Unlike child restraints with built-in harnesses, a booster seat relies on a vehicle safety belt to buckle the child in. Its purpose is to make the adult belt fit the child better. Booster seats are for kids who have outgrown their forward-facing restraints.
Our ratings help identify boosters most likely to provide good lap and shoulder belt fit. Safety belts are designed with adults in mind, not kids, but when a booster seat is doing its job, the vehicle belt will fit a child correctly. That means the lap belt willl lie flat across a child's upper thighs, not across the soft abdomen, and the shoulder belt will cross snugly over the middle of a child's shoulder.
The Institute puts the booster seats it tests into 4 categories:
IIHS assesses boosters using a special crash test dummy representing an average-size 6 year-old. Engineers measure how three-point lap and shoulder belts fit the dummy in each of the tested boosters under four conditions that span the range of safety belt configurations in vehicle models. An overall rating for each booster is then assigned based on the range of scores for the lap and shoulder belt measurements.