The Act of Union 1800, which unified Great Britain and Ireland, gave rise to a need to improve communication links between London and Dublin.

However between Weedon, Northamptonshire and Oakengates, Telford's Holyhead Road eschews the Watling Street corridor, picking up instead the major cities of Coventry, Birmingham, and Wolverhampton;[6] this routing being far more useful for communications.

In places he followed existing roads, but he also built new links, including the Menai Suspension Bridge to connect the mainland with Anglesey and the Stanley Embankment to Holy Island.

The road was designed to allow stagecoaches and the mail coach to carry post between London and Holyhead, and thence by mailboat to Ireland.

[7] These features include the following: In 1997, a section of bends on Telford's road between Tŷ Nant and Dinmael was by-passed by a modern cutting.

The A5 number disappears at the A41 near Edgware but the original road continues as the A5183 through Elstree, Radlett, St Albans, Redbourn and Dunstable.

A few miles north of Dunstable, the A5 regains its identity at the M1 motorway junction 11A, rejoining the old Roman Road and passing through Hockliffe before becoming a dual carriageway as it approaches Milton Keynes.

Continuing from the end of the M54, the route runs around Shrewsbury as the town's southern bypass (still as dual carriageway), combining for a stretch with the A49.

From Bangor the road crosses the Menai Suspension Bridge to Anglesey and then runs roughly parallel to the A55 expressway to the outskirts of the village of Valley where the A5 continues onto the Stanley Embankment.

The A5 traditionally terminated at Admiralty Arch (1822–24) on Salt Island, which was designed by Thomas Harrison to commemorate a visit by King George IV in 1821 en route to Ireland and marks the zenith of Irish Mail coach operations.

[10] This single carriageway stretch had 15 fatal and serious injury collisions between 2004 and 2006, and was rated as 'red'—the second highest risk band—in the EuroRAP report published by the Road Safety Foundation.