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Status Report, Vol. 42, No. 1 | January 27, 2007 Subscribe

Study shows fuel economy can be improved without compromising safety

Vehicles can be more fuel efficient without compromising safety. This is the main conclusion of a new study by injury epidemiologist Leon S. Robertson.

Automakers typically comply with U.S. fuel economy standards two ways, both of which have safety downsides. Making vehicles smaller means they use less fuel but generally aren't as crashworthy (the smallest vehicles on the road have the highest death rates). Adding small, light, and fuel-efficient vehicles to offset gas guzzlers raises overall fleet fuel efficiency but contributes to incompatibilities in crashes between smaller, lighter vehicles and bigger, heavier SUVs and pickups (see "Automakers' efforts reduce mismatch between cars and light trucks," Jan. 28, 2006).

A way to improve fuel economy and maintain vehicle crashworthiness is to use lighter materials that reduce vehicle weight but not size. Automakers traditionally haven't gone this route unless there were federal incentives to do so.

Robertson studied death rates in 67 car, van, and SUV models (1999-2002s) during 2000-04. First he looked at the effects of both vehicle weight and size on fuel use, finding that weight had a big impact. Heavy vehicles are the least fuel efficient (see "Proposed revision of fuel economy standards would be a win for safety," Feb. 25, 2006). Then Robertson studied the relationship between weight and death rates, controlling for other safety factors. Larger vehicles, good frontal offset crash test scores, and greater static rollover resistance were related to lower death rates. Weight increases also were associated with lower risk of death.

But considering all fatalities (pedestrians, motorcyclists, and people in other vehicles involved in crashes), Robertson found that weight increases beyond those associated with increased vehicle size raised the total risk. The protective effects of the increased weight for people in a given vehicle were offset by the higher risk to other people in crashes with those vehicles.

Robertson notes that weights varied within the same vehicle size groups as measured by turn distance. He concludes that reducing the weights of all vehicles in the study group to those of vehicles with the lowest weight per size would reduce fatality rates 28 percent and fuel use 16 percent.

"[F]uel economy is not incompatible with societal risk if reductions in vehicle weight are accomplished without reducing vehicle size," Robertson says. "More sensible fuel economy regulation that would not be adverse to safety could be achieved by setting a standard for minimum fuel economy dependent on vehicle size."

The federal government is moving in this direction. Standards issued in March 2006 require automakers to improve the fuel efficiency of SUVs, vans, and pickups by the 2011 model year. Fuel economy standards will vary by vehicle size, measured as wheelbase times track width, instead of applying a single fleetwide standard. Downsizing will be discouraged because standards will be more stringent for smaller vehicles. This should have the effect of raising these vehicles' fuel economy without compromising their safety.

"Robertson's study confirms that the new federal rule for light truck fuel economy is a good approach," says Institute president Adrian Lund. "Indexing these requirements to vehicle size makes it less likely that auto manufacturers will reduce the weights of their vehicles in ways that degrade occupant protection, and some weight reduction, especially among very heavy vehicles, could improve total safety by lowering the risk to other people on the road."

Something else that Robertson's study confirms is "the importance of choosing a vehicle that has good crash test performance, whatever its size and weight, and a vehicle with good stability and resistance to rollover," Lund says.

Pointing out a problem with Robertson's research, Lund says it "never addresses why the lower weight vehicles in each size group weigh less than others in the same group. In some cases, lower weights are due no doubt to smaller engines, so safety is partly affected by the performance of the vehicles. Weight reductions achieved solely by using materials that weigh less could have smaller benefits, or no net benefits, on societal risk of injuries."

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