Crash course in physics: Why kids need booster seats

The physics involved in car crashes don't change if families are just on a quick trip to the grocery store or in Grandma's car instead of their own. So safety experts advise parents to always use car seats or boosters to protect young children.

In a crash or sudden stop, passengers weigh their weight times the car's traveling speed. According to the Safety Restraint Coalition, which promotes the use of booster seats, this means that a 40-pound child in a car going 30 mph will exert 1,200 pounds of force — that's more than the weight of a grand piano — when he pushes against the seat belt in a crash.

In a crash, an adult's seat belt spreads the impact over the pelvis, chest and shoulders. While a seat belt of any sort is better than no restraint at all, children who are too short end up with the seat belt too high on their bellies and often, with the ill-fitting shoulder belt behind their back, experts say.

In a 2000 survey of 2,200 children statewide, researchers found half of children weighing 40 to 80 pounds had the lap belt on their abdomen; that jumped to two-thirds of children weighing 20 to 39 pounds. Only half of 40- to 60-pound kids (and just a quarter of those 20- to 39-pounds) had the shoulder belt over their shoulder, with almost one-third placing it behind their backs.

Forced into the soft tissue of the tummy, the lap belt can "pop bowels like a balloon," said Dr. Beth Ebel, a pediatrician at Children's Hospital and Harborview Medical Center who has done booster-seat research at the Harborview Injury Prevention and Research Center. "We've seen kids come in that had to have colostomy bags."

The lap belt can also damage other organs such as the liver and spleen.

Without a shoulder belt, the impact of a crash pushes the child's upper body forward so hard that his spine can tear apart or fracture, Ebel said. Head and neck injuries can result if the child's head hits the seat in front of him or whips backward without a protective headrest or high-backed seat.

A small child can shoot out from under a too-large belt, called "submarining," Ebel said. Unrestrained, the child can fly through the car or out of the vehicle.

"A booster fixes the problems by making the child appear more adult-like to the car," she explained. "It raises up the shoulder belt so it hits the sternum and shoulder and helps the lap belt stay low so the force is on the pelvis, not the belly."

Some kids may balk at sitting in a booster, especially if parents moved them out of a car seat already. Parents can emphasize the positives — it's easier for children to look out the window and it's more comfortable without an ill-fitting shoulder belt hitting them in the neck — but it comes down to parents deciding this is not negotiable, Ebel said.

"You know how they say to pick your battles with children?" she asked. "Car crashes are the leading cause of death for kids. Pick this battle and draw the line. This is the riskiest thing your child does."

— Stephanie Dunnewind, Seattle Times staff reporter