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). From 2010, articulated HGVs were diverted from the section between Cardigan and Fishguard because of this, and routed instead via the A478 road to Penblewin, then the A40 to Fishguard via Haverfordwest. However, there were still problems to some extent.[2]
The section of road in the Dulas valley between the River Dyfi near Machynlleth and Corris was built in the 1840s at the instigation of the local slate quarry owners to replace the old turnpike road on the opposite side of the valley. It may have utilised part of the formation of the Roman Sarn Helen. From 1859, the narrow-gauge Corris Railway followed the same route.
In 1989–90, Cardigan was bypassed south of the town with a new Priory Bridge over the River Teifi and a short 3-lane section between the bridge and the junction with the A478.
Between Penrhyndeudraeth and Porthmadog, the road passed over a mile-long embankment, known as The Cob. Until 2003, drivers had to pay a charge to cross The Cob.[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.[4] Work started on the project in 2010, the route of which passes under the Ffestiniog Railway, and then crosses over the Welsh Highland Railway. The original route over The Cob was renumbered as the A4971. The Porthmadog bypass was officially opened on 17 October 2011.[5]
The section between Porthmadog and Llanwnda has been improved. 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.
The preferred route for the £100m Caernarfon-Bontnewydd bypass had been announced; Caernarfon from the northern end of the Pen-y-groes bypass to the western end of the Y Felinheli bypass.[7] This bypass was under construction in 2021 and opened 19 February 2022.
