Tunnels typically run a third over budget, said a US report published last month.

According to the Journal of the American Planning Association in a study titled Underestimating Costs in Public Works Projects: Error or Lie, tunnels and bridges run over budget, on average, by 34%, second only to rail projects which ran 45% over estimated costs. Roads ran over budget by 20%.

Two hundred and fifty-eight projects were studied, including New York’s Holland Tunnel, completed in 1927 at a cost of $48M – the original estimates were 52% lower – and the Channel Tunnel, said in the report to be 80% over budget.

However, Boston’s Big Dig, an exercise to put the majority of the city’s major roads underground, started 15 years ago with a price tag of $4.5bn, but now costs $14.6bn. “You only find comparable cost overruns in the Third World,” said lead researcher Bent Flyvbjerg.

The report adds weight to the working party set up by the British Tunnelling Society and the Association of British Insurers to establish a joint code of construction practice. It has proposed that there be more openness in the tender process, more detailed risk assessment, and for the client not to automatically opt for the lowest bid.