"Carriageway" is UK expression so as a Canadian I'm not 100% sure, but as far as I'm aware, a "single carriageway" is a highway where all of the lanes are right next to each other, and a "dual carriageway" means that the two directions are separated by grass or some other barrier.

The definition of carriageway is the same. The difference in a singe carriageway means there is only room for one carriage, not two or more.