IIHS side crash test ratings and occupant death risk in real-world crashes

October 2011

Objective: To evaluate how well the Insurance Institute for Highway Safety (IIHS) side crash test ratings predict real-world occupant death risk in side-impact crashes. Methods: The IIHS has been evaluating passenger vehicle side crashworthiness since 2003. In the IIHS side crash test, a vehicle is impacted perpendicularly on the driver's side by a moving deformable barrier simulating a typical sport utility vehicle (SUV) or pickup. Injury ratings are computed for the head/neck, torso, and pelvis/leg, and vehicles are rated based on their ability to protect occupants' heads and resist occupant compartment intrusion. 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. Data were extracted from the Fatality Analysis Reporting System (FARS) and National Automotive Sampling System/General Estimates System (NASS/GES) for the years 2000-2009. Analyses were restricted to vehicles with driver side air bags 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 control 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 compared by overall rating. Results: 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. Compared with vehicles rated poor, driver death risk was 64 percent lower for vehicles rated acceptable and 49 percent lower for vehicles rated marginal. All 3 results were statistically significant. Among components, vehicle structure rating exhibited the strongest relationship with driver death risk. The vehicle registration-based results for drivers were similar, suggesting that the benefit was not due 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 marginally rated vehicles, also held for other crash types. Conclusions: Results show that IIHS side crash test ratings encourage designs that improve crash protection in meaningful ways beyond encouraging head protection side air bags, 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.