Even though some parents put their little ones in booster seats while in the car, they don't always require them to use one when they are carpooling with other kids -- that's according to a new survey published in this week's journal Pediatrics.

The research, conducted by the University of Michigan, found more than 30 percent of parents do not enforce the rule of booster seats when their kids are with another driver. Investigators also found 45 percent of parents do not require their little ones to use a booster when they're driving other children who don't have booster seats.

"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 Dr. Michelle Macy, a clinical lecturer of emergency medicine at the University of Michigan Medical School and a pediatrician at U-M C.S. Mott Children's Hospital. "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 car overcrowding and lack of time to coordinate booster seat switch offs are to blame for the lack of safety for kids in carpooling situations. Some parents seem to look the other way. But it's against the law, and many don't understand that.

"If parents don't have a booster seat for a child under eight, it's not safe," stressed Macy. "We realize life happens and things come up, but parents know they need to go by the laws or come up with options that are just as safe."

Most states require children to use a booster seat up to the age of eight. National recommendations encourage booster seats be used until a child grows to 57 inches, which is the average height of an 11-year-old, according to the study. Researchers also noted half of parents surveyed said they did not know the age limit on booster seats in their states and another 20 percent guessed incorrectly. That's why, study authors say, pediatricians should make it a top priority to let parents know about these laws.

"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.