Inclusion of single event crashes ensures that the longitudinal
barrier caused (or did not cause) the deployment of the airbag. Only
passenger vehicles and light trucks and vans (LTVs) were included;
all heavy vehicles were excluded from the analysis. For the purpose
of this study, a longitudinal barrier included concrete barriers, metal
beam guardrails, and cable barriers. Longitudinal barriers in NASS/
CDS are grouped into one of two categories: (a) concrete barriers,
and (b) other barriers. The latter category includes all types of steel
guardrail systems such as w-beam guardrails, box beam barriers,
and cable barriers. Each case was checked for proper barrier type
coding using crash scene photographs available online for the NASS/
CDS years 1997 through 2007. Any concrete barriers miscoded as
“other barrier”were reclassified accordingly and any steel barrier
systems miscoded as a concrete barrier were reclassified accordingly.
Any cases that were incorrectly coded as a barrier impacts (e.g., pole,
curb, or bridge structure impacts) were omitted from the data set.