Comparison of Child Body Dimensions with Rear Seat Geometry

Children who are too large for harness restraints but too small to obtain good restraint from a vehicle seatbelt alone should be seated in a belt-positioning booster. Boosters have been shown to significantly reduce abdominal injuries caused by seatbelts. This effectiveness may be due in part to the fact that boosters reduce the effective seat cushion length, allowing children to sit more comfortably without slouching. NHTSA recommends that children who do not use harness restraints use boosters until they are at least 145 cm tall. In this paper, data from several sources were combined to assess how well children fit on rear seat cushions. Data from NASS-GES were analyzed to determine the age distribution of rear-seat occupants. Anthropometric data from several sources were analyzed to determine the distribution of buttock-popliteal length, a measure of thigh length that is a key determinant of seat fit, as a function of age and gender. Second- and third-row cushion lengths were measured on a convenience sample of 56 late-model vehicles. Comparing the distribution of body size for rear-seat occupants with the seat cushion lengths showed that most cushions are too long for most rear-seat occupants, using commonly applied standards of seat fit. Given that most rear-seat occupants in the U.S. are children, rear-seat design standards should consider the smaller body dimensions and different restraint needs of this population.