Research shows that booster seats reduce the risk of injury 45% better than seat belts alone, for children between the ages of 4 and 8 (Effectiveness of Belt Positioning Booster Seats: An Updated Assessment, Pediatrics 2009). They also reduce the risk of death by 28% compared to restraining children with vehicle seat belts (Effectiveness of Child Safety Seats vs Seat Belts in Reducing Risk for Death in Children in Passenger Vehicle Crashes, Archives of Pediatrics and Adolescent Medicine 2006).
State law requirements vary for using car seats and boosters, but experts agree that booster seats should be used until seat belts fit correctly. Seat belts fit if a child can sit all the way back with knees bent naturally over the seat edge. The lap belt must stay snug on the leg or hip bones, with the shoulder belt against the chest and shoulder.