The A5 road across Anglesey, constructed by Thomas Telford starting in 1818, replaced an old turnpike road that dated back to 1765.

Key details regarding the route it replaced and the reasons for the change include:

• The Previous Route: The 1765 turnpike route ran through Bodedern and Llangefni. Telford’s new road was a more direct line across the island, designed to improve the speed and reliability of the Irish Mail.
• Engineering Failures of the Old Road: The road Telford replaced was described as “notoriously bad” and dangerous. It featured extremely steep gradients of up to 1 in 6½, sharp curves, and segments that were frequently potholed, boggy, or even covered by open water.
• The Four Mile Bridge Detour: At the western end of the island, before Telford built the Stanley Embankment (completed in 1823), the fastest route to Holyhead from the mainland of Anglesey required a “significant detour” via the old stone bridge at Four Mile Bridge (Pontrhydybont).
• Geopolitical Necessity: The replacement was driven by the Act of Union in 1800; the inclusion of Irish MPs in Parliament created intense political pressure for a high-quality link between London and the Dublin ferry at Holyhead.

While Telford utilized parts of existing roads for his project through much of England and mainland Wales, on Anglesey he predominantly built an entirely new road to ensure his strict engineering standards—such as a maximum gradient of 1 in 17—could be met

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