The A55 roughly follows the route of the Roman Road from Chester to Conwy. The Roman Route then avoided the coast and went over the mountains to reach Caernarfon.
Chester - Conwy
The original start point of the A55 was at The Cross in Chester, at the junctions of Bridge Street, Watergate Street (originally the A548), Eastgate Street and Northgate Street (both the A51). It headed south along Bridge Street and Grosvenor Street to cross the River Dee at the Grosvenor Bridge before turning to run west along Hough Green and into Wales. The bridge is now the A483 but most of this route is now the A5104.
The Chester Bypass was a major upgrade to tie in with the M53 (then numbered M531). This was completed in 1976, though it originally plugged into an earlier 1940s Eastern Bypass, which was by then part of the A41.
Most of the dualling between Chester and Conwy took place in the 1980s. The majority of this was actually done offline, to avoid small villages and some nasty bends and gradients. West of Broughton, the original A55 follows the B5125 through Hawarden, then moves on to the A5026 through Holywell before old and new roads rejoin. The St Asaph bypass, however, dates back to the 1960s, as does the road west of Abergele. The original line of the A55 west of Abergele is now numbered A547.
At Conwy, the A55 originally passed over Telford's Bridge (then a narrow toll bridge) and right up to the edge of the castle. By the 1950s, this had become such a bottleneck, that a new bridge was built slightly downstream, opening in 1958. The current route was the last section to be dualled, opening in 1991 following the construction of tunnels under the River Conwy by the immersed tube technique: sections of the tunnel are constructed on land before being sunk in the water and joined together.
Conwy - Holyhead
The coastal route from Conwy to Bangor was originally built by Thomas Telford, as a diversion from the older Sychnant Pass. Originally it ran right along the coastline and was single carriageway in places, but was upgraded in the 1930s to better suit motor traffic when the the two tunnels at Penmaenbach and Pen-y-clip were built. These tunnels speeded up journey times between Chester and Holyhead but needed second tunnels when the road was dualled in the 1980s.
The original endpoint of the A55 was on the A5 at Llandegai. When the Bangor Bypass opened, the A55 diverted south from its original alignment at Ty'n-yr-Hendre to meet the A5 at a more southerly point. That road then continued along the remainder of the bypass and over Britannia Bridge onto Anglesey.
In 2001, a major new dual carriageway was completed over Anglesey, under a 30-year shadow toll contract and partly funded by EU money. It was one of the last road projects in Wales to start before devolution. The A55 took over the route of the A5 Bangor and Llanfair PG bypasses (resulting in the A5 reverting to its original route via Menai Bridge), and along the new dual carriageway up to the ferry terminal.
Tai'r Meibion to Aber (J12-13)
The Aber to Tai'r Meibion was one of the first stretches of A55 to be dualled, in the late 1960s, by buiding a new westbound carriageway. the Eastbound carriageway in particular is sub-standard, and the section also has numerous property accesses.
On 11th July 2018 the Welsh Government announced a £25 million scheme (of which £14.9m is to be funded by the European Regional Development Fund) to improve safety and alleviate the risk of flooding by upgrading the 2.2km stretch to present day standards, providing safer accesses onto the A55, an improved drainage system to ensure future resilience to flooding, new footways to improve pedestrian access to the local bus service and new wildlife pipes installed under the A55.
They also plan to construct a new walking and cycling route will be constructed parallel with the eastbound carriageway to separate NMUs and to resurface the route with low noise surfacing to reduce road noise in this area.