It found that heavier people are up to 80% more likely to die in an accident than drivers of a healthy weight, with obese women most at risk.
The research, published in the Emergency Medicine Journal, found that fat people are propelled further forward during a collision because their additional soft tissue prevents the seat belt tightening immediately against their pelvic bones.
The study, which took place in the US, included 6,806 drivers involved in 3,403 collisions, of which 18% were classified as obese, 33% were overweight and 46% had a healthy weight.
Its findings show those who were most obese, with a Body Mass Index (BMI) of 40 or over, were 80% more likely to die in an accident than drivers of a healthy weight.
And obese women were found to be at greater risk than men, with a BMI of 35 and over roughly doubling the risk of death compared with women of normal weight.
With obesity on the rise in the UK as well as the US, it is hoped that car manufacturers will try to adapt their safety features to better protect larger Britons.
Education is needed to improve seat belt use among obese people. Clinical intervention could inform obese patients of the additional traffic injury risks and potential benefits of losing weight."
In the meantime, the AA advises drivers to adjust their seats according to their size to allow the air bag to protect them properly in the event of a crash.
However, the greater risk of death in a collision is certainly another reason for the one in four or so men and women who can now be described as clinically obese to take action to bring their weight down.