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Evaluation of belted and unbelted safety requirements

Although seat belt interlocks are now allowed as a compliance option, Federal regulations still require vehicles to meet occupant performance requirements with unbelted test dummies. Removing the test requirements with unbelted occupants might encourage the deployment of seat belt interlocks and allow restraint optimization to focus on belted occupants. The objective of this study is to compare the performance of restraint systems optimized for belted only occupants with those optimized for both belted and unbelted occupants using computer simulations and field crash data analyses.

In this study, two validated finite element (FE) vehicle/occupant models, including a mid-size sedan and a mid-size SUV were selected. Restraint design optimizations under standardized crash conditions with and without unbelted requirements were conducted for both vehicles on both driver and right front passenger positions. Results indicate that unbelted requirements do not affect the optimal seat belt and air bag design parameters in 3 out of 4 vehicle/occupant-side conditions, except for the SUV passenger side. Because knee bolsters generally do not significantly affect the injury risks for belted occupants in NCAP crash conditions, energy-absorbing (EA) components in the knee bolsters will likely be removed if unbelted requirements are eliminated.

To evaluate the field performance of restraints optimized with and without unbelted requirements, 55 frontal crash conditions covering a greater variety of crash types than those in the standardized crashes were selected and 1,760 FE simulations were conducted. Overall, compared to the optimal designs with unbelted requirements, optimal designs without unbelted requirements (mainly by removing the EA materials from the knee bolster) generated the same or lower total injury risks for belted occupants depending on statistical methods used for the analysis, but they also increased the total injury risks for unbelted occupants.

The study limitations include: crash pulses used for the field performance evaluation were from a vehicle different than the baseline models, only two vehicles were used in simulations, the design parameter ranges were relatively narrow, and the data analysis can be further refined. Nonetheless, this study demonstrated potential for reducing injury risks to belted occupants if the unbelted requirements are eliminated. Further investigations are necessary to confirm these findings because they can vary with the analysis methods used.

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