The Shortcomings of "Occupant Dynamics" Expert Evidence
Accident reconstrucion experts routinely give evidence during BC personal injury lawsuits when fault for a motor vehicle crash is at issue. One subset of such expert evidence is “occupant dynamic” evidence which seeks to explain how a passenger would be thrown around following a collision. While this evidence can have some value at trial it is accompanied with certain shortcomings. These were discussed in reasons for judgement released this week by the BC Supreme Court, Vancouver Registry.
In this week’s case (Byer v. Mills) the Plaintiff was one of two occupants in a vehicle which was involved in an at-fault collision. The central issue at trial was who the driver of the vehicle was. The Plaintiff was badly injured and had no recollection of who was driving. The second occupant of the vehicle died shortly following the crash. There were no independent witnesses addressing who was driving at the time of the crash and the Court had to decide this issue relying on circumstantial evidence.
In the course of the trial the Court heard evidence from an ‘occupant dynamic‘ expert. Ultimatley Mr. Justice Harris dismissed the Plaintiff’s lawsuit finding that, on a balance of probabilities, he was likely the driver therefore he was at fault for his own injuries. This decision was most influenced by lay witness evidence and the occupant dynamic expert testimony was of little value in this particular case. Mr. Justice Harris provided the following short but useful comment addressing the shortcomings of occupant dynamic evidence:
 The principles of occupant dynamics are helpful up to a point. Certainly, they assist in identifying the principal direction of force exerted on occupants. They are also helpful in identifying the point at which an occupant might be expected to make initial contact with the interior of the passenger compartment. In my view, in the circumstances of this collision, the predictive value of principles of occupant dynamics rapidly diminishes once the movement of the passengers is affected by contact with the interior of the compartment and with each other. At that point the situation becomes inherently dynamic and fluid. There are far too many variables involved to make accurate predictions of how the occupants and parts of their bodies would move once they start hitting each other. It must be remembered that if unrestrained an occupant would be traveling within the compartment at a speed of about 55 km/h. I am sceptical that any reliable prediction of how the occupants would interact with each other, with the interior of the passenger compartment and move within it can be undertaken.