U.S. can do better than simply adopt Canada's rear underride guard standard

February 26, 2016

The rear guard on this 2015 Vanguard trailer slowed the Chevrolet Malibu enough to prevent underride in the 30 percent overlap test at 35 mph. Vanguard is the second trailer manufacturer to pass this demanding IIHS test.

A proposed upgrade to rear underride guard regulations for tractor-trailers is a move in the right direction but isn't comprehensive enough to deliver the safety gains IIHS outlined in a 2011 petition for rulemaking, especially when it comes to preventing underride in offset crashes.

The National Highway Traffic Safety Administration (NHTSA) in December issued a notice of proposed rulemaking to require stronger underride guards to stop passenger vehicles from sliding underneath the backs of trailers and semitrailers in rear-end crashes. The notice responds to the Institute's petition to improve rear underride guards, as well as a 2014 request from the Truck Safety Coalition and Marianne Karth, a North Carolina mother whose daughters AnnaLeah, 17, and Mary, 13, died in an underride crash in 2013.

The proposal would align U.S. regulations with stricter ones in place in Canada since 2007. NHTSA estimates that 93 percent of new semitrailers sold in the U.S. already comply with the Canadian rules, based on information from the Truck Trailer Manufacturers Association. The agency estimates the rule would save one life and prevent three serious injuries a year.

IIHS crash tests show that compliance with the Canadian standard does not ensure guards can prevent underride when cars run into the outer ends of a trailer, where the underride guards are weakest  (see "Underride guards can be lifesavers, but most could be improved," March 14, 2013).

"We had hoped for a more a meaningful upgrade to the outdated standard for rear underride guards," says Adrian Lund, IIHS president. "As written, this proposal will have a minimal impact on safety. We urge NHTSA not to miss the opportunity to address a wider range of rear underride crashes."

In 2014, 371 of the 2,485 passenger vehicle occupants killed in large truck crashes died when the fronts of their vehicles struck the rears of trucks. Gaps in federal crash data make it difficult to pinpoint exactly how many of these crashes involve underride. A 2011 IIHS study of 115 crashes in which a passenger vehicle struck the back of a heavy truck or semitrailer found only about one-fifth involved no underride or negligible underride. Nearly half of the vehicles had severe or catastrophic underride damage, and those vehicles accounted for 23 of the 28 fatal crashes in the study.

When IIHS petitioned the federal government for a stronger underride guard standard, it asked NHTSA to include test procedures that would address protection in small overlap crashes. IIHS made the request after finding that in cases involving regulation underride guards, 30 percent involved crashes in which less than half of the passenger vehicle overlapped the trailer. In most of these, the guards' vertical supports didn't engage the passenger vehicle.

NHTSA declined to take up the issue, stating that offset crashes "appear to represent a small portion of the rear underride fatality problem."

IIHS believes the agency underestimates the scope of underride. NHTSA uses an estimate of the proportion of fatal crashes that involve severe underride that was derived from interviews taken long after the crash and thus may not be accurate. What is more, researchers interviewed people who were familiar with the crash but not necessarily driving the truck when it crashed.

Small overlap crashes

IIHS crash tests have demonstrated that rear guards can be designed to resist underride in small overlap crashes. So far, two trailers have successfully stopped underride in the toughest test, which involves 30 percent of the front of a car hitting the trailer at its outermost corner at 35 mph. The test configuration represents the minimum overlap under which the head of a person in a passenger vehicle would contact an intruding trailer if an underride guard fails.

Wabash trailer rear guard
(Photo courtesy of Wabash National Corp.)
Wabash recently introduced a new rear impact guard built to prevent underride in offset crashes. It includes extra vertical support posts on both ends of the guard and a longer, reinforced bumper bar. IIHS plans to test the guard this spring. An earlier model failed the 30 percent overlap test.

The first trailer to pass the 30 percent test was a 2012 model made by Canadian manufacturer Manac Inc. The Manac was the only trailer out of eight to pass. In January, IIHS evaluated a new design from Indiana-based Vanguard National Trailer Corp. The rear guard on the 2015 trailer prevented severe underride and intrusion into the striking car's occupant compartment. All injury measures taken from the dummy were good.

Manac and Vanguard are among the manufacturers voluntarily equipping trailers with rear guards that exceed safety standards.

"The performance of the Vanguard and Manac trailers shows there's more than one way to address underride crashes at the far edges of trailers," says Matthew Brumbelow, a senior research engineer at IIHS.

The Manac trailer's vertical supports are attached to a reinforced floor and located closer to the trailer's outer edges than on other models IIHS has evaluated. The design limited the potential for injuries to the dummy in the car and also reduced damage to the trailer itself.

On the Vanguard, extra vertical support tubes are located at the outermost ends of the underride guard and are reinforced with a triangle support gusset. In the test, the weld failed on the upper end of the tube where it attaches to the trailer sill, and the gusset tore free and bent. Still, the guard kept the Malibu from underriding the trailer.

Another new underride guard design looks promising. Wabash National Corp. in February announced a new rear impact guard engineered to prevent underride in small overlap crashes. The guard has two additional vertical support posts and a longer, reinforced bumper tube to "absorb energy better and deflect rear impact at any point along the bumper," Wabash says. The company notes that it has been building underride guards to exceed U.S. and Canadian standards since 2007. At Wabash's request, IIHS plans to test the new guard this spring.

Exempted trucks

NHTSA's proposal fails to mandate rear underride guards for more types of trucks, including ones with rear wheels set very close to the back of the trailer. IIHS research has shown that more than half of the truck units studied in real-world crashes were exempt from federal underride guard rules. Wheels-back trailers and single-unit trucks accounted for most of these exemptions.

Last year, NHTSA outlined a plan to require rear underride guards on single-unit trucks but didn't include wheels-back trucks (see "Rear underride guard mandate may extend to more trucks under NHTSA proposal," Nov. 10, 2015). A 2008-09 study by the University of Michigan Transportation Research Institute found that half of the wheels-back trailers involved in any fatal crashes had rear guards despite being exempt from the rules.

"The large number of wheels-back trailers with underride guards suggests it would be feasible to remove this exemption," Brumbelow says.

Attachment strength certification

The proposed rule would allow manufacturers to conduct certification tests on guards affixed to a rigid test fixture instead of actual trailers or sections of trailers that include the frame rails or cross beams. Trailer tests are more representative of real-world crash loads and can expose vulnerabilities at attachment points. It's not just the underride guard that must be strong. The points where the guards attach to the trailer also must be strong enough to withstand crash forces.

"Manufacturers can't just attach a stronger underride guard to a trailer without also reinforcing the underlying attachment structures," Brumbelow says. "In a real crash, deformation will happen at the weakest point. Trailer structures have to be able to resist as much loading as the guard does."

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