Flyvbjerg Megaproject Database: The 98% Overrun Source
Bent Flyvbjerg's Oxford megaproject database is the most-cited empirical source on megaproject performance. We give the sample, methodology and citation URL.
Average overrun by category (Flyvbjerg-Gardner 2024)
| Project category | Mean cost overrun | % overrun (any) |
|---|---|---|
| Solar power | +1% | about 50% |
| Wind power | +13% | about 55% |
| Fossil fuel transmission | +8% | about 60% |
| Roads | +24% | about 80% |
| Rail | +39% | about 80% |
| Bridges and tunnels | +34% | about 80% |
| IT (large) | +107% | about 80% |
| Olympic Games | +157% | 100% |
| Nuclear power | +120% | about 95% |
| Nuclear waste storage | +238% | about 95% |
From Flyvbjerg and Gardner (2024), How Big Things Get Done, Penguin Random House.
What counts as a megaproject
Flyvbjerg defines a megaproject as a project with a budget of 1 billion US dollars or more. The dataset also includes large projects below this threshold for categories where 1bn is unusual (some IT projects, smaller renewables). Olympic Games, defense procurement above the relevant defense-major threshold, and nuclear plants are always included.
The unit of analysis is the project, not the contract. Cost overrun is measured in real terms (inflation-adjusted) against the originally approved budget at the decision-to-build stage, not against later re-baselined figures. This is critical: many megaproject business cases re-baseline the budget partway through, which would otherwise mask the overrun.
How to cite
Source: bentflyvbjerg.com