An underride crash happens when a car slides beneath a semi-truck’s trailer instead of colliding with it the way two vehicles normally would. The results are often catastrophic: decapitation, crushed skulls, severed limbs. We’ve represented families through the worst version of this crash, and the pattern is always the same. The car looks survivable from the outside. It isn’t.
This page explains what makes underride different from an ordinary collision, what federal law does and doesn’t require of trucking companies and trailer manufacturers, and how we build these cases in Illinois.
What Is an Underride Accident?
An underride accident occurs when a passenger vehicle slides under the trailer of a semi-truck instead of striking it front-to-front or bumper-to-bumper. Trailers sit high off the ground. A sedan or SUV can pass beneath one, which means the trailer makes contact at windshield or roof height rather than at bumper height, right where the occupants are sitting.
Crash investigators and safety researchers generally group these crashes into three types, and the distinction matters because federal law treats them very differently.
Rear Underride
The most common version. A car strikes the back of a trailer, usually because the truck is stopped, parked, or moving much slower than the traffic around it. We cover the mechanics of these collisions in more depth in Rear-Ended by a Semi: Why These Claims Are Not Like Car Cases, but the short version is that a rear-end crash into a truck behaves nothing like a rear-end crash into a car. Rear underride typically shows up when:
- A truck stops suddenly or is parked on the roadway
- A truck’s rear lights are defective or obscured
- Poor visibility (fog, rain, darkness) hides the trailer until it’s too late
- The truck is traveling well below highway speed with no warning to drivers approaching from behind
Side Underride
Side underride happens when a car hits the length of a trailer rather than its back or front, usually while the truck is turning, crossing an intersection, or changing lanes. Common scenarios include:
- Truck turns at intersections
- Lane change accidents
- Trucks crossing highways from a side road or ramp
Side underride crashes are especially dangerous because, as we explain below, there is currently no federal requirement that trailers carry side guards at all. A trailer’s undercarriage is open air along most of its length, and a car that gets pulled into that gap has nothing engineered to stop it.
Front Underride: The Overlooked Third Type
Less discussed, but real. Front underride happens in the reverse direction: a heavy truck runs into the back of a smaller car and rides up over its trunk and rear seat. It’s the least common of the three, but it’s just as lethal when it happens, and it’s the one crash type where no federal standard, rear guard, side guard, or otherwise, applies at all. There is no equipment requirement governing what sits at the front of a semi to keep it from climbing over a car it hits from behind.
Why Underride Accidents Are So Deadly
Every modern car is built around a simple assumption: the thing it hits will make contact somewhere between the bumper and the hood. Crumple zones, airbags, and the safety cage are all engineered to activate against an impact at roughly that height. That assumption is what makes a 45 mph frontal collision survivable more often than not.
A trailer breaks that assumption entirely. Its underside sits well above bumper height, often close to where a car’s windshield and roofline begin. So instead of the bumper and crumple zone absorbing the hit, the trailer bed catches the car above them, at the A-pillar, the windshield, sometimes the roof itself. None of the safety engineering built into the car ever gets a chance to work. The occupant compartment is what makes contact, not the structure designed to protect it.
That’s the physics behind why underride injuries look the way they do: traumatic brain injury, skull fracture, decapitation in the worst cases. It isn’t that the crash forces are unusually extreme. It’s that the car’s safety systems never engage because the impact happens in the one place they weren’t built to handle.
The Insurance Institute for Highway Safety (IIHS) has put the annual death toll from underride crashes above 400. We’d treat that as a floor, not a ceiling. Underride isn’t always coded consistently in police crash reports, and a crash gets labeled as an “underride” only when investigators specifically note it, so the true number is likely higher. For families dealing with a fatal outcome, our guide to wrongful death and survival actions in Illinois walks through the legal options available.
Federal Underride Guard Requirements
Rear Guards (49 CFR 571.224 and 49 CFR 393.86)
Rear guards are the one piece of underride equipment federal law actually mandates, and it’s worth understanding how that mandate is split across two different regulations. 49 CFR 571.224 is a manufacturing standard: it requires most trailers with a gross vehicle weight rating of 10,000 pounds or more, built on or after January 26, 1998, to leave the factory with a rear impact guard. 49 CFR 393.86 is the companion in-use rule: it obligates the trucking company to keep that guard in safe, functioning condition for the life of the trailer, and rear impact guards must be checked as part of the trailer’s required annual inspection. A guard that was compliant on the day the trailer was built but has since rusted through, been bent in a loading-dock mishap, or removed and never replaced puts the carrier in violation regardless of what the guard looked like new.
The strength standard behind these guards has also gotten tougher. For years, the underlying test was built around a roughly 30 mph benchmark impact. In 2022, NHTSA finalized an upgrade to FMVSS 223 and 224 that raises the bar to 56 km/h, about 35 mph, requiring guards on newly manufactured trailers to provide more strength and energy absorption than before. That rule took full effect for new trailer production in July 2024. It’s a real improvement, but it only reaches trailers built after that compliance date. Older trailers already on the road were never required to be retrofitted to the new standard, and a trailer manufactured decades ago may still be running on guard hardware that was minimally compliant even by the old rules.
- The upgrade doesn’t reach the existing fleet: only newly built trailers have to meet the stronger 2022 standard
- Guard positioning matters as much as guard strength: a guard mounted too high can let a car’s hood duck underneath it before the guard ever engages
- Exemptions exist: certain specialized trailer types fall outside the equipment requirement entirely
- Pre-1998 trailers: some older trailers still on the road predate the guard requirement altogether
Side Guards
There is currently no federal requirement that trailers carry side underride guards. NHTSA opened an advance notice of proposed rulemaking on the question in 2023 and, as of this writing, is still in the comment-review stage of that process, with no timeline for an actual proposed rule, let alone a final one. Congress has tried to force the issue: the Stop Underrides Act, which would mandate side guards, has been reintroduced in the Senate and House multiple times, most recently in 2026, without passing. None of that amounts to a legal requirement today. If a trucking company or trailer owner installs side guards, it’s doing so voluntarily or under a local rule, not because federal law makes them.
That voluntary landscape is genuinely uneven. A handful of cities have required side guards on their own municipal fleets or on trucks working under city contracts, and some carriers install them on their own initiative, particularly for urban delivery routes where side-impact risk is higher. But “some cities require it on city trucks” is not the same thing as an industry-wide federal mandate, and we’d caution against any characterization of the law that blurs that line. The absence of a mandate is precisely why side underride crashes remain so lethal.
Common Causes of Underride Accidents
- Truck parked on the roadway without adequate warning triangles or hazard lights
- Truck without functioning lights or reflective tape, especially at night
- Inadequate or missing underride guards, whether by design gap or maintenance failure
- Truck crossing a highway from a side road, exposing its full length to oncoming traffic
- Truck driver misjudging the speed of oncoming or approaching traffic
- Low visibility conditions such as darkness, fog, or heavy rain
- Following too closely by the car driver, which shortens the reaction window to almost nothing
Proving Liability in Underride Accidents
Underride cases rarely have a single defendant. Because the crash sits at the intersection of driver conduct, carrier maintenance, and trailer engineering, the strongest cases usually name more than one party.
The Truck Driver
For negligent operation, improper parking, or failing to deploy the warning devices required by 49 CFR 392.22 when a truck is stopped on or near a roadway.
The Trucking Company
Carriers are vicariously liable for their drivers, but they also carry direct liability of their own for:
- Failing to maintain lights and reflectors
- Failing to maintain, inspect, or install an adequate underride guard as required by 49 CFR 393.86
- Negligent training on safety procedures around parking, stopping, and turning
Trailer Manufacturers
For designing trailers without adequate guards, or with guards that fail to prevent underride in crashes that were reasonably foreseeable. This is a genuinely separate legal theory from a negligence claim against the carrier, and it’s worth understanding why.
Guard Manufacturers
If the underride guard itself was defectively designed or manufactured, separate from any fault on the part of the trailer builder or the carrier that installed it.
Negligence vs. Product Liability: Two Different Theories
A negligence claim against the trucking company asks whether the carrier acted reasonably: did it inspect the guard, maintain it, replace it when damaged. A products liability claim against the trailer or guard manufacturer asks a different question entirely: was the product itself defective, in its design, in how it was built, or in the warnings that came with it, regardless of how carefully the carrier maintained it.
That distinction matters because meeting the federal minimum doesn’t automatically clear a manufacturer. A guard can comply with FMVSS 223 and still be the subject of a viable design defect claim if the evidence shows a safer, feasible alternative design existed and the manufacturer chose not to use it. Compliance with a minimum federal standard is a data point in that analysis. It isn’t a shield.
Proving the Guard Failed You
In practice, underride cases tend to fall into one of three fact patterns, and the litigation strategy shifts depending on which one we’re dealing with.
The first is the guard that was never there. Removed, rusted away, or missing from a pre-1998 trailer that was never required to have one. That’s typically a maintenance and negligence case against the carrier, built on inspection records, maintenance logs, and roadside inspection history.
The second is the guard that was present but inadequate, mounted too high, undersized, or bolted on in a way that let it tear loose on impact instead of absorbing the crash. That case usually points toward the trailer or guard manufacturer, and it requires an engineering analysis of how the guard should have performed against how it actually performed.
The third, and the hardest to prove, is the guard that was present, compliant, and properly maintained, and still failed. Those cases live or die on accident reconstruction and metallurgical or structural analysis of the failed guard itself, and they’re exactly the kind of case where a design defect claim against the manufacturer can succeed even though the carrier did everything it was supposed to do.
Illinois Law and Underride Accidents
Illinois follows a modified comparative negligence rule (735 ILCS 5/2-1116). Even if the car driver shares some fault, following too closely or driving too fast for conditions, recovery is still possible as long as that driver’s share of fault doesn’t exceed 50 percent. We’ve written more on how that threshold actually plays out in the Illinois 51 percent rule, which is the more precise way to think about where the cutoff falls.
The statute of limitations is two years under 735 ILCS 5/13-202, or two years for wrongful death claims under 735 ILCS 5/13-212. Given how many parties can be involved in an underride case, that two-year window fills up fast with investigation work. Waiting to call a lawyer costs you time you don’t have.
Damages in Underride Accident Cases
Because underride crashes cause such severe injuries or death, damages are typically substantial:
- Medical expenses: emergency care, surgeries, rehabilitation
- Future medical care: for survivors with permanent injuries
- Lost wages and earning capacity
- Pain and suffering
- Disfigurement: underride crashes often cause visible, lasting injuries
- Wrongful death damages: for surviving family members
Critical Evidence in Underride Cases
Underride cases are won or lost on physical evidence, and a lot of that evidence has a short shelf life. Trailers get repaired, guards get replaced, and inspection paperwork disappears into a carrier’s files where it’s a lot harder to get once litigation starts. We take steps early to preserve what matters, and our guide on preserving critical evidence after a truck accident covers the time-sensitive steps that apply broadly, on top of what’s specific to underride cases:
- Underride guard inspection: was it properly installed, sized, and maintained
- Lighting and reflector condition: were they functional at the time of the crash
- Accident reconstruction: speed, angle, and point of impact
- The truck’s location and reason for stopping, if it was stationary
- Warning device deployment: did the driver set out triangles or flares
Contact a Chicago Truck Accident Attorney
Underride accidents require aggressive investigation to identify all liable parties and prove negligence. Contact our office for a free consultation if you or a loved one was injured or killed in an underride crash. We’ll fight to hold negligent trucking companies and manufacturers accountable.
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