nearest practicable point. It may be appropriate to supplement it with a distance plate to diagram 572 (see section 18). A sign should not normally be sited more than 10% closer than the recommended distance, as this would be unlikely to provide sufficient warning. Where this is unavoidable, a distance plate should always be used, indicating the distance to the hazard to the nearest 10 yards.
1.21 Warning signs should normally be placed on the left hand side of the road, unless stated otherwise in the text (e.g. hazard markers to diagrams 560 and 561). However, site conditions sometimes make this impracticable. A warning sign might be placed on the right hand side on a left hand bend if it would otherwise be hidden from view, or if there would be no room for it on the left. If a sign is placed on the right hand side of the road, care must be taken to ensure that a driver would not be misled at night or in fog as to which side to pass. It will sometimes be appropriate to duplicate warning signs by providing them on each side of the road, as is recommended at the end of a dual carriageway, or on the approach to a roundabout on a high-speed road.
1.22 It is essential that drivers have an unobstructed view of traffic signs. The distance which should be kept clear of obstructions to the sight line, whether caused by foliage, other signs or street furniture is known as the clear visibility distance. The higher the prevailing traffic speeds, the greater this distance needs to be. It is important therefore that sight lines are properly maintained so that the intended warning is not compromised. Care in siting can minimise future problems of obscuration. Sight lines should not cross private land as it will be difficult to control the growth of vegetation or the placement of other obstructions. It is equally important that warning signs should not be placed where they will obstruct the view of other signs. e.g. advance direction signs. Such problems might be avoided by siting the sign further from the hazard, or on the right hand side of the road.
1.23 Appendix A specifies minimum clear visibility distances. These should normally be measured from the centre of the most disadvantaged driving lane. It is important that the full recommended sight line to the whole of the sign face is preserved. Trimming of foliage only in the immediate vicinity of the sign may not be sufficient; sign visibility should always be checked from the appropriate viewing distance.
MOUNTING
1.24 The normal mounting height measured to the lower edge of a warning sign is between 900 mm and 1500 mm above the carriageway alongside. The greater height should be used where road spray is likely to soil the sign. Where signs are erected above footways, or in areas likely or intended to be used by pedestrians (e.g. pedestrian refuges), a minimum headroom of 2300 mm is recommended, with 2100 mm as an absolute minimum. A clearance of 2300 mm should be maintained over a cycle track or shared cycleway/footway. When supplementary plates are used, the height should be measured to the bottom of the plate.
1.25 Plates should be separated from the sign or another plate by a vertical space not exceeding the x-height of the lettering.
1.26 Except where they support a luminaire, posts should never project above the top of the sign. This practice is unsightly, and needlessly increases visual intrusion and clutter.
MOUNTING MORE THAN ONE SIGN ON A POST
1.27 Research has shown that the greater the number of signs which drivers are presented with simultaneously, the greater the difficulty they are likely to have in assimilating the information. This problem in dealing with information overload increases with age, so that older drivers suffer disproportionately. Generally, therefore, not more than two signs should be mounted on one post. When a sign is accompanied by a supplementary plate, the combination of sign and plate should be regarded as one sign for this purpose. Exceptionally, three signs may be mounted on one post provided none requires a supplementary plate.
1.28 Warning signs should not be mounted on the same post as a STOP or GIVE WAY or terminal speed limit sign, nor mounted on a traffic signal post. When mounted with other types of sign, the triangular warning signs should always be mounted at the top.
1.29 Where two or more warning signs are erected together, the sign relating to the hazard first encountered should be placed uppermost. When a new sign is added to an existing post, it is important to ensure that the correct order is maintained, if necessary adjusting the position of the existing signs.
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