The Tradeoff Between Mass and Fuel Economy:
It cannot be wished away
21 Apr 2009 in Regulatory Science
It is a fact of physics that vehicles with greater mass do better in collisions. Wall Street Journal automotive columnist Joseph B. White explores this trade-off from an odd perspective -- one in which he seems to wish that it it weren't so.
In a new column, White writes as if the relationship between mass and safety is still a matter of debate. He begins by asking a question that seems to be rhetorical, then adds confusion about the nature of this relationship:
The U.S. government's push to decrease the nation's output of greenhouse gases by increasing the fuel efficiency of the cars Americans drive is rekindling an emotional debate: Does driving a small, fuel-efficient car make you more likely to die on the road?
Engineers and statistical analysts can point to data that suggest more-efficient cars don't necessarily put motorists at greater overall risk. But most of us care less about the "overall" risk than we do about ourselves. Driving a big Chevrolet Tahoe sport-utility vehicle makes many of us believe we are safer than we would be in a smaller car -- even if statistical measures across a large population of vehicles and all kinds of crashes suggest the margin of safety isn't quite as wide as SUV owners believe.
The answer to the first question -- will small, fuel-efficient cars make you more likely to die on the road -- is unambiguously "yes." Small may be fuel-efficient, but it also is more risky. White's subsequent appeal to unnamed "engineers and statistical analysts" prepared to show otherwise is hard to credit. The reason why drivers believe they are safer in larger cars is that, in this case, our intuition is consistent with reality.
The Obama Administration's decision to promote 35 mpg and greater corporate average fuel economy (CAFE) standards portends a world in which the US automotive fleet looks European -- almost all cars are small and trucks are rare. Once that transition is complete, collisions between small cars and SUVs will be rare events. Until then, however, occupants of small cars will face much higher risks of fatality and serious injury. (These risks will decline over time as the proportion of collisions involving large vehicles declines, but they never will go away.)
Unlike Europe, where fuel costs about $5 per gallon, traffic density will increase. Greater fuel efficiency will lead to more vehicle-miles traveled, and more vehicles will be needed to transport the same number of persons. Higher traffic density increases the likelihood of collision.
One of the myths about motor vehicle risk is that it is "voluntary" -- that is, drivers choose the amount of risk they bear by their vehicle selection and driving style. But motor vehicle risks is largely involuntary -- drivers cannot control the risks presented by other drivers.
A lesson about small-vehicle operation -- one that is not mentioned in White's column -- is that drivers of small, fuel-efficient vehicles can reduce their safety risks by highly defensive driving. This means not just being alert to the threats posed by drivers of other (and especially larger) vehicles, but also choosing lower-risk highways. For example, the risks of head-on collision can be very nearly reduced to zero by avoiding high-speed travel on roads in which such collisions are feasible, such as as highways lacking median barriers.
Making such adaptations will reduce safety risk for drivers of small, fuel-efficient vehicles, but it may require choosing a longer route. And ironically, more fuel.
