The National Highway Traffic Safety Administration (NHTSA) conducted a study to determine "the effect of changes in vehicle size on the crashes, injuries, and fatalities of passenger car and light truck occupants." In particular, the study focused on the effects on the rates of incapacitating injuries to drivers when a vehicle was reduced in weight by 100 pounds. An earlier (1991) study by NHTSA found that a 1000-pound reduction in vehicle weight (from 3,700 to 2,700 pounds) "resulted in increases of approximately 2,000 fatalities and 20,000 serious injuries each year."

This earlier study analyzed the impact of reduced vehicle weight for passenger cars involved in four types of crashes: rollovers, collisions with fixed objects, collisions with heavy trucks, and two-car collisions. Results showed "a significant increase in serious injuries" in two-car collisions, "a relatively large increase in rollover fatalities," and smaller increases in injuries in collisions with fixed objects and heavy trucks.

NHTSA believed a new study of this issue was important for several reasons. The 1991 study focused on the impact of weight reductions for passenger cars, but the number of light trucks and vans on the roads continues to increase. In addition, the use of safety belts continues to increase, and safety belts are "known to be particularly effective in preventing fatalities in rollover crashes." As such, "it is important to determine if the large increases in fatal rollover crashes will continue to occur as the average weight of passenger vehicles is reduced." The results of the new study appeared in NHTSA's Technical Report (January 1997), entitled "The Effect of Decreases in Vehicle Weight on Injury Crash Rates."

METHODS

Data for this study came from Illinois (for the calendar years 1990-1992) and Florida (for the calendar years 1991-1993). These two states were selected for two reasons: "both states experience a considerable number of crashes each year, thereby yielding large sample sizes," and "both states collect and record vehicle identification numbers" for vehicles involved in crashes. This second factor was particularly important, since researchers used vehicle identification numbers to obtain the weights of vehicles involved in crashes. Although data from only these two states were used in the study, it is reasonable to assume that the effects of vehicle weight reductions for passenger cars and light trucks and vans "should not differ dramatically from state to state." The data used in the study were also limited to crashes involving vehicles of the model years 1985-1993.

The study involved three "scenarios": (1) the effect of a 100-pound weight reduction on light trucks and vans, with the weight of passenger cars unchanged; (2) the effect of a 100-pound weight reduction on passenger cars, with the weight of light trucks and vans unchanged; and (3) the effect of a 100-pound weight reduction on all passenger vehicles, namely passenger cars and light trucks and vans. For each scenario, researchers analyzed changes in the rate of incapacitating injuries to drivers in four types of crashes: crashes between the vehicle and a fixed object, crashes between the vehicle and a heavy truck, crashes between the vehicle and a passenger car, and crashes between the vehicle and a light truck or van. For crashes involving two vehicles, injuries to drivers of both vehicles were included in the analysis.

The study focused on these four types of crashes because "these crashes are those for which changes in the vehicle's weight could be expected to be related to changes in the risk of driver incapacitating injury." The study did not "address possible environmental, economic, or other questions and issues that may be part of the picture involving a reduction in average vehicle weight." In addition, only incapacitating injuries to drivers were studied, rather than injuries to all occupants of the vehicle, because state police crash reports often do not list uninjured occupants. Assuming that the effect of a vehicle weight change is similar for all occupants of the vehicle, estimates on the safety impact of a vehicle weight reduction on driver injury "can be extended to estimating the safety impact for all occupant injuries."

RESULTS

The results of the study indicated that, when vehicle weights were reduced by 100 pounds, the estimated increases in the rates of incapacitating injuries to drivers ranged from 1.5 percent for light trucks and vans involved in crashes with fixed objects to 5.9 percent for light trucks and vans involved in crashes with passenger cars, where the weight of the car was unchanged. For crashes involving passenger cars with light trucks or vans, results showed that reducing the weight of one vehicle (either the passenger car or light truck or van) "had a small beneficial effect" on the vehicle that was not reduced in weight. In short, occupants of passenger cars "fared better" in crashes with light trucks or vans when the weights of the trucks or vans were reduced by 100 pounds. The occupants of light trucks or vans "fared better" in crashes with passenger cars when the weights of the cars were reduced by 100 pounds.

For the three scenarios examined in the study, the following findings were noted using the estimated percent increases in the rates of incapacitating injuries to drivers (and thus assumed to be true for all occupants of the vehicles):

• A 100-pound reduction in the average weight of light trucks and vans (with the weights of passenger cars unchanged) would result in an estimated increase of 1,795 incapacitating injuries each year.
• A 100-pound reduction in the average weight of passenger cars (with the weights of light trucks and vans unchanged) would result in an estimated increase of 8,804 incapacitating injuries each year.
• A 100-pound reduction in the average weight of all passenger vehicles (both passenger cars and light trucks and vans) would result in an estimated increase of 10,543 incapacitating injuries each year.