The road starts at a junction with the A40 in Haverfordwest and travels northwest to St David's to switch northeast through Fishguard, Cardigan, Aberaeron, Aberystwyth, Machynlleth and Corris.

Through the town of Fishguard, the road width in places is a very narrow single lane, leading to many traffic issues, especially with heavy goods vehicles (HGVs).

[3] In 2008 the Welsh Assembly Government published plans for the Porthmadog, Minffordd and Tremadog bypass, which would reduce the amount of through traffic in the town.

The new section bypasses Llanllyfni and Penygroes, in parts utilizing the old trackbed of the Caernarfon to Afon Wen railway line.

In April 2007, the 10 mi (16 km) new section had to be resurfaced in its entirety after it became apparent that the wrong type of stone had been used for the surface tarmac.