Crash Safety Study

Fact: Size – not weight – is a better determinant of automotive safety.

Safety is naturally among the top priorities for consumers and the automotive industry alike. However, automotive safety is a complex issue involving the study of many different types of single vehicle crashes (hitting obstacles, roll-overs, etc.) and multiple vehicle crashes. To further investigate, the Aluminum Association commissioned Dynamic Research, Inc. (DRI) to analyze the safety differences in a fleet of SUVs that vary by size and weight while also studying the safety of the driver in the other vehicle involved in the crash. This comprehensive approach considers safety in the context of providing protection for all passengers and vehicles on the road. Please read the executive summary for more details on this study.

Key Research Takeaways

• If weight is taken out of an SUV and size remains the same, there is less crash energy to absorb, while the vehicle structure remains in place to absorb the resulting energy.
• A lighter SUV is much less damaging to another vehicle in a crash, particularly a traditional passenger car.
• If the vehicle crush zone is increased, even by a few inches, it can have a very significant and positive safety benefit to all in every crash situation.
• Auto aluminum is beneficial in that it can be used to reduce the vehicle mass and slightly lengthen the vehicle without incurring a significant weight increase.

Methodologies

A numerical modeling approach was selected for this study; first 499 crashes were sampled from six calendar years (’97-’02). The accidents sampled approximately reflect the U.S. population of crashes and all 499 crashes involved a typical four-door SUV produced in North America. Of the total, 5 percent were rollover accidents, 12 percent were crashes with fixed objects, 50 percent were crashes with a mid-sized four-door passenger car and 33 percent involved crashes with another SUV. The virtual vehicles, equipped with seat-belted virtual crash dummies, and driver’s injuries were analyzed. The cumulative injuries were expressed as a Normalized Injury Cost referred to as Equivalent Life Units (ELU). An ELU of one represents the cost of a fatality.

Initially, driver injuries were assessed following 499 crash tests run on a baseline steel SUV and passenger vehicle. Next the SUV was reduced in mass by 20 percent and the stiffness was lowered to match the de-acceleration pulses the dummy experienced in the New Car Assessment Program tests. (An overall 20 percent curb weight reduction is typical of many aluminum intensive vehicles.) The exact same 499 crash tests were modeled and injuries assessed.

Finally, the SUV was returned to its original steel mass but with the lower stiffness used with lighter weight SUV. To absorb the same energy in the barrier tests, it was necessary to make the vehicle about 4.5 inches longer. With this approach it was possible to directly and independently assess the influence of mass and size on the overall safety of a fleet of vehicles.

Results

For the baseline set of accidents, both drivers suffered injuries with a total ELU of 8.0. However, there was a significant injury incompatibility between the two vehicles. When the mass of the SUV was reduced, the total ELU was reduced about 15 percent to 6.79, with the safety of the passenger vehicle being significantly improved. There is now less energy in the crash to be absorbed by both vehicles.

Slightly increasing the size of the SUV had a very significant impact, with a total ELU of 5.91 – a reduction of 26 percent. As is well known, a softer deceleration pulse leads to reduced injuries.

Conclusion

This study supports the fact that reducing the weight and varying the size of a vehicle provides measurable benefits in terms of reducing harmful energy transfer, injuries and making vehicles safer. Auto aluminum is a readily available lightweight material that can help automakers produce the bigger cars and trucks that consumers demand, without adding weight and compromising fuel economy or more importantly – safety.

While design studies of this type can oversimplify the complex world of automotive design and production, clearly a lighter, slightly larger vehicle is a safer vehicle for all concerned. In addition, the lighter weight vehicles consume less fuel and emit less harmful emissions. In short, everybody wins.

Many have long believed that size and weight were synonymous where safety was concerned. This study (and others, including a similar study by Honda) confirms that size, not weight, is a better determinant of vehicle safety – a fact that NHTSA has recognized in its new size-based CAFE standards for light cars and trucks.