Many parents may not be properly strapping their children into car booster seats, a new study suggests.

Most U.S. states now require that children who have outgrown traditional car seats use a car booster seat, which raises a child high enough so that the car seat belts can be positioned properly — with the shoulder strap across the shoulder and not the neck, and the lap belt across the hips.

However, even with booster seats, many children may still not be wearing their seat belts correctly, researchers report in the journal Accident Analysis and Prevention.

In observations of 564 children using car booster seats, the investigators found that two-thirds were improperly belted in. In most cases, the seatbelt shoulder strap was misplaced — pushed behind the child's back, under the child's arm or over the booster seat armrest, for example.

"Our findings clearly show that booster seats are not protecting children because of user error," lead researcher Dr. Joseph O'Neil, an associate professor of pediatrics at the Indiana University School of Medicine in Indianapolis, said in a written statement.

"Parents need to know how to safely place a child in a booster, supervise the buckling up of children who put themselves in the seat, and double check that the shoulder and lap belts restraining the children remain properly positioned during the drive," O'Neil said.

He and his colleagues based their findings on observations of parents who drove into parking lots at 25 fast-food restaurants and department stores throughout Indiana.

According to O'Neil, children should continue to use a booster seat until they are tall enough that their knees extend over the automobile seat at a 90-degree angle, ideally with their feet touching the floor.

As with infant car seats, he noted, booster seats should be placed in the back seat of the car.

Parents who need advice on using booster seats can go to online sources like the National Highway Traffic Safety Administration's Web site (www.nhtsa.gov) and Safe Kids USA (www.usa.safekids.org).

The NHTSA site also allows parents to search for local inspection stations where they can get help in installing and using car safety seats.

SOURCE: Accident Analysis and Prevention, May 2009.