7.20 The conspicuity of the bridge can be significantly enhanced by use of the marking to diagram 530.2. Where the yellow parts of the marking are made of retroreflective material, they may also be fluorescent (see para 7.6). The first of the two alternative types, with a cut-out for the signs, should normally be used on the lowest part of the bridge elevation. If the construction of the bridge is such that signs cannot be located in this position, the second alternative should be used, with the signs mounted above.
ARCH BRIDGES
7.21 Mandatory signs are not used at arch bridges, as the main risk to these comes from vehicles which, although low enough to pass through the central part of the arch, might strike the curved shoulder of the structure. See Chapter 5 (paras 22.5 to 22.10) for guidance on the use of road markings at arch bridges.
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531.1 Available headroom at arch bridge ahead
May be used with diagram 537.2, 572 or 573.
The height may be varied (see Appendix C).
The metric sign may be omitted but must not be used alone
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531.2 High vehicles to use middle of the road at arch bridge ahead
May be used only in combination with diagram 537. 7. An arrow may be added to the sign pointing either to the left or to the right
7.22 Arch bridges should be signed using diagram 531.1 and 531.2 (but see para 7.24). Signs to diagram 531.1 are always one size larger than normally required for a warning sign, with a minimum height of 750mm. This is because a smaller legend than usual has to be used in order to accommodate the arch symbol. These signs are intended for use in advance of the bridge, and should normally be used on both approaches at the standard siting distances (seeAppendix A). They should not be used on the bridge itself nor should they be incorporated in directional signs or the sign to diagram 818.5 where diagram 530 or 530A should be used. The signing on the approach to an arch bridge is shown in figure 73
23 The chord marking to diagram 532.2 or 532.2A, or if appropriate the double chord to diagram 532.3 or 532.3A, should be used on the bridge structure. It should always be reflectorised, to make it easier to see at night. If the double chord is used, the triangular signs at the side of the arch should be the appropriate size for the road (see Appendix A), and those at the top of the arch one size larger. The height indicated on the warning sign to diagram 531.1 will be that for the chord above the centre of the road.
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