National speed limit
|National Speed Limit Diagram 671.|
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The National Speed Limit sign (Diagram 671 in the TSRGD 2002) is used in the United Kingdom to denote the end of a local speed limit and the start of the national maximum speed limit for the class of vehicle being driven.
Originally in 1936 there was only one speed limit: 30 miles per hour.
This was applied only to roads with a system of street lighting (that is to say, generally, at the time, urban areas). If there were no street lights, there was no speed limit. However, on roads where street lighting was present, a legal order could be made to override the 30 mph speed limit - but repeater signs were required in this instance. This is still the case today for any speed limit over 30 mph (40 and 50 mph limits were introduced in the 1950s).
The derestriction sign survived the Worboys Report, and also the introduction of the 70 mph speed limit in 1965. The sign has remained the same ever since but repeater sign regulations were amended to provide two prescribed sizes in 1994 - 350 mm and 450 mm. The current guidance is that any national speed limit road requiring repeaters should use the 450 mm size repeater but this is not always adhered to in practice.
Before the adoption in that country of metric (km/h) speed limits, the Republic of Ireland had had a national speed limit of 60 mph (earlier, 55 mph). Following metrication, however, the notion of a "default" national speed limit, as applied in the United Kingdom, was abandoned in the Republic in favour of the explicit signing of specific speed limits on all roads.
Since Northern Ireland, as part of the United Kingdom, retains speed limits expressed in miles per hour the change from metric to imperial speed limits, and vice versa, is one of the clearer indications to travellers between the two parts of Ireland that they have reached the (otherwise largely unmarked) border.