a lamp intended to indicate either priority vehicles or a vehicle or a group of vehicles whose presence on the road requires other road-users to take special precautions, in particular, convoys of vehicles, vehicles of exceptional size and road construction or maintenance vehicles or equipment
Special warning lamps:
(a) Displaying a blue light may be used only on priority vehicles when carrying out an urgent mission or when in other cases it is necessary to give warning to other road-users of the presence of the vehicle; ...
