Underride guards are steel bars that hang from the backs of semitrailers to keep smaller vehicles from sliding underneath in a crash. When they don't work, the consequences can be deadly.

A new web video shows how the Insurance Institute for Highway Safety (IIHS) put underride guards to the test by crashing a midsize car into semitrailers from the eight biggest manufacturers.

"Inside IIHS: Understanding Underride" explains that IIHS embarked on this program after researchers found that underride crashes continued to kill people in passenger vehicles despite big improvements in crash protection. That's because a passenger vehicle's structure and airbags can't do their job when underride occurs.

The video shows how the tests were conducted and explains the results, which demonstrated that most trailers still need better guards.

The video is available on the IIHS YouTube channel and is part of the "Inside IIHS" series. Previous installments have focused on other IIHS tests, crash test dummies, the propulsion system used to power crash tests, and IIHS’s booster seat rating program.