Front-Seat Occupant Injuries in Rear Impacts: Analysis of the Seatback Incline Variable in NASS-CDS 2009-01-1200

Objective: This study addresses severe injury risks in rear impacts for front-outboard occupants using the seatback incline variable in NASS-CDS.

Methods: Severe injury risk (MAIS 4+F) was determined for front-seat occupants in rear impacts involving passenger cars from 1995–2006 NASS-CDS data. The risk of severe injury to front-seat occupants was determined as a function of the rotated position of the seatback and crash severity in three delta V ranges: <20, 20–30, >30 mph. The data was also analyzed for newer model vehicles (≥1997 MY) to assess changes with newer seats and head restraints. The effects of seatbelt use, occupant age and BMI (Body Mass Index) were also examined. Individual NASS-CDS electronic cases were also reviewed with MAIS 4+F injury. There were 25 injured occupants in rotated seats and 46 in non-rotated seats.

Results: Severe injury risk for front-seat occupants in rear impacts is lower with a rotated seatback in the most severe rear crashes. For delta V >30 mph, the risk of severe injury was 3.8-times greater when the seat had not rotated rearward (12.26% v 3.20%). For 20–30 mph delta V, the risk of severe injury was similar. In the lower-speed rear crashes the trend reversed, but the sample size was too small and confounding factors made it difficult to draw meaningful conclusions. Seatbelt use lowered the risk for severe injury by 43% in severe rear crashes; obesity and older age increase injury risks. The results of the individual case review shows that intrusion plays a substantial role in determining injury risk by supporting the seat upright or pushing it forward in severe rear impacts.

Conclusions: The relationship between a rotated or non-rotated seatback and injury risk is complex in rear impacts. Analysis of NASS-CDS data finds that a non-rotated seatback increased severe injury risks by 3.8-times for front-seat occupants in >30 mph delta V rear crashes. Analyses of other factors that could explain this increased injury risk other than a non-rotated seat were not suggestive of a cause for this difference other than seat rotation. There remains unanswered the extent to which inaccuracies in the coding of pre- and post-crash seat position may be a factor in some of the findings of this analysis.