Think about the 'dual' bit. They come in pairs. Individual way tagging can be done of course, but then which is the paired carriageway? Fetch all dual_carriageway ways then correlate by name/ref/geography? What if those aren’t consistent/cross boundaries? Did you know that carriageways in the UK typically have ref A and B? For Away from London, Back to London, usually. Can be seen from the emergency layby phone refs. Just because something is more frequently used, doesn't mean it is best. Plus mappers are free to tag as they see fit. My point about inferring from maxspeed=GB:nsl_dual remains. Are you thinking about urban dual carrigeways too with limit under national?