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Study confirms wisdom of linking fuel economy to a vehicle's footprint

Automakers can shed a few pounds to help meet fleetwide fuel economy standards without sacrificing safety if they concentrate their weight loss in the heaviest vehicles, a recent federal analysis concludes.

The study by the National Highway Traffic Safety Administration provides additional support for the approach the agency took when it updated fuel economy standards in 2010. Those standards are in effect for 2012-16 models and require the industry to reach an estimated fleetwide 34.1 mpg in the final year. NHTSA and the Environmental Protection Agency now are working on standards for model year 2017 and beyond.

The current standards are a departure from previous ones because they tie fuel economy to a vehicle's footprint, roughly equivalent to the square footage outlined by the wheels. A vehicle with a smaller footprint now has to adhere to more stringent fuel economy standards. That removes the incentive for automakers to simply sell more small cars as a way to meet fleetwide targets (see "Fuel economy and safety can be achieved at the same time," April 14, 2009 and "Overhaul of federal fuel economy program serves safety too," April 22, 2006).

The standards encourage automakers to use more efficient engine technologies, as well as hybrid and electric vehicles, to improve their fleetwide fuel economy. Manufacturers also make use of lightweight materials that can cut weight from a vehicle without changing a vehicle's footprint. The NHTSA study looks at what happens to safety when the last strategy is used.

It is an important question because, other things being equal, larger and heavier vehicles provide better occupant protection than smaller and lighter ones. Both size and weight play a role. Size is important because a longer crush space allows more crash energy to be absorbed before it reaches the occupant compartment. Weight matters because when two vehicles collide, the heavier one pushes the lighter one backward on impact, resulting in greater forces on the people inside the lighter vehicle.

Exactly how much of the advantage of bigger vehicles is due to size and how much to weight is hard to disentangle. The Institute's affiliate, the Highway Loss Data Institute, recently shed some light on the issue when it compared hybrid vehicles with their conventional counterparts and found that the odds of being injured in a hybrid are 25 percent lower than for people in nonhybrid vehicles. In effect, that study looked mostly at weight while controlling for footprint because the hybrids have identical structures to those of their conventional twins but are heavier, thanks to their battery packs (see "Hybrid advantage," Nov. 17, 2011).

In contrast to the HLDI study, which looked at injury rates for people in those vehicles, the NHTSA researchers were looking at the societal impact of weight variations in fatal crashes. That is, their analysis included not only fatalities of people inside a given vehicle, but also deaths among occupants of other vehicles that collide with it, as well as pedestrians.

The researchers derived their predictions from calculations of fatality rates by mass and footprint per billion vehicle miles traveled, using data on crashes in 2002-08 involving 2000-07 models. They took into account that all new models will have electronic stability control, a requirement as of 2012.

The study first looked at the effect on fatality risk of an across-the-board 100-pound weight reduction while maintaining vehicle footprint. Cutting 100 pounds from cars weighing less than 3,106 pounds would result in a 1.4 percent increase in fatalities associated with those cars, the researchers found. Hypothetical reductions in weight in other vehicle categories yield slight increases or decreases in fatalities, but those estimates aren't statistically significant. The overall effect for the entire fleet would be a 0.5 percent increase, though, again, the estimate isn't significant.

The researchers also examined what would happen if the weight reduction varied among vehicle classes with more of the decrease coming from heavier SUVs and pickups and less of it coming from small cars.

If the lightest cars drop only 70 pounds and heavier vehicles take a bigger cut, the fatality increase shrinks to 0.3 percent. A combination of weight reductions that takes only 18 pounds off the lightest cars wouldn't affect fatality rates at all. Finally, the researchers estimate that taking none of the weight decrease from the lightest cars and an even bigger chunk from the heavier ones, including 274 pounds from the heaviest SUVs and pickups, would shave off a 0.1 percent sliver of fatalities.

As the report notes, "any combination of mass reductions that maintain footprint and are proportionately somewhat higher for the heavier vehicles may well be safety-neutral or better."
