Truck Underride Accident in California: When a Car Goes Under a Semi-Trailer

By the Sky Law Group Editorial Team · Reviewed by Shakeal Masoud, Esq. (California State Bar No. 246309) · Reviewed November 11, 2026

Short answer: A truck underride crash happens when a car slides under the back or side of a semi-trailer. In California, the people hurt in the car may have claims against the truck driver, the trucking company, and sometimes the trailer owner or the maker of a guard that failed. Most injury lawsuits must be filed within two years (CCP §335.1), and the trailer itself is the most important piece of evidence. Call Sky Law Group at (844) 475-9529 — Hablamos Español.

Why an underride crash is different from a normal rear-end

A car’s crumple zones and airbags are designed to meet another vehicle at bumper height. A semi-trailer bed sits much higher, so when a car goes under it, the first contact can be the windshield and roof pillars. That is why underride crashes so often cause head, neck and brain injuries, even at moderate speed.

Underride also happens from the side, when a trailer turning, backing or changing lanes leaves an open gap a car cannot stop short of.

The safety rules that apply to trailers

Rear impact guards

Federal rules require most trailers to carry a rear impact guard, the steel bar you see hanging below the back doors (49 CFR §393.86). Manufacturers build those guards to federal standards FMVSS No. 223 and No. 224. In 2022, after the 2021 federal infrastructure law ordered an upgrade, the National Highway Traffic Safety Administration strengthened those standards so new guards must protect people in a car that hits the rear of a trailer at 35 mph, both head-on and at a 50% overlap. Trailers built on or after July 15, 2024 must meet the upgraded standard. Carriers must keep guards in working condition.

Side guards

At the time of writing, federal rules do not require side underride guards on trailers. Some carriers install them voluntarily.

Visibility and warning devices

Trailers must carry lighting and reflective conspicuity markings so other drivers can see them at night (49 CFR §§393.11 and 393.13). When a truck stops on a freeway shoulder or traffic lane, the driver must turn on hazard flashers and put out warning devices as soon as possible, and in any event within 10 minutes (49 CFR §392.22). Dirty or missing reflective tape and a dark trailer on the shoulder are common facts in nighttime underride crashes.

Who may be responsible

Party	How they may be involved
Truck driver	Sudden stop, unsafe lane change, turning or backing across traffic, or stopping without flashers and warning devices
Trucking company (motor carrier)	Responsible for its driver’s conduct on the job, and for inspecting and maintaining the guard, lights and reflective markings
Trailer owner or leasing company	May share responsibility for maintenance when the trailer belongs to someone other than the carrier
Trailer or guard manufacturer	Possible product liability claim if the guard was defectively designed or built (California strict product liability, Greenman v. Yuba Power Products (1963))
Public agency	Rarely, if a road defect contributed; a written claim is generally due within six months (Gov. Code §911.2)

For a deeper look at how liability is split among the driver, carrier, broker and others, see our guide on who is liable in a semi truck accident in California.

What the trucking company’s insurer will not tell you

The insurer’s first argument is usually simple: you hit the back of the truck, so you were following too closely (CVC §21703) or going too fast for conditions (CVC §22350). Even if part of that were true, it does not end your claim. California uses pure comparative fault, so your compensation is reduced by your share of fault, not erased (Li v. Yellow Cab Co. (1975) 13 Cal.3d 804). Our page on comparative negligence in California explains how that math works.

The insurer also knows the trailer tells the story. A bent, rusted or missing guard, burned-out lights or faded reflective tape can shift the focus to the carrier’s maintenance, which is why trailers are often repaired or re-dispatched quickly after a crash.

Where these crashes happen in Orange County

Heavy truck traffic runs every day on the 91 Freeway through Anaheim and Yorba Linda, the 57 Orange Freeway, the I-5 through Santa Ana and Irvine, and the 22/57/I-5 interchange known as the Orange Crush, a few minutes from our office on West Katella Avenue in Orange. Stop-and-go traffic near freeway transitions is a common setting for rear underride. Side underride is more common on surface streets near industrial areas, where trucks turn across several lanes or back into loading docks in the dark.

Freeway crashes are usually investigated by the California Highway Patrol; city-street crashes by local police or the Sheriff’s Department. Seriously injured people in central Orange County are often taken to UCI Medical Center in Orange, the county’s Level I trauma center. Our 91 Freeway semi truck crash guide covers that corridor in more detail.

Evidence to protect right away

• The trailer and its guard. Photos of the rear and sides, the guard, the lights and the reflective tape, plus the trailer number and the USDOT number on the cab.
• A preservation letter. A written demand that the carrier keep the trailer unrepaired, plus its electronic logging device data, engine data, dashcam video, and inspection and maintenance records (annual inspections are required by 49 CFR §396.17).
• Your car. Do not let it be scrapped. Its crash data and the crush pattern help show what happened.
• Medical records. Get checked even if you feel fine; head and neck injuries are not always obvious at first.

Our checklists on what to preserve after a semi truck accident and the first 48 hours after being hit by a semi truck walk through each step.

When a family loses someone

Underride crashes are too often fatal. In California, a spouse, children and certain other family members may bring a wrongful death claim (CCP §377.60), generally within the same two-year period. Our wrongful death lawyer page explains who can file and what the claim covers. For survivors with a head injury, see our brain injury lawyer page.

Sky Law Group handles truck crash claims across Orange, Anaheim, Santa Ana, Irvine, Fullerton, Yorba Linda, Garden Grove and the rest of Orange County. Our attorneys speak Spanish themselves, so families can talk through every step in the language they are most comfortable with. Learn more on our Orange County truck accident lawyer page.

Frequently asked questions

What is a truck underride accident?

It is a crash in which a car slides under the rear or side of a semi-trailer, so the trailer bed strikes the car’s windshield and roof area instead of its bumper.

Are trucks required to have rear underride guards?

Most trailers must have a rear impact guard under federal rules (49 CFR §393.86), and trailers built on or after July 15, 2024 must meet a stronger federal standard.

Who is liable if my car went under a trailer?

Depending on the facts, the truck driver, the trucking company, the trailer owner and sometimes the guard or trailer manufacturer.

Is it my fault if I hit the back of the trailer?

Not automatically. A sudden stop, a dark or unmarked trailer, or a missing guard can shift responsibility, and under California’s comparative fault rule a share of fault reduces your recovery rather than barring it.

Can I sue the manufacturer of the trailer or guard?

Possibly, if the guard was defectively designed or built. California applies strict product liability to defective products, but these claims need an early engineering review.

How long do I have to file in California?

Generally two years from the injury for a lawsuit (CCP §335.1), and generally six months for a written claim if a public agency is involved (Gov. Code §911.2).

What if the trailer was parked on the shoulder at night?

A stopped truck must use hazard flashers and put out warning devices within 10 minutes (49 CFR §392.22). Whether the driver did so, and whether the trailer’s lights and reflective tape worked, are key questions.

Is there a fee to talk with a lawyer?

Your first consultation with Sky Law Group is free. Call (844) 475-9529.

Can I talk to a lawyer in Spanish?

Yes. Our attorneys speak Spanish themselves. Hablamos Español.

Hurt in a crash with a semi-trailer? Call (844) 475-9529 for a free consultation. Hablamos Español.

Attorney advertising. This article is general information, not legal advice, and does not create an attorney-client relationship. Past results do not guarantee a similar outcome. Every case is different; no result can be guaranteed.