Vehicle Weight Alone Doesn't Account for Fatality Differences in Crashes Between Cars and SUVs, Study Shows

BUFFALO, N.Y. -- The difference in weight between two passenger vehicles, it turns out, is not the only "killer" factor in a head-on collision.

Researchers in the University at Buffalo's Center for Transportation Injury Research have found that even when a passenger car weighs more than an SUV, passengers in the car remain at higher risk of dying in a head-on collision than passengers in the SUV.

"It's clear that vehicle safety can't be evaluated on the basis of weight alone," said Dietrich Jehle, M.D., UB associate professor of emergency medicine, director of the research center and co-author of the study. "Mismatches in vehicle design and structural load path, which can allow an SUV to override a passenger car, can increase the severity and consequences of a crash."

All vehicles are designed to deliver the load of the impact to the structural frame, or crush zone, Jehle explained. Mismatches in this "structural load path," such as when one vehicle is higher than another, result in the forces being delivered more directly to the occupants.

"Moreover, disparities in bumper height may result in the passenger vehicle underriding, or 'submarining,' under the SUV, resulting in greater damage to the passenger compartment and its occupants," he said.

Results of the study appeared in a recent issue of The Journal of Trauma Injury, Infection and Critical Care. James Mayrose, Ph.D., UB research assistant professor of emergency medicine, was first author on the study.

Of all the causes of motor vehicle fatalities -- alcohol, speeding, not wearing a seatbelt, type of vehicle driven and other variables -- the relationship between vehicle type and fatalities has been studied the least, Jehle noted.

What is known is that as cars became lighter to achieve more fuel efficiency, fatalities and serious injuries increased. And as cars were getting lighter, they were sharing the roadways with a steadily increasing number of heavy vehicles. The Institute for Highway Safety estimated that by 1999, SUVs, vans and pickup trucks accounted for 34 percent of vehicles on U.S. highways.

Earlier research on vehicle fatalities showed that in head-on collisions between these two vehicle types, the vast majority of deaths occurred in passenger cars, and that there is a direct relationship between vehicle weight and fatalities. With increasing numbers of SUVs on the road, the weight-safety issue took on increasing importance.

To shed light on the issue, researchers in the Center for Transportation Injury Research, affiliated with UB's Department of Emergency Medicine at the Erie County Medical Center, set out to determine if this increase in deaths could be explained by vehicle weight difference alone.

Researchers obtained data for their study from the national Fatality Analysis Reporting System. They analyzed information on 633 crashes that occurred from 1995-99 in which there was at least one death. Only head-on collisions between a moving car and a moving SUV in which passengers were wearing seat belts were considered.

Results showed that of the 798 fatalities recorded, 76 percent were passengers in cars and 24 percent were passengers in SUVs. In fact, in crashes between small cars and large SUVs, the risk of death was 24 times greater in the passenger car than the SUV.

After assigning all vehicles into categories based on weight, it became clear that weight was not the only critical factor. When weights of car and SUV were equal, the difference in fatalities decreased but was still significant: 1.7 times greater risk of death in the passenger car versus the SUV.

In the 57 crashes where cars outweighed SUVs by an average of 234 pounds, occupants of cars still remained at greater risk of death, with 1.6 times greater fatalities in the larger car compared to the SUV.

"This study's findings make it clear that when controlling for weight, there is still a significant majority of deaths in the passenger car involved in a frontal crash with an SUV," Jehle said. "It is apparent that other factors besides vehicle mass play a significant role in the increase in deaths in the passenger vehicles."

"Future studies are needed to investigate how the structural design of SUVs can be modified in order to reduce the severity of injuries incurred by occupants of passenger vehicles involved in crashes with SUVs."

The research was funded in part by a grant from the Federal Highway Administration to the UB Center for Transportation Injury Research.