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.”

The researchers used a wide range of upper and lower belt anchorage locations and ranges of seat height, seat cushion angle, and seat back angles to test the seat belt geometry.Across individuals, an increase in BMI of 10 kg/m2 was associated with a lap belt positioned 43 mm further forward and 21 mm higher relative to the anterior–superior iliac spines of the pelvis. Each 10 kg/m2 increase in BMI was associated with an increase in lap belt webbing length of 130 mm. The worsening of lap belt fit with restricted foot position was slightly greater for obese participants. Obesity was associated with a more-inboard shoulder belt routing across a wide range of upper belt anchorage locations, and the shoulder belt webbing length between the D-ring and latch plate increased,” the study found.

As America’s obesity problem has grown, so has interest in its effect on occupant protection. The University of Buffalo has conducted two studies regarding weight and crash outcomes that have grabbed headlines. A 2010 examination of FARS data “identified obesity as a risk factor for death in a study of 155,584 drivers in severe auto crashes. In that study, they found that morbidly obese individuals are 56 percent more likely to die in a crash than individuals of normal weight,” according to an announcement of the study.

This spring, university researchers published a follow-up study showing that obese drivers were far less likely to wear their seatbelts, probably due to comfort issues – normal weight drivers are 67 percent more likely to wear a seatbelt. And The New York Times published a news article questioning the safety of obese airline passengers given the small space allotted to each passenger in economy class, and seats and seat belts designed to standards that are decades old and no longer reflect the body mass of many travelers.

The more interesting question is: What will automakers do about it? Current federal motor vehicle safety standards have recently been upgraded to protect heavier children in booster and child safety seats. But heavier adult occupants have not figured into any rulemaking. Obese adults place additional strain on the restraint systems and seat backs in ways that haven’t been and aren’t required to be tested. Obese dummies don’t exist, says Zellers.

In the UMTRI study, the authors pointed to the occupants as the problem:

“That is, occupant factors may be more important in determining the performance of the lap belt system than the system design variables. Increased BMI compounds the decrement in lap belt fit associated with foot position restriction. These results suggest that obese occupants will experience particularly poor lap belt fit in rear-seat conditions where lack of legroom causes elevated-knee postures.”

So far, the best some automakers can offer are seatbelt extenders, which don’t really address the slack and poor lap belt positioning issues. And it’s uncertain whether a seatbelt designed to protect larger occupants will work as well for smaller occupants.

Zellers says that obesity has been addressed sparsely at government and industry conferences, with countermeasures to protect the special population of the elderly taking more of the focus.

“There’s not as much research into protecting the obese, and I suspect it’s in part because of the stigma of obesity,” she says. “The attitude is: You can lose weight, you can’t fix being old.”

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