Abstract: The metallic guardrail barrier comprises a sustaining post and a spacing member extending parallel to the ground, secured at one side to the sustaining post and carrying at the other side of the rail. The spacing member includes a window presenting a quadrilateral shape with four sides defining as many vertexes. The spacing member is made of metallic material which is plasticizable under impelling loads, and one or more holes are provided on the spacing member in proximity of the vertexes of the quadrilateral window. Upon impact by a colliding vehicle, the spacing member is deformed in a controlled manner due to the fact that the quadrilateral window tends to collapse as an articulated quadrilateral.