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Status Report, Vol. 37, No. 4 | April 6, 2002 Subscribe

Improve fuel economy without negative safety consequences

The overall fuel economy of the passenger vehicles on U.S. roads has deteriorated in recent years, largely because of the surge in popularity of sport utility vehicles. This is prompting legislators and others to call for dramatic changes in federal fuel economy requirements. The potential environmental and safety implications of the proposed changes are being debated in Washington right now.

Back in 1973, Congress established the corporate average fuel economy (CAFE) program, which required each auto manufacturer to achieve an average of 27.5 miles per gallon across its car fleet by the 1985 model year. The legislation authorized the U.S. Department of Transportation to set CAFE standards for light trucks (sport utility vehicles, minivans, and pickups). Current requirements, which Congress froze in 1996, are 27.5 miles per gallon for cars, 20.7 for light trucks.

The debate going on now isn't the first one since CAFE began that has invoked calls for major changes (see Status Report special issue: fuel economy and safety, Sept. 8, 1990). But this time around the potentially negative safety effects of tougher CAFE requirements are attracting more attention.

At the request of Congress, last year the National Academy of Sciences studied the CAFE program, including an examination of its effects on vehicle occupant safety. The study report found that CAFE "has clearly contributed to increased fuel economy of the nation's light duty fleet during the past 22 years." Another finding is that "the downweighting and downsizing that occurred in the late 1970s and early 1980s, some of which was due to CAFE standards, probably resulted in an additional 1,300 to 2,600 traffic fatalities in 1993."

Fuel economy requirements can have adverse safety consequences because the vehicles that get the most miles per gallon typically are the smallest and lightest ones. They're also the least protective in crashes.

This points to the fundamental weakness of the current CAFE system — it doesn't force a vehicle manufacturer to improve the average fuel economy of its fleet by adopting fuelsaving technologies. Instead, a manufacturer can reduce vehicle weights and/or increase the sales of lightweight vehicles, including selling such vehicles at a loss. Another option is to decrease sales of heavier vehicles, but this isn't realistic. Manufacturers aren't likely to deliberately depress sales of the largest and heaviest vehicles, which are by far their most profitable products.

As the report from the National Academy of Sciences concluded, some earlier fuel economy improvements resulted from vehicle downweighting. If current CAFE requirements are dramatically ratcheted up, the likely result will be more downweighting or more sales of lighter vehicles and, once again, negative safety consequences.

Some advocates of tougher CAFE requirements acknowledge that the result might be lighter vehicles, but they claim the safety disadvantages can be offset by imposing tougher federal safety standards. However, the safety disadvantages of small, lightweight vehicles are inherent, and the laws of physics cannot be repealed by new safety standards.

Another claim is that people riding in small, light vehicles are at greater risk only because of collisions with heavier vehicles. According to this line of thought, eliminating the heavier vehicles would solve the problem. But it wouldn't. Occupants of small, light cars are at greater risk in all kinds of crashes, not just collisions with heavier passenger vehicles. Almost 60 percent of all deaths in the lightest cars occur in single-vehicle crashes, crashes with big trucks, and crashes involving more than two vehicles.

In crashes involving two passenger vehicles of differing weights, two effects are at work. The extra weight of the heavier vehicles reduces the risks for the people riding in those vehicles but also inflicts extra risks on the people in lighter vehicles. So there's a tradeoff. Because the benefits to occupants of the extra vehicle weight diminish as their vehicles get heavier and heavier, the very heaviest passenger vehicles (4,000 pounds or more) actually have a small negative effect on society. The additional harm they inflict on people in lighter vehicles in two-vehicle crashes more than offsets the benefits for their own occupants.

To avoid negative safety consequences and possibly reap some safety benefits, new fuel economy requirements should force the use of technologies and promote downweighting of only the heaviest vehicles. These goals could be accomplished by indexing the new requirements to vehicle weight, as detailed in the National Academy of Sciences report.

Under this plan, fuel economy requirements would slide according to vehicle weight. Because allowable fuel consumption would decrease as vehicles get lighter, there would be no incentive to downweight the lightest vehicles. The option of selling lightweight vehicles to offset sales of heavier ones would be removed. Instead, manufacturers would have to use innovative technologies to improve fuel economy.

To further enhance safety, the sliding fuel economy requirements could end at around 4,000 pounds. Each manufacturer's fleet of vehicles above this weight would be required to achieve a fixed fuel economy target. By holding all vehicles above this weight to a common value, manufacturers would be encouraged to reduce the weights of the heaviest vehicles.

The National Academy of Sciences report identified a number of vehicle and engine technologies that could be developed and implemented over 12 to 15 years to increase fuel economy without downweighting or sacrificing vehicle performance. If these technologies were applied to meet weight-based fuel economy requirements, small cars could go an estimated 11-12 percent farther on the same amount of fuel. Large cars could go 27 percent farther, and large pickups could go 38 percent farther.

But these technologies aren't cheap, and nothing in the current CAFE structure encourages manufacturers to apply the technologies to improve fuel efficiency. Instead automakers could — and probably would, at least to some extent — go the potentially cheaper routes of subsidizing lightweight vehicle sales and downweighting to meet tougher CAFE requirements. A weight-based approach would close off these routes because the requirements would go up as vehicles get lighter.

What's going on in Congress

A weight-based approach isn't part of any legislative proposal to toughen CAFE requirements. However, a bill approved by the House of Representatives directs the U.S. Department of Transportation to study this approach.

In the Senate, an ambitious plan that would have raised CAFE requirements to 36 miles per gallon for cars and light trucks was bypassed in favor of a more modest approach. The Senate voted to give the transportation department two years to develop new CAFE standards, directing the department to consider safety in the process but specifically exempting pickups from tougher fuel economy requirements.

From 1996 until this year, the transportation department was banned from considering new CAFE requirements. Now the Bush administration, citing the National Academy of Sciences, is asking Congress for "the necessary authority to reform the CAFE program." In a letter to Congressional leaders, Transportation Secretary Norman Mineta expressed concern "about the adverse impact the current CAFE program has had on safety."

A weight-based approach could reverse the adverse impact as well as improve fleet fuel economy. It's an idea worth considering.

Alternative CAFE structures: gallons of fuel per 100 miles driven, by vehicle weight

Current system graph

Current system: CAFE targets are 3.64 gallons per 100 miles (27.5 mpg) for cars and 4.83 per 100 (20.7 mpg) for light trucks, regardless of weight. An automaker selling SUV A, which uses more fuel than the target for light trucks, still can meet the federal standard by selling offsetting numbers of SUV B, which uses less fuel than the target. Sales of lighter vehicles thus offset heavier vehicle sales.

Weight-based system graph

Weight-based system: Line A indicates the current relationship between vehicle weight and fuel consumption. If line B were adopted as the federal CAFE standard, fleet fuel consumption could decrease by 10 percent. New fuel-efficient technologies would have to be used to achieve the reductions. The fuel economy of SUV B no longer could offset SUV A's because fleet fuel use would be indexed to weight.

Enhanced weight-based system graph

Enhanced weight-based system: Leveling off weight-based fuel economy requirements for the heaviest passenger vehicles (those weighing 4,000 pounds or more) would encourage manufacturers to downweight these vehicles — a safety plus because, as vehicles get heavier and heavier, the benefits of the added weight become small relative to the harm to others on the road.

How size relates to safety

Smaller vehicles consume less fuel, but don't protect their occupants as well as heavier ones. Driver death rates fall as vehicle size increases, up to a point.

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