Obesity carries a number of negative health effects, from heart disease to diabetes. New research from UC Berkeley says people also have a higher risk of dying from a vehicle crash as their waistline expands.

Researchers concluded that obese drivers involved in crashes are significantly more likely to be fatally injured than drivers with a normal body-mass index. The risk of a fatal accident went up steadily with higher BMIs.

The study, published in the journal "Emergency Medicine," analyzed 41,283 fatal crashes recorded in the National Highway Traffic Safety Administration's Fatality Analysis Reporting System. Drivers in these crashes were categorized by their BMI, which was derived from the height and weight recorded on their licenses.

Researchers looked at incidents where there was no significant difference in the size of the vehicles involved. They also accounted for data such as seat belt use, time of day, air bag deployment, type of collision, gender, and alcohol use.

The World Health Organization defines a normal BMI as falling between 18.5 and 24.9. The UC Berkeley study found that drivers with BMIs below 18 and those at "pre-obesity" levels of 25 to 25.9 had vehicle crash death rates that were comparable to drivers with normal BMIs.

By contrast, drivers with BMIs that fell into WHO's three classes of obesity were significantly more likely to be fatally injured in a crash. Researchers said a BMI of 30 to 34.9 were linked to a 21 percent increase in the risk of death. A BMI of 35 to 39.9 increased the risk of a fatality by 51 percent, while a BMI above 40 were 81 percent more likely than drivers with normal BMIs to die in a crash. In addition, obese women were more at risk of death than obese men.

"Vehicle designers are teaching to the test, designing so that crash test dummies do well," said Thomas Rice, a research epidemiologist at UC Berkeley. "But crash test dummies are typically normal size adults and children. They're not designed to account for our nation's changing body types."

According to the Centers for Disease Control, more than one-third of Americans are considered obese. The NHTSA currently uses crash test dummies to test the effect of crashes on adults and children. Although it does not use a dummy to simulate an obese adult, the company Humanetics Innovative Solutions—the only American manufacturer of crash test dummies—is developing such a model.

Previous studies have also highlighted the increased risk to obese drivers in vehicle crashes. An earlier UC Berkeley study, published in 2013, said the additional body tissue in obese drivers prevents the seat belt from fitting snugly. As a result, the lower body is propelled farther in a crash while the upper body is held back.

Another study determined that obese drivers are less likely to use seat belts than drivers with normal BMIs. Researchers at the University of Buffalo published a study in 2014 with the conclusion that normal BMI drivers were 67 percent more likely than morbidly obese drivers to wear a seat belt.

The most recent study was conducted by the Safe Transportation and Research Center at UC Berkeley. This center is affiliated with the school's Institute of Transportation Studies as well as its School of Public Health.