More and more states now require booster seats that raise a child up so the car's adult seat belt fits them properly, but the federal government doesn't have any guidelines for how booster seats should be designed, so the institute is trying to fill the gap with its annual ratings.
The institute took multiple measurements of 60 child booster seats to see where the seatbelt fell on a specially designed dummy the size of a 6-year-old. IIHS then observed how one seat ranked a "best bet" and another labeled "not recommended" reacted during crash tests.
On a booster with a good fit, the shoulder belt runs squarely over the middle of the child's shoulder and the lap belt sits flat across the thighs. On a booster with a bad fit, the shoulder belt slides off the corner of the child's shoulder and the lap belt is too high on the soft stomach. A bad fit at the shoulder can fling kids forward, and at the waist, can cause internal organ injuries.
"The main thing boosters are supposed to do is provide a good belt fit," said Anne McCartt of the Insurance Institute for Highway Safety. "So it is somewhat surprising that some boosters aren't doing that very basic job."
According to the institute, tests show that booster seats that are best bets position children well in relation to the seat belt no matter what type of vehicle they are used in. And the good news is that some of them cost as little as $20.
The institute said 11 seats were "not recommended," meaning parents should avoid buying them. Half of the seats on this list are 3-in-1 combination seats that convert from infant seats to toddler seats and then to booster seats. Their convertible shape places the lap belt too high and the shoulder belt too low.
"If parents have a booster that is on our not-recommended list -- they shouldn't throw the booster out," McCartt said.
Instead, keep using it until you can get a better one, because research shows children riding in boosters are 45 percent less likely to get hurt in a crash than children using seat belts alone. In other words, any seat is better than none at all.
There is one thing even safer than a booster seat, and that is a car seat just like toddlers use, but jumbo-sized for older kids.
Here's why: boosters don't attach to the car, and they can move around and get ejected in a car crash. By contrast, a car seat is tightly anchored to the car itself. It also has a five-point harness.
If your older kid doesn't want to wear one of these, just remind them that it's what race car drivers wear because it's the safest option available.