Previous studies have identified an elevated crash injury risk of 8-12 year olds restrained in seat belts compared to their younger counterparts in child restraints. This age group is of particular importance as they represent the transition age between those recommended to use an add-on restraint system such as a booster seat versus those recommended to use the adult seat belt system provided with the vehicle. In order understand the unique restraint needs of this particular age group, research is needed to compare their injury risk to other age occupants following best practice for restraint. Therefore the objective of this project was to compare the injury risk for children and adults who are age-optimally restrained (by seat row and restraint type) and understand the influence of the contributing factors to the risk.

Data were used from the Partners for Child Passenger Safety (PCPS) study and the National Automotive Sampling System (NASS). PCPS data from 1998-2007, collected from crashes reported to an insurance company in 15 states and DC, were used. NASS data from 2000-2009, collected from police reported towaway crashes throughout the US, were used. For both data sets, crashes were limited to vehicles of model year 1998 and newer. For NASS data, efforts were made to limit the crashes to those involving child occupants by identifying typical crash deformation classifications in child-involved crashes.

The AIS 2+ (PCPS and NASS) and AIS3+ (NASS) injury risks were calculated. For PCPS, the following age groups of rear seated occupants were compared:​ children <1 year of age in rear facing child restraints (RFCRS), children 1-3 years in forward facing child restraints (FFCRS), children 4-7 years in belt-positioning boosters, children 8-12 years in seat belts, and children 13-15 years in seat belts. In addition, the injury risks for children age 13-15 years in seat belts in the front seat were included. For NASS, injury risks were compared for the following rear seated age groups - 8-12 years, 13-15 years, 16- 24 years, 25-54 yrs, 55+ years – and front seated age groups - 13-15 years, 16-24 years, 25-54 yrs, 55+ years.

For the PCPS data, compared to children age 1-3 years in FFCRS, rear seated children 8-12 years were 1.9 times more likely to sustain an AIS2+ injury. For the NASS data, rear seated 8-12 year olds had a slightly lower AIS 2+ (2.4%) and AIS 3+ (0.92%) injury risk compared to 25-54 year olds in the front seat (3.2% and 1.2% respectively) (chosen as the reference due to the regulatory focus on this age and seat position) while rear seated 13-15 year olds had a similar injury risk to adults in the front seat. In addition to comparison of the overall injury risks, there are important differences in the body regions of injury that suggest different mechanisms of how the seat belt applies loads across age groups.