Several of the advanced vehicles examined by OTA will be extremely light. For example, one of the 2015 advanced conventional vehicles weighs less than 2,000 pounds. An examination of the basic physics of vehicle accidents and the large U.S. database on fatal and injury-causing accidents indicates that a substantial “down weighting” of the light-duty fleet will create some significant safety concerns, especially during the transition period when new, lighter vehicles mix with older, heavier ones.
Any adverse safety impacts, however, are unlikely to be nearly so severe as those that occurred as a result of changes in the size and weight composition of the new car fleet in 1970 to 1982.67 The National Highway Traffic Safety Administration concluded that those changes “resulted in (net) increases of nearly 2,000 fatalities and 20,000 serious injuries per year. ”
Many of those adverse impacts occurred because vehicles changed in size as well as weight, however, yielding reduced crush space, reduced track width and wheelbase (which increased the incidence of vehicle rollovers), and so forth. Reducing weight while maintaining vehicle size and structural integrity should have lower impacts.
The major areas of concern about vehicle “light weighting” are the following:
- ● Passengers in lighter vehicles tend to fare much worse than the passengers in heavier ones in collisions between vehicles of unequal weight, because heavy vehicles transfer more momentum to lighter cars than vice-versa. During the long transition period when older, heavier vehicles would remain in the fleet, lightweight vehicles might fare poorly.
Moreover, if the large numbers of light trucks in the fleet do not reduce their weight proportionately, the weight distribution of the fleet could become wider, which would cause adverse impacts on safety. Vehicle designers must balance the need to protect passengers from deceleration forces (requiring crush zones of lower stiffness), and the need to prevent passenger compartment intrusion (requiring high strength/high stiffness structure surrounding the passengers).
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68 Lighter vehicles will have lower crash energy in barrier crashes or crashes into vehicles of similar weight, so they will require a softer front structure than a heavier vehicle to obtain the same degree of crush (and same protection against 66 If the energy storage device is a battery, performance will likely be limited if the engine cannot be used. With a flywheel or ultra-capacitor, having adequate power is not a problem, but the EV range will be very short, perhaps no more than a few miles.
However, the roles are not truly independent; for example, the outer structure also works to avoid intrusion into the passenger compartment and the safety c-age may have to deform and dissipate crash energy in a very severe accident.
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