A breakaway support post (18, 210) is provided for a highway guardrail system (10) to enhance the safety of a vehicle impacting with either the rail face of the associated guardrail (16) or one end of the guardrail facing oncoming traffic. The breakaway support post may have upper and lower portions (26, 28) with a releasable coupling assembly (29, 211) disposed therebetween to maintain the upper and lower portions generally aligned with each other prior to the impact of a vehicle with one end of the associated guardrail. The breakaway support post may also have releasable coupling assembly (301) disposed between the upper and lower portions and a cable (303). The releasable coupling assembly may allow the upper portion to separate from the lower portion.