Laws on when the seats must be used vary by state, but guidelines from groups including the American Academy of Pediatrics recommend boosters from the time kids outgrow their car seats until they hit four feet, nine inches tall -- the height of the average 11-year-old.

The new report suggests that most parents of four- to eight-year-olds have a booster seat for their kids in their own cars, but don't always make sure boosters are used when they carpool.

"Until children reach a certain size, the seatbelt doesn't fit them properly enough to maximize the protection that seatbelts offer when you're bigger and taller," said Andrea Gielen, head of the Center for Injury Research and Policy at the Johns Hopkins Bloomberg School of Public Health in Baltimore.

Booster seats, she told Reuters Health, position smaller bodies to get the most benefit from the seatbelt in case of a crash.

"The study shows that parents are missing opportunities to protect their kids in the car on every ride," said Gielen, who wasn't involved in the research.

The new data come from an online survey of 681 U.S. parents conducted in early 2010.

Along with a range of questions related to kids' health and family dynamics, parents were asked about their own use of booster seats and how they felt about the seats when it came to carpooling.

Three-quarters of the parents said they used a safety seat for their own child in their car. Parents of younger kids, and those that lived in states that required the use of booster seats, were most likely to use boosters.

The majority of survey participants also said they either frequently or occasionally carpooled with another family.

Among parents who used a booster seat in their own car and said they carpooled, 79 percent reported that they would always ask another driver to put their kid in a booster seat. Just over half of them said they would make sure their kid was always in a booster when in the car with friends who didn't use the seats, according to the report, published Monday in the journal Pediatrics.

That may have to do with practical issues of transporting and passing off the seats, or with social norms regarding interactions between parents with different safety practices, researchers said.

"I wish I could say (the finding) was surprising," said Dr. Michelle Macy, from the University of Michigan in Ann Arbor, who worked on the study.

"But it does fit my experience that it can be a little bit of an awkward topic to broach with parents and it does require a little more planning."

Still, neither of those reasons should hold parents back from making sure their child is always in a booster seat, according to Dr. Mark Zonfrillo, a child safety researcher and emergency medicine doctor at The Children's Hospital of Philadelphia.

"If you were to send your child to one of their friend's houses and they needed some kind of medication, they had a chronic illness, you'd send them with the medication," said Zonfrillo, who wasn't involved in the new study -- and there's no reason booster seats should be any different.

"We know they're highly effective," he told Reuters Health. "They also happen to be the most convenient and portable of all child restraints, and they're really cheap."

The seats start at about $20, and researchers also pointed to car seat loner programs where families can get boosters for free.

Macy said that the findings also show the importance of pushing for state laws that meet guidelines for booster seat use, noting that most laws now only require boosters up to age eight -- when the majority of kids still won't be adequately protected by a seatbelt alone.

SOURCE: bit.ly/jsoh2P Pediatrics, online January 30, 2012.

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