Single file traffic' Supplementary Plate - Post Mount Diagram 518 R2/RA2This Single file traffic sign should only be used with triangular warning signs 516, 517, or 520. Used to inform drivers that the road narrows to the point where only one vehicle may fit in either direction. The sign can be customised by choosing an X height for the text that suits the road the sign is to be deployed on. It can also have an arrow, a distance such as 100 yds or a combination of both . To purchase a variant of this supplementary sign