Due to the disparate height of tractor trailers and automobiles, underride crashes are often the most catastrophic and fatal. In 1998, the National Highway Transportation Safety Administration mandated that all tractor trailers be equipped with underride guards. The mandate also increased the width of the bars from 60 inches to a minimum of 94 inches, in order to have the underride guards span the entire width of the tractor trailer.
While an NHTSA statistical study compiled in October 2010 showed that there was a slight, but not statistically significant, decrease in fatalities due to the newer underride guard standards, a study completed earlier this month by the Insurance Institute for Highway Safety highlights the grave safety risk that drivers sharing the road with tractor trailers still face.
The IIHS performed crash tests between underride guards and 2010 Chevy Malibus, the passenger vehicle with a five-star crash test rating from the NHTSA. Repeated tests showed that the underride guards failed to prevent vehicles from sliding under tractor trailers. The IIHS used the highest rated passenger vehicle to show that even in a car with top safety marks, a relatively low speed impact with a tractor trailer can still cause catastrophic damage. The rear guard prevented underride of the vehicle only tests in which the Malibu hit dead center in the back of the tractor trailer; nearly 80 percent of the tests resulted in severe damage, even decapitating the crash test dummies.
Under current certification, underride guards are not tested with welding, bolts, and trailers, showing that even if the guard meets NHTSA guidelines, there is no uniform mandate which insures the underride guard is installed and properly secured to the tractor trailer.