Daniel Convissor's Website

Corporate Average Fuel Efficiency: General Accounting Office Testimony

Automobile Weight and Safety

Before the Subcommittee on Consumer [sic]
Committee on Commerce, Science, and Transportation
United States Senate

April 11, 1991

GAO / T-PEMD-91-2

Our major conclusion is that it is not true that cars become more dangerous simply by getting lighter. The highest fatality rates occur in cars in the middle of the weight spectrum, particularly in single-car crashes. In two-car collisions, the occupants of the lighter car are at greater risk than the occupants of the heavier vehicle. However, our analysis suggest that if all cars became lighter, the increased vulnerability of lighter cars in two-car crashes would become more than offset--all things being equal--by the reduced threat from heavier cars....

Background
Two potential sources of ambiguity exist in this area. One concerns the car weight and safety relationship. There are at least three different ways that car weight can affect occupant safety. The first relates to stability, or the propensity to roll over. Some analysts maintain that lighter automobiles cause more occupant injuries and deaths because they are less stable than heavier cars, and thus roll over more easily. The second effect of size is the one that comes most easily to mind: occupant protection, or crashworthiness. It is clear that heavier automobiles offer more protection to their occupants in the event of a collision than do lighter cars. The third effect of size is as a force initiator that acts in much the same way as a weapon. Heavier cars subject their own occupants to more force at a given velocity than do smaller automobile when they hit a fixed object. Just as importantly, in the case of multi-car collisions, they impact the other vehicles with more force than would a smaller car. For example, a 2000 pound car needs to achieve a speed of 46 miles per hour in order to generate as much force at impact as a 3500 pound automobile traveling 35 miles per hour.

The second possible ambiguity stems from the imprecise use of terms like "downsizing" or "car size." For our purposes here, cars size refers only to car weight, not to length and width or other possible measures of size. Weight is the aspect of automobile size that is the most closely related to fuel efficiency.

For our data analyses, we asked NHTSA to make available to us the data from their Fatal Accident Reporting System (FARS)--which contains details of every highway fatality since 1975--together with information on the types of cars registered each year since then....

Preliminary Findings
there are two clear historical trends since the mid-1970s: highway fatality rates have decreased at the same time that the mean weight of cars has become smaller....

Single-Car Accidents
The weight categories delineate six equal-sized divisions of the population distribution of car weights (i.e., the number of cars in each category is roughly equal), with the smallest cars [in the first category and the largest cars in the sixth]....

fatality rates [per 100,000 autos] do not appear to be a simple linear function of car weight. The lowest fatality rates do occur in the largest car category, but the highest fatality rates occur in cars in the fourth weight category, not in the smallest weight group. The pattern is the same for nonrollover crashes, with the highest fatality rate again in the fourth category. For rollover crashes, the fatality rate in the four smallest weight categories is approximately equal, while the heaviest cars have a much lower fatality rate, only about one-third that of the other categories. Less than 20 percent of the fatalities in single car accidents occur in rollover crashes....

occupant fatality rates do generally decline as weight increases, with the largest cars providing the most protection. However, once again, the smallest cars are not the most dangerous; the cars with the highest occupant fatality rates occupy intermediate positions [the third position] in the weight distribution.

To this picture, [we add] the effects of car size in its third role, as an initiator of force. It shows the death rate for occupants of the "other" car in each accident; that is, for the vehicles that receive the force conveyed by different sized cars. Not surprisingly, there is a clear effect of automobile weight -- the number of "other" vehicle fatalities increases as the weight of one's own car gets larger. In this perspective, the smallest cars are the safest for the occupants of other vehicles, while the largest cars are the most dangerous....

This difference in perspective is an important one for policy. From the standpoint of an individual traveling in an auto that is involved in a two-car collision, it is safer to be in the heavier car. However, from the perspective of all passenger car occupants collectively, it is the overall fatality rate in two-car accidents that needs to be considered, and that rate could be at least somewhat reduced if the percentage of small cars on the road were increased....