July 31, 2002 ANN ARBOR -- A University of Michigan physicist and a research scientist are questioning the belief that bigger and heavier vehicles are automatically safer than other cars and trucks.
U-M physicist Marc Ross and Tom Wenzel, a research scientist at Lawrence Berkeley National Laboratory (LBNL), recently released a report which shows that vehicle quality is actually a better predictor of safety - both for the driver and for other drivers - than weight. The two will discuss their results at a briefing at LBNL's Washington, D.C. office on July 30.
"Safety is a challenging concept. It includes the design of the car itself, driver demographics and behavior, the kinds of roads, the time of day - a whole host of factors," Ross said. "What we need to do is move away from the idea that bigger and heavier vehicles are automatically safer."
Recent Senate hearings on Corporate Average Fuel Economy standards focused on the increased risk Americans would face if they had to give up their SUVs for vehicles that weigh less. "We set out to see whether that risk is real, whether SUVs really are safer than cars. The answer, by and large, is no," Ross said.
The first major result Ross and Wenzel found is that SUVs are no safer for their drivers than cars. Popular midsize cars, minivans and import luxury cars have the safest records, while SUVs are about as risky as the average midsize or large car, and are no safer than many compact and subcompact models. The researchers defined "risk" as the number of deaths per year per million vehicles.
Other studies have not considered combined risk, which looks at both risk to the driver of the model in question and risk to the drivers of all other vehicles involved in crashes with the model in question. The study found that, when measuring the combined risk, most cars are safer than SUVs, while pickup trucks are much less safe than all other types of vehicle.
"Clearly the characteristics of the drivers of certain types of vehicles also have a strong effect on their safety," Ross said. "However, it is not clear exactly what that effect is, and the age and sex of drivers do not fully explain these results." Some of the safest subcompacts also have a high fraction of young male drivers. At the other extreme, elderly drivers dominate certain large cars but there is no clear pattern suggesting that those cars pose especially high risk to drivers of other cars as a result.
The Chevy Corvette illustrates that both vehicle design and driver behavior can be important. Like drivers of other sports cars, Corvette drivers are much more likely to be killed than drivers of other types of cars. But although Corvettes are often driven dangerously, and are very risky for their own drivers, the risk to drivers of other vehicles is low in collisions with Corvettes. The low-slung design and plastic body of the Corvette probably account for its low risk to others.
To determine quality, Ross and Wenzel used quantifiable parameters such as new car price, used car price, Consumer Reports safety ratings, and country of origin. "It is extremely difficult to determine the inherent safety of a vehicle type or model because it is too hard to separate the contribution of driver characteristics and behavior from the contribution of vehicle design. We can say, however, that quality is a much better predictor of safety than weight," Ross said.
"It turns out that relatively inexpensive light cars do tend to be unsafe, but more expensive light cars are much safer, and are as safe as heavier cars and SUV models. In any event, the argument that lowering the weight of cars to achieve high fuel economy has resulted in excess deaths is unfounded. If designers pay careful attention to safety in vehicle design, smaller cars can be, and indeed have been, made as safe as larger ones," Ross said.
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