An impact head for a road guardrail (1) including cable routing means (2) adapted to form a convoluted path through which a cable can be threaded. The convoluted path that the cables (15) must follow through the impact head of the invention restricts movement of the cable (15) through the head, thereby providing sufficient friction to slow down the movement of the impact head during a vehicle impact. Also disclosed is a method of constructing a guardrail including the steps of slidably interconnecting a plurality of rails and attaching them to posts, positioning the impact head as claimed at one end of the slidably interconnected rails, threading at least one cable through the impact head and anchoring the cable to the ground.