The A5 has generally followed two main roads - the Roman Watling Street from London to Wroxeter, then Telford's Holyhead Road for the rest of the route, though various bypasses and improvements have taken place.
In 1810, Thomas Telford was given the job of improving the road from London to Holyhead, from where the sea crossing to Dublin could be made. The first half, from London to Shrewsbury, presented few problems, as the Romans had laid out their road (Watling Street) to Wroxeter some eighteen hundred years beforehand. Watling Street north of Rugby, however, was of poor quality and avoided the main cities of the West Midlands and so Telford's coaching road went via the modern A45, A41 and A464 through the centres of Coventry, Birmingham and Wolverhampton before rejoining the A5 at Oakengates, Shropshire.
However, the second half of the journey required some serious engineering to get through the mountains. The Holyhead Road was of vital economic and strategic significance in the recently-united United Kingdom of Great Britain and Ireland, connecting as it did the capital cities of the two principal islands in the Kingdom. However, in August 1848, just twelve and a half years after the Irish Mail first used the Menai Bridge, the raison d'être of Telford's road project disappeared with the transfer of the Irish Mail to the railway, using the Stephensons' newly-completed Chester & Holyhead line.
Marble Arch- M1
The A5 has always started at Marble Arch and headed northwest along Watling Street. There have been a few realignments, notably at Brent Cross, but otherwise the route is the same.
The A5 between the A41 Watford Bypass and M1 junction 9 was renumbered as the A5183 in the late 1970s, in order to encourage drivers to avoid the centre of St Albans as a through route. A section of the Redbourn Bypass runs on the old Hemel Hempstead - Harpenden railway.
Dunstable - Watford Gap
The Dunstable Northern Bypass opened on May 10th 2017.
The original alignment of the A5 through Milton Keynes is now the V4 and easily recognisable as the old road. It still forms a significant local link in the town, and continues to be referred to as the A5 by local residents, with the current bypass known as the "A5D" (ie: A5 diversion).
The A5 has never run along Watling Street immediately north of Watford Gap, as quicksand in the local area rendered the road impractical for a coach route. A small section of the Roman road was reclaimed in 1959 when the first section of the M1 was built. The A5 mainline was originally a TOTSO to this spur, but was converted into the current roundabout in 1997 when the International Freight Terminal at Daventry was built.
Crick - Telford
The Atherstone bypass opened in December 1963.
At Wall, the A5 originally left the Roman road and briefly multiplexed with the A38, before heading due west to the village. The straight route along Watling Street has never been used by motor traffic; the modern A5 bypasses Wall and the construction of the M6 Toll has completed changed the road network.
The A5 has never followed Watling Street through what is now Telford. From the east, it ran to the centre of St Georges, then turned left to rejoin Telford's Holyhead Road (originally the end of the A41, then the A464) to the south, which took the mainline back up to the Roman road. In 1932 it was diverted further east on a bypass of St Georges. The construction of Telford and the M54 changed the alignment of the A5 again, to run slightly east to meet the new motorway and the A464.
Telford - Chirk
The original route of the A5 ran on what's now the B5061 to Wroxeter, the B4380 to Emstrey, and the A5112 and A458 through the town. The first Shrewsbury Bypass was completed in 1933, and was single carriageway with at-grade junctions. It is now an extension of the B4380. The current bypass dates from 1994.
The Nescliffe Bypass opened in March 2003.
Between West Felton and Chirk, the A5 runs mostly on a completely different route, in order to bypass Oswestry and provide a high quality route that can cater for both itself and the A483. From the Queen's Head, the original route ran on what's now the B5009 to Gobowen, then briefly running along the current route before taking the B5070 across the Welsh border through Chirk to meet the modern alignment to the north of the village.
Chirk - Holyhead
The imposing geography of North Wales has meant that the A5 largely follows Telford's road from here to Holyhead. The main improvement is a realignment at Ty Nant that opened in 1996, avoiding a dangerous bend.
South of Llandegai, the original route of the A5 carried straight on to meet the end of the A55 in the village. The current rerouting dates from the opening of the Bangor Bypass. The A5 moved from running on the Menai Bridge to the Britannia Bridge when the latter was rebuilt to cater for motor traffic, but moved back in 2000 when the A55 was extended across Anglesey to the Holyhead Ferry terminal.
The last section of the A5 in Holyhead ran along Victoria Road and ended at the original ferry terminal to Dublin. The terminal has now expanded and moved, and this road is now the A5154, though as the docks are gated off, the road is of lesser use.
Original Author(s): Tom, Viator, Steven, Bealach na Ba, T1(M), Lez Watson