Underride Crashes: Why Side Guards Aren't Required Yet

Most truck accident claims turn on who caused the collision. An underride claim often turns on something that happened after the collision was already underway — whether the trailer itself was built and equipped to stop a passenger vehicle from sliding underneath it, and whether federal law even required that it be.

Quick answer: Federal law has required rear underride guards on trailers since 1996 — FMVSS No. 224 requires the guard, FMVSS No. 223 sets its strength standard — and a July 2022 rule raised the protection standard from 30 mph to 35 mph impacts. Side underride guards are not federally required at all, though the Stop Underrides Act 2.0, reintroduced in Congress in February 2026, would change that — it is proposed legislation, not current law. A side underride claim without a specific regulatory violation to point to still runs on ordinary design-defect and negligence theories, using available guard technology and industry practice as evidence of what a reasonably careful manufacturer or carrier should have done.

Why Underride Crashes Are Different

An underride crash occurs when a passenger vehicle slides partially or fully beneath a tractor-trailer, typically the rear or side, striking the trailer’s undercarriage at roughly windshield height rather than being absorbed by the vehicle’s bumper-height crumple zones and frame. Because passenger vehicles are engineered around a bumper-height impact — that is where the crumple zones, airbags, and structural reinforcement are concentrated — an underride collision routinely produces catastrophic head, neck and upper-body injuries even at speeds that would be survivable in an ordinary rear-end collision.

This is precisely why the government has regulated guards — physical barriers mounted on a trailer specifically to stop a car from sliding underneath — as a distinct category of truck safety equipment.

Rear Guards Are Required. Side Guards Are Not.

This asymmetry is the single most important fact in this area of law, and it is not intuitive.

FMVSS No. 224, in effect since 1996, requires new trailers and semitrailers with a gross vehicle weight rating of 10,000 pounds or more to be equipped with a rear impact guard meeting the strength and energy-absorption requirements of FMVSS No. 223. A rear underride claim against a trailer that lacked a compliant guard, or whose guard was damaged, missing, or improperly maintained, can point directly to a specific federal safety standard the vehicle was required to meet.

Side underride guards have no equivalent federal mandate at all. A side underride crash — arguably just as catastrophic as a rear one — is not currently governed by any FMVSS requiring a side guard, even though the technology to build one exists and has been used elsewhere, including under European Union requirements that have applied for years.

The Rear Guard Standard Just Got Stronger

A final rule published July 15, 2022 upgraded FMVSS Nos. 223 and 224 in a way that matters directly to any claim involving a trailer built before that rule took effect versus after:

• The old standard was built around protecting occupants of compact and subcompact cars in impacts up to 30 mph, using a peak-load test of 100,000 newtons applied at specific points on the guard.
• The updated standard raises that to 35 mph impact protection, requiring the guard to withstand a uniformly distributed load of 350,000 newtons while limiting deflection to no more than 125 millimeters — a materially more demanding test across the whole guard rather than at isolated points.

Which standard applied to a specific trailer depends on its manufacture date relative to the rule’s compliance deadlines — a threshold factual question worth establishing early, since it determines what the manufacturer was actually required to build.

Side Underride Guards: Proposed, Not Required

The Stop Underrides Act has been introduced in Congress repeatedly — in 2017, 2019, 2021, and again in February 2026 as the Stop Underrides Act 2.0 — and would require rear, side, and front underride guards on commercial trailers. It has not become law. NHTSA’s own side underride guard rulemaking remains at an early “analyzing comments” stage as of this writing, and a 2024 report to Congress confirmed no federal mandate for side underride guards currently exists.

This matters for how a side underride claim actually gets built: there is no regulation to point to as the standard the trailer failed to meet, the way a rear underride claim can point to FMVSS 223/224. See our explainer on defective product claims for how a design-defect theory works generally in the absence of a specific applicable regulation.

Building the Claim Without a Side Guard Mandate

A side underride claim can still succeed, but it has to be built on evidence rather than a regulatory citation:

• Available technology. Side underride guard designs exist and have been commercially deployable for years — evidence a manufacturer or fleet operator could have installed one is central to the claim.
• Industry and international practice. The European Union has required side underride protection on trailers for an extended period, which can support an argument that the risk and the remedy were both well known.
• Cost relative to the trailer’s value. A design-defect claim is generally stronger where the safety measure’s cost was modest relative to the product’s overall value — a factor our guide to defective product claims discusses in the broader risk-utility framework courts apply.

Who Can Be Held Responsible

• The trailer manufacturer, for a rear guard that didn’t meet the applicable FMVSS strength standard when built, or for a side guard omission where the design-defect theory above applies
• The motor carrier or trailer owner, for operating a trailer with a damaged, corroded, or missing rear guard, or for failing to maintain one in compliant condition — relevant to the same third-party claims described in our guide to suing a trucking company after an 18-wheeler accident
• A leasing or maintenance contractor, where trailer upkeep was outsourced and the guard’s condition was neglected

The Evidence That Decides These Claims

• The trailer’s make, model and manufacture date, to identify which version of FMVSS 223/224 applied
• The guard’s physical condition at the time of the crash — corroded, bent, previously damaged in an earlier incident, or missing entirely
• Maintenance and inspection records for the specific trailer
• Accident reconstruction establishing the impact speed, angle, and point of contact, since the guard’s expected performance depends on all three
• For a side underride claim, evidence of guard technology’s availability, cost, and adoption elsewhere at the time the trailer was manufactured

Practical Steps

1. Identify the trailer’s manufacture date as early as possible, since it determines which FMVSS 223/224 standard actually applied.
2. Preserve the guard itself and photograph its condition before any repair, salvage, or disposal of the trailer.
3. Request maintenance and inspection records for the specific trailer, not just the tractor.
4. For a side underride crash, do not assume no claim exists because no specific regulation was violated — the design-defect and negligence theories above may still apply.
5. Get an accident reconstruction expert involved early, since impact speed and angle materially affect what a guard was reasonably expected to withstand.
6. Confirm the trailer’s owner and any maintenance contractor, since responsibility for a guard’s physical condition may sit with a party other than the motor carrier itself.

Sources & Further Reading

• 49 CFR § 571.223 (FMVSS No. 223, Rear impact guards) and 49 CFR § 571.224 (FMVSS No. 224, Rear impact protection) — the federal equipment and installation standards for rear underride guards
• NHTSA final rule, 87 Fed. Reg. 41398 (July 15, 2022) — upgrading FMVSS 223/224 to the 35 mph, 350,000 N distributed-load standard
• Stop Underrides Act 2.0, H.R. 7354 / S. 3775 (introduced February 2026) — proposed legislation that would require side and front underride guards; not enacted as of this writing
• NHTSA, Report to Congress on Underride Protection (2024) — confirming the absence of a federal side underride guard mandate
• See our guides to suing a trucking company after an 18-wheeler accident for the broader framework of carrier and third-party liability, and defective product claims for the design-defect theory a side underride claim generally has to rely on