5.7 Where a high-speed dual carriageway road ends at a roundabout, signs to diagram 520 should be erected on both sides of the carriageway approximately 100 metres before the roundabout. A sign to diagram 521 should be erected approximately 50 metres after the roundabout. Warning signs to diagram 510 should also be used as appropriate (see paras 2.13 to 2.15).
5.8 Where speeds on the dual carriageway road are high, but the length of dualling is so short that it would not be possible to site the signs in accordance with figure 5-2, they may be sited at not less than one half the normal distance from the end of the central reservation, provided they are still duplicated on the right hand side of the carriageway.
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The distance may be reduced
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The distance "½" may be varied to "¼"
5.9 On a single carriageway road which widens to a dual carriageway for a length of at least 400 metres, an advance sign to diagram 818 should be provided. Siting distances, normally measured back from the nose of the central hatch marking at the start of the dual carriageway, are similar to those indicated in Appendix A for warning triangles. If the dual carriageway is shorter than 400 metres, no advance sign should be provided. The sign to diagram 818.1A may be used to warn drivers that a dualled length is short and so might not be long enough to permit overtaking. The sign may display distances of "¼ mile" or "½ mile" only.
5.10 Further advance warning up to a distance of two miles may also be given by a sign to diagram 818.1. This sign may be used to encourage drivers to delay overtaking until the dual carriageway is reached. Distances greater than two miles are not permitted, as this might result in impatient drivers trying to overtake on an unsuitable length of road.
5.11 For guidance on the appropriate x-height for these signs, see table 5-2.
| 85th percentile speed (mph) |
x-height (see note) (mm) |
|---|---|
| Up to 30 | 75 |
| 31 to 40 | 100 |
| 41 to 50 | 125 |
| Over 50 | 150 (200) |
NOTE: The larger size shown in brackets may be used on high standard single carriageway roads where speeds are high (e.g. on 10m wide carriageways).
5.12 "Keep left" signs (diagram 610) should always be placed at the start of the central reservation, and at any gaps. See Chapter 3 for further guidance.
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