In particular, booster seats appeared to sharply cut the risk of injuries to the abdomen and spine.

"Besides providing the first evidence of an overall benefit, this study demonstrated the virtual elimination of 'seat belt syndrome' for children who used the booster seat," says Dr. Dennis R. Durbin, lead author of a study appearing in the June 4 issue of the Journal of the American Medical Association and an attending physician in the emergency department at the Children's Hospital of Philadelphia. "We did not expect to see that dramatic a result."

"We certainly stand behind any belt-positioning booster," adds Stephanie Tombrello, executive director of SafetyBeltSafe U.S.A. "We recently did a five-step test event at a community event with 50 children ages 3 to 12, and only one of them passed the five-step test, a 12-year-old. Every other one needed to use the booster. The boosters really make a big difference."

Booster seats elevate the child so the vehicle's lap or shoulder belts fit him or her properly. The National Highway Traffic Safety Administration (NHTSA) recommends that children between the ages of 4 and 8 use a booster seat, unless they are 57 inches tall.

So far, 18 states and the District of Columbia have passed laws requiring booster seats for children over the age of 4, reports Cathy Chase, director of state affairs for Advocates for Highway and Auto Safety in Washington, D.C. Other states are in the process of upgrading their laws.

Durbin and his colleagues are hoping the study results will spur legislators to act quickly and decisively.

"We are certainly hoping that our results could be used by legislators and by parents to set a new standard for what is considered normal for how we protect children in this age group in the car," Durbin says. "I think parents often look to their state laws for guidance and they figure if it's not the law it must not be important. Over [age] 4, lots of parents are under the mistaken impression that the child is big enough to sit with a seat belt. We are hoping that our information can be used both by parents and legislators and safety advocates to get the word out there that there is a step between the car seat and the seat belt."

The current study is part of the Partners for Child Passenger Safety (PCPS) project, a collaboration between State Farm Mutual Automobile Insurance and the Children's Hospital of Philadelphia.

Durbin's group had previously shown that children aged 2 to 5 in different kinds of child restraints, including booster seats, did better than when they wore seat belts alone.

For the latest study, the authors collected data on car crashes in 15 states that had involved children between the ages of 4 and 7. The data came from State Farm Insurance claim records and a telephone survey. Ultimately, the researchers collected information on 3,616 crashes involving 4,243 children, which was then extrapolated to represent 56,593 children in 48,257 crashes.

All of the vehicles were model year 1990 or newer; 53 percent were passenger cars, 20 percent were minivans; 18 percent sport-utility vehicles; 6 percent pickup trucks, and 2 percent large passenger/cargo vans. This was roughly equivalent to the distribution of vehicles in general.

Injuries occurred among 1.81 percent of the children, with the percentage higher (1.95 percent) for those in seat belts and lower (0.77 percent) for those in belt-positioning booster seats. The odds of injury were 59 percent lower for children in the 4-to-7-year age bracket in belt-positioning boosters than in seat belts. Furthermore, children in booster seats had no injuries to the abdomen, neck, spine, back or lower extremities, while children in seat belts alone had injuries in all body regions. The effectiveness of the booster seats was the same regardless of whether the children used both the lap and shoulder portion of the seat belt or just the lap portion.

Children in belt-positioning booster seats were also more likely to sit in the back seats of vehicles, which is safer than the front. "It would appear as though parents who use boosters are really understanding how to keep their kids safe," Durbin says. "Not only are they using proper restraining, but they are using it in the correct place in the car."

Although the study covers kids up only to age 7, there's no reason to think that booster seats wouldn't benefit many 8-year-olds as well. "We didn't have any 8-year-olds in the study that were using booster seats, [but] we would certainly expect 8-year-olds to benefit," Durbin says.

That protection could extend to even older children, Tombrello says. "Our experience is that in many cases children can't sit properly until they're 10 or 12. We're not suggesting that laws should specify age. The important thing is that all people ride correctly."

And for the 4-to-7-year age bracket, riding correctly should be easy. "This is a pretty simple one for parents," Durbin points out. "The seats do not have to be installed in the car. They're small, highly portable and very affordable."

More information

For more on child passenger safety, visit the American Academy of Pediatrics, SafetyBeltSafe U.S.A. or Advocates for Highway and Auto Safety.