Abstract: A plastic standoff block having a substantially H-shape cross-section that is used for mounting a guardrail barrier to a vertical support post. The standoff block has horizontally disposed dual compression-resisting zones comprised of two back-to-back cavitated block modules joined by a common vertically disposed support base. In accordance with one embodiment, the support base extends outwardly from the modules to form a peripheral shoulder for a plurality of elongated horizontally disposed struts that buttress the modules against transverse buckling.