A breakaway support post for a guardrail includes an upper post member and a lower post member. The upper and lower post members are overlapping and configured such that the upper and lower post members are non-rotatable relative to each other about an axis extending in an axial impact direction. In one embodiment, a tensile fastener extends in the axial impact direction and connects overlapping portions of the upper and lower post members. In another embodiment, a shear fastener extends transversely to the axial impact direction and is the only connection between the upper and lower post members. In another aspect, a guardrail assembly includes first and second rail sections, with a deforming member deforming the first rail section as it moves relative to the second rail section. Methods of using and assembling a guardrail assembly are also provided.