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UMTRI research: Protecting rear-seat passengers

February 17, 2016

Riding in the rear seat of a vehicle is generally safer than riding in the front seat, but the advantage isn’t as big as it once was, say UMTRI Biosciences researchers. 
Associate research scientist Jingwen Hu explains that the rear seat of a vehicle accommodates a more diverse range of passengers than the front seat—everyone from infants, toddlers and young teens to seniors. Optimizing occupant crash protection for this wide range of ages and body sizes is a bigger challenge than protecting the adult-sized occupants in front seating positions. Hu explains why. 
“Based on the crash injury data for rear-seat occupants, the head is the most commonly injured body region for young children, while abdominal injury is disproportionally high for 6–8 year-old children, and the chest is most vulnerable in older adults,” says Hu. “These injury trends indicate different injury mechanisms for the wide range of rear-seated occupants.” 
And different injury mechanisms require different types of protection. 
With funding from the National Highway Traffic Safety Administration (NHTSA), Hu and UMTRI colleagues Jonathan Rupp and Matt Reed collaborated with ZF TRW to develop and test a suite of advanced restraint systems for rear-seat occupants. 
Advanced Restraint Systems 
Advanced restraint systems include such devices as three-point seat belts with pre-tensioners (which pull the seat belt at the beginning of a crash to remove belt slack) and load limiters (which release webbing gradually during a crash to prevent excessive loading to an occupant’s chest). Load limiters have been widely used in vehicle front seats but are rarely available for rear seats due to the lower occupancy rates. Other advanced restraint systems include four-point seat belts, inflatable belts, roof airbags, and self-conforming rear-seat air bags (SCaRAB), all of which are designs currently focusing on rear-seat occupants. 
In the UMTRI/TRW study, researchers conducted a series of three sled crash tests to establish the baseline performance of the current rear-seat restraint system. The tests also allowed researchers to investigate a variety of advanced restraints and to test the final optimal restraints.  For the sled tests, Hu and his team used four different crash-test dummies—representing a six-year-old child, small female, medium-sized male, and large male—and subjected them to two crash pulses (soft and severe), selected based on standard NHTSA 35-mph frontal crash tests from 25 small cars. Two impact angles (0° and 15°) were varied in each series of crash tests. 
The physical tests in the first two series were used to calibrate sophisticated computational models of the occupants and vehicle. Hu says that once good correlations between the tests and simulations were achieved, he used the validated models to perform design optimizations, or strategies to improve protection for rear-seat occupants of all sizes. 
“Computational models can be rapidly ‘crashed’ thousands of times with many different restraint configurations to find the best way to protect occupants,” explains Hu. 
Adaptability is Key Factor 
The UMTRI/TRW researchers found that advanced restraints have to adapt to occupant size and crash severity to achieve the best occupant protection. For instance, because field data show that the chest is the most commonly injured body region for rear-seated adult occupants, the researchers attempted to reduce the amount of force impacting the chest while keeping the head from pitching too far forward and impacting the front seat. 
Children 6-12 years old, however, have a different problem. Their smaller body size and often slouched posture in vehicle seats can result in poor seatbelt fit and the risk of submarining, or sliding under the seatbelt in the event of a crash. 
In addressing these various injury situations, the UMTRI/TRW researchers found that under the soft crash pulse, an optimal advanced belt-only design (three-point belt with pretensioner and load limiter) met all of the injury criteria; under the severe crash pulse, the advanced seatbelt must be combined with the SCaRAB airbag to meet all of the injury criteria.  
Final results of the crash tests show that the advanced restraints, especially those with SCaRAB airbag can significantly reduce the injury risks to the head, neck, and chest to the rear-seat occupants with different sizes and under different crash severity. Hu emphasizes, however, that this study used only a single vehicle model, thus future work is necessary to translate similar results to the whole vehicle fleet. 
Still, the UMTRI study demonstrates that properly optimized seatbelts and airbags can significantly improve protection for rear-seat vehicle occupants. 
These results are very promising, says Hu, and they come at the right time. 
“With the recent popularity of services such as Uber and Lyft, the growing attraction of self-driving vehicles, and the steady increase in the older population in the U.S., rear-seats may become more important than ever before,” says Hu. “Now is a good time to improve rear-seat occupant protection.”  
--Joyce Daniels, UMTRI 
Photo: A series of sled tests shows the baseline performance of the current rear-seat restraint system (top left) as well as performance of advanced restraints including three-point seatbelt with pretensioner and load limiter, four-point seatbelt with pretensioner and load limiter, self-conforming rear-seat air bag (SCaRAB), airbag in roof, and inflatable seatbelt. Credit: Courtesy of UMTRI Biosciences.