Tractor Trailer Underride Investigations Expert Article

In this article, vehicle engineer Christopher Roche provides an overview of heavy truck underride incidents. His discussion includes a description of underride events, why they occur, relevant US and international standards, and how these incidents are typically investigated from a forensic standpoint.

Tractor Trailer Underride Expert Investigations
Image source: IIHS

Overview of Tractor Trailer Underride Investigations

What is Underride?

The term ‘underride’ in a heavy truck context is most commonly used to describe a vehicle collision in which a passenger vehicle enters the open space beneath the frame of a tractor trailer. This phenomenon occurs when there is a geometric incompatibility of two vehicles’ crash absorbing structures.

The crash absorbing structure of many passenger vehicles, generally strongest at or below hood level, is lower to the ground than the structural members of a trailer. Regardless of whether the impact is to the rear or the side of the trailer, the mismatch between crash absorbing structures of the vehicle and the vehicle of the trailer frequently results in increased passenger compartment intrusion, particularly at shoulder and head level.

These crashes result in a far higher rate of serious and/or fatal injuries than would be expected from non-underride incidents of similar magnitude.

Prevalence of Underride Incidents

According to the Government Accountability Office (GAO) report in March 2019, using Fatality Analysis Reporting System (FARS) data from 2008 through 2017, an average of 219 fatalities occurred annually due to underride crashes involving one or more trucks. However, these numbers are likely to be low as the Insurance Institute for Highway Safety (IIHS) suspects that underride crashes are typically underreported or misrepresented due to the FARS inclusion criteria employed.

Underride can occur when a passenger vehicle strikes the rear or the side of a tractor trailer. An analysis by the IIHS in 1997 of crashes involving passenger cars and large trucks identified that underride crashes involved the front of the truck 57% of the time, 22% involved the rear, and 20% the side of the truck.

Current United States Underride Regulations

Federal Motor Vehicle Safety Standards (FMVSS) 223 (equipment) and 224 (vehicle) regulate what vehicles need an underride guard and the minimum performance requirements of a guard.

Figure 1: Rear Impact Protection

These standards were enacted in 1953, revised in 1998 to reduce the risk of passenger compartment intrusion, and updated again in 2022 to match more stringent Canadian rules. They require the guard structure to be no more than 22” (560 mm) above ground level and 12” (305 mm) in front of the rearmost portion of the truck/trailer, and to withstand the prescribed loading parameters. The load requirements for underride guards are approximately equivalent to the force of a compact car striking at 35 MPH in a full overlap collision.

The IIHS introduced its own testing and rating of underride guards and developed the TOUGHGUARD award for guards that can prevent passenger car underride in three rear impact conditions: full overlap, 50% overlap, and 30% overlap.

The European Union has had regulations in place for side underride guards since 2003. There is currently no US requirement for side guards, but the National Highway Traffic Safety Administration (NHTSA) has issued an Advance Notice of Proposed Rulemaking (ANPRM) for “Side Impact Guards for Combination Truck Trailers.” 

According to IIHS data, in 2015 nearly 20% of the passenger vehicle occupants killed in two-vehicle crashes with a tractor-trailer died when their vehicles struck the side of a tractor-trailer.

IIHS recently tested a newer product that protects passenger vehicle occupants from the dangers of side impact underride collisions at speeds of up to 40 mph.

Figure 2: Test of Side Underride Guards

Investigating Underride Incidents

The goal of many underride investigations is to determine if there was a failure in the passenger vehicle’s safety systems or the tractor trailer’s underride guards or lack thereof that contributed to the severity of the crash. The foundational step in any underride investigation is a comprehensive reconstruction of the crash. Reconstruction allows investigators to determine delta-V (change in velocity), or the severity of the collision.

With an established delta-V, investigators can compare vehicle damage, injuries, and passenger compartment intrusion, against anticipated values for crashes of a similar magnitude. In instances where injuries and intrusion exceed anticipated values, there may be cause to investigate potential defects in safety systems. Failed underride guards should be inspected to determine if the failure was caused by an excessive collision force or a potential defect in design, manufacture, or assembly.

Robson Forensic provides the full range of expertise to conduct comprehensive investigations into underride incidents and other complex crash scenarios. Within Robson Forensic you can find technical experts specializing in crash reconstruction, vehicle crashworthiness, materials science, injury biomechanics, human factors, roadway engineering, toxicology, and much more.

Submit an inquiry to connect with a technical expert to discuss your case.

Featured Expert

Christopher D. Roche, Automotive Engineer & Crash Expert

Christopher D. Roche

Automotive Engineer & Crash Expert
Christopher Roche is an automotive engineer with nearly 30 years of professional experience working for major vehicle manufacturers, specializing in the design of vehicle structures to protect… read more.


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