On a dual carriageway, lanes in one direction are separated from lanes in the other direction by a central reservation. There may be a safety barrier along this central reservation.
Some dual carriageways are very similar to motorways, with slip roads to join and leave. However, motorway regulations do not apply and you may come across slow-moving traffic such as cyclists or farm tractors.
Driving - the essential skills - pg 177
On a two-lane dual carriageway you should stay in the left-hand lane. Use the righthand lane for overtaking or turning right. After overtaking, move back to the left-hand lane when it is safe to do so.
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.
Some slip lanes don't converge with the main carriageway immediately. This is generally much easier. The lane may, however, become a compulsory exit lane further along.