Ooooo, that's what that piece of metal is doing on truck trailers. I always assumed it was a step to access the trailer and clearly that's all it's really good for, anyways.
Fun fact: Jayne Mansfield, famous actress and one of the earliest Playboy Playmates, and her then boyfriend who was driving, were both decapitated and killed when the rear ended one of these trailers back in 1967, before they started putting these guards on them. The publicity from her death was a big contributor these things becoming standard.
Her daughter, Mariska Hargitay (the actress who plays Olivia Benson on Law and Order SVU) was three years old at the time, and was in the back seat when it happened. She only managed to survive because she was laying down in the back seat when they struck the trailer.
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