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
Saving Gas, Saving Lives
If a typical car could somehow drop 10 percent of its mass, its fuel economy would increase by anywhere from 3 percent to 8 percent. (The larger value applies if the size of the engine is also reduced to keep acceleration performance the same.) Designers are keenly aware of this relation, and after federal fuel-economy standards were first enacted, automakers virtually eliminated the heaviest cars (those weighing more than 4,000 pounds) from their new-vehicle fleets. These behemoths went from 40 percent of sales in 1975 to just 3 percent in 1980, after which only a dwindling number could be found in showrooms. But in the late 1980s the sales of heavier "light trucks," including many of the SUVs used as substitutes for cars, began to increase. By 2002, the fraction representing light trucks weighing more than 4,500 pounds reached 32 percent.
People often feel very secure and protected when driving such mammoth vehicles. And indeed, the tendency of such large masses to resist sudden deceleration adds some degree of safety. But deceleration is a relatively small danger compared with the three phenomena that most frequently kill and maim people in a serious crash: intrusion of another vehicle or roadside object into the passenger compartment, rollovers and failure of the restraints to keep the occupants from hitting hard interior surfaces.
The key factors that determine whether a life-threatening intrusion will take place are the strength of the passenger compartment and the height and stiffness of the collision partner. Automakers could thus use stronger materials and more compatible designs to reduce casualties in two-vehicle collisions; adding mass is not necessary. For example, designers could incorporate light honeycomb-sandwich panels or fiber-reinforced materials to help sever the connection between mass and strength.
The best strategies for avoiding rollover are to lower a vehicle's center of gravity and to increase its track width (the distance between the right and left tires). Electronic stability control, a relatively new technology, also helps by providing automatic braking separately at the four wheels in such a way as to inhibit rollover. Once such a mishap occurs, the crush resistance of the roof normally determines whether seat-belted occupants get hurt. And, of course, whether you are belted in will affect whether you end up being ejected during a crash, which would likely result in serious or fatal injury.
Restraints, both safety belts and air bags, and interior padding provide important protection to occupants in rollovers—indeed, they serve this critical function in all types of crashes. Side-curtain air bags, which reduce head contact with windows, are becoming more common, and advanced seat belts, ones with pretensioners and load limiters, are also being incorporated in many models. Under research are four-point seat belts, which hold occupants in position in side-impact crashes better than do today's three-point, lap-and-shoulder belts.
Electronic stability control, side-curtain airbags and advanced seat belts tend to be included in heavier, more expensive vehicles. But such cutting-edge safety equipment need not add much mass, particularly if attention is paid to this goal. And although lighter vehicles are often at a disadvantage in crashes with heavier ones, a general reduction in weight across all vehicle types would not compromise safety, in part because the relative masses of colliding vehicles wouldn't change, but also because a substantial majority of casualties in motor vehicle crashes are unrelated to the masses of the vehicles involved.
Just what then does control safety on the road? Driver behavior and environmental factors contribute most to whether a serious accident takes place and to the speed of the impact. Whether the occupants of the car or truck involved are properly wearing their safety belts influences the type and extent of injuries, as do the age and physical condition of these people. But it is vehicle design that above all affects the severity of the consequences of a given crash.
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