, because the standard is a floor, not the ceiling of reasonable safety engineering.
The Design Defect Theory Against the Trailer Manufacturer
The design defect case against the trailer manufacturer begins with identifying feasible alternative designs that would have prevented the underride. Several alternative guard designs have been developed that provide better protection in offset and corner-impact scenarios, including longer guards that extend closer to the trailer's width, stronger guard cross-sections, and designs that maintain their energy-absorbing geometry under real-world impact loads.
The risk-utility test, used in a majority of jurisdictions, balances the probability and gravity of harm the design poses against the burden of adopting an alternative. When a guard that could have prevented a fatality costs modestly more to produce and was technically available to the manufacturer at the time of manufacture, the risk-utility balance typically favors the plaintiff. The expert must be prepared to quantify the cost differential, describe the alternative design in engineering terms, and explain why the defective design was selected over the safer option.
In a manufacturing defect theory, the focus shifts to whether the specific guard that failed met the manufacturer's own engineering specifications. A guard that buckled at a load well below its design specification fails in its own terms, independent of whether the standard was adequate.
IIHS Test Results and the Rule 407 Problem
The Insurance Institute for Highway Safety has conducted rear underride guard testing beyond the FMVSS 224 protocol, including offset-impact and small-overlap scenarios. Those results are publicly available and show dramatic differences in guard performance across manufacturers and trailer designs.
The evidentiary challenge is Rule 407, which limits admissibility of subsequent remedial measures to prove negligence or culpable conduct. If the manufacturer improved its guard design after the plaintiff's trailer was manufactured, evidence of that improvement may be excluded under Rule 407 as evidence of negligence.
The IIHS testing data is distinguishable from subsequent remedial measures if the testing occurred before or during the period when the defective trailer was manufactured. If the test results predate manufacture, they establish that the manufacturer had access to information showing alternative designs outperformed its own, which is evidence of what was knowable, not of post-incident remediation. Frame the IIHS data as prior knowledge evidence rather than subsequent remedial measure evidence to preserve admissibility.
The Trailer Manufacturer as a Distinct Defendant
In underride cases, the trailer manufacturer is a separate defendant from the motor carrier that operated the vehicle. The motor carrier may own the trailer, may lease it, or may pull a shipper-owned trailer under a one-trip lease. Regardless of the operational relationship, the trailer manufacturer's product liability exposure is based on the design and manufacture of the guard, not on the driver's or carrier's conduct.
Name both. The carrier may be liable in negligence for operating a trailer with a guard that failed to meet regulatory requirements, for failure to inspect, or under statutory employment theory. The trailer manufacturer faces the design defect case. These theories are independent and can both succeed at trial even if the damage award is allocated between them.
For related discussion of the discovery strategies in commercial truck cases, see our resources on truck and motorcycle litigation. Recent settlement patterns in high-value trucking cases appear in our case law and settlements coverage.