In a Vehicle Rollover, Beware the "Outside Arc"

Risk of death, injuries greatly magnified for those in SUVs

By Lois Baker

Release Date: July 14, 2006

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The direction in which a vehicle rolls over when it careens out of control determines whether the driver or front-seat passenger is at greater risk of serious injuries or death.

BUFFALO, N.Y. -- If a vehicle carrying a driver and front-seat passenger careens around a corner and rolls over, which occupant would sustain the worst injuries and have the least chance of survival?

It depends on which way the vehicle rolls over, with the person in the "outside arc" position, the side that hits the hardest, having the highest risks, according to a study conducted by researchers at the Center for Transportation Injury Research (CenTIR) at the University at Buffalo.

"Inside arc" and "outside arc" refer to the positions of the occupants in relation to the direction of the rollover.

"When a vehicle rolls onto the driver's side, for example, the right-front passenger would be on the outside arc and would experience the strongest initial impact, while the driver would be on the inside arc, where the impact would be less," said lead researcher Dietrich Jehle, M.D, associate professor of emergency medicine in the UB School of Medicine and Biomedical Sciences..

"In a rollover onto the passenger side, the 'arc' positions and risk of serious injury and death are reversed," said Jehle, who presented the study findings at a recent meeting of the Society for Academic Emergency Medicine in San Francisco, Calif.

Results showed the risk of death was 1.6 times greater for outside-arc occupants than inside-arc occupants.

"For SUVs, which are more prone to rollover due to their higher center of gravity, the chances that the outside-arc occupant would be killed or injured severely were even greater," Jehle said.

"The study results are a call to car manufacturers to increase roof strength of vehicles at higher risk for rollover, particularly SUVs," he noted, "and to install airbags designed to protect the head."

Some models already include "side curtain" airbags, which are designed either to drop vertically from the panel above the side windows or horizontally from the side panel.

The study analyzed only single-vehicle crashes involving a driver and a front-seat passenger, all of whom were wearing lap-shoulder belts. The study sample included all such cases in the National Accident Sampling System Crashworthiness Data System from 1992-2002. During that period, 735 crashes met the criteria. In these crashes, 48 people died and 192 sustained moderate to severe injuries.

Rotational torque and roof crush are critical factors determining injury and death in rollover crashes, resulting in more head and neck injuries than in other types of crashes, the data showed.

Roof crush, as the phrase implies, refers to how much the roof caves in during the crash.

Rotational torque is the force that develops as the vehicle rolls from one side to the other, creating an arc, similar to the motion of a windshield wiper. The person on the outside arc undergoes the most rotational torque during a rollover.

"The more distance the passenger travels from the initial pivot point of the rollover, the greater the rotational torque and the greater risk of injury," said Jehle.

"Adding stronger roofs, upper side airbags and seatbelt retractors that activate with rollover would decrease the severity of impact experienced by the outside-arc occupant and might reduce the number of head injuries and deaths in rollover crashes," he said.

The research was conducted in conjunction with the Calspan/UB Research Center and Erie County Medical Center.

Additional researchers on the study were James Mayrose, Ph.D., UB research associate professor of emergency medicine and mechanical and aerospace engineering; Joseph Kuebler, a UB medical student, and Peggy Auinger, statistician.

The study was funded in part by a grant from the Federal Highway Administration.

The University at Buffalo is a premier research-intensive public university, the largest and most comprehensive campus in the State University of New York. The School of Medicine and Biomedical Sciences is one of five schools that constitute UB's Academic Health Center.