22nd June 2026

NAO report: Estimating and monitoring the costs of building roads in England

National Audit Office (published March 2007)

It looked at motorway and major road schemes in England and found:

  • Around £3 billion in cost overruns across nearly 200 road schemes
  • Average cost overruns of about 40% on many projects before later adjustments
  • Systemic problems with early cost estimates and optimism bias in forecasting
  • The Treasury had to revise how it accounted for costs because estimates were consistently too low

A notable line from the findings was essentially that poor cost control across schemes had created a multi-billion pound overspend burden for taxpayers.

NAO £3 billion roads overspendDownload