When properly designed, manufactured and used, seatbelts help to protect occupants from injury. The mechanical engineer is able to inspect, investigate and render an expert opinion regarding the analysis of seatbelt design, performance and use, as well as identify the factors contributing to the occupant’s injuries.

Common Issues the Mechanical Engineering Expert Addresses Regarding Seatbelts Includes


  • Occupant’s use of seatbelt
  • Seatbelt function and design
  • Seatbelt performance
  • Mechanical malfunction/failure of seatbelt system component
  • Non-deployment of seatbelt pre-tensioner(s)
  • Non-deployment of seatbelt pre-tensioner(s)
  • Unintended release of seatbelt buckle
  • Operation of seatbelt retractor


A Mechanical Engineer

Seatbelt Expert Witness

  • Reviews post-accident airbag condition
  • Inspects airbag for physical evidence
  • Evaluates seatbelt performance
  • Determines seatbelt mechanical malfunctions
  • Evaluate seatbelt system design
  • Reviews developmental testing of seatbelts
  • Verifies compliance with required standards for seatbelts
  • Evaluates seatbelt hardware
  • Tests seatbelt components to evaluate design and performance

Seatbelt Case Study

Case Synopsis: One afternoon, a vehicle driver and his girlfriend were returning from picking up his paycheck when he reportedly hit a dip in the road and lost control. The vehicle rotated and the right side tires slide into the curb, causing it to begin to rollover twice. During the rollover, the driver was ejected through the vehicle’s soft-top and came to rest on the ground and suffered severe injuries to his entire body. The girlfriend, in the right front seat, experienced minor injuries to her head and right shoulder. According to the police report, the right front occupant was wearing her seatbelt while the driver was not.

The defendant in the case retained an expert who proffered a seat belt defense (SBD) claiming that the driver was not using the available lap/shoulder seatbelt, and had he been wearing it, he would not have been ejected and would have been well protected, with only minor injuries sustained from the crash.

Expert Analysis: The vehicle was not available for inspection and there were only limited photographs. These photographs were sufficient to show only minor damage to the roll cage, and the occupant space was well preserved. Given the crash severity and the vehicle damage, a belted occupant should have been reasonably protected in the crash.

The driver maintained he was wearing his seatbelt at the time of the crash. This was based on the fact he always wore his seatbelt. The girlfriend confirmed the driver always wore his seatbelt and that it was on at the time of the crash. The driver testified he knew he had it on when he left his employers’ business because his boss, a police officer, walked out to his vehicle with him. This was confirmed by his boss who testified he would have told the driver to put his seatbelt on if it was not engaged prior to leaving. The investigating officer testified that he was surprised to see the driver ejected from the vehicle, upon arriving on the scene, as they just passed each other a minute prior to the crash and saw the driver belted.

Given the substantial testimonial evidence that the driver was wearing his seatbelt before the crash, an analysis was performed to determine how the driver could have come unbelted and ejected. Several theories were investigated, but one provided the most likely scenario to explain the driver’s ejection. The vehicle was equipped with a seatbelt buckle of a design known to have the ability to be released by impacting the rear side of the buckle, i.e. the side adjacent to the driver. Since the rollover was initiated by a right-side impact, the driver would move toward the buckle, providing an opportunity to impact the rear of the buckle case with his hip. It was also reported that the driver typically wore his cell phone on his right hip. Impact to the buckle case by a rigid object, such as a cell phone, increases the potential for it to release.

Conclusion: Given a plausible alternative to explain the driver’s ejection had he been belted as numerous people testified, the case settled prior to trial.