14.19 The size of the terminal sign to diagram 670 or 671, as shown in tables 14-1 and 14-2, depends upon the approach speed limit, not the limit being signed. Unless it is impracticable to do so (see Notes to the tables), the standard size of sign should be used on each side of the carriageway. This also produces a more balanced appearance. The signs should be sited so that they can be seen in good time to allow drivers to adjust their speed by the time they pass them. Obstructions to visibility such as vegetation, street furniture, buildings, bends, humps, other topographical features etc., need to be taken into account when determining the terminal point for a speed limit order. "Countdown" signs giving advance indication of a change in the speed limit are not prescribed and must not be used. Nor should the sign to diagram 818.2 be used to indicate a speed limit ahead. Any development that takes place after a speed limit order has been made might require re-siting of the terminal signs to maintain adequate visibility, and amendment of the order. Table 14-3 shows the minimum clear visibility distances that should be maintained for terminal signs. These are based on speed differential rather than simply the approach speed limit and 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.
14.20 Guidance on the illumination of speed limit terminal signs can be found in paras 14.54 to 14.58, and on their mounting and co-location with other signs in paras 14.60 and 14.62.
| Type of junction and location of sign | Speed limit on approach road (mph) (higher limit) | ||||
|---|---|---|---|---|---|
| 30 | 40 | 50 | 60 | 70 | |
| Size of sign (mm) | |||||
| Signal-controlled junction (signs viewed only from turning vehicles) | 600 | 600 | 600 | 600 (750) | 600 (750) |
| Signal-controlled junction (other signs) | 600 | 600 | 750 | 750 | 900 |
| Side road approach to a priority junction | 600 | 600 | 600 | 600 | 750 |
| Entry into side road at a priority junction | 600 | 600 | 600 | 600 (750) | 600 (750) |
| Approach to a roundabout | 600 | 600 | 600 | 750 (600) | 900 |
| Exit from a roundabout | 600 | 600 | 600 | 600 (750) | 600 (750) |
| NOTE 1: The larger alternative sizes shown in brackets might be appropriate where there are high-speed flow conditions such as a diverge lane at a priority junction or spiral markings on a large roundabout. The smaller alternative size of 600 mm shown in brackets for the approach to a roundabout may be used where the 85th percentile speed is less than 50 mph. | |||||
| NOTE 2: Where the terminal sign indicates a higher speed limit, this should generally be the 600 mm size. | |||||
| NOTE 3: Where the width of the verge is insufficient to allow the provision of a 600 mm sign shown in the table or referred to in Note 2, a 450 mm sign might have to be used. | |||||
| NOTE 4: Where signs of different diameters are to be mounted back to back (and a backing board is not used), consideration should be given to using the larger diameter for both, as this should result in a neater and less cluttered assembly. | |||||
| NOTE 5: The prescribed size of 300 mm is used only for repeater signs (see table 14-4) and never as a terminal sign. The prescribed 1500 mm sign is for use only at road works (see Chapter 8, Part 1). | |||||
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