Truck Underride Guards Fail in Crashes so IIHS Wants New Rule

New crash tests and analysis by the Insurance Institute for Highway Safety (IIHS) show that underride guards on tractor-trailers can fail in relatively low-speed crashes. If they do the result is often a fatal accident. IIHS is therefore petitioning the National Highway Traffic Safety Administration (NHTSA) to require stronger underride guards that will remain in place during a crash and to mandate guards for more large trucks and trailers. The underride guard rules were last changed in 1996.

In 2009, 70% of the 3,163 people who died in all large truck crashes were occupants of cars or other passenger vehicles. Underride makes death or serious injury more likely since the upper part of the passenger vehicle’s occupant compartment typically crushes as the truck body intrudes into the vehicle safety cage.

NHTSA recently unveiled the most rigorous crash safety standards in history as part of a new five-star safety ratings program, a spokesperson replied to AutoInformed.

“We have continued looking for ways to strengthen crash safety standards, however, including the current truck underride standard. In 2009, NHTSA identified underride performance in rear corner impact crashes as a part of the current standards that needed improvement, and IIHS’s report is consistent with our findings. As a result of NHTSA’s 2009 review, the agency initiated an in-depth field analysis to determine how we can improve current federal motor vehicle safety standard requirements. We expect to complete our review in 2012,” NHTSA said in a written reply.

“The front-end structures of cars are designed to manage a tremendous amount of crash energy in a way that minimizes injuries for their occupants,” said Adrian Lund, Institute president.

“Hitting the back of a large truck is a game changer. You might be riding in a vehicle that earns top marks in frontal crash tests, but if the truck’s underride guard fails — or isn’t there at all — your chances of walking away from even a relatively low-speed crash aren’t good,” Lund said.

Lund said IIHS has studied the underride crash problem for more than 30 years, including mid-1970s crash tests demonstrating how then-current guards were ineffective in preventing underride.

In the latest study the IIHS analyzed case files from the Large Truck Crash Causation Study, a federal database of about 1,000 real-world crashes in 2001-03, to identify crash patterns leading to rear underride of heavy trucks and semi-trailers with and without underride guards. Underride was a common outcome of the 115 crashes involving a passenger vehicle striking the back of a heavy truck or semi-trailer. Only 22% of the crashes didn’t involve underride or had only negligible underride, a finding consistent with prior studies. In 23 of the 28 cases in which someone in the passenger vehicle died, there was severe or catastrophic underride damage, meaning the entire front end or more of the vehicle slid beneath the truck.

The National Highway Traffic Safety Administration (NHTSA) has estimated that about 423 people in passenger vehicles die each year when their vehicles strike the backs of large trucks. More than 5,000 passenger vehicle occupants are injured.

The IIHS study raised questions about how and why underride guards failed and at what speeds, so the Institute conducted crash tests evaluating 3 semi-trailer rear guards complying with US rules. Two of the trailers also are certified to Canadian requirements, which are more stringent than the United States when it comes to strength and energy absorption. The tests involved crashing a 2010 Chevrolet Malibu into the rear of parked trailers.

The goal wasn’t to evaluate the Malibu’s crashworthiness. The midsize sedan is an Institute Top Safety Pick and earns a 5-star safety rating in NHTSA’s New Car Assessment Program.

“The aim was to see if some underride guards perform better than others and to identify what crash speeds and configurations produce different types of failure,” Lund says. “Damage to the cars in some of these tests was so devastating that it’s hard to watch the footage without wincing. If these had been real-world crashes there would be no survivors.”

Decapitation is a serious threat in underride accidents. In 3 of the crash tests the heads of the dummies in the car made contact with either the intruding trailer or the car’s hood after it tore free and pushed into the occupant compartment.

One such test involved a Hyundai trailer whose underride guard bent forward, sheared its attachment bolts, and broke after the Malibu hit it in the center rear at crash into a Wabash trailer with a strong guard. The occupant compartment is intact.

In contrast, a Wabash trailer outfitted with a guard certified to Canadian specifications successfully prevented underride of the Malibu’s passenger compartment in a center-rear test at 35 mph. The trailer was made by Wabash National Corp. Its guard was the strongest of the 3 evaluated.

“Strong attachments kept the Wabash guard in place so it could engage the Malibu, allowing the car’s structure to absorb and manage the crash energy,” Lund says. “In the real world, this would be a survivable crash.”

IIHS also ran offset crash tests with overlaps of 50% and 30% to find out what happens when a car hits the trailer with only part of its front instead of head-on.

In a 35 mph test with a 50% overlap, the guard on a Vanguard trailer allowed severe underride. The trailer was made by Vanguard National Trailer Corp., and the guard is certified to US and Canadian standards. In contrast, the Wabash trailer’s guard successfully prevented underride in the same test. The outcome for the Wabash was different when the overlap was reduced to 30%. The struck end of the guard bent forward, and there was severe underride.

This test shows that even the strongest guard left as much as half of the rear of the trailer vulnerable to severe underride. The guard only worked as intended when the striking car engaged the center.

Offset tests severely stress guards’ unsupported outboard ends. The vertical frame supports that attach guards to their trailer chassis are closer to guards’ centers than ends. Preventing underride in narrow overlap crashes like these might mean devising a new way of attaching guards to trailers to utilize the side rails, in addition to requiring manufacturers to conduct compliance tests with guards on trailers.

“Under current certification standards, the trailer, underride guard, bolts, and welding doesn’t have to be tested as a whole system,” Lund says. “That’s a big part of the problem. Some manufacturers do test guards on the trailer. We think all guards should be evaluated this way. At the least, all rear guards should be as strong as the best one we tested.”

Another problem is that regulatory gaps allow many heavy trucks to forgo guards altogether. When they are present on exempt trucks, guards don’t have to meet 1996 rules for strength or energy absorption.

“Underride standards haven’t kept pace with improvements in passenger vehicle crashworthiness,” Lund says. “Absent regulation, there’s little incentive for manufacturers to improve underride countermeasures, so we hope NHTSA will move quickly on our petition.”