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Status Report, Vol. 48, No. 2 | March 14, 2013 Subscribe

Rear underride crashes are easier to address than front or side ones

In this 2002 fatal underride crash, a Chevrolet Impala was hit by another vehicle from behind, lost control and struck the back of a tractor-trailer parked on the shoulder.

A hurried driver looks over his shoulder as he tries to merge onto the freeway, failing to notice traffic stopped ahead of him. He plows his van into the back of a tractor-trailer.

A Chevrolet Prizm slams into the side of a tractor-trailer as it makes a U-turn from the opposite direction at a traffic signal while both vehicles have the green light.

The driver of a logging truck sees a Ford Explorer coming toward him in his lane on a rural, undivided highway. Both vehicles move into the other lane at the last minute and crash head-on.

All of these examples were taken from a federal database of truck crashes, and each resulted in the death of the passenger vehicle driver. In the first, the outcome may well have been different if the truck had been equipped with a stronger rear underride guard such as those already on some trailers. In the second two crashes, which involved the side of one large truck and the front of the other, potential solutions exist, but they aren't as readily available.

Crashes involving the rear of a large truck account for about one-fifth of fatal underride cases, Institute researchers found in 1997. Another fifth are side crashes, while the majority are frontal ones. Unlike front and side underride, rear underride fatalities are often preventable, and there is a framework in place to address the problem.

"We already have a regulation on rear underride guards, so we should make sure that regulation is effective," says Matthew Brumbelow, an IIHS senior research engineer.

Still, with so many underride crashes involving the fronts and sides of large trucks, should guards surround trucks completely?

In side crashes, underride guards have the potential to save lives. An IIHS analysis of crashes in which passenger vehicles hit the side of large trucks found that out of 143 crashes in which the truck side impact produced the most severe injury, more than half would not have been as severe if there had been side underride guards on the truck.

For side guards to work, several hurdles would have to be overcome. For one thing, many trailers have sliding axles that can be adjusted depending on the load, making it difficult to position a side guard so that it won't interfere with the wheels. In addition, side guards that are strong enough to prevent underride would add a lot of extra mass to a trailer — much more than a rear guard, which doesn't have to cover as big an area — and in the trucking industry, any additional pounds can affect the bottom line.

The European Union requires side guards, but they are intended to protect only pedestrians and bicyclists. Because of this, they are much weaker and lighter than they would need to be to protect people in passenger vehicles.

Front underride guards, which are required in the EU to protect vehicle occupants in crashes with combined speeds of about 35 mph, also might prevent some deaths. An earlier Institute study of fatal truck crashes in Indiana found that 9 out of 44 front underride crashes might have been survivable in the absence of underride (see "FARS undercounts fatal large truck-car underride crashes," Feb. 15, 1997).

However, in most of the crashes studied, front underride guards would not have changed the outcome. In crashes involving a passenger vehicle and the front of a large truck, the truck is typically moving toward the other vehicle. The enormous difference in mass between a tractor-trailer and a car, SUV or pickup means that there is a high probability such a crash will be fatal at even moderate speeds, underride or no underride.

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Rear underride guards could do better

IIHS crash tested eight trailers to see if their underride guards could stop a car from sliding underneath. Only one passed the toughest test, a 30 percent overlap.

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