[K.S.H]

Motorways don't have to be 3 lanes, they can be 2 lanes
Dual carraigeways can be more than 2 lanes wide
Motorway has a speed limit of 70MPH (for cars)
Dual carraigeways have the national speed limit unless stated otherwise but can be anything ie 30/40/50mph
The difference between them is one is designated as a motorway and this has got the blue signs, used to have a sign on every junction telling you that you were joining a motorway and what the restriction were but these aren't there anymore