Effects of Shoulder Belt Limit Forces on Adult Thoracic Protection in Frontal Collisions

Three-point restraint systems have been installed in vehicles since the early 1960s. However, it wasn't until the automatic protection rule became effective for 1987 Model Year vehicles that manufacturers began installing 3-point restraints with force-limiting shoulder belts and frontal airbags for the driver and right front passenger. This was the first time that all vehicle manufacturers had to certify that their cars would meet the 50th percentile, adult male protection requirements in the 48 km/h frontal, rigid-barrier test specified in FMVSS 208.

To assess the effectiveness of these certified 3-point restraint systems, a search was done of the 1988-2005 NASS data for 3-point belted, front outboard-seated, adult occupants in passenger vehicles that were equipped with airbags and that were involved in frontal, towaway collisions. These data showed that i) half of the occupants with AIS ≥ 3 chest injuries were in collisions with a ΔV ≤ 40 km/h; ii) for older occupants (50+ years), half experienced their chest injuries at ΔVs ≤ 34 km/h; and iii) the chest injury rate for the older occupants was more than double that of the younger occupants. An analysis was done to estimate the effectiveness of various levels of shoulder belt limit loads in reducing chest injuries to older occupants. The result of the analysis indicated that a 2.5 kN shoulder belt limit load would substantially reduce shoulder belt-induced AIS ≥ 3 chest injuries in 99 percent of frontal collisions to all adult, front outboard seated occupants whose normalized bone strengths are greater than 0.4.