Weighing in on In-Car Traffic Injuries

Given the increase in average body size (and weight), it is not surprising that the automotive safety community is questioning the impact of obesity on the performance and assessment of occupant protection systems.

This issue was recently addressed by David Viano and colleagues, who work at ProBiomechanics LLC, a Michigan firm specializing in occupant kinematics and injury causation, published last month in Traffic Injury Prevention.

The authors investigated the relationship between fatality and serious injury risks for front-seat occupants by body mass index (BMI) using a matched-pair analysis. They also developed a simple model for the change in injury risk with obesity which includes the normal mass (m) and stiffness (k) of the body resisting compression during a blunt impact. For a given impact severity, the risk of injury was assumed proportional to compression. Energy balance was used to determine injury risks with increasing mass.

Data for 1993-1004 was analyzed from the National Automotive Sampling System Crashworthiness Data Set (NASS-CDS), an ongoing study of more than 5,000 accidents each year, in which trained investigators look at wrecked vehicles, read through police reports, and talk to accident victims. Occupant injury was divided into normal and obese categories. A matched-pair analysis was carried out. Driver and front-right passenger fatalities or serious injuries (MAIS 3+) were analyzed in the same crash to determine the effect of obesity.

Based on the model, an obese occupant (BMI = 30-35 kg/m2) has 54-61% higher risk of injury than a normal BMI occupant (22 kg/m2). Matched pairs showed that obese drivers have a 97% higher risk of fatality and 17% higher risk of serious injury than normal BMI drivers. Obese passengers have a 32% higher fatality risk and a 40% higher risk than normal passengers. Obese female drivers have a 119% higher risk than normal BMI female drivers and young obese drivers have a 20% higher serious injury risk than young normal drivers.

These data add to several prior publications highlighting the increased risk for overweight and obese drivers and passengers in automobiles.

Viano and colleagues also estimated how much extra ballast the family of Hybrid III crash test dummies would need to represent an obese or morbidly obese occupant. According to these assessments, the smallest crash test dummies need proportionately more ballast to represent an obese or morbidly obese occupant in the evaluation of safety systems. The 5% female Hybrid III (BMI = 20.4) and needs 22 kg of ballast to represent an obese and 44.8 kg to represent a morbidly obese female, while the 95% male needs only 1.7 and 36.5 kg, respectively.

The authors conclude that

Obesity influences the risk of serious and fatal injury in motor vehicle crashes. The effect is greatest on obese female drivers and young drivers. Since some of the risk difference is related to lower seatbelt wearing rates, the comfort and use of seatbelt extenders should be examined to improve wearing rates by obese occupants. Also, crash testing with ballasted dummies to represent obese and morbidly obese occupants may lead to refined safety systems for this growing segment of the population.

As also noted recently by Ben Zarzaur and Stephen Marshall, Surgeons at the University of Tennessee Health Science Center, Memphis, in The Journal of Trauma, the combination of obesity and not using a seat belt is particularly deadly.

Most importantly, David Schlundt and colleagues from Vanderbilt University just reported in OBESITY based on data from the 2002 Behavioral Risk Factor Surveillance System Survey, that seatbelt use declines as BMI increases, with approximately 55 percent of extremely obese individuals say that they do not use a seatbelt.

Consequence of these finding for counseling our obese patients: buy cars where seatbelts fit your size or buy seatbelt extenders AND USE THEM!

Sounds like a class-action waiting to happen!

Edmonton, Alberta