Using Biomechanics to Support or Refute a Seat Belt Defense

Expert Institute Expert

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— Updated on June 23, 2020

Using Biomechanics to Support or Refute a Seat Belt Defense

Biomechanics Expert Witness

Whether upholding or challenging a seat belt defense, a biomechanical engineer can help evaluate and demonstrate the validity of such a defense. The seat belt defense is currently allowed in only some states; however, that group has been expanding with the addition of Idaho in 2014 and Texas in 2015. In fact, the 2015 ruling in Texas Nabors Well Services Ltd. v. Romero, 456 S.W.3d 553 (Tex. 2015) called the preclusion of seat belt evidence “a vestige of a bygone legal system and an oddity in light of modern societal norms”— essentially calling for other states to consider a change.

The specifics of a seat belt defense varies from state to state, but in most cases, the key issues are whether or not a seat belt was used, and whether or not a seat belt would have prevented the injuries sustained in an accident. In some states, it is also important to determine what other injuries, if any, would have been sustained if the occupant had been properly using a seat belt.

With their ability to conduct a seat belt use and effectiveness analysis, a biomechanical engineer is often the best expert to address these key issues.

Qualifications of the Biomechanical Engineer

It is important to have the right expert when addressing a seat belt defense case. Vehicle accident reconstructionists and medical doctors are often considered to testify; however, the biomechanical engineer is the most qualified expert to opine on this topic.

It is understandable that one might consider having a vehicle accident reconstructionist testify in a seatbelt defense. The reconstructionist should be able to address the accident’s severity as well as the resulting vehicle dynamics, and determine whether or not the seat belt was used. However, most of the time the reconstructionist will not be qualified to discuss the injuries sustained, nor will he/she be qualified to address the important issue of whether or not the injuries would have been prevented by a seat belt.

One might also consider hiring a medical doctor to testify. Most medical doctors, however, do not have the biomechanical training necessary to evaluate the forces in a specific collision. Medical doctors can often opine about whether they see similar injuries to belted occupants in their practice. But medical doctors often have insufficient knowledge of the specific collision to reasonably opine about whether a seat belt would or would not have prevented a specific injury in the subject case.

Thus, a biomechanical engineer is uniquely qualified to fill in the gaps between the accident reconstructionist and the medical doctor. A biomechanical engineer is qualified to analyze collision forces on an occupant both with and without a seat belt, and estimate the likelihood of specific injuries in each scenario. Both Forensic Biomechanics[1] and the Reference Manual on Scientific Evidence[2] provide additional information about how a biomechanical engineer can be used in court, and how to determine if they are qualified.

This section describes some of the methods involved in a seat belt use and effectiveness analysis.

Was a Seat Belt Used, Or Not?

The first issue to address in a seat belt defense is whether or not the seat belt was used. Simply relying upon a police report to determine seat belt use is insufficient. A study looking at data from NASS (National Automotive Sampling System) found that police reported belt use was wrong 12.3% of the time when compared to detailed government crash investigations.[3]

There are better ways to determine seat belt use in a crash. One option is to download the vehicle’s airbag module or “black box.” A CDR (crash data retrieval) report often contains information about whether the front seat belts were buckled at the time of a crash. Combined with a forensic analysis of seat belt evidence, this kind of data is often very powerful evidence of the use, or non-use, of a seatbelt.

Forensic evaluation of seat belt evidence should be performed whenever the subject vehicle is available for inspection. Evidence on the belt system including the latch plate, D-ring, and webbing can provide very good physical evidence of whether a seat belt was used or not.[4] In some cases, the evidence may show that a seat belt was used, but it was used improperly; for example – putting the shoulder belt behind the back or under the arm.

In such cases, a seat belt defense may still be used to argue that proper use of the seat belt would have prevented injuries. For example, proper use of the shoulder belt could reduce head excursion and reduce forces on the abdomen, potentially preventing injuries to those body regions. Another benefit of a forensic evaluation of the seat belt is that the belt system can be tested to show whether or not it was functional at the time of the collision. In some states, showing that the seat belt system was available and functional at the time of the accident is a required component of a seat belt defense.

Would the Seat Belt Have Prevented Injuries?

The second issue in a seat belt defense is whether a seat belt would have prevented the injuries sustained by the plaintiff. The general principle is that seat belts are effective at preventing serious injuries unless the crash is too severe. But the specific analysis is more complicated than that.

Some injuries, like sprains and strains, are too minor to be prevented in many crashes by a seat belt. The analysis begins with a standard biomechanical analysis of an automotive crash, essentially determining how the plaintiff’s injuries occurred. First, a vehicle accident reconstruction must be performed to determine the impact severity and how the vehicle moved during the crash. This can be done either by a separate accident reconstructionist, or the biomechanical engineer (depending on their qualifications).

Next, the biomechanical engineer performs an occupant kinematics analysis, which essentially determines how the occupant’s body moved throughout the crash.

Finally, the biomechanical engineer analyzes the forces on the body to determine how and when injuries occurred. For example, in a rollover accident where an unbelted occupant was ejected, the analysis might involve determining if the injuries occurred inside the vehicle or as a result of ejection and impact with the ground. In a frontal collision, the analysis might involve determining what contacts with the vehicle interior caused the injuries.

After the standard biomechanical analysis, the alternative scenario is considered— determining what would have happened if the occupant had been wearing a seat belt. The same accident reconstruction information is used, but the occupant kinematics and injury analyses change due to the interaction of the seat belt. At this stage the biomechanical engineer can determine if the injuries sustained by the plaintiff would or would not have been prevented by the seat belt, and if any new or different injuries would be likely to occur under the belted scenario.

Examples of Cases with a Weak Seat Belt Defense

A biomechanical engineer can be used to support or refute a seat belt defense depending on the facts of the case. That being said, an example of a weak seatbelt case might play out as follows: an unbelted driver impacts the rear of a semi-truck trailer and goes under the rear of the trailer, sustaining a head injury. In this case, the rear of the trailer intrudes into the occupant compartment, resulting in the occupant’s head directly contacting the rear of the trailer at high speed. As shown in the following crash test video, a seat belt cannot prevent, or even slow down, the high speed contact of the driver’s head with the back of the trailer— therefore there is no biomechanical basis for a seat belt defense related to the head injury.

Another example of a weak seat belt defense is a rollover crash where there is catastrophic roof crush. Consider a case in which an unbelted occupant has been ejected from a vehicle prior to the occurrence of substantial roof crush and sustained a serious neck injury outside of the vehicle. While a seat belt would likely prevent ejection of the occupant, there may still be substantial risk of injury. Roof crush in particular has been correlated with increased risk of injury, among other factors. For belted occupants in rollovers seated in an area where there is catastrophic roof crush, there can be a substantial risk of serious head, neck, and chest injuries, or even asphyxia.[5] In this situation, a seat belt cannot prevent the significant risk of neck and other serious injuries— depending on the jurisdiction of the case, a seat belt defense is thus ineffective.

Examples of Cases with a Strong Seat Belt Defense

An example of a strong seat belt defense might include a scenario in which there is, for example, a 25 mph (40 kph) frontal impact with unbelted occupants in the front and back seats. The unbelted driver’s head goes over the top of the airbag resulting in a head injury, and the driver’s knees impact the dashboard with the full mass of his unrestrained pelvis resulting in a left femur fracture and a right hip dislocation. The unbelted rear passenger moves forward, into and over the driver’s seatback, resulting in head and neck injuries.

A belted driver in the same accident would be restrained, and have their head directed into the center of the airbag— preventing the head injury. While it doesn’t completely prevent knee contact, the restraint of the lap belt reduces the forces on the legs, preventing the femur fracture and hip dislocation. A belted rear passenger is restrained, preventing head contact on the front seat.

This case is a very strong seat belt defense, because the injuries sustained by the unbelted occupants would be prevented by the use of a seat belt. The following video demonstrates the difference between the belted and unbelted scenarios.

Likewise, here is an example of a strong seat belt defense in a rollover case: a vehicle rolls over 1.5 times (6 quarter turns), and the occupant is ejected. The occupant sustains fatal pelvis and abdominal injuries due to being crushed by the vehicle during the process of ejection. The vehicle occupant compartment sustained damage but was relatively well maintained during the crash.

Had the occupant been wearing a seat belt, they would have been contained within the vehicle and likely would have survived. Moreover, they wouldn’t have sustained any serious pelvis or abdominal injuries. When the vehicle occupant compartment is well-maintained, rollover cases with ejection tend to make strong cases for a seat belt defense.

In conclusion, a biomechanical engineer is uniquely qualified to evaluate and testify about whether a seat belt would or would not have prevented the injuries in the specific case. The expert uses the case specific vehicle dynamics to analyze the occupant kinematics, conclude how the subject’s injuries occurred, and determine whether a seat belt would have changed the forces on the occupant in a way that would have prevented their injuries.


[1] Hannon P, Knapp K. Forensic biomechanics. Lawyers & Judges Publishing Company, 2006.

[2] Federal Judicial Center, National Research Council. Reference Manual on Scientific Evidence, 3rd Edition. The National Academies Press, pp. 900-902, 2011.

[3] Schiff MA and Cummings P. Comparison of reporting of seat belt use by police and crash investigators: variation in agreement by injury severity. Accident Analysis & Prevention 36.6 (2004): 961-965.

[4] Raymond DE, Bir CA, Begeman PC, Chien HC, Wheeler JB. Forensic determination of seat belt usage in automotive collisions: development of a diagnostic tool. SAE Technical Paper; 2006.

[5] Storvik SG, Campbell JQ, Wheeler, JB. Asphyxia in Motor Vehicle Crashes: Analysis of Crash-Related Variables Using National Automotive Sampling System Crashworthiness Data System and Forensic Case Studies. American Journal of Forensic Medicine and Pathology, (in press) 2017.

Expert Witness Bio E-084238

Biomechanics Expert WitnessThis expert specializes in injury biomechanics and has testified in cases involving automotive collision injuries, seat belt use and effectiveness, industrial/occupational injuries, worker’s compensation claims, and sport/recreational accidents. He holds a Masters Degree in Mechanical Engineering and is a PhD of Biomechanics. He is an Editorial Board Member for the Journal of Forensic Biomechanics and sits on the ASTM committees for Snow Skiing and Sports Equipment & Facilities. Currently, this expert is a Senior Biomechanical Engineer at a private consulting firm.
BA, Physics, The Colorado College
MS, Mechanical Engineering, University of Colorado
PhD, Mechanical Engineering & Biomechanics, Colorado School of Mines
Certified, ACTAR Accredited Reconstructionist
Member, American Society of Testing and Materials
Member, Editorial Board – Journal of Forensic Biomechanics
Published, 50+ articles and presentations
Former, Research Engineer, Conrad Technologies Inc.
Former, Lead Engineer, GESAC Inc.
Former, Research Engineer, IT Shows Inc.
Former, Biomechanical Engineer, Vector Scientific Inc.
Current, Senior Biomechanical Engineer, a Private Consulting Firm

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