Traffic Signs Manual/Chapter 4/2013/7

by Department of Transport (UK)

7BRIDGES AND OTHER STRUCTURES

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HUMP BRIDGES

528 Hump bridge ahead

May be used with diagram 511, 572, 573, 575 or 782

7.1 The sign to diagram 528 should be used where a hump bridge is so severe that unless drivers are forewarned they might lose control of their vehicles. If visibility is inadequate, double white lines or a hazard line should be laid in accordance with the normal criteria (see Chapter 5). If the hump bridge hides a further hazard, another sign indicating this should be provided on the same post (with the sign indicating the first hazard encountered placed uppermost). If there is a risk of long vehicles grounding on the bridge, diagram 782 (see para 21.13) should be used also, mounted below diagram 528. They should be sited, with a distance plate, on the approach to a junction at which vehicles can divert, and repeated at the standard siting distance from the structure (see Appendix A).

BRIDGE PARAPET OR OTHER OBSTRUCTION

528.1 End of bridge parapet or other obstruction

7.2 Signs to diagram 528.1 should be used where a bridge parapet, abutment or other obstruction is immediately adjacent to, or encroaches onto, the carriageway. This includes any parapet or abutment on the off side where it would be a hazard to drivers overtaking or passing a temporary obstruction on the near side. In addition to the risk to passing road traffic, damage to a parapet can also result in considerable danger to rail traffic. It is therefore particularly important that signs to diagram 528.1 on parapets or abutments of bridges are correctly installed and maintained to a high standard. The use of these signs helps to make the vulnerable parts of a structure more conspicuous. If the obstruction is accompanied by a narrowing of the carriageway, "road narrows" signs to diagram 516 or 517 (see section 4) and edge lines should also be used. The signs should be used as in the diagram, sloping downwards towards the carriageway. The Regulations permit the use of yellow material which is both retroreflective and fluorescent (see para 7.6).

OPENING BRIDGE

529 Opening bridge ahead

May be used with diagram 572, 573 or 773

7.3 The sign to diagram 529 should be used in advance of opening bridges (lifting or swing). If movement onto a bridge is controlled by wig-wag signals to diagram 3014, a plate to diagram 773 (see section 21) should be added.

TUNNELS

529.1 Tunnel ahead

May be used with diagram 572, 573, 575 or 773

Figure 7-1 Name and length of tunnel on a motorway

When indicating a tunnel on an all-purpose road, the sign has black characters on a white background with a black border The sign is not prescribed by the Regulations and requires authorisation

7.4 The sign shown in diagram 529.1 is for use in advance of tunnels. If the tunnel is controlled by wig-wag signals, the sign should be supplemented by a plate to diagram 773 (see section 21).

7.5 The Road Tunnel Safety Regulations 2007 require that a tunnel’s length should be indicated on a sign at the portal. This should be done using a non-prescribed variant of diagram 2401 or 2928 (see figure 7-1 and para 1.5.). It is not appropriate to use diagram 529.1, where the supplementary plate indicates the distance ahead to the hazard.

LOW BRIDGES 7.6 Incidents in which road vehicles strike low bridges present a serious hazard to both rail and road users. There is a significant number of reported bridge strikes each year. It takes only a relatively small amount of force to displace bridge girders sufficiently to derail a train. Serious damage can also be caused to arch bridges. It is therefore particularly important that the signs and markings on low bridges are correctly installed and maintained to a high standard. The use of black and yellow hazard markings helps to make the vulnerable parts of a structure more conspicuous. The yellow parts of these markings may also be reflectorised, making them much brighter at night, and fluorescent. The latter greatly increases conspicuity in dull weather and at dusk (see para 1.33 for further guidance on fluorescence). However, fluorescence is visually intrusive and is therefore best confined to structures that are known to be at risk.

7.7 The standard minimum clearance over every part of the carriageway of a public road is 16'-6" (5.03 m). Where the clearance over any part is less than this, signs should be provided. Where a dual carriageway road has hard shoulders with restricted headroom, this should be separately signed, supplemented with the hazard markings in diagram 530.2 or the chord markings used at arch bridges (see paras 7.23 and 7.24).

MEASUREMENT OF HEIGHT

7.8 All bridges and other structures with a headroom of less than 16'-6" (5.03 m) should be clearly signed. The Regulations allow heights to be shown in either imperial units or both metric and imperial units. It is strongly recommended that both units are displayed on signs, especially on main routes and roads used frequently by foreign vehicles. Metric signs alone must not be used; either two signs or a combined metric/imperial sign must be used. Imperial and metric heights should be calculated separately.

7014.1 Reduction in bridge headroom ahead

The height may be varied

7.9 The bridge height should be re-measured when works on the road or to the bridge might have affected the available headroom. Permanent signs should be changed if necessary and a temporary sign to diagram 7014.1 may be erected for a period of 6 months following completion of the works.

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

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

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

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.

7.16 At arch bridges, where the available headroom is indicated by signs to diagram 532.2, 532.2A, 532.3 or 532.3A, an alternative route may be signed using a sign to diagram 818.5, at the x-height indicated in Appendix A. As with mandatory height restrictions, the sign should be placed where the driver can see it in good time to make the necessary manoeuvre and full route continuity signing should be provided.

NON-ARCH BRIDGES

7.17 Mandatory signs should normally be used at non-arch bridges, as they can give more effective protection than warning signs. Traffic regulation orders are not required for mandatory height limit signs at a bridge, tunnel or similar structure over the highway (see direction 7(3)), but are required for limits imposed for environmental reasons.

629.2 Mandatory height restriction

629.2A Mandatory height restriction (metric and imperial units)

These signs may be used with diagram 530.2.
The height may be varied (see Appendix C)

7.18 Signs to diagram 629.2 or 629.2A may be used, but the latter, which shows both imperial and metric dimensions, is preferred. The signs should be mounted on the bridge, as a driver seeing a bridge with no height limit indicated might well assume the headroom to be at least 16'-6". They may also be placed at the roadside in advance of the bridge in accordance with direction 8 (in most cases a sign being required on each side of the road), but as it is an offence for drivers of overheight vehicles to pass these signs, they should not be placed where they would prevent legitimately-required access to premises or side roads. Even when diversion signing has been provided, it is still necessary to give proper advance warning of the height restriction. Signs to diagram 530 or 530A are not appropriate, as they give no indication of the prohibition. Diagram 818.3 (or diagram 818.4 if it is considered necessary to include the diversion route) should be used; it will sometimes be possible to indicate an escape route for drivers who have missed the diversion signs by siting this sign in advance of the last diversion or turning point before the bridge. If it is much further from the bridge than the usual warning distance indicated in [[Traffic Signs Manual/Chapter 4/2013/A|Appendix A, another sign to diagram 818.3 or 818.4 may be required at the appropriate place. Diagram 818.3 or 818.4 may also be used to indicate a restriction in a side road by including an arrow on the sign (the legend “ahead” must not be used with the arrow). The signing on the approach to a non-arch bridge is shown in figure 7-2.

7.19 Although warning signs to diagram 530 or 530A may be mounted on non-arch bridges as an alternative to mandatory signs, this practice is not recommended.

530.2 Reduced headroom at a hazard

May be used only in combination with diagram 530, 629.2 or 629.2A

End of diversion route for overheight vehicles.

NOTE: Black and yellow markings on bridge should not extend over footway.

Signing at Structure

Diagram 629.A plus conspicuity markings to diagram 530.2

Prohibitory Signs, if appropriate (see para 7.18)

Diagram 629.2A

Note: If access is required to premises by a single sign to diagram 818.3

Advisory sign in advance of last turning opportunity if needed

Diagram 818.3

Advisory signing in advance of start of diversion route for overweight vehicles

Diagram 2002 or 2003 (Diagram 2107 or 2108 on non-primary routes)

Diagram 818.4

Notes:

(1) Either diagram 2002 etc, or 818.4, or both types of advisory signing, may be used as appropriate.

(2) Continuity signing is essential on the diversion route.

Figure 7-2 Signs on approach to a non-arch bridge with a diversionary route

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.

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

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.

Signing at structure

Diagram 532.2A or 532.3A

Road markings is needed (see Chapter 5)

Final warning sign

Diagram 531.1 (see para 7.22 for location)

Diagram 531.2 if needed

Additional sign in Advance of last turning
turning opportunity, if needed

Diagram 531.1 plus 572

Advisory signing in advance of start of diversion route for overheight vehicles.

Diagram 2002 or 2003 (Diagram 2107 or 2108 on non-primary routes

Diagram 818.5

Notes:

(1) Either diagram 2002 etc. or 818.5, or both types of advisory signing, may be used.

(2) Continuity signing is essential on the diversion route.

Note: Black and yellow markings on bridge should not extend over footway

Figure 7-3 Signs on approach to an arch bridge with a diversionary route

532.2 Maximum headroom at centre of arch bridge indicated by signs to diagram 530

The black and yellow markings may be omitted. The metric sign may be omitted.

The height may be varied (see Appendix C)

532.2A Maximum headroom at centre of arch bridge indicated by a sign to diagram 530A

The black and yellow markings may be omitted. The height may be varied (see Appendix C)

532.3 Maximum headroom at side and in centre of arch bridge indicated by signs to diagram 530

The black and yellow markings shown in diagram 532.2 may be added.
The metric signs may be omitted. The height may be varied (see Appendix C)

532.3A Maximum headroom at side and in centre of arch bridge indicated by signs to diagram 530A

The black and yellow markings shown in diagram 532.2A may be added.
The height may be varied (see Appendix C)

7.24 The chord marking should indicate the available headroom over a width of not less than 3 metres. The height indicated on the warning signs, determined in accordance with paras 7.8 to 7.12, should be the lowest headroom between the chord limits. On a narrow bridge where the headroom decreases rapidly away from the centre, a 3 metre chord marking is appropriate. On a wide bridge with only a gradual reduction in headroom it might be possible to increase the chord width to 6 metres or more without excessively restricting the signed height, thus maintaining two-way operation for all vehicles that can pass under the bridge. In these circumstances, the warning sign to diagram 531.1 is unlikely to require the supplementary plate to diagram 531.2. Where 16'-6" (5.03 m) headroom is available over much of the carriageway width, but not all of it, diagram 532.3 or 532.3A should be used with the central signs varied to 16'-6" (5.0 m) and the signs on the side chords indicating the clearance available at the near side channel (see paras #7.10 and #7.11 for measurement of clearances). Black and yellow markings shown in diagram 532.2 or 532.2A should then be used over those parts of the carriageway where the clearance is below 16'-6", to give greater emphasis to the restricted height. It is recommended that the yellow parts of the marking should be retroreflective; they may also be fluorescent (see para 7.6). The chord markings shown in diagrams 532.3 and 532.3A may be used in circumstances other than described above, particularly on wide roads where there are two lanes in one or both directions, with the side chord positioned over the left hand lane. Side chords are also useful where vehicles with a height well below that shown for the centre of the road might strike the bridge if keeping close to the left hand side of the carriageway.

COMPOSITE BRIDGES

7.25 Some bridges originally built as arches have been adapted with the addition of girders or beams. Where the arch is the lowest part, the whole structure should be signed as an arch bridge. Black and yellow striped plates (to highlight the profile of the arch) should be suspended from the bridge beam, together with further plates on the arch itself.

7.26 Where the beam is lower than any part of the arch, the whole structure should be signed as a non-arch bridge and plates with black and yellow markings to diagram 530.2 suspended from the arch at the height of the beam. Similarly, where two adjacent beam bridges have different headroom, the plates should be located on the higher bridge at the height of the lower one.

7.27 Experience has shown that these suspended plates will themselves be struck from time to time and that rigidly-mounted aluminium substrates are not suitable. Rubber or other flexible material should be used for the backing, suspended by means of chains or hinges fixed securely to the bridge structure by a method agreed with the bridge owner. The plates should not be fixed rigidly by screws or bolts to the face of the bridge, as there is a greater risk than with flexibly-suspended plates of them being dislodged and falling onto vehicles on the road beneath. The use of rubber-backed plates will help to avoid annoyance to nearby residents from the noise of hanging metal plates striking the bridge structure in wind or vehicle slipstream. It is recommended that the yellow parts of the marking should be retroreflective; they may also be fluorescent (see para 7.6). When the signs are lit, the plates should also be lit whenever practicable. This is particularly helpful where a girder bridge is followed by a more restrictive arch bridge.

OVERHEAD POWER CABLES

7.28 For guidance on signing overhead power cables, see paras 21.9 to 21.12.

OVERHANGING BUILDINGS

530 Available headroom with imperial and metric units shown on separate signs

May be used with diagram 530.1, 530.2, 572 or 573. The height may be varied (see Appendix C). The metric sign may be omitted or placed to the right of the imperial sign, but must not be used alone

530A Single sign showing available headroom in both metric and imperial units

May be used with diagram 530.1, 530.2, 572 or 573.
The height may be varied (see Appendix C)

530.1 Building overhanging part of carriageway

May be used only in combination with diagram 530 or 530A. A distance, an arrow or both may be added.
"building" may be varied to "buildings" or to "structure"

7.29 Where an overhanging building or other structure causes a restriction over part of the carriageway, the plate to diagram 530.1 should be used with diagram 530 or 530A. A distance may be added. Where the building is in another road, an arrow may be added, pointing horizontally to the left or to the right. Details are given on the working drawing (see para 1.8).