The A55 or North Wales Expressway is a primary route across north Wales. Originally designed and constructed as a through route by Thomas Telford in the early nineteenth century, the road ran from Chester to Bangor and has been progressively upgraded over the years, by-passing many towns and villages. It was extended from Bangor to run parallel to the A5 Holyhead Road (nprn 402349) across Anglesey into Holyhead Docks (nprn 41260) by 2001. The road is now dual carriageway throughout apart from a section across the Britannia Bridge (nprn 34614).

Notable engineering features along the route include the 1932 Penmaen-bach Tunnel (nprn 419882) and the 1989 new Penmaen-bach Tunnel (nprn 403560); the 1930s Pen-y-Clip alignment (nprn 91423) and the 1993 new Pen-y-Clip Tunnel (nprn 91426); Telford's Conwy suspension bridge (nprn 43083), the 1958 replacement Conwy road bridge (nprn 85483) and its 1991 replacement tunnel crossing of the River Conwy (nprn 409064). On the Anglesey route extension is Robert Stephenson's Britannia Tubular Railway Bridge, adapted for road traffic in 1980.

B.A.Malaws, RCAHMW, 17 February 2014.