1.27 All proposed assemblies should be critically examined to ensure that the intended messages are clear. Ambiguity may result in difficulty enforcing a traffic regulation order. Where a supplementary plate with the legend "End" (diagram 645) is used to indicate the termination of a prohibition or restriction, particular care is necessary to ensure that it is clear which sign it applies to when there is more than one.
1.28 Where a speed limit sign is erected on the same post as a clearway sign accompanied by an "End" plate, the plate should be butted directly up to the lower edge of the clearway sign. The speed limit sign should be mounted at the top of the assembly with a space equal to twice the width of the red border between the roundels to ensure there is no ambiguity.
1.29 Generally no assembly should exceed 4 m in height above ground level. This may be exceeded to improve visibility of the signs at particularly difficult sites, but account should always be taken of the potential environmental impact of tall and cluttered sign assemblies.
1.30 It should also be borne in mind that high-mounted signs may receive little light from car headlamps, particularly on dipped beam. Where such signs are not directly lit but rely on reflectorisation to be seen at night, they are likely to be less conspicuous and less legible.
BACKING BOARDS
1.31 To improve conspicuity against a complex or dark background, a regulatory sign may be mounted on a grey or yellow backing board (direction 42, as amended by the Traffic Signs (Amendment) General Directions 2004). A backing board can also make for a neater assembly, e.g. when a sign requires a supplementary plate, and also eliminates the risk of the plate becoming misaligned. A yellow backing board must be rectangular in shape (except when a speed limit terminal sign is mounted with a town or village boundary sign; see para 14.51), but a grey board may be non-rectangular, e.g. to enable a circular sign to be bracketed off a lamp column. A backing board must not itself be provided with a border, nor give the impression of being an additional border. Where it seems that a sign is not being noticed by drivers, it should be checked to ensure that it is well-sited, not obscured by vegetation or other obstructions, and is of the appropriate size and in good condition. Only then should the use of a yellow backing board be considered.
1.32 A yellow backing board may be reflectorised to increase its conspicuity at night, although this is not usually necessary for regulatory signs. In most cases these are lit when placed on lit roads, or are mounted parallel to the kerb, and on unlit roads reflectorisation of the sign is usually sufficient to ensure night-time conspicuity. A yellow backing board may also be fluorescent; this greatly increases conspicuity in dull weather and at dusk. Fluorescence can also be particularly effective in drawing attention to signs mounted in deep shadow, e.g. below overhanging trees. However, fluorescence is visually intrusive and should be used with discretion. The modern true yellow materials are less garish than the original yellow-green type and are much to be preferred.
1.33 There are potential disadvantages to the use of backing boards. The larger overall size of the assembly can sometimes obstruct sight lines. A backing board can deprive non-rectangular signs of a primary recognition aid: their distinctive silhouette. Yellow backing boards can be especially environmentally intrusive, and their over-use could eventually devalue their attention-attracting benefits. A less garish way of increasing a sign’s conspicuity is simply to provide a standard sign of larger size. Not only will this be more noticeable than a smaller sign, but it will also improve legibility and hence reading distance, which a yellow backing board cannot. Detailed guidance on the correct design and use of backing boards can be found in Chapter 7 (paras [[../../Chapter 7/2013/14#14.19|14.19]] to [[../..//Chapter 7/2013/14#14.24|14.24]]). Guidance on the use of backing boards for speed limit signs is given in this chapter (paras 14.50 to 14.53).
ILLUMINATION
1.34 For general guidance on sign illumination see paras 2.5 to 2.7. For illumination of speed limit signs see paras 14.54 to 14.58.
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