Government Press Release - Department of Transport - June 2004.
In order to assist motorcyclists in identifying potentially dangerous drivers, it is now compulsory for anyone with a lower than average driving ability, or who may exhibit poor eyesight and/or lack of forward/rearward observation and overtaking ability, to display a warning flag.
This flag comprises a red cross on a white background, attached to the top of at least one door of their vehicle.
For drivers of exceptionally low ability, who may not even realise how bad they are behind the wheel, additional flags are required.