Previous crash tests showed common guard failure, allowing vehicles to slide beneath the trailer and crush the cabin.
The Insurance Institute for Highway Safety has praised five semitrailer manufacturers for going beyond federal safety regulations to prevent deadly underride crashes.
The IIHS began performing underride crash tests in 2011. Six years of lab impacts and analysis uncovered apparent deficiencies in trailer design and government regulations.
"Our research told us that too many people die in crashes with large trucks because underride guards are too weak," said IIHS research chief David Zuby. "So we designed crash tests to replicate scenarios where guards have failed in real-world crashes."
Semitrailers from eight different companies were each put through three tests. All easily passed the full-width test and 50-percent overlap. Only one trailer, built by Manac, held up against a challenging 30-percent overlap.
Great Dane, Stoughton, Vanguard and Wabash all reworked their designs and asked for a second test, all of which passed the 30-percent overlap.
"IIHS isn't a regulatory agency, and other than safety, there was no incentive for semitrailer manufacturers to make improvements," Zuby added. "When we started testing, we weren't sure how they would respond. These companies deserve a lot of recognition for their commitment to addressing the problem of underride crashes."
The IIHS says 427 fatalities out of 2,646 passenger vehicle occupants killed in large truck crashes in 2015 died when the front of their vehicles struck the back of the truck, an increase of 39 percent from 2011.