The guards are a safety measure for passenger vehicles when they crash into the back of a tractor-trailer, but the Institute says new crash tests and analysis show the guards can fail in relatively low-speed crashes.
The consequences of a car sliding under a big rig trailer can be deadly and decapitation is a possibility.
Underride makes death or serious injury more likely since the upper part of the passenger vehicle's occupant compartment typically crushes under the trailer.
Institute President Adrian Lund says some test videos were difficult to watch, knowing what would happen in a real crash.
(The Associated Press contributed to this report.)
