1.30 Generally no assembly should exceed a height of 4 m above ground level. All proposed assemblies should be critically examined to ensure that the intended warnings are clear. Account should always be taken of the potential environmental impact of tall and cluttered arrays of signs.
1.31 It should also be borne in mind that high-mounted signs may receive little light from vehicle 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 legible (see also para 1.36).
BACKING BOARDS
1.32 To improve conspicuity against a complex or dark background, a warning sign may be mounted on a grey or yellow backing board (direction 42). 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, but a grey board may be non-rectangular, e.g. to enable a triangular sign to be bracketed off a lighting column (see Chapter 1). 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 foliage 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.
1.33 A yellow backing board may be reflectorised to increase its conspicuity at night. This should not usually be necessary on unlit roads, although it might sometimes be helpful on lit roads, particularly where the sign itself is unlit. It 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 new true yellow materials are less garish than the original yellow-green type and are much to be preferred.
1.34 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 triangular 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 14.19 to 14.24).
ILLUMINATION
1.35 On unlit roads, reflectorisation generally produces an adequate level of sign luminance in the illumination from a vehicle’s headlamps. In areas of street lighting, however, much higher levels of luminance are required to ensure that signs are always adequately conspicuous. Reflectorised materials cannot guarantee luminance levels comparable to those provided by direct lighting. Modern microprismatic materials achieve high luminances for many drivers in defined situations, but not for all drivers in all circumstances.
1.36 Except in Northern Ireland, the current Regulations therefore require most regulatory and warning signs to be directly lit when sited within 50 metres of a street lamp which forms part of a system of street lighting. In most other circumstances, reflectorisation alone will be satisfactory. However, some signs are sited where they will not receive adequate illumination from headlamps, and it might then be prudent to provide direct lighting regardless of the regulatory requirements. Examples include signs mounted unusually high above the level of the carriageway, or on the off side of the road. Retroreflection is also less effective where the sign is presented at a large angle to the direction of oncoming traffic.
1.37 Illumination requirements are referenced as Item 5 in the tables associated with each diagram illustrated in the Regulations, and the detailed requirements set out in Schedule 17. Most warning signs are required to be illuminated in accordance with Schedule 17 Item 1 or Item 4. Item 4 allows a simple alternative between reflectorisation and direct lighting, wherever the sign is sited. Item 1 signs are required to be directly lit if they are sited within 50 metres of a street lamp forming part of a system
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