Truck Underride Crashes and Guard Requirements

An underride crash happens when a smaller vehicle slides beneath a truck or trailer. The trailer can enter the passenger compartment instead of striking the vehicle’s front-end crash structure. Guards are designed to limit that intrusion, but rear and side protection are different systems (IIHS truck-underride research).

Rear underride guards are the horizontal metal bars mounted below the backs of tractor-trailers. They are designed to engage the front of a smaller vehicle during a rear-end collision and limit how far it slides beneath the trailer. A guard that performs as intended can keep the trailer from entering the passenger compartment, allowing the car’s crash structure, seat belts and airbags to provide protection (NHTSA’s final rule).

Federal rules require rear guards on many trailers and set minimum requirements for their dimensions, installation, strength and energy absorption. Those requirements do not guarantee protection in every crash. The amount of the car that strikes the guard, the impact speed, the guard’s design and the condition of its attachments can affect the result (Federal Motor Vehicle Safety Standard No. 223, Federal Motor Vehicle Safety Standard No. 224, IIHS crash testing).

Passenger Protection from Rear Underride Guards

A trailer’s bed and supporting frame can sit above the front of a passenger car. Without an effective barrier below that frame, the car’s front end can travel underneath it. In a severe underride collision, the trailer reaches the windshield and occupant compartment, where people are sitting. Federal safety research calls this passenger compartment intrusion (NHTSA’s final rule).

The rear guard creates a lower contact surface. Its horizontal bar connects through supports and attachments to the trailer’s structure. When a car strikes the bar, those connections transmit forces into the trailer. The route the force follows is called a load path. A guard needs both a suitable horizontal member and attachments capable of carrying those loads (Federal Motor Vehicle Safety Standard No. 223, NHTSA’s final rule).

Depending on its design, the guard can also absorb energy as it deforms. Deformation does not necessarily mean the guard failed. Federal standards expressly address energy absorbed through permanent bending. The purpose is to limit underride while managing crash forces, not to leave the bar untouched after a collision (Federal Motor Vehicle Safety Standard No. 223).

Keeping the trailer out of the passenger compartment changes how the car responds to the impact. Its front-end structure can crush and absorb energy, while its restraints work within the space intended for occupants. When the car slides beneath the trailer, the impact can bypass those front-end structures and crush the occupant compartment instead (IIHS crash testing).

Federal Rear Underride Guard Requirements

Two National Highway Traffic Safety Administration standards govern the rear-guard system on covered new trailers:

• Federal Motor Vehicle Safety Standard No. 223, “Rear impact guards,” addresses the guard itself, including strength, energy absorption, labeling and installation instructions.
• Federal Motor Vehicle Safety Standard No. 224, “Rear impact protection,” addresses the trailer, including guard placement and installation (Federal Motor Vehicle Safety Standard No. 224, Federal Motor Vehicle Safety Standard No. 223).

The Federal Motor Carrier Safety Administration has separate requirements for rear protection on vehicles in service. Its rule, 49 CFR § 393.86, generally requires covered trailers manufactured on or after January 26, 1998, to meet the guard and installation standards in effect when the trailer was manufactured (FMCSA’s rear-protection rule).

Trailers Covered by Rear Guard Rules

The modern trailer requirements generally cover trailers and semitrailers with a gross vehicle weight rating of at least 10,000 pounds. Gross vehicle weight rating is the manufacturer’s rated maximum loaded weight, not the trailer’s weight on the day of a crash (NHTSA’s final rule).

The rules contain defined exceptions. Standard No. 224 excludes pole trailers, pulpwood trailers, certain low-chassis and special-purpose vehicles, wheels-back vehicles, road-construction controlled horizontal-discharge trailers and temporary living quarters. These categories depend on the vehicle’s construction and the regulation’s definitions. An ordinary freight trailer does not become exempt simply because its owner describes it as special-purpose equipment (Federal Motor Vehicle Safety Standard No. 224).

An exemption from Standard No. 224 also should not be read as a universal exemption from every rear-protection requirement. Section 393.86 contains a separate set of provisions for certain older trailers and other motor vehicles. Those provisions use different dimensions and exceptions (FMCSA’s rear-protection rule).

Rear Guard Placement Requirements

For a covered trailer under the modern standards, the guard must meet these basic dimensional requirements:

These measurements come from Standard No. 224’s installation requirements and Standard No. 223’s horizontal-member requirement. The regulations include limited allowances for rounded corners (Federal Motor Vehicle Safety Standard No. 224, Federal Motor Vehicle Safety Standard No. 223).

Standard No. 224 also specifies the condition in which installation dimensions are measured: the trailer rests on level ground, unloaded, with a full fuel supply and the tires and any air suspension set according to the manufacturer’s recommendations. A photograph taken on uneven pavement or after a collision is not the same as a measurement under those conditions (Federal Motor Vehicle Safety Standard No. 224).

Rear Guard Strength and Energy Absorption

Standard No. 223 uses controlled force tests to evaluate a guard’s resistance and deformation. These are quasi-static tests, meaning a testing device applies force slowly under specified conditions. They are not a requirement to crash a passenger car into every trailer model. The guard can be tested on an appropriate rigid fixture or a complete trailer (Federal Motor Vehicle Safety Standard No. 223).

Under the upgraded requirements, the guard must resist:

• A 50,000-newton force at each specified point-test location.
• A uniformly distributed force of at least 350,000 newtons across the horizontal member.

The guard must resist those forces without deflecting more than 125 millimeters, approximately 5 inches, and without eliminating a load path that existed before testing began (Federal Motor Vehicle Safety Standard No. 223).

For guards subject to the ordinary energy-absorption requirement, the standard requires at least 20,000 joules of energy absorption through permanent deformation within the first 125 millimeters of movement. It also controls ground clearance at the supports after testing. The rule provides an alternative for guards that resist a distributed load greater than 700,000 newtons, as well as specified treatment for hydraulic and tanker-trailer guards (Federal Motor Vehicle Safety Standard No. 223).

These technical requirements distinguish a rear impact guard from a bar that merely looks substantial. Its performance depends on the bar, supports, attachment hardware and the trailer structure receiving the load. Standard No. 224 requires installation according to the guard manufacturer’s instructions on a suitable vehicle (Federal Motor Vehicle Safety Standard No. 224, Federal Motor Vehicle Safety Standard No. 223).

2022 Rear Guard Standards

NHTSA’s 2022 final rule strengthened the rear-guard standards using requirements similar to Canada’s. The upgrade was designed to protect occupants of compact and subcompact passenger cars striking the rear of a trailer at 35 mph in two configurations:

• A centered impact in which the car’s full width overlaps the trailer.
• An offset impact in which 50 percent of the car’s width overlaps the trailer (NHTSA’s final rule).

“Overlap” describes how much of the passenger vehicle’s width engages the trailer. A 50 percent overlap collision involves half the car’s width. It does not mean half the trailer is struck (IIHS underride ratings).

The rule became effective January 11, 2023, but its mandatory compliance date was July 15, 2024. Manufacturers could comply early. Those dates serve different purposes, so a trailer manufactured during the transition period should not automatically be treated as subject to the upgraded requirements merely because it was built after January 2023 (NHTSA’s final rule).

The upgrade did not impose a general retrofit requirement on existing trailers. For a trailer already in service, its manufacture date and the standards applicable at that time remain relevant. Older equipment still must satisfy the rear-protection and maintenance rules that apply to it (FMCSA’s rear-protection rule, NHTSA’s final rule).

Trailer Corner Impacts Against Rear Guards

A car that strikes the center of a guard can engage a different part of its supporting structure than a car that clips the end of the bar. In a small-overlap collision, the impact may load the horizontal member outside its vertical support. That outer portion can bend forward without adequately stopping the car (IIHS crash testing, NHTSA’s final rule).

The Insurance Institute for Highway Safety demonstrated this problem in its 2013 rear-underride crash testing. A 2010 Chevrolet Malibu struck parked trailers at 35 mph. All eight tested trailer designs prevented underride in the centered, full-width configuration. Seven passed the 50 percent overlap test. Only one, a Manac design, passed the 30 percent overlap test (IIHS crash testing).

Those results describe the designs tested at that time. They should not be presented as the performance of every current trailer made by those manufacturers. Later designs improved, and IIHS created its Toughguard award to recognize better rear-underride protection (IIHS Toughguard program, IIHS underride ratings).

The current IIHS underride-rating program explains the full-width, 50 percent and 30 percent overlap configurations. It also allows a specified quasi-static test for the 30 percent overlap evaluation instead of an actual vehicle crash. Recognition applies to listed trailer models, manufacture periods or guard options, not automatically to every trailer carrying a manufacturer’s name (IIHS underride ratings).

As of October 11, 2026, IIHS lists ten manufacturers with qualifying Toughguard trailers: Fruehauf, Great Dane, Hyundai Translead, Kentucky Trailer, Manac, Stoughton, Strick, Utility, Vanguard and Wabash. The listed model, date and equipment conditions still control; the manufacturer’s name alone does not establish that a particular trailer qualifies (current Toughguard coverage).

NHTSA’s 2022 upgrade did not establish a mandatory 30 percent overlap performance requirement. In its 2024 response to a reconsideration petition, the agency retained that decision, explaining that it had not found the proposed requirement reasonable and practicable for all covered trailer types under the governing federal safety law. That regulatory decision does not mean stronger corner-impact protection is unavailable (NHTSA’s reconsideration decision, NHTSA’s final rule).

Limits of Rear Underride Protection

Impact Speed and Rear Guard Protection

The 35 mph protection objective is tied to specified impact configurations and the research supporting the standard. It is not a promise that every compliant guard will prevent intrusion at that speed in every collision. Nor is it a certification of occupant protection in a 65 mph impact. NHTSA’s rule separately discusses research into higher-speed protection (NHTSA’s final rule).

Impact speed also differs from a vehicle’s earlier travel speed. In an actual collision, investigators need to determine the vehicles’ motion at contact. The guard’s response must be evaluated against that reconstructed event, not simply the highway speed limit. NHTSA’s underlying crash research examined relative impact velocity as part of its analysis (NHTSA’s final rule).

Rear Guard Inspection Defects

A guard’s original design cannot compensate for a connection that is no longer secure. Broken or missing fasteners, cracked welds, cracked supporting metal and other damage compromising attachment are expressly listed as rear-guard inspection defects (Federal periodic inspection standards).

A damaged bar also may no longer occupy its required position. Inspection standards address excessive height, inadequate width, excessive setback and insufficient vertical size, along with missing guards (Federal periodic inspection standards).

Injuries When a Rear Guard Holds

Preventing the trailer from entering the occupant compartment does not eliminate the rear-end impact. The passenger vehicle still experiences crash forces. IIHS testing found substantially different occupant-compartment damage and injury measurements when guards held compared with tests in which the car underrided the trailer, but a successful guard is not a guarantee against injury (IIHS crash testing).

Side Underride Guard Protection

A rear guard protects the opening beneath the trailer’s back. It does not create a barrier along its sides. A vehicle striking the side can therefore underride a trailer even when its rear guard is intact. IIHS treats side underride protection separately from rear-guard performance.

IIHS tested an AngelWing side guard in 2017 at 35 mph and later at 40 mph. In the 40 mph test, a 2009 Chevrolet Malibu struck the center of a 53-foot dry van trailer. The guard bent but kept the car from traveling underneath; the seat belt and airbags restrained the driver-seat dummy. Its measurements indicated a low likelihood of injuries in a similar crash (IIHS side-guard testing).

A separate 35 mph test without a side guard ended with the car lodged beneath the trailer and part of its roof torn off. These tests show what the tested equipment achieved in specified conditions. They do not establish that every side guard will prevent intrusion at every speed, angle or contact point (IIHS test results).

Side Underride Guards Under Federal Law

Federal rear-guard standards should not be read as a side-guard requirement. IIHS’s current truck-safety overview describes side guards as a separate protection measure and discusses the federal side-guard analysis. A device used to improve a trailer’s aerodynamics is not evidence, by itself, that the trailer has crash-tested side-underride protection.

Side Underride Fatality Estimates

NHTSA’s June 2024 report to Congress examined underreporting in federal crash data. It estimated that side-underride fatalities were about 80 percent higher than reported in the databases and applied a 1.8 adjustment factor. The report estimated an average of 89 deaths annually within its defined scope: occupants of light passenger vehicles in two-vehicle crashes that struck and underrided the side of a tractor-trailer (NHTSA’s side-underride report).

The same report describes the April 2023 cost-benefit analysis, which estimated that side guards could save about 17 lives and prevent about 69 serious injuries annually if all trailers in the fleet were equipped. That calculation addressed a narrower target population and impact speeds of 40 mph or less. The potential benefit estimate is not the total number of side-underride deaths, and the report describes an advance notice of proposed rulemaking rather than a final side-guard mandate (NHTSA’s analysis and rulemaking account).

Stop Underrides Act 2.0

Lawmakers introduced H.R. 7354 and S. 3775 on February 4, 2026. The introduced Stop Underrides Act 2.0 would direct rulemaking for side guards on new trailers, semitrailers and single-unit trucks. Its proposed passenger-vehicle performance standard addresses perpendicular side impacts at closing speeds up to and including 40 mph. These provisions describe proposed legislation, not requirements created by the existing rear-guard standards.

The proposal also calls for reconvening the Advisory Committee on Underride Protection, a public research website, studies of underride incidents and the 2022 rear-guard rule, review of crash classifications, and free identification training for law enforcement. The House’s recorded latest action is a February 5, 2026 referral to the Subcommittee on Highways and Transit. The bill’s text and legislative record should be checked before treating any proposed obligation as law.

Rear Guard Inspection and Maintenance

Rear-guard compliance continues after manufacture. Federal motor-carrier rules require systematic inspection, repair and maintenance of vehicles under a carrier’s control, with required parts and accessories kept in safe and proper operating condition (Federal inspection and maintenance requirements).

Inspection records, driver vehicle inspection reports and repair records can document earlier damage. Those records should be considered alongside the physical truck-crash investigation evidence when evaluating how the guard performed (federal maintenance requirements).

Rear impact guards are included in the minimum periodic inspection standards in Appendix A to Part 396. For covered modern trailers, a vehicle does not pass that inspection if its guard is missing, insecurely attached or outside the listed dimensional requirements (Federal periodic inspection standards).

An inspection record and a physical examination answer different questions. The record can document what was inspected or repaired at an earlier time. Examining the guard, its fasteners, welds and supporting structure shows their condition after the crash. Neither should substitute for the other when evaluating how the system performed (Federal periodic inspection standards, Federal Motor Vehicle Safety Standard No. 223).

Rear Guard Evidence in Crash Investigations

The conduct that caused a collision and the equipment that affected its severity are separate parts of an investigation. A rear guard does not prevent a driver from reaching the trailer. Its safety function begins when contact occurs, by limiting how far the passenger vehicle travels underneath it (NHTSA’s final rule).

A focused investigation can examine:

• The trailer’s identification, manufacture date and vehicle category.
• The guard’s manufacturer, design and installation instructions.
• The horizontal bar, supports, fasteners, welds and surrounding trailer structure.
• Inspection reports, repair records and any documented earlier damage.
• The point of contact, amount of overlap, impact speed and extent of passenger-compartment intrusion.

These subjects connect the applicable requirements to the mechanisms evaluated in federal standards and IIHS testing. They are investigative considerations, not findings about any particular crash (FMCSA’s rear-protection rule, IIHS crash testing, Federal periodic inspection standards, Federal Motor Vehicle Safety Standard No. 223).

A guard bent or detached after a collision does not, by itself, establish that it violated a federal standard. The collision may differ from the standard’s test conditions, and some guard deformation is contemplated by the energy-absorption requirements. Conversely, meeting a minimum manufacturing requirement does not establish how the guard performed in a particular corner impact or whether its attachments were properly maintained (NHTSA’s reconsideration decision, Federal periodic inspection standards, Federal Motor Vehicle Safety Standard No. 223).

Responsibility requires a separate assessment of the applicable law and the evidence. Guard performance, regulatory compliance and the cause of an occupant’s injuries should not be collapsed into a single conclusion based on a crash photograph.

Underride Guard Design Evidence

Design drawings, attachment instructions, the actual trailer structure and relevant crash tests can help explain why a guard performed as it did. Toughguard recognition is specific to listed models, manufacture periods or guard options. A comparison with a recognized design can inform the engineering analysis, but does not by itself establish a defect or resolve a products-liability claim (IIHS model-specific ratings).

Side Guard Liability Questions

Side-guard questions require a separate review of the equipment, the collision and the governing law. Neither the absence of a federal mandate nor a successful test of a different guard determines responsibility in a particular case. The investigation should identify what equipment was present, what protection it could provide and how that relates to the injuries (side-guard test conditions).