Hard to answer this question without further information but if it is a dual carriageway to which the national speed limit applies, you will only get a national speed limit sign as you go onto the dual carriageway, i.e a circular sign, white background with a black band diagonally across, this would be a 70mph limit for a car or car derived van. It would be a lower speed for larger vans and other vehicles. If the dual carriageway is governed by a lower speed limit this would also be displayed as you went onto that road this again would be a cicular sign with a red border with the speed limit thereon i.e 50 or whatever. If this is the case there should be repeater signs every so often with the posted speed thereon these are a similiar sign but are smaller. Hope this helps.