Strong underride guards on the rear of tractor-trailers have helped prevent underride in previous crash tests by the Insurance Institute for Highway Safety, and the institute’s new tests also show how a well-built guard can prevent a passenger vehicle from sliding beneath the side of a semitrailer.

IIHS tests conducted this spring marked the first time the institute has evaluated a side underride guard. IIHS ran two 35-mph crash tests: one with an AngelWing side underride protection device from Airflow Deflector Inc. and a second test with a fiberglass side skirt intended to improve aerodynamics, not to prevent underride. The results were dramatically different, IIHS said.

In both tests, a mid-size car struck the center of a 53-foot-long dry van trailer. In the AngelWing test, the underride guard bent but didn't allow the car to go underneath the trailer. As a result, the car's air bags and safety belt could properly restrain the test dummy in the driver seat. In the second test with no underride guard for protection, the car ran into the trailer and kept going. The impact sheared off part of the roof, and the sedan became wedged beneath the trailer. In a real-world crash like this, any occupants in the car would likely sustain fatal injuries, according to IIHS.

The institute has been testing rear underride guards for several years. In March, IIHS announced the first winners of the new ToughGuard award recognizing rear underride guards designed to prevent underride in a range of crashes into the backs of tractor-trailers. So far, five North American semitrailer manufacturers have qualified for the award.

The latest tests illustrate the importance of countermeasures to prevent side underride crashes, too, IIHS said. In 2015, 301 of the 1,542 passenger vehicle occupants killed in two-vehicle crashes with a tractor-trailer died when their vehicles struck the side of a tractor-trailer. This compares with the 292 people who died when their passenger vehicles struck the rear of a tractor-trailer. Because of gaps in federal crash data, IIHS researchers can’t determine exactly how many of these crashes involved underride, but they estimate that underride occurs in about half of fatal crashes between large trucks and passenger vehicles.

A 2012 IIHS study found that strong side underride guards have the potential to reduce injury risk in about three-fourths of large truck side crashes resulting in a fatality or serious injury to a passenger vehicle occupant. This proportion increased to almost 90% when restricted to crashes with semitrailers.

Federal law requires large trucks to have rear underride guards but not side underride guards. At least three U.S. cities — Boston, New York and Seattle — mandate side guards on city-owned or contracted trucks as part of Vision Zero initiatives to eliminate crash deaths and injuries, particularly among pedestrians and bicyclists.

“Our tests and research show that side underride guards have the potential to save lives,” said David Zuby, IIHS executive vice president and chief research officer. “We think a mandate for side underride guards on large trucks has merit, especially as crash deaths continue to rise on our roads.”

The wheels on a tractor and trailer offer some underride protection if a passenger vehicle were to strike them. With no side underride guard, only 28% of a 53-foot trailer’s length would be protected from underride. With the AngelWing side underride guard in place, 62% of the trailer’s length would be protected. Side underride guards can be retrofitted to existing semitrailers.

“With the rise of injuries and fatalities due to truck crashes, along with the need for greener technologies, Airflow Deflector's AngelWing side underride guards are an integral part of a long-term eco-friendly and cost-effective safety strategy that will benefit the public at large,” said Robert Martineau, president of Airflow Deflector.