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.