4.2. Restraint Usage and Airbag Deployment Rates
Restraint usage and airbag deployment proportions were determined directly from the suitable NASS/CDS cases after the application of
the associated statistical weighting factors. In the analysis that follows,
the term‘ airbag restrained’ indicates that an airbag was available to the
occupant, not that the airbag necessarily deployed. Seat belt usage rates
were determined for the entire data set and two subsets: (a) airbag
restrained occupants and (b) non-airbag restrained occupants. Airbag
deployment rates in crashes with longitudinal barrier were determined
using the airbag restrained occupant data subset. Airbag deployment
rates were also examined as a function of crash severity using the
equivalent barrier speed (EBS) metric. Although delta-V is the preferred measure of crash severity, delta-V is difficult to estimate for
longitudinal barrier crashes (Smith & Noga, 1982). In addition, delta-V
was not available for a majority of the suitable cases. Due to the uncertainty in the delta-V estimates for this crash mode, the authors opted not
to pursue a multiple imputation approach involving vehicle delta-V.
Instead, for this portion of the analysis, cases were only included if
the EBS was known. EBS can be determined based on the crush of
the subject vehicle. EBS avoids many of the difficulties associated with
delta-V computations for vehicles impacting objects of unknown stiffness such as guardrails. Two airbag restrained occupant subgroups
were also analyzed based on type of barrier impacted: (a) concrete
barrier or (b) other barrier. Data from these subgroups were then used
in a two-way contingency table analysis to determine if differences
in airbag deployment rates existed by barrier type.