VEHICLE-ACTIVATED SIGNS
1.12 Regulation 58(7) permits certain warning signs (diagrams 504.1, 505.1, 506.1, 507.1, 510, 512, 512.1, 512.2 and 513) when displayed by means of light-emitting characters or symbols also to display below the sign, and at the same time, the legend "SLOW DOWN" in characters not less than one quarter of the height of the triangle. The signs will be triggered by vehicles exceeding a pre-determined safe speed on the approach to a junction or bend. They should be used only to supplement fixed signing, and not as a substitute for it. Vehicle-activated signs should not be considered until the fixed signing and road markings have been checked to ensure that they comply fully with the guidance in this chapter and in Chapter 5 in terms of correct size, siting, visibility and condition.
SIGN SIZES
1.13 Warning signs are normally prescribed in five sizes. The normal minimum size is indicated in the diagrams, with alternative sizes in brackets. All sizes are in millimetres unless stated otherwise. Signs need to be of a size appropriate to the prevailing traffic speed on the road on which they are used. On roads with a 30 mph speed limit, the smallest prescribed size of warning triangle (normally 600 mm) is usually adequate. On roads where speeds are higher, signs need to be larger. This enables them to be detected at a greater distance and ensures that drivers have sufficient time to recognise and assimilate the warning and take any necessary action before the hazard is met. The largest signs are for use on motorways or high-speed roads. Warning signs which are not appropriate for such roads are not generally prescribed in the largest (1500 mm) size. Appendix A details the appropriate size of sign for various speed ranges, based on the 85th percentile approach speed.
1.14 Where special amenity considerations apply, or there are physical constraints on the width of sign that can be accommodated, the next smaller size can be substituted. It should however be borne in mind that smaller signs are likely to be seen later, and do not become legible until drivers are closer to them, giving less time to react.
1.15 If the accident record suggests that drivers are failing to notice the warning, or seeing it too late to take the necessary action, the next larger size can be used. Conspicuity can also be increased by the use of yellow backing boards (see paras 1.32 to 1.34).
1.16 Many warning signs are accompanied by supplementary plates. Appendix B recommends appropriate x-heights to match the size of the plates to the size of the triangle they are used with, and to ensure adequate legibility. There are restrictions on which plates may be used with individual signs; the plates prescribed for use with each sign are indicated below each diagram illustrated in this chapter. Detailed drawings showing the correct layouts for all permitted variants have been produced by the Department for Transport (see para 1.8).
1.17 A special sign (diagram 7014) is prescribed to warn drivers of a permanent change in the road layout ahead (see also para 1.11). Several variants are prescribed, e.g. "GAP CLOSED AHEAD", "NEW TRAFFIC SIGNALS AHEAD" and "NEW ZEBRA CROSSING AHEAD". The x-height of the sign may vary between 50 and 200 mm (i.e. the capital letter height varies between 70 and 280 mm). The appropriate x-height at a specific site will depend upon the speed of traffic, with the 50 mm x-height suitable for speeds up to 30 mph and 150 or 200 mm for 70 mph. Intermediate sizes should be used for speeds between these extremes.
1.18 The Traffic Signs (Amendment) (No.2) Regulations and General Directions 2011 prescribe a new sign (diagram 7014.1) to be used in the event of either a temporary or permanent reduction in headroom at a bridge. This sign previously required authorisation. The x-height may be between 50 mm and 200 mm, with the same selection criteria as for diagram 7014 (see para 1.17).
1.19 The minimum x-height of the "ANIMAL DISEASE" sign, diagram 574, is determined in a similar manner to diagram 7014 as described in para 1.17, except that the minimum size is 40 mm. This size may be used on narrow rural roads where speeds are low and space is restricted.
SITING
1.20 In general, the greater the speed of approach, the further in advance of the hazard the sign needs to be placed. This is to ensure that drivers have the necessary time to respond to the warning. Appendix A sets out recommendations for the distance from the hazard at which a sign should be sited. If it is impracticable to place a sign within about 10% of the recommended distance, it should be sited further upstream of the hazard at the
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