This study presents (1) a case of an injury to an unbelted passenger and (2) the possibilities of proving the occurrence of injuries to traffic accident participants. We demonstrate the case of an injury to a passenger who failed to fasten her seat belt, and question whether her injuries would have been equally serious if she had fastened her seat belt. Theoretical bases and methods for the interdisciplinary procedure of medical examiners using the PC Fortis program and technical analysts of traffic accidents using the PC Crash program are presented. Furthermore, individual practical steps are documented, the result showing that the injuries to the passenger would have occurred, but, to a minimum extent, i.e., 6.9% of the original injuries, which would have not exceeded the legal limit for damage to health.