Too Big to Crash?

According to the Centers for Disease Control one third of Americans over the age of 20 are over-weight, another third are obese, but the world of occupant protection is stuck in the 50th percentile. A latest study – by researchers at the University of Michigan Transportation Institute – builds on other research showing that obese occupants are more likely to suffer injuries or a fatality in a crash, and finds that poor belt fit is a factor in those outcomes.

Effects of Obesity on Seat Belt Fit, published in Traffic Injury Prevention, studied a population of 48 men and women, with nearly half obese, as defined by a body mass index (BMI) of 30kg/m2 or greater. The static laboratory testing, using a seat manufactured in the 1990s that is still typical of current designs, shows broadly that the greater the BMI, the higher the lap belt portion rides on the lower body and the more slack is introduced to the shoulder belt restraining the upper body.

“In order for the seat belt to work effectively, the lap belt needs to rest snugly, low on your hip bones.  Obese people have significantly more fatty abdominal tissue which compromises belt fit – making it hard to position the lap belt on the pelvis properly,” says SRS biomechanical engineer Salena Zellers. “In addition, fatty abdominal tissue compresses under seat belt loading, increasing the occupant’s excursion.   Compression of the abdominal tissues also introduces slack in the restraint system - never a good idea.”

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