Probably the most abused rule regarding lights & reflectors on trailers (including a-frame towed cars) is that regarding “Retro-Reflective Rear Triangles”.
Size is everything!
Specifically, each triangle should be red, be equilateral, 160mm along each edge (making it 140mm tall), and it must meet the photometric and colorimetric requirements laid down in EC regs or UNECE regs. It must carry either EC approval marks or UNECE approval marks including IIIA (class). https://eur-lex.europa.eu/legal-content/EN/ALL/?uri=CELEX:32008L0089
With positioning, the triangles must be fixed vertically with apex to the top and the apex needs to be no more than 400mm from the outside edge of the trailer body. A matching pair is required, fitted between 350mm and 1500mm from the ground. They must not be fitted to a moveable panel (which includes on to, or inside, a boot lid of a towed car).
The above rules most likely render the majority of marker boards supplied by a-frame suppliers, effectively illegal.
OTHER LIGHTS.
Category O1 (unbraked) and O2 (braked) trailers must replicate the lights of the towing vehicle, including (if 1600mm wide or over) white front position lamps. They must also carry the registration mark of the towing vehicle.
From Oct 2012, all braked trailers are required to be fitted with (at least one) operational reversing light(s).
There is NO requirement for an a-frame towed car to carry any additional signage but, it’s not illegal.
Additional sidelights or reflectors are required on trailers over 5mt long. (Boat trailers are exempt from this and from the requirement for front position lamps).
