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Dual Carriageways
1. As stated above the speed limit for dual carriageways is 70 mph and if entering a dual carriageway from a road that has the national speed limit eg. a bypass, then there is no need to sign it with further derestriction signs at the start of the dual carriageway. The only signs that you will see are the blue background with white lettering and white border to inform you that a dual carriageway is ahead eg 'Dual carriageway 2 miles ahead' or 'Dual carriageway ahead' or 'Dual carriageway for ½ mile'.
2. If however the dual carriageway has a different limit to the one you are on, and this maybe a 60mph speed limit (remember that dual carriageways are classified as 70mph), there needs to be a speed limit sign erected on the nearside and one on the central reserve informing the driver what the new limit is. Repeater signs will be required within 400 metres of the speed limit sign, then at intervals of no more than 500 metres on alternate sides of the carriageway or 800 metres on the same side of the carriageway - tables--signs (2)
3. When you enter a new speed limit and leave the old limit there will be one sign on the nearside and one on the central reserve. If you are leaving a dual carriageway into a side road then one speed limit sign on the nearside is all that is required within 20 metres of the junction to inform you of the new limit. If however that sign is say 40 metres from the junction and the dual carriageway is subject to the national speed limit, this will mean that the road you turn into will be subject to the national limit also for a distance of 40 metres, if the road is a two way single carriageway road it will be 60mph.
4. If you turn onto a dual carriageway with a national speed limit (70mph) from a national speed limit, for example a two way single carriageway then no signs are required as you are turning from one national speed limit onto another. If however you are turning onto a dual carriageway with a national speed limit from a road that has either a 20/30/40 or 50 mph speed limit, then 2 x national speed limit signs are required on each side of the carriageway ie. white roundel with a black stripe. If the speed limit on the dual carriageway is lower than the national speed limit but higher than the road that you are on then there only need be a repeater sign on the dual carriageway within 100 metres of the junction, if the repeater sign is more than this then 2 x speed limit signs are required on each side of the carriageway. If the speed limit is lower on the dual carriageway than the road that you are on, then 2 x signs on each side of the carriageway are required.
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