The team of researchers studied data from the US Fatality Analysis Reporting System [FARS] for 1996 to 2008. This is operated by the National Highway Traffic Safety Administration and records all fatalities arising within 30 days of a traffic collision.
During this period, details of 57,491 road traffic collisions were recorded in the system.
The researchers sought out collisions involving two passenger vehicles, and for which the impact of the crash was the most harmful component of the crash, resulting in the deaths of one or both drivers.
They also ensured vehicles involved in the crash were of similar size and type. They selected 3,403 pairs of drivers for whom data on weight, age, seat-belt use and airbag deployment were available.
Almost half of these drivers [46%] were of normal weight; one in three was overweight; and almost one in five [18%)] were obese.
The analysis showed that risk of death increased the more obese the driver was, according to the World Health Organization [WHO] classification, which categorises obesity from levels I to III.
At level I, obese drivers were 21% more likely to die; at level II they were 51% more likely to do so; and at level III they were 80% more likely to do so than drivers of normal weight.
When broken down by gender, obese women were at even greater risk. At level I they were 36% more likely to die; at level II they were more than twice as likely to do so; and at level III they were almost twice as likely to die.
Referring other research studies, the authors present a hypothesis that the lower body of obese drivers is propelled further forward on impact before the seatbelt engages the pelvis, because of the additional soft tissue which prevents the belt from fitting snugly, while the upper body is held back.
Also, obese drivers may be more likely to have underlying health problems that may contribute to the greater risk of death. But car design may need to change too, they venture.