What are the federal rules for truck rear underride guards?

Last updated October 7, 2026.

Under 49 CFR 393.86, most trailers and semitrailers rated at 10,000 pounds or more and built on or after January 26, 1998 must have a rear impact guard meeting federal safety standards. The bottom of the guard may be no more than 22 inches off the ground, and the guard must reach to within 4 inches of each side of the trailer. Older vehicles have their own, looser guard rule.

An underride crash happens when a car slides under the back of a trailer, so the trailer's frame meets the passenger compartment instead of the bumper. When a guard is missing, bent or set too high, the federal rule becomes part of the truck accident claim, alongside the ordinary question of who caused the crash.

How is an underride crash investigated in Las Vegas?

Preserve the trailer. The guard is physical evidence. Send a written request to the carrier and its insurer to keep the trailer and guard unrepaired until inspected.
Identify the trailer. Its manufacture date decides whether the 1998 rule applies or the older rule for vehicles built after 1952. The trailer's identification plate and registration usually show this.
Measure the guard. Height from the ground, distance from each side, distance from the rear edge, and the height of the horizontal bar are all measurable against the rule.
Request the carrier's maintenance file. Carriers must inspect, repair and maintain their vehicles and keep records (49 CFR 396.3); see truck maintenance rules and records.
Look at how the crash happened. Why the car hit the trailer still matters: a trailer stopped in a travel lane, missing lights, or sudden braking can point to the truck's side; following too closely points to the car's side (NRS 484B.127).
Apply Nevada's fault rules. Each party's percentage is set and recovery is reduced by your share (NRS 41.141). See who can be held responsible for a truck accident in Nevada.

The guard requirements

Requirement	Trailers and semitrailers built on or after January 26, 1998 (10,000 lb or more)	Other vehicles built after December 31, 1952
Guard required	Yes, meeting FMVSS No. 223, with the vehicle meeting FMVSS No. 224	Yes, if the rear body or chassis is more than 30 inches off the ground when empty
Maximum height of the guard's bottom edge	22 inches across the full width	30 inches when empty
Width	Within 4 inches of each side; not beyond the side	No more than 18 inches from each side
Set-back from the rear	Within 12 inches of the rear extremity (at heights of 22 inches or more)	No more than 24 inches forward of the rear extremity
Bar height	At least 100 mm (3.94 inches)	Substantially constructed and attached by bolts, welding or similar
Not covered	Pole trailers, pulpwood trailers, low chassis, special purpose and wheels back vehicles, certain road construction trailers, driveaway-towaway	Truck tractors, pole and pulpwood trailers, certain construction trailers, driveaway-towaway

What do investigators measure on a guard?

The federal rule is written in measurements, which makes it unusually testable. After an underride crash, the questions are concrete:

Height. For a trailer built on or after January 26, 1998, is the bottom edge of the horizontal bar 22 inches or less from the ground at every point across its width (49 CFR 393.86(a)(3))? Guards with rounded corners may curve upward within 10 inches of the sides.
Width. Does the bar reach to within 4 inches of each side of the trailer, without sticking out past the side (393.86(a)(2))?
Set-back. At heights of 22 inches or more, is the rear surface of the bar within 12 inches of the back of the trailer (393.86(a)(4))?
Bar size. Is the horizontal member at least 100 mm (3.94 inches) tall at every point (393.86(a)(5))?
Standards. Was the guard certified to Federal Motor Vehicle Safety Standard No. 223, with the trailer meeting No. 224, as of the date the trailer was built (393.86(a)(1))?

Measurements taken soon after the crash, before the trailer is moved or repaired, carry the most weight. Photographs should show a tape measure against the guard and the ground, plus the trailer's identification plate.

The rule also reaches older vehicles. For a vehicle built after 1952 that is not covered by the 1998 rule, a guard is needed when the rear body or chassis sits more than 30 inches off the ground empty, and that guard must be substantially built and attached by bolts, welding or similar means (393.86(b)(1), (b)(2)).

What changes the answer

The manufacture date. The stricter 22-inch rule applies to trailers and semitrailers built on or after January 26, 1998. Older vehicles fall under the 30-inch rule in 49 CFR 393.86(b).

The type of vehicle. Pole trailers, pulpwood trailers, low chassis vehicles, special purpose vehicles, wheels back vehicles, certain road construction trailers and driveaway-towaway operations are excluded from the 1998 rule (49 CFR 393.86(a)(1)). Some of them can meet the older rule if their body or chassis provides comparable protection (393.86(b)(3)).

Maintenance. A guard that met the standard when built can be bent or weakened over time. The carrier's duty to inspect, repair and maintain its vehicles (49 CFR 396.3) and the driver's daily inspection report (49 CFR 396.11) are the records to check.

Product questions. If a guard failed even though it was intact and properly mounted, its design or manufacture may be at issue. Nevada keeps joint and several liability for products made, distributed, sold or used in Nevada (NRS 41.141(5)); our post on accidents caused by defective parts covers product claims generally.

Why the car hit the trailer. Following too closely, speeding or distraction on the car's side will be raised by the defense; a trailer stopped without lights or reflectors raises questions on the truck's side. Our post on rear-end collision claims explains how rear-end fault is usually argued.

Interstate or intrastate. Nevada has adopted the federal safety rules for many intrastate carriers by regulation (NAC 706.297); see whether federal rules apply to a Nevada company that never leaves the state.

For example: a guard set too high

Imagine a hypothetical driver named Rafael whose sedan slides into the back of a semitrailer that stopped suddenly in heavy traffic near the Las Vegas Strip. The hood goes under the trailer. Investigators photograph the guard and measure its lower edge at 27 inches, with one side bracket bent.

The trailer was built in 2014, so the 22-inch rule applies. Maintenance records show the bent bracket was noted months earlier and never repaired. The claim now covers both the sudden stop and the guard's condition, with the carrier's maintenance duty at the center.

In this hypothetical, Rafael's own following distance is also examined, and any share of fault on his side would reduce his recovery under NRS 41.141. The numbers here are illustrative only.

Common mistakes in underride cases

Letting the trailer go back into service. Once repaired, the guard's condition at the time of the crash is lost.
Not measuring. The rule is about inches; photographs with a tape measure in frame matter.
Ignoring the manufacture date. It decides which version of the rule applies.
Focusing only on fault for the impact. Even if the car struck the trailer, a defective guard can change how badly people were hurt.
Missing record deadlines. Driver inspection reports are kept only 3 months (49 CFR 396.11).

What to do this week

Write down the trailer's company name, USDOT number and trailer number.
Send a preservation request covering the trailer, the guard and maintenance records.
Gather photographs of the trailer's rear and the guard, from the scene or from witnesses.
Get the crash report from the investigating agency.
Read our guide to what happens after a truck accident in Las Vegas.
Check the retention periods in what records a trucking company must keep, so your requests arrive in time.

Frequently asked questions

Do truck tractors need rear underride guards?

The older rule in 49 CFR 393.86(b) excludes truck tractors. The 1998 rule applies to trailers and semitrailers rated at 10,000 pounds or more.

How low must the guard be?

For trailers and semitrailers built on or after January 26, 1998, the bottom edge may be no more than 22 inches above the ground at any point across its width (49 CFR 393.86(a)(3)).

How wide must the guard be?

Its horizontal member must reach to within 4 inches of the vehicle's side extremities without extending beyond them (49 CFR 393.86(a)(2)).

Does the rule apply to trailers built before 1998?

Yes, in a different form. Vehicles built after December 31, 1952 that are not covered by the 1998 rule need a guard when the rear sits more than 30 inches off the ground empty, with the guard's bottom no higher than 30 inches (49 CFR 393.86(b)).

Are side underride guards required?

Section 393.86 is about rear impact guards. It does not set a side guard requirement.

Who is responsible for a bent guard?

The carrier has a duty to inspect, repair and maintain vehicles under its control (49 CFR 396.3). Who else may share responsibility depends on who owned and maintained the trailer.

Does a defective guard mean the truck driver caused the crash?

Not by itself. Fault for the impact and the guard's condition are separate questions, and Nevada sets each party's percentage (NRS 41.141).

If you or someone you love was hurt in an underride crash in the Las Vegas Valley, attorney Maysoun Fletcher can explain how the guard rules fit your case. Call 702-333-6339 for a free consultation.