Side impact crashes accounted for 27 percent of passenger vehicle occupant deaths in the United States in 2009. Although the fronts and rears of most passenger vehicles have substantial crumple zones, the sides have relatively little space to absorb impact for ces or limit occupant compartment intrusion. Side airbags help to absorb impact forces and are highly effective in reducing driver death risk, but must work well with vehicle structures to maximize occupant protection. The Insurance Institute for Highway Safety (IIHS) has been evaluating passenger vehicle side crashworthiness since 2003. In the IIHS side crash test, a vehicle is impacted perpendicularly on the driver (left) side by a moving deformable barrier weighing 1,500 kg (3,300 lb) and traveling at 50 km/h (31 mi/h). Dimensions of the barrier, especially height, are designed to simulate the front of a typical SUV or pickup. Injury measures are taken from 5th percentile female test dummies in the driver and left rear seating positions, and injury ratings are computed for the head/neck, torso, and pelvis/leg based on biomechanical and crash research. Vehicles also are rated based on their ability to protect occupants’ heads and resist occupant compartment intrusion. These component ratings are combined into an overall rating of good, acceptable, marginal, or poor. A driver-only rating was recalculated by omitting rear passenger dummy data.

To evaluate how well IIHS side crash test ratings predict real-world occupant death risk, data were extracted from the Fatality Analysis Reporting System (FARS) and National Automotive Sampling System/ General Estimates System (NASS/GES) for years 2000-09. Analyses were restricted to vehicles with driver side airbags with head and torso protection as standard features. The risk of driver death was computed as the number of drivers killed (FARS) divided by the number involved (NASS/GES) in left side impacts and was modeled using logistic regression to estimate the effect of crash test rating while controlling for the effects of driver age and gender and vehicle type and curb weight. Death rates per million registered vehicle years were computed for all outboard occupants, and these were compared across the overall test rating for each vehicle.

Based on the driver-only rating, drivers of vehicles rated good were 70 percent less likely to die when involved in left side crashes than drivers of vehicles rated poor, after controlling for driver and vehicle factors. Driver death risk was 64 percent lower for vehicles rated acceptable compared with poor and 49 percent lower for vehicles rated marginal compared with poor. All three results were statistically significant. The vehicle registration-based results for drivers were similar, suggesting the benefit largely was due to crashworthiness improvements and not to differences in crash risk. The same pattern of results held for outboard occupants in nearside crashes per million registered vehicle years and, with the exception of marginal-rated vehicles, also held for other crash types. This suggests design changes that improved side crashworthiness also benefited occupants in other types of crashes. Among component ratings, the vehicle structure rating exhibited the strongest relationship with driver death risk. In sum, results show that IIHS side crash test ratings encourage designs that improve crash protection in meaningful ways beyond encouraging head protection side airbags, particularly by promoting vehicle structures that limit occupant compartment intrusion. Results further highlight the need for a strong occupant compartment and its influence in all types of crashes.