A rigid ram, joined to a structural element of a mechanism which receives and transmits the impact of a vehicle, is displaced along a deformable metallic profile of open section in the form of “U”, “C”, “Σ” or “Ω”, the ram having a partial or total intersection with the transversal section of the deformable metallic profile, and producing thereto plastic deformations which are propagated along the deformable metallic profile as the ram is longitudinally displaced along said deformable metallic profile.