FEATURE ARTICLE
Safer Vehicles for People and the Planet
Motor vehicles contribute to climate change and petroleum dependence. Improving their fuel economy by making them lighter need not compromise safety
Thomas P. Wenzel, Marc Ross
Nothing Simple About Safety
Although serious auto wrecks are relatively rare events, the consequences are momentous enough that many consumers study the safety characteristics of the different vehicles they are contemplating for purchase. Buyers often consider the protective equipment offered and scrutinize the results of crash tests when deciding which model to choose. Such information is widely available. The National Highway Traffic Safety Administration regularly conducts crash tests and rates vehicle models under its New Car Assessment Program. The Insurance Institute for Highway Safety also performs such tests and publicizes the results in various ways, including on its Web site. Consumer Reports publishes a Safety Assessment rating, which is a combination of the NHTSA's findings and results of handling and braking tests conducted by that magazine's parent organization, Consumers Union.
As important as these studies are, one must accept that they have limited value. After all, no test procedure can replicate all of the conditions that come into play when things go wrong on the road. To address this rather fundamental shortcoming, we and other investigators have sifted through a great deal of real-world data to better reveal how driver and environmental conditions influence crashes—and how well safety devices and vehicle design protect occupants when an accident happens. The yardstick we and others use for this purpose takes the form of the fatality or serious-injury rate for various vehicle types or for specific makes and models. Unfortunately, in these kinds of analyses it is difficult, often impossible, to judge the relative contributions of driver behavior, environmental conditions and vehicle attributes to the overall fatality or injury rate.
For our studies, we calculated the risk of driver fatality using the Fatality Analysis Reporting System, or FARS, the federal database of all deaths stemming from accidents on public roadways. "Fatality risk," in our formulation, is simply the number of driver deaths divided by the number of registered vehicles operating during the year under examination. Actually, our procedure is a little more complicated, because we typically analyze a range of several years and because we normally do not have registration records for the entire period (the full complement of these data being too costly to obtain). So we estimate the total number of registered-vehicle years indirectly by making some straightforward deductions from the information contained in our limited records (such as the model year of each registered vehicle). Using this approach, we can easily calculate fatality risk for a particular type of vehicle or for an individual make or model.
Researchers at the Insurance Institute for Highway Safety, as well as some others, use this same risk metric for many of their studies. We were, however, interested in exploring not only the danger to someone operating a particular type or model, but also the risk that such a vehicle poses to other drivers. That is, we were keen to find out the total burden society bears by encouraging one sort of motor vehicle over another. Usually investigators only present and discuss the risk to occupants of the car or truck in question—as if society at large has no stake in the mayhem caused by some vehicles as long as those riding in them aren't themselves killed.
Because we used actual crash statistics, the values we calculated reflect the risk of involvement in a crash and the speed at which it occurs, which hinge primarily on environmental factors and the behavior of the driver, as well as the risk of fatality once a serious crash has taken place, which depends on belt use, vehicle design and driver frailty. As such, our use of the word "risk" here is just shorthand for "risk as a car or truck is really driven."
One of the most important things we found in our studies is that drivers are just as safe in cars as they are in SUVs and pickups. This result is easily explained: Although the risk of dying in a collision is often lower in SUVs and pickups, their high center of gravity makes them more susceptible to rollovers. Fortunately, the recent trend of manufacturers to produce so-called crossover SUVs, which are lower and sometimes wider than conventional truck-based SUVs, has led to large reductions in the rollover risk.
Among cars, large ones present only slightly lower risks than midsize models, compact cars or even the safer subcompacts. The small differences are within the statistical uncertainty of the analysis. What really surprised us, however, was the wide range in risk among different models of subcompact cars—the worst ones having an average fatality risk that is three times that of the best ones.
It is not hard to find examples of a small car, a large car and an SUV that all pose about the same amount of risk to their drivers. On the other hand, in terms of the risk to drivers of other vehicles, SUVs and pickups are much more dangerous than cars or minivans. Common sense explains why: Most conventional SUVs are merely carlike cabins bolted onto the frames of pickup trucks, which include two longitudinal steel beams that can act like spears or fork tines when striking a car, often overriding a bumper in a frontal crash or punching through a door in a side impact. These high, rigid structures make the designs of pickup trucks and truck-based SUVs fundamentally incompatible with those of cars, which is why researchers in the field of traffic safety refer to the "aggressivity" of such designs.
These experts have considered various ways to reduce the aggressivity of SUVs and light trucks towards cars. Strategies include redesigning truck fronts and bumpers to make them lower or "softer" and raising the door sills of cars to prevent the intrusion of truck bumpers and frame rails. Our analysis of car-based crossover SUVs that lack rigid frame rails and have lower fronts indicates that these newer designs are much less dangerous than truck-based SUVs (they also provide roughly 3- to 4-miles-per-gallon better fuel economy for a given interior volume).
We also found, not surprisingly, that the aggressivity of pickup trucks increases with their rated carrying capacity. The largest pickups are nearly six times more aggressive to other vehicles than the average car. Indeed, our study uncovered a shocking statistic: We calculate that during the 14 years the average one-ton pickup truck is driven, it has almost a 1 percent chance of killing someone. (Only one-third of this huge risk falls on the person driving the truck.)
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