A barrier (1) installed on a generally upright structure, the structure being generally elongate and protruding from a surface to rise to a first height above the surface, wherein: the barrier has a front surface (10), which lies on a first side of the upright structure, and a rear surface (13), which lies either on the same side, or on the opposite side, of the upright structure, the front and rear surfaces being connected to each other; the barrier rises to a second height above the surface, the second height being higher than the first height; and neither of the front and rear surfaces of the barrier significantly protrudes in a direction which is generally perpendicular to the plane of the upright structure than the other of the front and rear surfaces of the barrier.
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