The road begins at Cadle in Swansea and heads westwards as a southern bypass to Loughor. It continues over the Loughor bridge into Bynea, Carmarthenshire. It continues through the centre of Llanelli and traverses near the south coast of Carmarthen Bay until it reaches Pembrey. After Pembrey, the road bears northwards and continues in a northerly direction towards Carmarthen. At the junction with the A40/A48 at Pensarn, the road is severed by the new A40 bypass, with the road instead resuming at the junction with the A40/B4300. Here, the road resumes and continues through the eastern part of Carmarthen then heads north to Bronwydd Arms. It then turns to a northwest direction heading to Cynwyl Elfed and then heads northwards again. After the village of Llangeler, it turns west and continues westwards until the junction with the A487 in Cardigan.
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