Oncoming vehicles in middle of road' Supplementary Plate - Post Mount Dia. 575 R2/RA2This sign supplementary plate is used where the layout of the road may force larger vehicles into the middle of the road. An example of this may be where a road has many tight turns that would force arctics to swing wide to clear verges. The sign can be supplied with a number of different text sizes (known as X heights) to suit the road it is being installed on. You would normally see this sign face alongside sign diagrams 512, 512. 1, 512. 2, 512. 3