>>1590558*
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/771873/traffic-signs-manual-chapter-1.pdf7.2. Backing boards
7.2.1.Where it is impossible to avoid a poor or distracting background, it can be partially screened using a suitable backing board as illustrated in Figure 7-1. Using a backing board can also result in a neater assembly than a collection of individual signs, but their use can sometimes result in over intrusive assemblies, so should be used sparingly. Advice on the use of backing boards can be found in Chapters 3, 4 and 7.
7.2.2. Yellow backing boards are intrusive; they should be used sparingly, and not as a matter of course. They can reduce the attention drivers give to other, more important, signs and over use could eventually devalue their attention-attracting benefits. Similarly grey backing boards can be intrusive especially if used as an inexpensive means to attach small signs to widely spaced supports.
Figure 7-1 Effective use of backing boards
