Brumbelow 1 EFFECTS OF SEAT BELT LOAD LIMITERS ON DRIVER FATALITIES IN FRONTAL CRASHES OF PASSENGER CARS

In the mid-1990s, seat belt load-limiting devices were introduced on many new passenger vehicles equipped with front airbags. These devices are intended to reduce belt-induced injuries such as rib fractures by allowing forward movement of occupants’ torsos when belt loads exceed some threshold. Load limiters have been shown to reduce thoracic injury risk in controlled experiments with cadavers and in fullwidth rigid barrier frontal crash tests. The Insurance Institute for Highway Safety has evaluated many vehicles equipped with load limiters in 64.4 km/h (40 mi/h) frontal offset crash tests. Results indicate that in some crash circumstances the amount of forward movement allowed by load limiters could increase the risk of head injury from contacts with vehicle interior components. Thus, although load limiters perform well in rigid barrier tests with high deceleration, short duration, and low intrusion, the forward movement they allow in crashes with longer duration and higher intrusion may increase head injury risk. To examine the effects of load limiters on driver fatality risk in real-world crashes, the present study compared rates of belted driver deaths per vehicle registration before and after load limiters were added to seat belts. Study vehicles were restricted to models and years with no other significant design changes. Fatality rate comparisons for passenger cars with and without load limiters suggest these devices have not reduced fatality risk and even may have increased risk. Also presented in this study is a review of a small number of cases from the National Automotive Sampling System that illustrate how increased occupant forward movement can contribute to head injury risk even in vehicles with front airbags. INTRODUCTION Seat belts are the single most important safety feature of any passenger motor vehicle. They have been estimated to have saved more lives since 1960 than all other crashworthiness design features combined (National Highway Traffic Safety Administration (NHTSA) 2005). However, many studies have shown that seat belts can contribute to thoracic injuries under certain loading conditions, especially among older occupants (Augenstein et al., 1999; Dalmotas, 1980; Hill et al., 1992; Niederer et al. 1977; Patrick and Andersson, 1974). Several patents filed as early as the 1950s and 1960s described methods of limiting the magnitude of belt loads to reduce the risk of these injuries (Viano, 2003). The major drawback of these technologies is that they must sacrifice occupant coupling to the vehicle by allowing forward movement of the occupant’s torso, increasing the risk of head or chest contact with the steering wheel or other vehicle interior components. As a result, it was not until front airbags were installed as standard safety equipment that automobile manufacturers began to equip production vehicles with seat belt load limiters in large numbers. Although airbags provide additional occupant protection against contacts with the vehicle interior, they may not eliminate the risk associated with large amounts of increased forward movement in many serious frontal crashes. Prior Research on Load Limiters Cadaver tests – Cadaver testing has examined the potential of load-limiting seat belts in combination with airbags to mitigate thoracic injuries. Kent et al. (2001) conducted seven cadaver tests and found a 40 percent reduction in the average number of rib fractures for belts that limited loads to 3.5 kN compared with standard belts that did not limit loads. Cadaver subjects averaged older than 60 years and were positioned to avoid potential hard head contacts. Crandall