Diagram 671 is the sign every road user likes to see, the national speed limit! This makes the road user aware that the speed restrictions have been lifted and the road is now subject to the national speed limit for that type of carriageway.
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8.2.1. A terminal sign to diagram 670 (S10‑2‑1, see Figure 8-1) should be used to indicate the beginning of a maximum speed limit (except for 20 and 40 mph zones). Diagram 670 is not used to indicate the national speed limit; the appropriate sign is diagram 671 (S10‑2‑2, see Figure 8-2). All speed limits, except those for tramcars, are signed in miles per hour.
8.2.2. Schedule 10 General Direction 4 requires the placing of a terminal sign as near as practicable to the point where the speed limit begins or ends. The sign to diagram 671 should be used where a speed limit ends and the national speed limit applies, otherwise the end of one speed limit is indicated by the start of another by diagram 670. There is no specific requirement to provide two terminal signs, one on each side of the carriageway, where the speed limit changes along a length of road. However, in most situations it is recommended that two signs are provided, particularly on motorways and rural dual carriageway roads. Drivers need to be SPEED LIMITS 69 fully aware of the speed limit both for reasons of road safety and enforcement, particularly where the sign indicates a reduction in the speed limit. There is always the possibility that a single sign might become damaged or obscured by vegetation and require a more rigorous maintenance regime. A single sign could also be obscured by high‑sided vehicles, particularly on roads with more than one traffic lane in the same direction. Any decision by the traffic authority to use a single sign should be underpinned by robust risk analysis. Where a single sign is to be used this could be supplemented by a speed limit road marking (see 8.18.1). Where the speed limit changes to the national limit, two signs are likely to be required as there is no appropriate road marking and repeater signs are not used where there is no street lighting. A driver not aware of the higher limit might be a hazard to other road users and encourage dangerous overtaking. A single sign is likely to be appropriate on narrow roads, particularly in rural locations and in urban areas where a 20 mph speed limit is introduced. Where a single carriageway road has a central traffic island, a terminal sign may be erected on that island rather than on the opposite side of the road. Figure 8-3 shows examples of siting terminal signs.