Where a single carriageway has four or more lanes, use only the lanes that signs or markings indicate.
137. Multi-lane carriageways - Dual carriageways
On a two-lane dual carriageway you should stay in the left-hand lane. Use the right-hand lane for overtaking or turning right. After overtaking, move back to the left-hand lane when it is safe to do so.
138. Multi-lane carriageways - Dual carriageways
On a three-lane dual carriageway, you may use the middle lane or the right-hand lane to overtake but return to the middle and then the left-hand lane when it is safe.