Underride Crashes and Guard Rules

An underride crash occurs when a passenger vehicle slides beneath the body of a large truck, and the truck’s undercarriage crushes the passenger compartment. Federal standards FMVSS 223 and 224 require rear underride guards on trailers of 10,000 pounds or more, but no federal law requires side underride guards. IIHS crash testing has shown that even required rear guards often fail in offset impacts, and Congress has repeatedly considered, without passing, stronger protections through the STOP Underrides Act.

What Is an Underride Crash?

An underride crash happens when a car strikes the side or rear of a large truck trailer and the height gap between the road and the trailer floor, often 4 feet or more, lets the car slide directly under the trailer body. The top of the car is sheared off at passenger compartment height. In severe underrides, the entire front of the car disappears beneath the trailer, with the engine, dashboard, and firewall crushing the occupants. Seatbelts and airbags are designed for frontal or lateral impacts with another vehicle’s structure and cannot protect against this type of intrusion.

The Insurance Institute for Highway Safety (IIHS) has documented 500 to 600 deaths per year from underride crashes in which a passenger car struck a tractor-trailer from the rear or the side. That is roughly two deaths every day from a crash type that existing technology can largely prevent. Underride cases involving guard failures and regulatory violations are part of truck crash litigation in Kentucky.

Rear Impact Guard Standards

Two federal standards govern underride protection:

• FMVSS 223, “Rear impact guards,” is an equipment standard that sets strength and energy absorption requirements for rear underride guards. Guards must sit low and wide enough to impede a striking passenger vehicle, withstand a 30 mph impact, and absorb energy to further protect occupants.
• FMVSS 224, “Rear impact protection,” is a vehicle standard that requires all new trailers and semi-trailers with a gross vehicle weight rating (GVWR) of at least 10,000 pounds to carry a rear impact guard meeting FMVSS 223.

These standards were first issued in 1996. In 2022, following a congressional mandate in the Bipartisan Infrastructure Law, NHTSA upgraded FMVSS 223 and 224 to require rear impact guards to protect occupants of compact and subcompact passenger cars striking the rear of trailers at 56 km/h (35 mph) in both full-width and 50% overlap configurations. The updated rule became effective in January 2023.

30% Overlap Gap

NHTSA’s 2022 rule covered full-width and 50% overlap crashes but left out the 30% overlap configuration, even though IIHS research shows that 30% overlap crashes are a common and deadly real-world scenario. Safety groups petitioned NHTSA to reconsider, calling the final rule “inadequate and dangerous” for leaving this configuration unprotected. NHTSA denied the petition in 2024. In crash litigation, a 30% overlap underride into a trailer without a TOUGHGUARD-rated guard can be strong evidence that the trailer fell short of a recognized safety standard, even if it technically complied with federal minimums.

IIHS Crash Testing Results

Since 2011, IIHS has conducted crash tests of rear underride guards on semi-trailers. Each test sends a midsize car at 35 mph into the back of a parked semitrailer in three configurations: full-width (center impact), 50% overlap, and 30% overlap. To earn the IIHS TOUGHGUARD award, a trailer must prevent underride in all three tests.

When IIHS began testing in 2011, most guards failed in offset impacts. A guard that survived a full-width center impact would buckle and fold when struck at 50% or 30% overlap, letting the car slide under the trailer. Over the following decade, the IIHS testing program and the 2022 NHTSA rulemaking pushed trailer manufacturers to build stronger guards. Nine trailer manufacturers have earned the TOUGHGUARD award for some or all of their trailer models: Great Dane, Hyundai Translead, Kentucky Trailer, Manac, Stoughton, Strick, Utility, Vanguard, and Wabash.

TOUGHGUARD trailers are the voluntary high end of the market. Trailers built before the 2023 effective date of the upgraded rule stay on the road for decades. Many have rear guards that comply with the pre-2023 standard but would fail an offset impact test, so they provide little protection in the most common real-world underride configurations.

Side Guard Gap in Federal Law

Rear underride guards have been federally required since 1996, but no federal requirement exists for side underride guards on trailers. A passenger car that drifts or is forced into the side of a trailer during a lane-change crash, a T-bone intersection collision, or a merge has almost no structure between its roof and the trailer’s undercarriage.

IIHS ran side underride guard tests in 2017. A midsize sedan struck the center of a 53-foot semitrailer at 35 mph. With only an aerodynamic side skirt on the trailer, the car slid completely under, and the impact sheared off part of the roof and wedged the car beneath the trailer. When the same test was run with an AngelWing side guard installed, the guard bent but the car did not go under, and occupants would likely have sustained only minor injuries. A 2022 NHTSA side underride analysis estimated that side guards could save approximately 17 lives and prevent 69 serious injuries each year, but found the cost-benefit calculation negative under its central assumptions and declined to mandate side guards at that time.

A June 2024 NHTSA Report to Congress on Side Underride Protection found that side underride fatalities were approximately 80% higher than the number reported in fatal crash databases, because law enforcement often fails to identify and record underride events accurately.

Stop Underrides Act 2.0

The STOP Underrides Act, which would require rear, side, and front underride guards on commercial trailers, has been introduced in Congress in 2017, 2019, 2021, and again in 2026. In February 2026, Senators Gillibrand and Luján and Representatives Cohen, DeSaulnier, and Ross introduced the Stop Underrides Act 2.0 (H.R. 7354 and S. 3775), which would:

• Require NHTSA to finalize rulemaking mandating side underride guards on commercial trucks
• Restart the DOT’s Advisory Committee on Underride Protection
• Require DOT to publish underride research on a public website
• Instruct NHTSA to review and correct underride misclassifications in its FARS crash database
• Mandate free law enforcement training on identifying and documenting underride crashes

Underride provisions were included in the 2021 Bipartisan Infrastructure Law, but a complete side underride mandate has not cleared Congress. Until it does, carriers remain free to operate trailers with no side underride guards at all.

Legislative Gap as a Liability Theory

The absence of a federal side guard mandate does not insulate a carrier or trailer manufacturer from liability when a person dies in a side underride crash. Courts evaluate product liability and negligence claims against what a safe vehicle design would include. IIHS testing has shown that side guards work, NHTSA has acknowledged the fatality problem, and Congress has repeatedly tried to mandate the guards. A carrier that chose not to equip trailers with available side guard technology faces significant exposure to negligence and failure-to-warn claims.

Liability Theories When Guards Fail

Negligence and the Standard of Care

For rear underride crashes involving trailers built after January 2023, a guard that fails under the current FMVSS 223 and 224 standard is a direct violation of federal law. For pre-2023 trailers, the questions are whether the guard met the standard in effect at the time of manufacture and whether the carrier or trailer owner kept the guard in proper condition. A corroded, bent, or detached rear guard that was never repaired is direct carrier negligence regardless of manufacture date.

Products Liability for Defective Design or Manufacture

Trailer manufacturers can be held strictly liable when a guard fails in a way that a properly designed guard would not. When IIHS testing shows that TOUGHGUARD-rated guards prevent underride in configurations where a competing product fails, that testing can be offered as evidence in products liability litigation against the manufacturer of the failing guard.

Negligent Maintenance

Under 49 CFR Part 396, carriers must systematically inspect, repair, and maintain the motor vehicles under their control, and prior inspection records can show whether guard damage predating the crash was known and ignored. The driver vehicle inspection reports required under Part 396 are part of that record, alongside the trailer’s crash investigation evidence.