A guardrail support, attachment and positioning block used to space guardrails from support posts is disclosed. The positioning or spacer block is a generally rectangular block having corded-out cavities to reduce weight and tabs or projections for cooperatively engaging the sides and top of a support post as well as the edges of the guardrail during roadway safety barrier system installation. Webbing within one or more of the cavities may be used for additional structural support. To optimize the strength to weight characteristics of the spacer block, the spacer block may be formed by low-pressure injection molding. The spacer block may also be formed from virgin and/or recycled plastic material and/or include virgin or recycled rubber material, such as that obtained from the regrind of used tires, and/or another elastomeric materials from other sources.