In recent years, automobiles have been associated with nearly 29,000 traffic fatalities annually in the United States, including the deaths of both automobile occupants and others involved in collisions with automobiles.The National Highway Traffic Safety Administration (NHTSA), a unit of the Department of Transportation (DOT), has the lead role in federal government efforts to reduce the number of traffic crashes and to minimize their consequences. Some types of automobiles have higher fatality rates than others. (For example, small cars generally have higher rates than large cars.) In addition, some categories of drivers are more likely to be involved in serious crashes than others. (Young drivers, for example, have higher involvement rates than other drivers.) However, as we previously reported, one cannot conclude from differences in fatality rates that some types of cars, or some types of drivers, are in fact more dangerous than others, because driver and automobile characteristics are highly related (GAO, 1991). For example, since small cars have a disproportionate percentage of young drivers, do the high fatality rates for those cars stem from vehicle characteristics or the recklessness with which they are operated? The goal of this report is to isolate the independent effects of important crash-related factors on the likelihood of injury in a collision. The report focuses on the most important predictors of occupant injury in a collision: crash type and crash severity, automobile size, safety belt use, and occupant age and gender. The report is concerned with both crashworthiness (protecting automobile occupants) and aggressivity (protecting other roadway users struck by automobiles). The report also considers prospects for improving the safety of automobile occupants. This report is one of three GAO reports examining automobile safety. One of these, Highway Safety: Factors Affecting Involvement in Vehicle Crashes (GAO, 1994), examines the independent effects of driver characteristics and automobile size on crash involvement. Another, Highway Safety: Reliability and Validity of DOT Crash Tests (GAO/PEMD-95-5), looks at the extent to which results from the crash test programs conducted by NHTSA accurately predict injury in actual automobile crashes. Since the mid-1960’s, both the number of traffic fatalities and the fatality rate per registered vehicle have sharply decreased in the United States. The fatality rate for automobile occupants has declined by 36 percent since 1975. The continued emphases on reducing drunk driving and increasing safety belt use, along with the introduction of antilock brakes, air bags, and other safety enhancing features, have increased the chances that this favorable trend will continue. Nonetheless, in 1991 automobiles were associated with nearly 29,000 traffic deaths in the United States.About 22,000 of the automobile-related fatalities were automobile occupants (about 10,000 killed in single-car collisions and 12,000 in multiple-vehicle collisions), about 2,500 were occupants of other types of vehicles (for example, light trucks, vans, or motorcycles) involved in collisions with automobiles, and approximately 4,000 were pedestrians or cyclists hit by automobiles. (See table 1.1.) Different models of automobiles appear to make very different contributions to this fatality toll. For instance, the Insurance Institute for Highway Safety (IIHS) has reported that the most “dangerous” automobile models have occupant fatality rates more than nine times higher than the “safest” models. Further, some of the automobile characteristics associated with this variation in fatality rates are well known. For instance, sports cars have higher fatality rates than station wagons, and small cars have higher fatality rates than large cars. However, this does not mean that types of automobiles with high fatality rates are necessarily more dangerous than those with low fatality rates. This is because different types of drivers prefer particular types of automobiles, affecting both the number of collisions involving particular autos and, perhaps, the probability of serious injury in the event of a collision. For example, young drivers are much more likely to be involved in fatal accidents than others—drivers age 16 to 20 are involved in fatal accidents at a rate three times higher than that for drivers age 45 to 54—thereby inflating the fatality rate for types of cars preferred by young drivers. Similarly, some types of automobile occupants are more likely to be seriously injured in a collision than others. For example, in collisions in which at least one vehicle was towed from the accident scene, NHTSA (1992a) recently estimated that women automobile occupants are about 36 percent more likely to be hurt than men occupants in similar collisions. This suggests that types of cars with a disproportionate number of women occupants may have higher fatality rates than other cars. As automobiles abruptly stop or change direction in a collision, occupants continue moving in the original direction of travel. This independent movement of an occupant within a rigid vehicle that is decelerating more quickly than its occupant provides several opportunities for injury. First, some occupants are injured by being ejected (either partially or totally) from the vehicle. Ejection substantially increases the risk of serious injury—ejected occupants are three to four times more likely to be killed in a collision than occupants who do not leave the vehicle. Second, occupants can collide with the interior of the vehicle or other objects intruding into the passenger compartment. This “second collision” (following the “first collision” of the automobile striking an object) is understandably worse if it occurs at high speed, involves impact with a sharp or unyielding portion of the car’s interior, or involves contact with part of another vehicle or a roadside object that has penetrated the passenger compartment. Finally, because different portions of an occupant’s body decelerate at different rates, internal injuries can be caused by the “third collision” of soft tissues against hard, bony structures. For example, in high-speed collisions, the skull decelerates more quickly than the brain, potentially causing injury to the brain as it strikes the hard skull. Automobiles, and federal automobile safety regulations, are designed to protect their occupants from these dangers in several ways. One way is to attempt to reduce the deceleration forces acting on occupants. Deceleration forces can be reduced by designing the structure of a vehicle to absorb as much energy as possible before the crash forces are transmitted to the passenger compartment or by giving the occupant more time to slow down, thereby reducing the maximum force level the occupant is subjected to. The latter can be accomplished by starting the deceleration period more quickly (for example, by designing safety belts that begin holding back the occupant sooner) or by increasing the total deceleration period (for example, by lengthening the front end of the vehicle). Another way cars may protect their occupants is by encasing them in a protective compartment that preserves a living space and prevents the intrusion into the passenger compartment of striking vehicles or other objects (such as light posts or trees) that a car hits. Third, automobiles are designed to keep their occupants both in the vehicle and away from interior surfaces. This is most obviously accomplished through the use of safety belts, but a number of other components are also intended to keep the occupant in the vehicle, including door latches and windshields that are reinforced to eliminate potential ejection routes. In addition, automobile interiors are designed to absorb energy from the occupant and to limit the occupant’s movement rather than serve as a rigid barrier. Energy absorbing steering columns are one example. There is little doubt that cars from recent model years, as a group, are safer than automobiles from past model years and that some of this improvement can be attributed to federal government safety regulations. For instance, in comparing the crash test results of cars from model years 1980 and 1991, NHTSA researchers found that one set of scores measuring injury potential had declined about 30 percent during the intervening years (Hackney, 1991). Similarly, Evans (1991b) estimated that the total effect of nine federal motor vehicle safety standards enacted by 1989 had been to reduce the occupant fatality rate by about 11 percent. The objective of this report is to examine the independent effects of a number of factors—crash type, crash severity, automobile weight and size, safety belt use, and occupant age and gender—on the risk of injury in an automobile crash. We are concerned with both crashworthiness and aggressivity. Crashworthiness refers to the extent to which automobiles protect their occupants in a collision, and aggressivity refers to automobile characteristics that affect the safety of the occupants of the other vehicles in a collision. We restricted the scope of the study in several ways in order to obtain a clear picture of the most important phenomena. First, we looked only at the safety of automobile occupants and the dangers automobiles pose to occupants of other vehicles. We were not directly concerned with factors affecting the safety of occupants of other types of passenger vehicles, such as pickup trucks, vans, minivans, and multipurpose vehicles; we considered these vehicles only as they affect the safety of automobile drivers in two-vehicle collisions. In our judgment, concerns about the safety of light trucks and other passenger vehicles differ significantly from that of automobiles. Light trucks and other passenger vehicles are less stable and thus roll over more frequently than automobiles, and they have been subject to less stringent safety regulations than automobiles. Second, our statistical analysis focused on model year 1987 and later cars, because the safety experiences of those cars are more likely to apply to today’s new cars than are the safety experiences of older ones. Finally, we considered in our analysis only the injury experiences of drivers, not those of automobile passengers or factors specifically affecting the safety of child occupants. (Roughly half of the automobiles on the road have no occupants other than the driver.) To meet our objective, we reviewed technical reports from NHTSA and other sources and consulted auto safety experts and representatives of automobile manufacturers. We also conducted our own statistical analyses of traffic safety databases obtained from NHTSA.