The present invention relates to a structure for supporting an information or surveillance device above a road. The structure includes: at least one post extending upwards from the ground to an elevated position, at least one overhead beam which is supported by said post, the overhead beam extending over a horizontal distance above the road, at least one information or surveillance device being connected to said overhead beam and being positioned above the road, a catwalk extending along the overhead beam, wherein said catwalk comprises a guard rail assembly which is movable between: an operating position in which the guard rail assembly extends upwardly from a floor of the catwalk and protects personnel from falling from the catwalk, and a retracted, non-operating position in which the guard rail assembly is concealed behind the overhead beam when viewed from a direction of approaching traffic on the road.