Because seatbelts are designed to provide optimal protection for adults, they do not provide sufficient protection for children. Seatbelts depend on the person's bone structure to operate properly, spreading the forces of a crash over the hips, shoulders, and chest, keeping the occupant in place so that the head, face, and chest are less likely to strike the inside of the vehicle. Correct seatbelt fit is not usually achieved until a child is 9 years old, the age at which the child's thigh is long enough for the child to sit against the seat back, the hips are sufficiently developed to anchor the belt, and the child's height is sufficient for the shoulder belt to fit properly over the shoulder and sternum.
In 1996, the Safety Board examined the performance and use of occupant protection systems for children. The Safety Board reviewed 120 accidents in which at least one vehicle had a child passenger younger than age 11 and in which at least one occupant was transported to the hospital. This sample included 46 children who were restrained in child restraint systems, 83 children restrained in seatbelts, and 65 children who were unrestrained, for a total of 194 children. The Safety Board found that none of the fatally injured children were children who were in the appropriate restraint and who used it properly. Children inappropriately restrained by seatbelts had higher overall injury severity, including 5 fatal injuries, than children properly restrained. Among the unrestrained children, almost 30 percent suffered moderate or worse injuries, including 5 fatalities. Children in high severity accidents tended to sustain injury, which makes proper restraint even more important in such accidents.
Using a seatbelt without a booster seat can result in serious injury to children. Without a booster seat, the lap belt can ride over a child's stomach and the shoulder belt can cut across a child's neck. As this position is uncomfortable, children frequently remove the shoulder portion of the adult seat belt, increasing their risk of head injury. According to a study by Partners for Child Passenger Safety, children placed in seatbelts are 3.5 times more likely to suffer significant injury and 4 times more likely to suffer head injuries. However, when children use booster seats, there is a 50 percent to 75 percent reduction in serious injuries.