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 (F) having a direction that is transversal to the extension axis (Z) 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 (F) which is collimated that is, limited in terms of divergence; the light beam (F) has a first angular opening (P1) on a vertical plane and a second angular opening (P2) 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 (P1) is at most 30°; the second angular opening (P2) 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 (F) having a direction that is transversal to the extension axis (Z) 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 (F) which is collimated that is, limited in terms of divergence; the light beam (F) has a first angular opening (P1) on a vertical plane and a second angular opening (P2) 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 (P1) is at most 30°; the second angular opening (P2) 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 (F) which is collimated that is, limited in terms of divergence; the light beam (F) has a first angular opening (P1) on a vertical plane and a second angular opening (P2) 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 (P1) is at most 30°; the second angular opening (P2) is at most 100°. <EOS>