The findings prompt the researchers to consider whether car design might need to change to afford greater protection to the considerable proportion of obese people in the population - currently around a third of all US adults.
The researchers used 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 submitted to the system.
The researchers looked for collisions in which two passenger vehicles were involved, and for which the impact of the crash was the most harmful component of the incident, resulting in the deaths of one or both drivers.
They also looked for collisions in which both parties had been driving vehicles 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.
Two thirds were male, and almost one in three was aged between 16 and 24; one in three was not using a seat belt properly - lap or shoulder only, rather than both - and in over half (53%) of cases, the airbag deployed.
For years, we’ve been told that being overweight is a failure of willpower. The most likely scenario for people with a large amount of weight to lose is that even if they shed it by dieting, it goes back on after they stop, plus more.
This yo-yo behaviour has led psychiatrists at the University of Connecticut to question whether obesity is a form of addictive behaviour – and if that’s the case, the solution needs to start in the mind.
The US study was the inspiration behind the TV programme Obese: A Year To Save My Life. On the Sky1 show, principles of cognitive behavioural therapy, an NHS-approved type of psychotherapy used to treat addiction, were applied to nine morbidly obese people.
The results were startling. The show’s fitness trainer Jasse Pavelka specialises in working with patients who have weight-loss surgery. He believes obesity goes hand-in-hand with trauma. ‘You realise people who are dangerously overweight have emotional problems,’ he says.
Nine men and women, who all used food as a crutch, took part. One woman turned to food after she was sexually abused in childhood, another began overeating after her parents divorced, another man was unhappy in his relationship.
The analysis showed that risk of death increased the more obese the driver was, according to the World Health Organization 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.
According to Australian government health statistics being obese increases the chance of injury.
The Australian Institute of Health and Welfare said in its report, “Obesity and Injury in Australia: a Review of the Literature”, that in spite of the findings being mixed, the probability of falls, trips or stumbles rise with obesity. However, the Institute’s James Harrison says the increased risk of injury from falls in the obese may be somewhat offset by the better cushioning body fat provides.
There are also indications of a link between obesity and an increase of injuries at work. One study showed that adults with a high body mass index were twice as likely as adults with normal range BMI to have a driving injury.
Another study found that sleep-disorder breathing conditions - such as sleep apnea - among obese people increased the risk of driver involvement in traffic accidents causing an injury. People who are obese are at higher risk of developing sleep apnea.
The report said obesity was associated with impaired postural balance, even in relatively young individuals. Surprisingly, obesity was linked with a reduced risk of hip and wrist fractures in older adults.
“In contrast, obesity has been associated with a greater risk of distal forearm and wrist fractures in children” the report said. “This has been attributed to an increased risk of falling, compounded by decreased bone density in obese children, who are more likely than other children to be sedentary.”
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.