LOSING WEIGHT TO SAVE LIVES: A REVIEW OF THE ROLE OF AUTOMOBILE WEIGHT AND SIZE IN TRAFFIC FATALITIES

Marc Ross and Tom Wenzel

July 2001

Executive Summary

Reductions in vehicle weight will be necessary to dramatically increase vehicle fuel economy and address concerns about global warming. The purpose of this report is to explore the relationship between vehicle weight and fatalities in traffic accidents. One of the most interesting possibilities is to use new technologies to reduce vehicle weight while maintaining vehicle size to protect occupant safety.

Two-vehicle crashes are the largest source of traffic fatalities, accounting for 43% of traffic fatalities in 1999. Fatalities in car-to-car crashes have sharply declined even while the number of cars on the road has gradually increased. Car-to-car head-on fatalities dropped 35% in the 1980s and did drop another 25% in the 1990s. If one looks at fatalities in new cars only, the decline is even more rapid-an 80% decrease for 1980-97! The consequences of car-to-car head-on fatalities have been revolutionized by protection technology, motivated in part by the standardized crash test. Seat belts and air bags are increasingly effective in protecting occupants. Powerful computer-assisted efforts also enable safety improvements in the design and manufacture of vehicle structures.

Light trucks crashing with cars now cause many more fatalities than cars crashing with cars. Collisions where trucks strike cars on the side are now the largest cause of fatalities in two-vehicle crashes. Over two thousand lives would be saved annually by establishing "compatibility" between cars and light trucks. This means reducing the mass differential between cars and those light trucks used as car substitutes by making the heavier vehicles lighter. Compatibility in height and stiffness is also required-for example, for the front of the striking vehicle (truck) and the side of the struck vehicle (car). Compatibility involves both vehicles: The lighter cars would not be made still lighter, but would be made larger in selected ways.

Such changes can be achieved with the help of mass-reduction technologies. First, the basic structure of light truck car-substitutes that are now body-on-frame would instead be unibody (like today's cars) or perhaps space frame. Second, the use of lightweight materials (such as high- and ultrahigh-strength steels, aluminum, and engineering plastics) would be emphasized. Third, high-efficiency propulsion systems would be much lighter. These technologies include: (1) small displacement engines with a much higher ratio of power-to-displacement; (2) automatic transmissions that function smoothly without a torque converter (with sophisticated motor-shifted standard transmission or continuously variable transmission); and (3) on-shaft starter-generators with a 42 volt (V) electrical system, enabling idle-off and other modest hybrid-drive capabilities without a heavy battery.

There are also many deaths in one-vehicle crashes-31% of all fatalities are from collisions with stationary objects like trees, parked cars, and utility poles. Fatalities in these crashes have also declined, perhaps for similar reasons as the decline in car-to-car crash fatalities, although the decline has not been as rapid. Progress could continue in the face of mass (but not size) reduction because cars that are "overweight" for their size do not appear to offer significant added protection.

Some 12% of fatalities in cars and light trucks occur in "non-collision" events, mostly rollovers following driver loss of control. The likelihood of a rollover is increased by certain design features, such as high center of mass, narrow track width, and softness of suspension with respect to roll. The likelihood is also increased by a high load, like passengers and luggage in an sport utility vehicle (SUV) with its high floor. The historical correlation in cars of light weight with rollovers is not inherent, but a matter of design. In new model cars, this correlation has essentially disappeared. With rollover tests and standards, design changes will also be made to reduce rollovers in light trucks.

In Europe, efforts to develop extremely light/small vehicles (by U.S. standards) are leading to the development of relatively effective protection systems for that vehicle class, although it appears that the safety technology may increase both cost and weight. The European studies we have seen have not examined the option of maintaining or even increasing size while reducing vehicle weight using new lightweight materials.

Our conclusions regarding the impact of reducing vehicle weight on vehicle safety are:

(1) The high mass ratio of most "light" trucks over cars is an inherent aspect of incompatibility in crashes. Reduction in the mass and other incompatibilities of the light trucks would result in a major decrease in car fatalities and in fatalities overall.
(2) Moreover, increasing size in selected ways, such as increased crush space and stiffness at the sides, is likely to be a powerful technique for reducing traffic fatalities.

In a nutshell, from a safety perspective the United States needs to resolve the incompatibility of light trucks with cars and it needs to continue development and adoption of powerful crash mitigation and avoidance technologies. Making heavier vehicles lighter (but not smaller) and making lighter cars larger (but not heavier) would not only increase safety but also increase fuel economy. We project a fuel economy increase of over 50% in association with these safety measures.

This work was supported by The Energy Foundation. Prepared for the U.S. Department of Energy under LBL Contract No. DE-AC03-76SF00098. Report Number LBNL-48009