A system for illuminating a carriageway in a case of fog, comprising: a road safety barrier which is arranged along a carriageway; at least a fog density detecting device which is arranged along the road safety barrier; a plurality of light sources which are borne by the road safety barrier, which are distributed along the road safety barrier and which are orientated so as each to emit a light beam having a direction that is transversal to the extension axis of the road safety barrier; a control unit which is connected to the plurality of light sources and to the at least a fog density detecting device which is configured to activate the light sources of the plurality of light sources when the at least a fog density detecting device detects a fog density value of above a threshold value; the system being characterised in that: the plurality of light sources are orientated so as to illuminate the horizontal signage arranged on the carriageway; each light source of the plurality of light sources is designed to emit a light beam which is collimated that is, limited in terms of divergence; the light beam has a first angular opening on a vertical plane and a second angular opening on a horizontal plane; the vertical plane being the one passing through each light source of the plurality of light sources; the horizontal plane being the one passing through each light source of the plurality of light sources; the first angular opening is at most 30°; the second angular opening is at most 100°. a road safety barrier which is arranged along a carriageway; at least a fog density detecting device which is arranged along the road safety barrier; a plurality of light sources which are borne by the road safety barrier, which are distributed along the road safety barrier and which are orientated so as each to emit a light beam having a direction that is transversal to the extension axis of the road safety barrier; a control unit which is connected to the plurality of light sources and to the at least a fog density detecting device which is configured to activate the light sources of the plurality of light sources when the at least a fog density detecting device detects a fog density value of above a threshold value; the system being characterised in that: the plurality of light sources are orientated so as to illuminate the horizontal signage arranged on the carriageway; each light source of the plurality of light sources is designed to emit a light beam which is collimated that is, limited in terms of divergence; the light beam has a first angular opening on a vertical plane and a second angular opening on a horizontal plane; the vertical plane being the one passing through each light source of the plurality of light sources; the horizontal plane being the one passing through each light source of the plurality of light sources; the first angular opening is at most 30°; the second angular opening is at most 100°. the plurality of light sources are orientated so as to illuminate the horizontal signage arranged on the carriageway; each light source of the plurality of light sources is designed to emit a light beam which is collimated that is, limited in terms of divergence; the light beam has a first angular opening on a vertical plane and a second angular opening on a horizontal plane; the vertical plane being the one passing through each light source of the plurality of light sources; the horizontal plane being the one passing through each light source of the plurality of light sources; the first angular opening is at most 30°; the second angular opening is at most 100°.