Truck underride guards: how they work, what the rules cover and how to check them
Where federal rules stop, how to choose a labeled guard and how to inspect it
Safety Behind the Wheel Foundation built this guide from Insurance Institute for Highway Safety (IIHS) tests and research, federal vehicle safety and motor carrier regulations, state rules from Texas, Ohio and Pennsylvania, and published underride research. Every source is listed at the end.
What underride is and why a guard helps
Underride happens when a smaller vehicle slides beneath the body of a truck or trailer. A car’s bumper, hood and frame are built to absorb crash energy. In an underride collision, those strong parts may pass beneath the trailer. The trailer’s rear edge can then strike the upper part of the car, where people sit. Researchers call this structural incompatibility [1].
A rear impact guard is a steel bar that hangs from the back of a large truck. It keeps a car from moving underneath in a rear collision [5]. A side underride guard does the same job along the side of a trailer.
The research in this article points to three questions about any guard: how low it hangs, how strong it is, and how well it is attached to the trailer [4].
How often these collisions happen
In 2024, 5,340 people died in collisions involving large trucks, and 62 percent were occupants of cars and other passenger vehicles [11].
Not every one of those collisions involves underride. In 2015, of 1,542 passenger vehicle occupant deaths in tractor trailer crashes, 301 involved the trailer’s side and 292 its rear [7]. In 2021, IIHS counted 488 passenger vehicle occupant deaths in crashes involving the side of a tractor trailer [8].
These counts use different years and definitions. A review of police reports from 29 states also found that the national fatal crash database undercounts side underride collisions by a factor of about three [12].
Rear guards: what the federal rules require
Three federal rules shape rear guards. Federal Motor Vehicle Safety Standard (FMVSS) No. 224 says which new trailers need a guard and where it must sit [2]. FMVSS No. 223 sets how strong the guard must be [9]. A motor carrier safety rule applies the requirement to trailers on the road [3].
The rules use gross vehicle weight rating, or GVWR. This is the most a vehicle may weigh when fully loaded, as set by its maker. The rear guard rule covers trailers and semitrailers with a GVWR of 4,536 kilograms, or 10,000 pounds, or more [2, 3]. A semitrailer rests part of its weight on the tractor that pulls it.
Comparison compiled by Safety Behind the Wheel Foundation
What the federal rear guard rules require
| Feature | What the rule text says | Why it matters |
|---|---|---|
| Weight | Trailers with a GVWR of 10,000 lb or more [2, 3] | Lighter trailers are not covered |
| Height | Bottom edge no more than 22 in. (560 mm) above ground [2, 3] | A high bar lets a car’s nose slip under |
| Width | Ends within 4 in. (100 mm) of the trailer’s sides [2, 3] | The outer ends need protection too |
| Position | Rear face within 12 in. (305 mm) of the trailer’s back edge [2, 3] | Keeps the bar at the very end |
| Label | “DOT,” maker’s name, month and year of manufacture [9] | Lets you check certification |
The strength tests push on the guard with set forces: 50,000 newtons at each of two test points, and at least 350,000 newtons spread across the bar [9]. A newton is a metric unit of force. Fifty thousand newtons is about 11,000 pounds.
Trailers the rule leaves out
The standards exclude pole trailers, pulpwood trailers and road construction trailers with controlled horizontal discharge. They also exclude special purpose vehicles, low chassis vehicles, wheels back vehicles and trailers built as temporary living quarters [2]. The motor carrier rule also exempts driveaway towaway operations [3]. Two terms in plain words [2]:
- Wheels back: the rear tires sit within about 12 inches of the trailer’s back edge.
- Special purpose: the trailer carries work equipment that fills certain space in transit, or has a loading platform that folds out from the rear.
The 2022 update and what it leaves out
Both standards carry an amendment dated July 15, 2022 [2, 9]. This article does not restate each change. For exact start dates, read the current text on the eCFR site, or ask the maker which version a guard meets.
IIHS says today’s federal rules already require every guard to withstand its full width and 50 percent overlap crash tests [5]. Overlap is how much of the car’s front lines up with the guard, so a low overlap means a hit near the trailer’s outer corner.
The IIHS TOUGHGUARD award recognizes guards that stop underride in three tests. In each, a midsize car at 35 mph hits the back of a parked semitrailer. The setups are full width, 50 percent overlap and 30 percent overlap [5]. The corner test has been the hard one to pass [6].
Why corners matter
In 2013, IIHS tested trailers from eight makers. In the 30 percent overlap test, every trailer but one failed [6]. The car hit outside the guard’s vertical attachment bar, and the unsupported end bent forward [6]. On the trailer that passed, the supports sat about 18 inches from the edge. On the others, they averaged 28 inches [6].
That test predates the 2022 amendments, but it shows that where the supports sit matters as much as bar strength.
What research found in real collisions
A study of 115 real collisions from the Large Truck Crash Causation Study, a detailed review of truck crashes, looked at how guards performed. Almost half of the passenger vehicles had severe or catastrophic underride damage. They accounted for 23 of the 28 collisions with an occupant killed [4].
On trailers with guards that met one or both federal standards, deformation or failure was frequent. Causes were weak attachments, frame buckling and bending of the guard’s outer end in narrow overlap collisions [4]. The authors also found fault with the rule. It allowed too much ground clearance. Its test conditions allowed designs that fail in narrow overlap crashes. And certifying guards apart from the trailers they ride on produced weak attachments [4].
That study covered guards built under the standards in place since 1998 [4]. The lesson holds for newer guards too: height, outer ends and mounting are where guards fail.
Trucks and trailers the rear rule does not reach
Protection differs by vehicle type. In the same study, most truck units qualified for at least one exemption. The largest groups were trailers with small wheel setbacks and single unit straight trucks. Dump trucks were an especially hazardous category [4].
A single unit truck has its cab and cargo body on one frame, as box and dump trucks do. The federal rear guard rules cover trailers and semitrailers, so they do not reach these trucks [2, 4].
Comparison compiled by Safety Behind the Wheel Foundation
Which vehicles the federal rear guard rule covers
| Vehicle | Federal rear guard rule | What to do |
|---|---|---|
| Trailer of 10,000 lb GVWR or more, built on or after Jan. 26, 1998 | Covered, unless an excluded type [2, 3] | Check the DOT label |
| Single unit truck, such as a box or dump truck | Not covered by the trailer rules [2, 4] | Ask your state agency about other rules |
| Trailer under 10,000 lb GVWR | Not covered [2] | Ask the maker what protects the rear |
| Excluded trailer types | Not covered [2] | Ask the maker how the rear is protected |
| Any truck’s side | No federal side guard rule found [7, 8] | Treat a side guard as extra equipment |
If your trailer’s GVWR is under the threshold, or it is an excluded type, no federal rule asks for a rear guard. The same goes for a box or dump truck. Closing that gap is a choice, and the maker can advise.
Side underride guards: the research so far
In 2017, IIHS reported that federal rules require underride guards on the rear of tractor trailers but not on the sides [7]. In 2023, the National Highway Traffic Safety Administration (NHTSA) asked for public comment on a cost and benefit analysis of side guards [8]. No side guard rule appears in the sources reviewed.
Studies of the benefit use different crash sets, so results differ.
- Crash case review. An IIHS researcher reviewed 206 truck side collisions. In 143, the side impact caused the worst injury to a passenger vehicle occupant. Guards could have reduced injury severity in 76 of those, including 38 of the 49 serious or fatal cases [1].
- A 40 mph test. In 2017, IIHS hit the center of a 53 foot dry van trailer with a car at 40 mph. The side guard bent, but the car did not go under, and the airbags and belt held the test dummy [7].
- Federal estimate. NHTSA estimated that guards on all large trucks would cost $973 million to $1.2 billion and prevent 17 deaths and 69 serious injuries a year [8].
- IIHS estimate. IIHS projected 159 to 217 passenger vehicle occupant deaths prevented per year. The range depends on whether guards cover the area forward of the rear axle or the full trailer length. It said NHTSA’s estimate leaves out many relevant collisions, including ones with more than two vehicles and ones above 40 mph [8].
- Police report review. A 2013 review estimated 202 side underride deaths a year with the truck’s body pushing into the passenger space. It found serious nonfatal injuries in these wrecks to be rare [12].
One researcher notes that the strength and placement of a side guard remain technical challenges [1]. Treat any single number as an estimate.
How state rules reach trucks that stay in state
The federal rear guard rule sits in Part 393 of the federal motor carrier safety regulations [3]. States choose how to apply those parts to trucks that stay inside their borders [13, 14, 15]. Three examples show the range, as of October 2026.
Comparison compiled by Safety Behind the Wheel Foundation
How three states apply federal truck rules
| State | What the rule does | Who it covers |
|---|---|---|
| Texas | Adopts federal parts 390 to 393 and others, as amended through Sept. 1, 2022 [13] | In state commercial vehicles that meet set criteria, such as over 26,000 lb [13] |
| Ohio | Adopts federal parts 390 to 397, effective Sept. 26, 2024 [14] | Motor carriers in intrastate commerce [14] |
| Pennsylvania | Adopts federal parts 390 to 396 [15] | Motor carriers in intrastate commerce [15] |
Texas uses weight and use thresholds [13]. Ohio and Pennsylvania speak more broadly of carriers in intrastate commerce [14, 15]. Check your own state’s agency page before you assume a rule does or does not apply to your truck.
Choosing compatible, tested equipment
Carriers, contractors, tradespeople and drivers usually buy guards through trailer makers and dealers, vehicle makers’ parts departments and truck equipment distributors. Start with your trailer’s maker, because a guard has to match the frame it bolts to. RV owners and others who tow heavy trailers should ask whether their trailer falls into an excluded group, such as temporary living quarters [2].
A certified rear guard must carry a label with the maker’s name, the month and year of manufacture, and the letters DOT, which serve as the maker’s certification [9]. The guard maker must also supply printed instructions that list the vehicles the guard fits and how it attaches to each [9]. Those instructions are your compatibility check. The guard maker certifies the guard. The carrier or owner then keeps it in working order [9, 10].
No federal side guard standard appears in the sources reviewed, so there may be no DOT label to look for [7, 8]. Ask the maker for crash test evidence and installation limits in writing. Never drill, weld or cut a trailer frame or guard yourself.
Inspecting and caring for guards
Federal inspection rules say parts and accessories, including those covered by Part 393, must be in safe and proper operating condition at all times [10]. Safety Behind the Wheel Foundation gives fleets the same advice for every guard. Carriers must also keep inspection, repair and maintenance records for each vehicle they control for 30 consecutive days [10].
Use this routine before long trips and after any backing mishap:
- Walk to the back of the trailer. Confirm a guard is there and its DOT label is readable [9].
- Look for bent, cracked or sagging bars. Dock strikes, curbs and backing mistakes can bend a guard.
- Check every bolt, weld and bracket where the guard joins the trailer. The research found weak attachments and frame buckling [4].
- Measure from the ground to the bar’s bottom edge, which should be no more than 22 inches. Check that the bar’s ends come within 4 inches of the trailer’s sides [2, 3].
- Repair or replace damaged parts using the guard maker’s instructions. Do not bend a guard back and keep driving.
- Write down the date, what you found and what you fixed [10].
What drivers, families and local leaders can do
Guards lower risk, but they do not make a collision safe. IIHS tests rear guards at 35 mph and one side guard at 40 mph [5, 7]. Real crashes can be faster. So keep a generous gap behind trucks and stay out of the space beside trailers. See our guides to sharing the road with trucks and safe following distance.
If you manage a fleet, add the guard to your inspection list. Ask which box trucks and dump trucks have rear protection, since the federal trailer rule does not cover them [2, 4]. If you serve on a school board or town council, ask the same of vehicles your agency owns or hires. If you see a damaged guard, note the company name on the vehicle and tell the carrier. Your state highway safety office can point you to the right agency.
Frequently asked questions
Do all trucks and trailers have underride guards?
No. Federal rules require rear guards on trailers and semitrailers of 10,000 pounds GVWR or more, with several excluded types [2, 3]. They do not cover single unit trucks such as box and dump trucks [2, 4]. The sources reviewed show no federal side guard rule [7, 8]. Some trucks carry guards by choice or under other rules, so ask the owner.
Can a guard that meets the federal standard still fail?
Yes. In real collisions, guards that met one or both federal standards often deformed or failed. Causes included weak attachments, frame buckling and outer end bending [4]. In 2013 IIHS tests, all but one trailer failed the 30 percent overlap test [6]. The standards were amended in 2022, so ask for current test evidence on any guard you buy [2, 9].
What does the IIHS TOUGHGUARD award mean?
It marks a rear guard that stopped underride in three crash tests at 35 mph: full width, 50 percent overlap and 30 percent overlap [5]. It is a voluntary recognition by a research group, not a federal certification. The DOT label is the federal certification mark [9].
Can I add a guard to my own trailer or box truck?
Start with the trailer maker or a trailer dealer. A guard’s printed instructions list the vehicles it fits and how it attaches [9]. Guards certified apart from the trailer can end up with weak attachments [4]. Do not drill, weld or improvise. Ask your state agency whether any rule applies to your truck.
Does a guard mean I can follow a truck more closely?
No. Rear guard tests use a 35 mph crash [5]. A guard may help in a wreck, but it cannot stop one. Room to stop and staying out of blind spots remain your best protection.
The bottom line
Recommendation from Safety Behind the Wheel Foundation
Safety Behind the Wheel Foundation urges fleets and owners to choose labeled, tested guards that match the trailer and to inspect them often. Underride guards look simple, but their performance depends on height, strength and attachment. Federal rules set a floor for many rear guards. They leave out single unit trucks, some trailer types and, in the sources reviewed, the sides of every truck.
If you buy, run or repair trucks and trailers, choose guards that are labeled, matched to your trailer and checked often. If you share the road with them, keep your distance.
Related articles
Sources and further reading
- 1Brumbelow ML. Potential benefits of underride guards in large truck side crashes. Traffic Injury Prevention 2012;13(6):592-599. PubMed record. Data year: 2006 to 2008. pubmed.ncbi.nlm.nih.gov/23137090
- 2Legal Information Institute, Cornell Law School (text of the Code of Federal Regulations). 49 CFR 571.224 – Standard No. 224; Rear impact protection. Amendment note dated July 15, 2022; accessed October 2026. law.cornell.edu/cfr/text/49/571.224
- 3Legal Information Institute, Cornell Law School (text of the Code of Federal Regulations). 49 CFR 393.86 – Rear impact guards and rear end protection. Latest amendment note dated February 19, 2026; accessed October 2026. law.cornell.edu/cfr/text/49/393.86
- 4Brumbelow ML, Blanar L. Evaluation of US rear underride guard regulation for large trucks using real-world crashes. Stapp Car Crash Journal 2010;54:119-131. PubMed record. Data year: Large Truck Crash Causation Study cases (years not stated in the abstract). pubmed.ncbi.nlm.nih.gov/21512906
- 5Insurance Institute for Highway Safety. Truck underride guard ratings. No update date shown; accessed October 2026. iihs.org/…/truck-underride
- 6Insurance Institute for Highway Safety. New crash tests: Underride guards on most big rigs leave passenger vehicle occupants at risk in certain crashes. Published March 14, 2013. Data year: 2013 (crash tests). iihs.org/…/new-crash-tests-underride-guards-on-mo…
- 7Insurance Institute for Highway Safety. Side guard on semitrailer prevents underride in 40 mph test. Published August 29, 2017. Data year: 2015 (fatality counts). iihs.org/…/side-guard-on-semitrailer-prevents-und…
- 8Insurance Institute for Highway Safety. NHTSA study underestimates benefits of side underride guards for trucks. Published June 13, 2023. Data year: 2021 (fatality counts). iihs.org/…/nhtsa-study-underestimates-benefits-of…
- 9Legal Information Institute, Cornell Law School (text of the Code of Federal Regulations). 49 CFR 571.223 – Standard No. 223; Rear impact guards. Amendment note dated July 15, 2022; accessed October 2026. law.cornell.edu/cfr/text/49/571.223
- 10Legal Information Institute, Cornell Law School (text of the Code of Federal Regulations). 49 CFR 396.3 – Inspection, repair, and maintenance. Accessed October 2026. law.cornell.edu/cfr/text/49/396.3
- 11Insurance Institute for Highway Safety. Fatality Facts 2024: Large trucks. Posted June 2026. Data year: 2024. iihs.org/…/large-trucks
- 12Padmanaban J. Estimating side underride fatalities using field data. Annals of Advances in Automotive Medicine 2013;57:225-232. PubMed record. Data year: police reports from 29 states (years not stated in the abstract). pubmed.ncbi.nlm.nih.gov/24406960
- 13Legal Information Institute, Cornell Law School (Texas Administrative Code). 37 Tex. Admin. Code § 4.11 – General applicability and definitions. Last amended effective January 8, 2023. law.cornell.edu/…/37-Tex-Admin-Code-SS-4-11
- 14Legal Information Institute, Cornell Law School (Ohio Administrative Code). Ohio Admin. Code 4901:2-5-03 – Adoption and applicability of U.S. department of transportation safety standards. Most recent effective date September 26, 2024. law.cornell.edu/…/Ohio-Admin-Code-4901-2-5-03
- 15Legal Information Institute, Cornell Law School (Pennsylvania Code). 67 Pa. Code § 231.1 – General information and requirements. Page shows amendment effective April 24, 2010; accessed October 2026. law.cornell.edu/…/67-Pa-Code-SS-231-1
This article reflects the safety priorities of commercial vehicle underride protection and fleet equipment safety. It is general education, not legal, insurance or professional equipment advice. Federal and state rules change, so verify details with official sources. For your own trucks and trailers, contact your state motor carrier or highway safety agency, your trailer or truck maker, or a qualified repair shop. Reviewed October 2026.
Cite this article: Safety Behind the Wheel Foundation. “Truck underride guards: how they work, what the rules cover and how to check them.” drivewithcare.org, https://drivewithcare.org/commercial-underride-guards-rear-side-rules/. Reviewed October 2026.
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