The rule for this is that any dual carriageway is national speed limit unless either road signs tell you otherwise. If you enter a dual carriageway and there is no lighting and no speed repeater signs then it is NSL, (70). If the limit is below this then it must legally have repeaters for the speed limit every few hundred meters. If there are street lights and it is a built up area (driveways and pavements etc) such as a city or town then you should assume it is 30mph unless signposted otherwise