The A40 is a major road connecting London and Fishguard. On designation in 1922-3 the A40 between Ross-on-Wye and Abergavenny ran via Skenfrith on what is now the B4521; it was re-routed through Monmouth in about 1935, taking over the route of the former A48 between Ross on Wye and Raglan. The A40 crosses the Wales - England border two miles north of Monmouth. As a through trunk road it avoids congested areas and major conurbations along the south Wales coast, which are served by the A48 (nprn 412594) and the M4 motorway (nprn 417517). From Monmouth the A40 passes via Raglan, Abergavenny, Brecon, Llandovery, Llandeilo, Carmarthen and Haverfordwest, approaching Fishguard from the south. There are inevitable re-alignments and improvements (e.g. road was re-routed along the former A483 between Llandovery and Llangadog and at Bwlch, see nprn 91236), by-passes (e.g. at Brecon and Carmarthen) and lengthy stretches of new dual carriageway alignment such as between Monmouth (including the Gibraltar Tunnels), Raglan and Abergavenny, and to the west of Carmarthen.

B.A.Malaws, RCAHMW, 12 February 2014 and 26 December 2022.