Large truck crashes account for a substantial portion of the fatalities and serious injuries occurring in modern passenger vehicles designed for good frontal crash protection. Incompatibilities in mass, stiffness, and ground clearance present challenges in improving crash outcomes for passenger vehicle occupants. A recent Insurance Institute for Highway Safety study of cases from the Large Truck Crash Causation Study (LTCCS) found that rear underride guards meeting US federal requirements still can allow severe passenger vehicle underride, often resulting in serious or fatal injury. The study identified patterns of real-world guard failure, but the impact speeds necessary to produce these failures could not be determined. Also, due to the LTCCS case selection requirement that each crash produce an injury, differences among the large number of guard designs and resulting crash performance and injury risk could not be compared. The current study used a series of six crash tests to investigate these issues.

Crash tests were conducted in which the front of a midsize sedan impacted the rear of a semi-trailer equipped with an underride guard. Three trailer/guard designs were evaluated in various conditions. Each guard design was certified to the US Federal Motor Vehicle Safety Standard (FMVSS) 223 requirements, and two also met the more stringent Canadian Motor Vehicle Safety Standard (CMVSS) 223 regulation. Quasi-static tests were conducted to determine the compliance margins.

In a full-width test at 56 km/h, the guard design built only to the US requirements failed catastrophically at the points of attachment to the trailer, allowing severe underride and trailer contact with the dummy’s head. The second guard failed in 50 percent overlap tests at 40 and 56 km/h, producing underride to the base of the sedan’s windshield in the first test and to the dummy’s head in the second. The third guard was able to prevent underride in full-width and 50 percent overlap tests at 56 km/h but failed when the overlap was reduced to 30 percent.

The minimum force requirements of FMVSS 223 are too low to prevent guard failure in full-width crashes. CMVSS 223 is an improvement over the US regulation, but its requirements also should be strengthened because underride still can occur in offset crashes. Both standards should require quasi-static tests to be conducted with guards attached to a trailer. The current standards allow tests using a rigid fixture, so even well-designed guards could be attached to a trailer such that they fail to prevent underride due to weakness of the trailer chassis or attachment mechanism.