The Academy of Pediatrics and experts at Failure Analysis, a California firm that analyzes what went wrong in accidents, recommend against the use of any booster seats currently on the market once a child reaches 40 pounds. This usually happens when a child is 55 inches tall. For a child under 55 inches tall, The Academy of Pediatrics recommends the use a lap belt, and to put the shoulder strap behind the child. The worst alternative is to put the shoulder strap under one arm. As a driver, check to make sure that everyone in the car is buckled in before you drive, even if only driving a short distance.
Most accidents do occur on surface streets within five miles of home.
Children have been seriously injured by being propelled against a loose-fitting belt. Your question highlights a poignant situation. Automobile accidents are responsible for more deaths in middle childhood than any other cause. There are an additional 100 children injured in car crashes for every child who dies. Air bags are a case in point.