In the wee hours of June 29, 1967, Jayne Mansfield was being driven to New Orleans in a 1966 Buick Electra. She was sitting up front with her driver and her lover, while her children (including actress-to-be Mariska Hargitay) slept in the back seat.
Around 2:30am, Mansfield's car slammed into the back of a big rig. The collision forced the Buick underneath the tractor-trailer and sheared off most of the roof. All three of the adults in the front seat were killed instantly, while the children in the back escaped largely unharmed.
After an extensive investigation, federal regulators introduced a new requirement for big rigs: the now-ubiquitous red-and-white striped underride guard, commonly known as the "Mansfield bar". It's designed to keep cars from sliding underneath tractor-trailers during front-end collisions and prevent deaths like those of Mansfield and her companions.
New tests conducted by the Insurance Institute for Highway Safety show that Mansfield bars work fairly well in cases where vehicles strike big rigs squarely on their back ends. However, the IIHS says that "in crashes involving only a small portion of the truck's rear, most trailers fail to prevent potentially deadly underride".
The IIHS has apparently known about this problem for some time: it petitioned the National Highway Traffic Safety Administration to toughen standards for underride guards back in 2011. NHTSA hasn't yet responded to the IIHS request, but trailer manufacturers have already begun installing sturdier bars on their own. That's because our neighbors to the north upped standards for Mansfield bars used in Canada in 2007.
As you'll see from the video embedded above, cars fare reasonably well when at least 50% of a vehicle's front end hits the underride guard. When that figure drops to 30%, however, the failure rate soars.
For more details on the types of trailers that performed well -- and not so well -- check out the IIHS press release
[h/t John Voelcker]