This study aims to fill a knowledge gap by conducting a targeted investigation on the belt fit and posture of a cohort of 60 adolescent and young adult females (10–21y) in a rear seat vehicle environment. Specifically, the influence of thoracic and abdominal anthropometric variation on posture and belt fit outcomes will be investigated. Adolescent outcomes will be compared to relevant anthropomorphic test devices (ATDs) to compare posture and belt fit metrics.