A new US study finds that obese drivers are much more likely to die in road accidents than drivers of normal weight. The authors say car designers need to make their vehicles safer for obese drivers, especially given that around one in three American adults is obese.

Transport safety researchers Thomas Rice of the University of California at Berkeley and Motao Zhu of the University of West Virginia, write about their findings in the 21 January online issue of the BMJ publication Emergency Medicine Journal.

They suggest the most obese drivers are 80% more likely to die in a car crash than their normal weight counterparts, with obese women at much higher risk than obese men.

For their study, Rice and Zhu analyzed figures from the Fatality Analysis Reporting System (FARS). This database, which is kept by the National Highway Traffic Safety Administration, records all road deaths arising within 30 days of a traffic collision.

The study covered the period 1996 to 2008, during which the database recorded nearly 57,500 road traffic collisions.

Rice and Zhu searched for crashes involving two cars of similar size and type, where one or both drivers were killed.

On this basis they found records of just over 3,400 crashes that also included details on the drivers’ weight and age, whether they had used a seat belt, and whether airbags were deployed.

The records showed nearly half (46%) of the drivers were of normal weight, one in three was overweight, and nearly one in five (18%) was obese.

The researchers used the World Health Organization definition of Body Mass Index, a benchmark of obesity which takes a person’s weight in kilos and divides it by the square of their height in meters. A BMI of 30 and over is obese.

When they analyzed the figures in terms of risk of death, the researchers found drivers with a BMI between 30 and 34.9 had an increased risk of death of 21% compared to drivers of normal weight (BMI 18.5 to 24.9).

Drivers with a BMI of 35 to 39.9 had an increased risk of death of 51%, and the most obese, with a BMI of 40 and over, had an increased risk of death of 80%.

Obese female drivers were at even greater risk. For the most obese female drivers (BMI 35 to 39.9), the risk of dying in a car crash was double that of a normal weight female driver.

These figures were independent of other influencing factors such as age, and use of alcohol.

There were also no significant differences among types of vehicle, collision or use of seat belts. However, in nearly a third of fatal crashes the drivers had not been properly belted.

An interesting result was that underweight male drivers (BMI under 18.5) were also more likely to die in a car crash than their normal weight counterparts.

In trying to explain these results, Rice and Zhu note that other studies have found the lower body of heavier drivers travels further forward in a crash before the seatbelt engages the pelvis, while the upper body is held back. This is because the frame of the obese driver has more padding or abdominal fat, which increases the time it takes for the belt to tighten on impact.

Another reason for obese drivers having a greater risk of death, could be because they are more likely to have underlying health problems.

Nevertheless, the authors urge manufacturers to reconsider the way they design cars to take into account the obese driver:

“The ability of passenger vehicles to protect overweight or obese occupants may have increasingly important public health implications, given the continuing obesity epidemic in the USA,” write Rice and Zhu.

“It may be the case that passenger vehicles are well designed to protect normal weight vehicle occupants but are deficient in protecting overweight or obese occupants,” they note.

Written by Catharine Paddock PhD