Parents don't always use booster seats for children, even though they're required in 47 states. Only half insisted their child use a booster when riding in the family car with boosterless friends.

Grade-schoolers are supposed to be riding in booster seats. But anyone who's ever chauffeured a bunch of second-graders can tell you that the day will come when you don't have enough boosters to go around. Faced with this obvious safety risk, most parents (including this one) buckle up the kids without boosters, and pray.

That's confirmed by a new survey in this week's Pediatrics, which found that parents are good at using booster seats when driving 4- to 8-year-olds in the family car, with 76 percent using boosters.

But they were not so good when carpooling. Only half make their children use booster seats when they are riding in the family car with friends who don't have boosters, and 21 percent let their own child ride booster-less when in someone else's car.

Some parents resorted to obviously unsafe choices, including buckling two children in one seat belt, or putting a child in the cargo area.

That means a lot of children are being exposed to the risk of serious injury. Children in this age range still aren't big enough to be safely restrained by car seat belts. Using a booster to position the lap and seat belt properly reduces injury risk by half.

The 681 parents surveyed said they didn't use booster seats for a number of reasons, including difficulty getting seats from other parents, and being unable to fit enough seats in the car.

Legal pressure clearly helps; parents were far more likely to use boosters if they live in one of the 47 states that require them for children up to 4 feet 9 inches in height. (Here's an earlier NPR story on state efforts to require booster seats.)

I've never known a child injured in a car crash, even though it's the most common cause of death in children ages 3 to 14. But I figured I should find out what can happen to a child if they're in an accident without a booster seat. It's not pretty. In this 2005 study, eight children ages 4 to 8 who were wearing only seat belts suffered serious abdominal injuries. Five broke their spine. And four are permanently paralyzed. That sure got my attention.

The National Highway Transportation Safety Administration spells out the details for safe use of boosters in children ages 8 to 12. [Copyright 2012 National Public Radio]