of street lighting. However, certain exceptions are permitted in paragraph (4) in column (3) of Item 1. These include road works signs on a road where the permanent speed limit is 40 mph or less and signs marked with an asterisk in column (2), provided they are placed on a single carriageway road which is not a principal or trunk road and has a speed limit of 30 mph or less. Signs that qualify for this exemption from direct lighting need only be reflectorised, although they may be directly lit. It is recommended that high-performance microprismatic retroreflective materials are used where an asterisked sign is reflectorised instead of being directly lit. Materials that offer performance little better than conventional beaded sheetings are unlikely to be adequate.
1.38 All warning signs, including those used at street works and road works must therefore be either reflectorised or directly lit, except for the overhead black and yellow hazard markings and white chord markings used on railway bridges and similar structures, where this is optional. It is recommended that signs that are directly lit are also reflectorised in order to maintain some degree of illumination in the event of failure of the lighting. Where a sign is reflectorised, all parts of the sign face not coloured black must be reflectorised (regulation 19). Partial reflectorisation is unlawful, as is partial lighting.
MAINTENANCE
1.39 Over a period of years, signs gradually become faded and their retroreflective properties diminish. This will reduce both conspicuity and legibility, by day and by night. Excessively discoloured or faded signs (e.g. white backgrounds which have become grey or brown, or red borders faded to pink) and signs where the legend or graphic is peeling cannot be fully effective and need to be replaced. Guidance can be found in TD 25, in Volume 8 of the Design Manual for Roads and Bridges (see para 1.3).
1.40 Signs should be cleaned at intervals appropriate to the site conditions. Signs located where they are subject to heavy soiling from passing traffic, or algae growth (a common problem with signs beneath tree canopies) will need more frequent cleaning. Neglect reduces the external contrast between the sign and its surroundings, making it less likely to be noticed by drivers. It also reduces the internal contrast between legend and sign background, making the sign more difficult to read. Moreover, it seriously reduces light transmission through the retroreflective medium. Dirty signs will be far less effective at night. Older drivers are particularly disadvantaged; the ageing process of the eye means that progressively more light is required to maintain the same legibility performance. Dimmer signs take longer to recognise and to read, reducing the time available for drivers to take appropriate action.
1.41 The importance of maintaining the necessary clear visibility distance is emphasised in paras 1.22 and 1.23. Regular inspection, particularly in summer when the rapid growth of foliage and other vegetation is most likely to cause obscuration, will ensure early detection of any problems.
1.42 A reference number may be placed on the back of a sign in a contrasting colour in characters not exceeding 50mm in height (direction 42(3)). It is unlawful, as well as distracting and unsightly, to place reference numbers on the sign face or on the front of a backing board.
TEMPORARY SIGNS
1.43 Certain signs are intended to be displayed only during transient conditions. These include diagrams 551.1 (Migratory toad crossing), 554 (Flood or No smoking), 554.2 (Ice), 556 (Uneven road), 557 (Slippery road) and some applications of 562 plated with 563 (Other danger). They should be removed when the danger has passed. Many temporary signs are specifically required by the Directions to be removed when the problem they warn of no longer applies, including diagram 574 "ANIMAL DISEASE" and variants of diagram 7010.1 "ROAD AHEAD CLOSED" etc. Diagram 7014 warns of a permanent change in road layout or new traffic signals etc, this sign must be removed within three months of completion of the works (direction 37). Diagram 7014.1 warns of a reduction in headroom at a bridge. Where this reduction is temporary, the sign may be retained only for the duration of the reduction. When a permanent reduction occurs, resulting in headroom of less that 16′– 6″ (5.03 m), the sign may be retained for a period of six months (see para 7.9). Authorities that fail to comply with their statutory responsibilities to remove redundant signs not only devalue the signs but contribute to sign clutter.
1.44 Regulation 53(1)(e) permits a traffic authority to provide a temporary sign to warn of a temporary hazard caused by works being executed on a road,
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