While most parents with small children use booster seats, nearly a third of them do not enforce this rule when their child is riding with another driving, such as when carpooling.

A study conducted by child health experts at University of Michigan’s C.S. Mott Children’s Hospital, also revealed that 45 percent of parents do not require their kids to use a booster when driving other children who do not have one.

“The majority of parents reported that their children between the ages of four and eight use a safety seat when riding in the family car,” says Michelle Macy, M.D., M.S., a clinical lecturer of emergency medicine at the University of Michigan Medical School and an investigator with the U-M Child Health Evaluation and Research Unit. “However, it’s alarming to know that close to 70 percent of parents carpool, and when they do, they’re often failing to use life-saving booster seats.”

Researchers believe practical barriers, including limited vehicle space and difficulties making arrangements with other drivers, lead parents to abandon safety seats when carpooling.

Results of the study were published online in Pediatrics.

Most state laws require children to use a booster seat, many until children are 8 years old. National recommendations encourage the use of booster seats until a child reaches 57 inches, which is the average height of an 11-year-old.

Placing a child in an adult seat belt prematurely can cause shoulder and lap belts to fit improperly, negating the life-saving benefits of seatbelts.

“Therefore, parents who do not consistently use booster seats for kids who are shorter than 57 inches tall are placing children at greater risk of injury,” says Macy. “Parents need to understand the importance of using a booster seat for every child who does not fit properly in an adult seat belt on every trip.”

Study authors suggest that social norms may be set by state booster seat laws, as parents are motivated to follow guidelines set forth by law. State booster seat laws were associated with higher safety seat use, regardless of carpooling, even though half of parents surveyed admit to not knowing the age cited in their state booster seat law and another 20 percent guessed incorrectly.

“According to current recommendations most children should be using booster seats beyond the age cited in state laws. As many parents may not even be aware of current booster seat recommendations, pediatricians should make it a priority to share this vital information with them,” says Macy.