Big Dig Boston: Central Artery / Tunnel Project
The Central Artery / Third Harbour Tunnel Project (CA/T) was the most expensive highway project in US history. Original 1985 cost estimate of 2.6 billion USD (GAO); 2007 cost of 14.8 billion USD (Massachusetts State Auditor); total including interest on Massachusetts bonds is projected to exceed 24 billion by 2038.
How much did the Big Dig cost?
The Big Dig finished at about 14.8 billion US dollars in construction cost in 2007, up from an original 1985 estimate of 2.6 billion, an overrun of roughly 469% in nominal terms (220% in real terms, per Flyvbjerg). Counting interest on the Massachusetts bonds that financed the state share, the total cost is projected to exceed 24 billion dollars by 2038, making it the most expensive highway project in US history.
Why the cost grew so much
- Massive scope expansion. The 1985 estimate did not include the Ted Williams Tunnel, the I-93 north reconfiguration, the Charles River bridges, or the surface restoration of the old elevated artery footprint. By 2000 all four had been added.
- Unprecedented urban underpinning. The project required underpinning live high-rise buildings, an active subway, and the South Station rail terminal while keeping them all in service. That deep urban work, through difficult and incompletely mapped ground conditions, was a major driver of the cost growth.
- Inflation across a 16-year build. Construction-cost inflation 1991-2007 was significantly above general CPI; about 20-25% of total overrun was inflation.
- Quality and safety failures. The 2006 ceiling collapse, leaks, and use of substandard concrete added cost in remediation and litigation.
- Originally a 70/30 federal/state split that became 50/50 after federal caps. Massachusetts had to absorb the unanticipated state share via bond issuance, hence the projected 24bn including interest.
Sources
- Massachusetts State Auditor, report 2007-0510-3C2 (June 2008), p. 1: total cost $14.798 billion as of May 2007 (archived copy). Read 8 October 2026.
- US GAO, GAO/RCED-96-65R (January 1996), p. 1: 1985 estimate of $2.6 billion. Read 8 October 2026.
- FHWA project profile: Central Artery/Tunnel.
- US Government Accountability Office reports on CA/T (GAO/RCED-00-141 and follow-ups).
- Greiman V. (2013). Megaproject Management: Lessons on Risk and Project Management from the Big Dig. Wiley.
Related
Frequently Asked Questions
How much did the Big Dig cost?
The Big Dig (the Central Artery/Tunnel Project) finished at about 14.8 billion US dollars in construction cost in 2007, up from an original 1985 estimate of 2.6 billion (GAO). Counting interest on the Massachusetts bonds that financed the state share, the total cost is projected to exceed 24 billion dollars by 2038. It is the most expensive highway project in US history.
How much over budget was the Big Dig?
The cost rose from a 2.6 billion US dollar estimate in 1985 (GAO) to 14.798 billion as of May 2007 (Massachusetts State Auditor), about 469% more in nominal terms. Measured in real terms against the decision-to-build estimate, Bent Flyvbjerg puts the overrun at 220%. The increase came from scope expansion, inflation over a long build, unforeseen ground conditions, and remediation of quality and safety failures.
Why did the Big Dig cost so much?
Four main drivers: massive scope expansion (the 1985 estimate excluded the Ted Williams Tunnel, the I-93 north reconfiguration, the Charles River bridges, and surface restoration); unprecedented urban underpinning of live buildings, an active subway, and South Station; construction-cost inflation across a 16-year build; and quality and safety failures including the 2006 ceiling collapse and substandard concrete.