Adult car passengers adopt a wide range of sitting postures while travelling. This user study compared a stationary and a driven scenario of 45 minutes each, and a stationary scenario over time, to investigate the potential influence study scenarios and time have on sitting postures and shoulder belt fit of rear seat passengers. A machine learning algorithm tracked head, upper sternum, and shoulder belt positions from video data of 13 participants, captured using two 3D cameras. The results showed small differences in the average head and sternum positions and position ranges when comparing the two scenarios. The average belt position was similar in both scenarios, but the shoulder belt tended to move closer to the neck in the driven scenario for participants with specific body shapes. Average belt, head and sternum positions were similar during the first three minutes, as for the total 45 minutes of the stationary scenario, but the position ranges were wider over the total 45 minutes. To conclude, a simplified short stationary test may be used to estimate the average postures and belt fit of passengers, but longer dynamic tests are needed to capture variations in posture and belt fit of passengers with specific body shapes.