A breakaway coupling for joining a generally vertical pole structure to a supporting foundation is provided. The breakaway coupling is provided with a multi-sided, elongated body, which may be square-shaped, hexagonal, or generally polygonal in shape. The multi-sided, elongated body is provided with an upper end and a lower end, an outer surface, and is machined to have an internally threaded bore that extends between the upper end and the lower end of the multi-sided, elongated body. The outer surface of the multi-sided, elongated body of the breakaway coupling is provided with at least one horizontally disposed, circumferential groove extending partially into the multi-sided, elongated body toward the internally threaded bore and also extending to a critical notch depth.