In an underride crash, a passenger vehicle goes partially or completely under a truck or trailer, increasing the likelihood of death or serious injury to the passenger vehicle occupants. A 1997 Institute study of fatal crashes between large trucks and passenger vehicles estimated that underride occurred in half of these crashes (Braver et al., 1997). Of the underride crashes, 57 percent involved the front of the truck, 22 percent involved the rear and 20 percent the side. 

A federal rule to upgrade the rear-impact guard standard for new trailers took effect in January 1998. Several types of trucks are exempt from the rule, including single-unit trucks, trucks with rear wheels set very close to the back of the trailer and various types of special-purpose trucks. In 2010, the Institute studied how guards built to comply with the federal standards are performing in real-world crashes and found many fail, allowing severe passenger vehicle underride and resulting in serious or fatal injury (Brumbelow & Blanar, 2010).

As a result of this research and some initial crash tests that confirmed the problem, the Institute petitioned the federal government to require stronger rear underride guards on large trucks and improve performance test procedures. Citing this petition, NHTSA issued a proposed upgrade to the rear underride regulations for tractor-trailers in 2015 (Office of the Federal Register, 2015) and finalized the rule in 2022 (Office of the Federal Register, 2022). The final rule largely aligns U.S. regulations with stricter ones in place in Canada since 2007.

NHTSA denied the Institute’s requests to require guard designs with improved performance in offset crashes, to certify guards when attached to a trailer instead of a rigid fixture and to consider reducing the number of trailer types that are exempt from the standard. NHTSA previously decided not to extend underride regulations to single-unit trucks (Office of the Federal Register, 2015).

NHTSA estimated its final rule will have minimal effect because 94 percent of new trailers already have guards that meet the Canadian standard. Institute crash tests have shown that underride guards built to this standard work well when passenger vehicles strike the center of the trailer’s rear but don’t necessarily prevent underride in crashes involving only a small portion of the truck’s rear.

Many trailers already have underride guards that go beyond the new requirements, thanks to the Institute’s own testing program. The IIHS TOUGHGUARD award, introduced in 2017, recognizes rear underride guards that prevent underride in three crash test scenarios — full-width, 50 percent overlap, and 30 percent overlap — using a midsize car traveling at 35 mph into the back of a semitrailer. Today, nine North American trailer manufacturers, including the eight largest, produce guards that qualify for the award. Most of the companies have made the improved guards standard on all new trailers.

Truck underride guard evaluations

Stronger underride guard requirements could prevent deaths and injuries in rear- and side-impact crashes.

There is no requirement for front or side underride guards in the United States. European Union regulations have required front underride guards on large trucks since 2003. A 1998 Institute study of fatal truck crashes in Indiana found that 9 out of 44 front underride crashes might have been survivable had underride not occurred (Braver et al., 1998). Side underride guards could be even more effective. A 2012 Institute study found that in the U.S. strong side underride guards have the potential to reduce injury risk in about three-fourths of cases where a passenger vehicle occupant sustained a serious injury from an impact with the side of a large truck (Brumbelow, 2012).

IIHS has conducted two tests of a side underride guard. The AngelWing guard, made by Airflow Deflector Inc., succeeded in blocking a midsize car traveling 35 mph from traveling underneath the trailer (Automotive World, 2017). In a later test, it prevented underride at 40 mph (IIHS, 2017). The AngelWing doesn’t cover the whole length of the trailer, but in combination with the wheels, it provides protection along 62 percent of it. As required by Congress, NHTSA plans to publish research and consider requiring side underride guards (NHTSA, 2022).