An underrun guard for a road transport vehicle is disclosed comprising a beam intended to be supported across the rear of the vehicle by a support mechanism which yields under a force which would otherwise be likely to cause damage to the vehicle. The support mechanism comprises at least one first for attachment to the beam and at least one second plate for attachment to the vehicle, and a clamping member for clamping the plates together while permitting relative sliding movement between said first and second plates when a substantial force is applied to the bean. A high friction element may be sandwiched between the first and second plates to increase the sliding friction generated on relative movement of the plates, thereby increasing the reaction force restaining such movement.
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