plane are safer in a child safety seat than in a parent's arms. The FAA and the AAP recommend that all children under 40 lb use a child safety seat on a plane. Booster seats cannot be used on airplanes because they don't have shoulder belts. Parents should not put children into safety seats with thick winter coats on. The coat will flatten in an accident and the straps will not be snug enough to keep the child safe. An alternative would be placing a coat on the child backwards after buckling the child in. Straps on the harness should be snug on the child, parents should not be able to pinch the straps away from the shoulders of the child. The straps also need to be placed at the proper height for the child. Seat placement A study of car crash data from 16 U.S. states found that children under the age of 3 were 43% less likely to be injured in a car crash if their car seat was fastened in the center of the back seat rather than on one side. Results were based on data from 4,790 car crashes involving children aged 3 and younger between 1998 and 2006. According to data, the center position was the safest but least used position. However, economist Steven Levitt (see below) has demonstrated that car seats do not reduce fatalities when compared to regular seat belts. The move from having car seats in the front passenger seat to having them in the back seat, facing backwards, may make it easier for a busy, distracted parent to leave an infant in the car. Each year, between 30 and 50 infants die of heat illness and hypothermia in the United States after being left in a car. Law Baby car seats are legally required in many countries, including most Western developed countries, to safely transport children up to the age of 2 or more years in cars and other vehicles. Other car seats, also known as "booster seats," are required until the child is large enough to use an adult seat belt. This is usually, but not always, when the child is tall. The child needs to meet five criteria before moving out of the booster seat, including the child's seating position, shoulder belt position, lap belt position, knee position, and ability to sit properly for the length of the trip.