|Distance:||9.2 miles (14.8 km)|
|Meets:||Unclassified, A40, A4242|
|Route outline (key)|
The B4312 connects the Carmarthenshire coastal village of Llansteffan with the county town and the rest of the world.
It starts on an unclassified road in the middle of Llansteffan High Street, at the point where Church Road heads off to the south. It heads northeast along the High Street before zigzagging across a narrow valley to leave town.
The road then follows the Towy Estuary northwards for over a mile before turning inland at Morfa Bach. After going through Llangain it heads back towards the river, turning away sharply at almost the last minute, although here the road is a good height above the river. The B4312 then heads through the outer suburbs of Carmarthen and crosses the A40 bypass at a GSJ. One movement is not allowed here: eastbound A40 onto the B4312.
A short distance further a traffic-light-controlled T-junction is reached. This is the pre-bypass A40 and the B4312 has taken over the old route in both directions. Turn left for a full-access GSJ with the A40; turn right to head into the centre of town. In this direction the road passes a traffic island with an obelisk on it – a war memorial to Lieutenant General Sir Thomas Picton who died at Waterloo – before the road wheels right at a roundabout to end on the A4242 on the bank of the Towy on the edge of the town centre.