The Institute 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 (see "IIHS crash test research demonstrates car-into-truck underride problem, solutions," March 29, 1977).
Federal rules put in place in 1953 required interstate carriers to have rear underride guards meeting specifications for ground clearance, setback, and width, but not strength, energy absorption, or attachment methods.
The National Highway Safety Bureau, predecessor to the National Highway Traffic Safety Administration (NHTSA), indicated in 1967 that it would develop a new standard, but the agency abandoned the effort in 1971 even though the National Transportation Safety Board recommended that energy-absorbing underride and override barriers on trucks, trailers, and buses be required.
In 1977 the Institute demonstrated that a 30 mph crash of a Chevrolet Chevette into a tractor-trailer with a rear guard meeting the U.S. rule resulted in severe damage to the car's occupant compartment. The Institute petitioned NHTSA for a new standard.
It took the agency nearly 20 years to publish new rules. The upgrade took effect in 1998 and resulted in lower and wider underride guards under Federal Motor Vehicle Safety Standard 224. Another standard, Federal Motor Vehicle Safety Standard 223, introduced quasi-static test requirements specifying minimum levels of strength and energy absorption (see "Finally: new rule 43 years after the last action to reduce underride," March 2, 1996). The standards cover new trailers but exempt many types of heavy trucks used in everyday commerce including straight trucks, wheels-back trucks, and special-purpose trucks. The result is that the majority of trucks on the road aren't subject to underride rules.
Meanwhile, the passenger vehicle fleet has changed dramatically since NHTSA wrote the standards. Regulators then were concerned that "overly rigid guards could result in passenger compartment forces that would increase the risk of occupant injuries even in the absence of underride." The agency also recognized the need for balancing energy absorption with guard strength because "the more the guard yields, the farther the colliding vehicle travels and the greater likelihood of passenger compartment intrusion."
The Institute's latest analysis indicates that guards too weak to adequately mitigate underride are a bigger problem than overly stiff guards.