NHTSA study underestimates benefits of side underride guards for trucks

June 13, 2023

The National Highway Traffic Safety Administration (NHTSA) should reconsider its dramatic underestimate of the number of lives that could be saved by requiring side underride guards for large trucks, the Insurance Institute for Highway Safety said in a recent regulatory comment.

Requiring side underride guards could save more than 10 times as many lives as NHTSA projects, IIHS Senior Research Engineer Matthew Brumbelow wrote in the comment.

In an underride crash, a smaller vehicle crashes into a truck and goes completely or partially underneath it. This makes serious injuries to people riding in the smaller vehicle more likely. Federal regulations require tractor-trailers to have underride guards on their rears, but not on their sides.

NHTSA requested comments on its preliminary cost-benefit analysis of requiring side underride guards on all new trailers and semitrailers. The report estimated that equipping all large trucks in the U.S. fleet would cost between $973 million and $1.2 billion and would prevent 17 fatalities and 69 serious injuries per year.

However, that estimate excludes many types of crashes that are likely relevant.

For example, it excludes crashes that involve more than just a single passenger vehicle and a single tractor-trailer and those that occur at speeds greater than 40 mph.

NHTSA also used speed limits and police-estimated pre-crash speeds rather than actual velocity changes gathered from event data recorders for its calculation.

Using an alternative method, Brumbelow projected that a side underride guard mandate could prevent 159-217 passenger vehicle occupant fatalities per year, depending on whether it required protection forward of the rear axle or along the full length of the trailer.

It would also likely save many of the pedestrians, bicyclists and motorcyclists who are killed each year in crashes involving the sides of tractor-trailers.

Past IIHS crash tests have shown that aftermarket side underride guards can prevent vehicles from sliding underneath truck trailers at speeds as high as 40 mph. It's possible that they also work at higher speeds, especially in nonperpendicular crashes where the speed relative to the trailer is lower than the travel speed.

In 2021, there were 488 passenger vehicle occupant fatalities in crashes involving the side of a tractor-trailer.

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