discretion. The modern true yellow materials are less garish than the older yellow—green type and are much to be preferred.
14.53 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 circular 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. Detailed guidance on the correct design and use of backing boards can be found in Chapter 7 (paras 14.19 to 14.24).
ILLUMINATION OF SIGNS
14.54 Traffic signs must be illuminated in accordance with regulation 18. Specific requirements for speed limit signs are set out in Schedule 17, items 10 and 11. Item 10 requires terminal signs to diagrams 670, 671, 672 and 673 sited on trunk and principal roads (other ”A” roads) to be illuminated by internal or external lighting throughout the hours of darkness when they are located within 50 m of a street lamp (which does not have to form part of a system of street lighting). It is insufficient to place a sign on a lighting column; a separate means of illuminating the sign face directly must be provided. However, during any time overnight that the street lamp is switched off, the sign need only be reflectorised (i.e. the sign lighting may be turned off at the same time as the street lamp). Even where the street lamp is illuminated throughout the hours of darkness, it is recommended that the sign is reflectorised in addition to being directly lit, as a precaution against a power failure.
14.55 Other terminal signs to diagrams 670, 671, 672 and 673, i.e. those on unlit roads and those on lit roads that are not trunk or principal roads, must be either directly lit or reflectorised (Schedule 17, item 11). Again, it is recommended that where a sign is directly lit it is also reflectorised. Where a road has a system of street lighting, but is not a trunk or principal road, it is recommended that speed limit terminal signs are directly lit. Although this is not a specific requirement of the Regulations, it would minimise the risk of drivers failing to see them. Repeater sign illumination is also specified in Schedule 17, item 11; signs may be either directly lit or reflectorised. It is unusual for such signs to be directly lit, although this is permitted.
14.56 20 mph zone signs to diagrams 674 and 675 may be either directly lit or reflectorised. It is recommended that where the signs are directly lit, they are also reflectorised.
14.57 Terminal speed limit signs erected in pairs must each be illuminated by the same means (i.e. internal illumination, external illumination or reflectorisation). This applies to terminal signs on each side of the road and to those erected at the same junction for the same purpose.
14.58 Where a sign is reflectorised, reflecting material must be applied to the whole sign face except those parts which are coloured black (regulation 19). The optional reflectorisation of yellow backing boards is dealt with in para 14.52.
SIGN DESIGN
14.59 Speed limit signs must conform in appearance to the diagrams prescribed by the Regulations. The numerals in diagram 670 must be in Transport heavy alphabet (Schedule 13 Part II) and be of the correct size, as specified on the working drawing P670 (see para 1.9), and be positioned within the roundel as shown on that drawing.
SIGN MOUNTING
14.60 Research has shown that the greater the number of signs that drivers are presented with simultaneously, the greater the difficulty they are likely to have in assimilating all the information. The problem of dealing with information overload increases with age, so that older drivers suffer disproportionately. Terminal speed limit signs should not therefore normally be co—located with other signs, except for simple boundary signs to diagram 2402.1 (see figure 14-22). At junctions where signs are needed to indicate the start or end of zonal restrictions or a requirement to stop or give way, these may be co—located with terminal speed limit signs if separate mounting would cause one sign to obstruct the driver's view of another. In such cases signs should be placed in the following order from top to bottom:
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