August 26, 2002
Berkeley Lab Science Beat Berkeley Lab Science Beat
Is Bigger Safer? It Ain't Necessarily So
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Those who think that driving big is driving safe, or that lightweight, fuel-efficient vehicles are inherently more dangerous than their heavyweight counterparts, need to think again. A researcher with Lawrence Berkeley National Lab (Berkeley Lab) has teamed with a researcher from the University of Michigan in a unique risk analysis study which shows that, contrary to conventional wisdom, vehicle quality is a much more important safety factor than weight for the drivers of vehicles involved in a crash.

"Most cars are safer than the average sports utility vehicle [SUV], while pickup trucks are much less safe than all other types. Minivans and import luxury cars have the safest records," states the report, "An Analysis of Traffic Deaths by Vehicle Type and Model," which was prepared by Tom Wenzel, an energy analyst with Berkeley Lab's Environmental Energy Technologies Division, and Marc Ross, a professor in Michigan's Applied Physics Department.

In the collaborative Berkeley/Michigan study, researchers looked at the risk to drivers of a particular vehicle, the risk that that vehicle imposed on drivers of other vehicles, and the combined risk (the sum of the two). Their analysis evaluated the highway dangers posed by the most popular vehicle models from 1995-1999 and showed that bigger is not necessarily safer.

In the ongoing debate over automotive fuel economy, the argument has been made that increasing fuel efficiency will require a decrease in vehicle weight, which in turn will result in a higher rate of traffic fatalities -- as lighter-weight vehicles are inherently more unsafe.

To test the validity of this argument, Wenzel and Ross compared the risk of death in traffic accidents based on vehicle type and model. They looked at driver deaths per year per million vehicles sold for model years 1995-1999, using as their data source the annual census of traffic fatalities published by the National Highway Traffic Safety Administration. Their analysis is among the first to look at "combined risk," which encompasses all the drivers involved in a crash.

"A shortcoming of many safety analyses has been that only risks to drivers of a given kind of vehicle are evaluated while the risks imposed on others are ignored," says Wenzel. "We focused on the risk not only to occupants of the vehicle model in question in all types of crashes, but also on the risk to the drivers of other vehicles involved in crashes with the model in question."

Looking at combined risk is a more accurate means of evaluating a type of vehicle or model for its contribution to overall danger on the highway. By limiting their analysis to recent vehicle models, Wenzel and Ross were able to obtain enough statistics on sales and fatalities to be confident that their calculated risks "reflect the true risk of the vehicle model and not a statistical aberration."

Wenzel and Ross determined that SUVs are no safer for their drivers than the average midsize or large car and not much safer than many of the most popular compact and subcompact car models. Drivers of pickup trucks are at even greater risk than drivers of SUVs. When the combined risk is considered, SUVs and pickup trucks are revealed to be significantly more dangerous than just about any car.

"The safest SUV, the Suburban, has at least a 40 percent higher combined risk than the three safest midsize and large cars, the Avalon, Camry, and Accord," the scientists state in their report.

The Chevy Suburban was found to be the safest SUV model as determined by a combined risk analysis of traffic fatalities, but not nearly as safe as a Camry or an Accord.  

That vehicle quality is a more important safety consideration than weight was evident in the wide range in risks between different subcompact and compact models. The safest small cars, the Volkswagen Jetta and Honda Civic, were shown to be twice as safe as the comparably sized Chevrolet Cavalier, Ford Escort, and Dodge Neon.

"In looking at all vehicles, cars designed by Honda and Toyota consistently are safer, and weigh less, than comparable cars designed by domestic manufacturers," Wenzel says.

Wenzel and Ross did take into consideration the age and sex of the typical driver of a specific vehicle model and how that vehicle is normally driven. For example, sports cars, as driven, are extremely risky to their drivers, who typically are aggressive young males. For minivans, the least risky of all vehicles, the story is much different.

"They are seldom driven by young males and tend to be driven more carefully than other vehicle types," says Wenzel. He notes that a portion of the high risk ratings for pickup trucks can be attributed to their use in rural areas where speeds are high and road conditions poor.

However, Wenzel and Ross found no evidence that driver age and sex were responsible for SUVs and pickup trucks garnering higher risk ratings than cars. In the future, they plan to study whether other driver characteristics, such as use of seatbelts or drug and alcohol use, may explain some of the results of this study.

"All the evidence in our study shows that vehicles can be, and in fact are being, made lighter and more fuel efficient without sacrificing safety," says Wenzel. "The argument that lowering the weight of cars to achieve high fuel economy has resulted in excess deaths is unfounded."

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