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Status Report, Vol. 49, No. 11 | December 23, 2014 Subscribe

Laying the groundwork for safety improvements for back-seat occupants

When it comes to advances in occupant protection, the front seat has gotten much of the attention while vehicle restraint system improvements for people who ride in back haven't kept pace. A new study by the Institute and The Children's Hospital of Philadelphia examines the characteristics of back-seat occupants injured in crashes in late-model vehicles to help zero in on ways to make rear seats safer.

Researchers analyzed real-world data on crashes during 2007-12 involving restrained occupants in the front and rear rows of 2000 and newer model passenger vehicles. Data were taken from two federal crash databases, the National Automotive Sampling System-Crashworthiness Data System (NASS-CDS) and the Fatality Analysis Reporting System (FARS). The former is a nationally representative sample of police-reported tow-away crashes on public roads, while the latter is a census of all crashes on public roads resulting in a fatality within 30 days of the crash.

Among all passenger vehicle occupants in crashes during 2007-12, 12 percent were seated in back. Children younger than age 13 accounted for 56 percent of back-seat occupants, teenagers ages 13-19 made up 19 percent, and adults accounted for 21 percent of occupants in the back.

Although more than half of rear-row occupants were younger than 13, they accounted for only 19 percent of serious injuries in crashes and 24 percent of crash deaths. Adults were overrepresented among rear-row occupants with serious or fatal injuries, and they had lower rates of restraint use. Only 70 percent of 20-54 year-olds and 86 percent of people 55 and older were restrained. This compares with 99 percent of infants, 96 percent of 4 to 8 year-olds and 93 percent of 9 to 12 year-olds.

Restraint use is an important risk factor for injury. The risk of serious injury was nearly 8 times higher among unrestrained rear-row occupants as compared with those using restraints.

The relative risk of death was lower for restrained children up to age 8 in the rear compared with front passengers but was higher for restrained children ages 9 to 12 seated in back. It's not clear that the front seat is safer for preteens. The finding could be due to sparse data on kids this age seated in front. Among occupants ages 13 to 54, researchers couldn't find evidence of a difference in death risk in the rear compared with the front seat.

Belted adults age 55 and older seated in the back had the highest risk of any age group of sustaining a serious or fatal injury in a crash, and they had a higher relative risk of death when seated in the back as compared with the front. The finding is consistent with previous research indicating that adults in the rear are more likely than adults in the front to sustain chest injuries, along with some evidence of an elevated risk of head and neck injuries for restrained women seated in the rear compared with the front (see "Safer vehicles benefit all occupants," Feb. 20, 2014).

After controlling for occupant age and gender, the relative risk of death for restrained rear occupants was significantly higher than that of front occupants in model 2007 and newer vehicles and significantly higher in rear and right-side impact crashes.

"That doesn't mean that the rear seat in newer vehicles is less safe than in older model vehicles," says Anne McCartt, the Institute's senior vice president of research and one of the authors of the study. "The risk of fatal injury for rear occupants is similar across all of the model years we examined. Instead, the disparity reflects the fact that the front seat is getting safer."

Consumer crash-test information programs largely focus on drivers and front-seat passengers because that's where most people ride and most deaths occur in crashes.

"Enhancing safety in the rear seat where children and adults alike sit is challenging," says Dr. Dennis Durbin, director of the Office of Clinical and Translational Research at The Children's Hospital of Philadelphia Research Institute and the main author of the study. "You have to be sure that changes that help one group won't harm another or interfere with the installation of child restraints."

The study's findings suggest that rear-seat occupants could benefit from some of the same technologies used to protect drivers and front passengers. Front airbags, side airbags and knee airbags, plus features that ready safety belts when a crash is imminent and limit the amount of energy that is transferred to an occupant are among these innovations.

For belted teens and adults up to age 54, sitting in back is no riskier than sitting in the front seat, the study found. However, things change after age 55. Adults this age and older had the highest risk of any age group of sustaining a serious or fatal injury in a crash, and they had a higher relative risk of death when seated in back as compared with the front. The findings suggest a need for advanced restraint systems tailored for the back seat, such as inflatable safety belts (above right) like the ones Ford offers on some models. (Inflatable belt photo: Ford Motor Company)

Belt pretensioners tighten up slack when triggered by vehicle sensors and retract the belt almost instantly in a crash. Load limiters manage the force that belts apply to occupants' chests in a crash by allowing some of the webbing to spool out when the forces exceed levels that can cause injuries.

There is some question about the benefits of load limiters. Prior IIHS research on front-seat occupants has indicated in certain severe crashes, including small overlap front crashes, shoulder belts with load limiters may spool out too much, allowing occupants to move enough to strike hard surfaces inside the vehicle (see "Allowing belts to loosen in crashes may be contributing to injuries," Oct. 13, 2007).

Another option would be to develop additional advanced restraint systems tailored for the back seat, such as inflatable safety belts that manage crash forces and help control head excursion. Ford offers optional inflatable belts in certain rear seats of the Ford Explorer, F-150 SuperCrew, Fusion, Flex and Taurus, plus the Lincoln MKT and MKZ.

When deployed, inflatable belts aim to reduce chest injuries by distributing crash forces more widely over the body than with conventional belts. The inflated belt also provides support for the head and neck to prevent excessive motion. When vehicle sensors determine that a severe collision is occurring, the belt's airbag fills with cold compressed gas and expands sideways across the occupant's body.

Side curtain airbags protect people in the back in side crashes, and in front crashes some cars deploy one or both side curtains, depending on the crash angle. There are no vehicles on the market with rear-seat airbags for protection in a full-front crash. The Scion iQ microcar has a rear-window curtain airbag that deploys in a rear crash.

Under current regulations, the performance of restraint systems for rear seats in frontal impacts is evaluated only in static tests, not crash tests. Assessing the risk of injury to rear-seat occupants isn't required, and IIHS and government crashworthiness programs focus on evaluating occupant protection for people seated in front. In contrast, vehicle evaluation programs in Australia, China, Europe and Japan take into account protection for children in crash tests.

The National Highway Traffic Safety Administration is weighing changes to the New Car Assessment Program to focus on rear-seat restraint system performance. Options include creating a crashworthiness rating for rear-seat child occupant protection and running frontal tests using a 5th percentile hybrid III dummy representing a 108-pound, 5-foot tall woman or child seated in back.

Extending safety belt laws to back-seat occupants also could help. In states that require belt use in all seating positions, 84 percent of back-seat passengers ages 8 and older were observed using safety belts in 2012, compared with 67 percent of back-seat passengers in states that require only front-seat belt use, a federal National Occupant Protection Use Survey found. Primary belt laws also encourage people to buckle up. Under a primary law, a police officer can stop a driver solely because they or their passengers aren't using belts. Thirty- three states and the District of Columbia have primary belt laws, but only 16 of these states cover back-seat occupants, too.

Allowing belts to loosen in crashes may be contributing to injuries

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