Side impact motor vehicle crashes: driver, passenger, vehicle and crash characteristics for fatally and nonfatally-injured rear-seated adults

Initial point of impact and crash side relative to occupant seating position plays an important role in injury severity for rear-seated occupants involved in a side crash. Angle crashes and sideswipe in the opposite direction were associated with higher injury rates than sideswipe in the same direction. Occupants seated on the same side of the crash were more likely to have severe/fatal injury compared to occupants seated on the opposite side or in the middle seat after controlling for seat belt status. Although not specific to rear-seated occupants, this is consistent with previous studies that report a higher probability of injury in occupants seated on the near-side compared to occupants seated on the far-side (Newgard et al. 2005; Ryb et al. 2009). Raneses and Pressley (2015) have reported similar findings of rear-seated adult passengers who were same-side seated having higher mortality using FARS data.

Occupants of larger vehicles were less likely to be injured in a side-crash. This is consistent with previous studies conducted on front-seated passengers and drivers (Anderson 2008; Gayer 2004; Farmer et al. 1997). Larger vehicles have been reported to be more protective of front-seated drivers in collisions in side crashes (Gabler 2003; Vander Lugt D 1999).

Seat belt use was highly protective for serious/fatal injury for side crashes after controlling for seating position and crash impact type for side crashes. This is consistent with previous studies that report seat belt use being highly protective (Mayrose and Priya 2008; Zhu et al. 2007). A previous study of fatal collisions using FARS data demonstrated an exponential increase in mortality over the adult age span and an increased use of seatbelts in older occupants who were also more likely to die (Raneses and Pressley 2015). However, our study that included both fatal and nonfatal crashes and a younger population contains a much smaller proportion of fatal injuries and indicates that seatbelt use is highly protective for rear-seated occupants involved in same-side crashes. This may be due to the fact that our study population included both fatal and non-fatal crashes and our population was much younger compared to the previous study. In our study, mortality was examined in a category with serious injury specifically due to the small proportion of deaths captured in the NASS/GES dataset.

Previous studies report that newer model vehicles are protective for front-seated occupants (Ryb et al. 2011; Wenzel 2013; Glassbrenner 2012). In our study, model year was associated with less severe injury, with newer cars generally being safer compared to older cars only when analyzed as continuous variable. However, when analyzed categorically (Ryb et al. 2011), newer vehicles were not significantly associated with reduced injury severity in rear-seated occupants. This finding is likely due to improvements in vehicle engineering that focused on the driver with fewer safety improvements specifically geared to rear-seated occupants (Durbin et al. 2015, Mitchell et al. 2015).

Previous studies note that vehicle side crash ratings are protective for front-seated occupants (Teoh and Lund 2011). In our study of rear-seated occupants, side crash ratings were available for a small number, about one-fifth, of vehicles involved in a same-side crash and were not found to be associated with injury severity.

This study has limitations. Firstly, NASS/GES injury data uses officer reported KABCO and may not accurately reflect moderate and minor injuries. There are reports that use of KABCO injury rates may result in misclassification for degrees of severity of injury (Farmer 2003). To address this issue, we collapsed injury categories into serious/fatal injury vs. no or minor/possible injury instead of examining each injury severity level separately. Secondly, other studies reported that the effect of belt usage on severe/fatal injury can be overestimated, due to nonfatal crash victims being more likely to report restraint use inaccurately to law enforcement (Clark 2003; Robertson 1992). Because estimation of mortality using NASS/GES needs to be interpreted with caution (Clark 2003), we combined serious/fatal injury into one category which may be less biased. It is possible that the NASS/GES sampling scheme may underestimate fatality in older populations when compared to FARS. In this study, we believe that the finding of categorical age in older populations showing less severe/fatal injury in the older occupants compared to the middle – aged occupants is a dataset anomaly. In contrast, when examined continuously, older age was associated with increased severe/fatal injury. Thirdly, the majority of vehicles, especially older vehicles were not rated for side crash safety. In the subpopulation analysis of vehicles with safety ratings, we did not control for other factors that might be potential confounders for safety ratings and injury severity for rear-seated occupants due to a small sample size. Furthermore, driver alcohol involvement was categorized mainly based on law enforcement officer report instead of actual blood alcohol level as the majority of drivers did not have a blood alcohol concentration reported in the dataset. Lastly, we did not have information on side air bag deployment, which may have an impact on injury severity in side crashes.