A signalling system for vehicles, comprising: one or more optical sensors (11a-f) for removable attachment to a respective vehicle light, which optical sensors (11a-f) are configured to generate a signal responsive to detecting emitted light from the vehicle light to which it is attached, the system being arranged responsive to receiving a signal from the one or more optical sensors (11a-f) to output a signal to an external signalling unit to cause a corresponding light on said external signalling unit to operate; wherein plural optical sensors (11a-f) are provided, each connected to a control unit by means of respective lengthwise conductors, the control unit comprising circuitry for relaying the sensor signals to corresponding lights of an external signalling unit ; a trailer board for removable mounting to the rear of a vehicle , the trailer board carrying on a rear surface a plurality of signalling lights , and wherein the control unit is configured to relay sensor signals received from the sensors to a corresponding signalling light on the trailer board ; characterised in that the system comprises one or more proximity sensors adapted for retrofitting to the trailer board , and means for relaying signals produced by the proximity sensors to an audible or visual unit which may be carried within the vehicle , and in that each optical sensor (11a-f) comprises a light detector surrounded by a wall having an end surface adapted for removable attachment to a vehicle light, and wherein the surrounding wall is formed of a deformable material with a substantially planar end surface which, in use, is removably attached to a vehicle light by suction. one or more optical sensors (11a-f) for removable attachment to a respective vehicle light, which optical sensors (11a-f) are configured to generate a signal responsive to detecting emitted light from the vehicle light to which it is attached, the system being arranged responsive to receiving a signal from the one or more optical sensors (11a-f) to output a signal to an external signalling unit to cause a corresponding light on said external signalling unit to operate; wherein plural optical sensors (11a-f) are provided, each connected to a control unit by means of respective lengthwise conductors, the control unit comprising circuitry for relaying the sensor signals to corresponding lights of an external signalling unit ; a trailer board for removable mounting to the rear of a vehicle , the trailer board carrying on a rear surface a plurality of signalling lights , and wherein the control unit is configured to relay sensor signals received from the sensors to a corresponding signalling light on the trailer board ; characterised in that the system comprises one or more proximity sensors adapted for retrofitting to the trailer board , and means for relaying signals produced by the proximity sensors to an audible or visual unit which may be carried within the vehicle , and in that each optical sensor (11a-f) comprises a light detector surrounded by a wall having an end surface adapted for removable attachment to a vehicle light, and wherein the surrounding wall is formed of a deformable material with a substantially planar end surface which, in use, is removably attached to a vehicle light by suction. <EOS>