7.10 Care should be taken to ensure that vehicles of the maximum length permitted by the Construction and Use Regulations will be able to pass safely under the bridge. This is particularly important where the road dips or hogs sharply or is on a curving alignment under the bridge. Changes in gradient might affect the headroom, e.g. the effective clearance will be reduced for a long wheel base vehicle spanning a dip. Where the road passing under a structure is on a sag curve, the headroom should be measured along the carriageway over a 25 m chord.
7.11 The imperial figure shown on signs to indicate the available headroom should be at least 3 inches less than the measured height to allow a safety margin. If the resulting figure is not a multiple of 3 inches, it should be rounded down to the nearest lower multiple of 3 inches.
Example 1: measured height 15'-2 subtract 3" to create a safety margin 14'-11 round down to nearest multiple of 3" and sign as 14'-9".
Example 2: measured height 14'-6, subtract 3" to create a safety margin 14'-3 and sign as 14'-3" (rounding down not required as already expressed to the nearest 3")
Thus, the maximum headroom that will normally appear on a sign is 16-0".
METRIC HEIGHTS
7.12 To obtain the metric figure shown on signs, the bridge height should be measured to two decimal places, rounding down to the nearest 0.01 m. The following method is then used to calculate the appropriate signed height:
(i) if the second decimal digit is 8 or 9, delete it and sign the bridge with the remaining whole number and the first decimal digit,
(ii) if the second decimal digit is 7 or less, delete it and reduce the first decimal digit by 1. Sign the bridge with the remaining whole number and first decimal digit, as reduced,
e.g. measured height 4.19 m, sign as 4.1 m,
measured height 4.17 m, sign as 4.0 m.
The height shown on the sign must be to only one decimal place. The maximum headroom that will normally appear on a sign is 4.9 m.
DIVERSION SIGNING
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818.3 Mandatory height limit at low bridge ahead
"Low bridge" may be varied to "Height limit" or omitted (with the remaining legend placed below the roundel). An arrow may be added or substituted for the distance (with "ahead" omitted in each case).
The distance may be varied (see Appendix C).
Details of the permitted variants are shown in the table below the diagram in the Regulations
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818.4 Mandatory height limit at low bridge ahead and indication of alternative route
An arrow may be added above the roundel. "Low bridge" may be varied to "Height limit" or omitted, with the location varied as appropriate or changed to a distance (see Appendix C). "ahead" may be added after the distance when no arrow is shown. The alternative route may be varied as appropriate and may include a reference to a motorway junction.
A diversion route symbol may be added. Details of the permitted variants are shown in the table below the diagram in the Regulations
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818.5 Available headroom at low bridge ahead and indication of alternative route (Advance sign for an arch bridge)
An arrow may be added above the diagram 530A symbol. The location may be varied as appropriate or changed to a distance (see Appendix C). "ahead" may be added after the distance when no arrow is shown. The diagram 530A symbol may be varied to diagram 530. The alternative route may be varied as appropriate and may include a reference to a motorway junction. A diversion route symbol may be added. Details of the permitted variants are shown in the table below the diagram in the Regulations
7.13 In order to reduce the risk of the driver of an overheight vehicle being confronted with an impassable bridge, it is important that properly planned diversion route signing is provided, certainly where a structure has a history of repeated strikes. In such a case, it is not sufficient to rely on warning signs alone. Diversion route signing may be incorporated into standard junction advance direction signs and flag-type signs, or separately mounted signs to diagram 818.4 or 818.5 may be provided (see paras 7.15 and 7.16), or both types of signing may be used together. The sign to diagram 818.3 is used to give advance warning of a non-arch bridge, but without indicating an alternative route. This sign might be appropriate in advance of any diversion route signing or on the immediate approach to a bridge (see para 7.18).
The Regulations prescribe both map-type and stack-type advance direction signs for indicating alternative routes avoiding a low bridge (diagrams 2002 and 2003 on primary routes; 2107 and 2108 using Schedule 16 item 32 variants on non-primary routes). Prohibitory roundels are used where the restriction is mandatory (see para 7.17), otherwise the triangular warning signs shown in diagram 2002, or the dual height warning sign shown in diagram 530A, are used. The flag-type signs at the junction should also incorporate a warning sign, as in diagram 2027, or prohibitory sign as appropriate. Directional signs with a prohibitory roundel must also include a distance plate unless the restriction commences at the junction. For design details, see Chapter 7 paras 4.16 to 4.27, 5.60 to 5.81, 6.12 to 6.15, 7.6 to 7.10 and 11.16.
7.15 An alternative route avoiding a mandatory height restriction may be signed using a sign to diagram 818.4, at the x-height indicated in Appendix A. The sign should be placed where the driver can see it in good time to make the necessary manoeuvre, before the advance direction sign for the next junction at which the routes advised are signed. It is essential that full route continuity signing is provided to the point where the original route is resumed.
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