Curious definition of dual. Implies two roads, parallel to each other. Both Department of Transport and Council Speed Awareness course instructor confirmed to me that on-slips are national single carriageway limit, up to the point that the grass separator ends and they become the dual carriageway road. Doesn’t stop everyone going faster than that of course. Official stance is that merging at 60 mph is closer and therefore safer to average traffic speed of lane 1.