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.