Correctly worn seat belts help to contain occupants in their seats and allow most of the collision forces associated with a rapidly decelerating vehicle to be applied across the chest and pelvis. A properly fitted shoulder belt should lie securely across the center of the chest and shoulder, while the lap belt should lie securely across the upper thighs or pelvis. A lap belt should not be worn across the stomach, where it would allow collision forces to be applied to the vulnerable abdominal organs. It is particularly important that pregnant women position their lap belt low across their hips, as wearing it across their stomach would increase the risk of injury not only to themselves but also to their unborn child in the event of a collision
While a properly worn seatbelt is effective in minimizing injuries, many newer vehicles go one step further by adding seatbelt pretensioners that tighten up slack in the belt during a collision. The pretensioner tightens the belt almost instantaneously in the event of a crash, pulling the occupant against the seat in order to minimize occupant motion and to ensure proper occupant positioning in the event of airbag deployment. In general, the seatbelt, pretensioner, airbag, and vehicle’s sensors work together in an effort to ensure occupant safety. In some cases, the vehicle’s black box may record information regarding seatbelt usage (i.e. buckled/unbuckled), as well as other information that may be useful in the investigation of a collision.