i have been wondering this really exciting problem for ages...

do u assume on dual carriageway that it is 70 unless stated otherwise, or only if it has the black diagonal sign before it? as there is a piece of dual carriageway near my house in biggin hill that is a couple of hundred metres long, but the road before it and after is a 30... and there are always police in the bay with speed guns.. so is it a 30.. or a 70?

katie x