Abstract: The device comprises at least one reflective flag carried by a lateral rod in its turn carried by a lower bracket. The bracket is constituted by a strip of stainless steel sheet or other weather-resistant material, shaped as an arcuate hairpin for gripping the upper part of the guard rail resiliently. In one embodiment, the flag is mounted on the rod so as to be pivotable and is biassed by a spring into a position in which it projects towards the carriageway.Preferably the flag includes a frame in which replaceable reflective sheets are inserted.