The Insurance Institute for Highway Safety has awarded its top underride guard rating to eight major trailer manufacturers, Great Dane, Hyundai Translead, Manac, Stoughton Trailers, Strick Trailers, Utility Trailer Manufacturing, Vanguard National Trailer, and Wabash National, representing 80% of trailers on the road.

To qualify for the IIHS ToughGuard award, trailers must have rear guards that prevent underride of a midsize car in three test modes: full-width, 50% overlap, and 30% overlap. In each test, a midsize car travels at 35 m.p.h. toward the back of a parked semitrailer.

In the full-width test, which all trailers were able to pass in the initial round of testing, the car strikes the center of the guard head-on. In the 50% overlap, which all but one trailer passed initially, half of the car's front end strikes the guard. In the 30% overlap, the toughest evaluation, 30% of the car's front strikes the corner of the trailer.

Trailers on U.S. roads are required to have underride guards that meet federal safety standards and also meet a more stringent Canadian regulation. Despite this, IIHS research showed that even guards that met these requirements could buckle or break off in a crash.

At first, only one trailer underride guard was able to pass the 30% overlap test in testing. However, all other companies made updates to their guards and requested retests. By the time IIHS announced the ToughGuard award last year, five of the eight guards met the criteria. Today, all eight designs meet the criteria. Some of the manufacturers have made the improved guards standard on all new trailers, while in other cases they are optional.

In the full-width test, which all trailers were able to pass in the initial round of testing, the car strikes the center of the guard head-on. In the 50% overlap, which all but one trailer passed initially, half of the car's front end strikes the guard. In the 30% overlap, the toughest evaluation, 30 percent of the car's front strikes the corner of the trailer.

"We're pleased that all the major manufacturers responded positively to our underride tests," says David Zuby, IIHS chief research officer. "By improving their guards, these companies have demonstrated a commitment to the safety of passenger vehicle occupants who share the road with their trailers."