Telford highway to Holyhead found intact under the A5

Early 19th-century Britain's most famous road - the original trans-Britain highway built by the civil engineer Thomas Telford to improve communication between London and Dublin after the act of union in 1800 - is alive and well, according to a major survey just completed by archaeologists.

Early 19th-century Britain's most famous road - the original trans-Britain highway built by the civil engineer Thomas Telford to improve communication between London and Dublin after the act of union in 1800 - is alive and well, according to a major survey just completed by archaeologists.

The 18-month-long, in-depth survey has revealed that the highway's original fabric is far more intact today than anyone had imagined. Telford himself regarded the road as his greatest engineering achievement. The survey is the first of its type to be carried out in Britain.

Of the most technologically challenging part of the road - the stretch through the hills and mountains of North Wales - archaeologists had assumed that relatively little had survived. Academics, studying the transport aspects of the Industrial Revolution, had long believed that the original road had largely been obliterated to make way for the present modern one - the A5 - and that most of the original embankments, walls and revetments had also been substantially demolished or replaced.

But now the archaeological survey - funded by the Welsh Assembly's heritage organisation, Cadw - is revealing that, in Wales, around 40 per cent of the highway survives essentially as Telford built it.

Telford's original embankment terraces still allow the modern A5 to cling precariously to hillsides. His original dry stone walls still survive, defining the highway's boundaries. And the archaeological survey has revealed that the modern road surface is built on top of - not as a replacement for - Telford's foundations and original surfacing.

What's more, around 80 per cent of Telford's original ancillary buildings - bridges, toll houses and weighbridges - still survive in one form or another, as do dozens of gravel storage "lay-bys" often dug into the hillsides.

The London-Holyhead highway was the most sophisticated and advanced highway of the Industrial Revolution. It replaced a road which included stretches described as "a miserable track, circuitous and craggy, full of terrible jolts, round bogs and over rocks where horses broke their legs". Whereas the old road had twisted and turned and had gradients of one in six, Telford's new highway was relatively straight with no gradients in excess of one in 17.

The new road, initiated in 1815, took 11 years to build. Although Telford's highway helped to integrate North Wales into the economy of the Industrial Revolution, it was actually commissioned for very mundane political reasons.

London-Holyhead was the only channel of communication with Dublin and, after the act of union, large numbers of politicians and civil servants had to use the route regularly. The road part of the journey took 41 hours, and the road-weary Irish politicians raised the problem in Parliament in London. It was their wish for a more comfortable journey which got the highway built at a cost of more than £500,000.

But, then as now, the construction of this early 19th-century version of a motorway caused almost as much bureaucratic pen-pushing as engineering work. For there were constant arguments about land ownership, compensation and unfulfilled contracts. There were scores of compensation claims from local landowners in North Wales alone.

The archaeological survey has also made use of historical material, including the recently rediscovered notebooks of one of Telford's two chief supervising engineers. Significantly, the archaeological record and the notebooks, when compared with Telford's original plans, combine to reveal that the supervising engineers - two brothers by the name of Provis - changed and improved crucial local details of the scheme.

"We've been surprised at how extraordinarily well Telford's road has stood the test of time," said Lancaster University archaeologist and landscape surveyor Jamie Quartermaine, who led the surveying team. "The archaeological evidence we've collected shows that much of his road is still largely intact today."