Cliff notes: Data analysis shows most public projects vastly underestimated costs and overrated project benefits like ridership. Unintentional errors would balance out, but the errors almost always skewed in one direction. Strongly suggesting intentional lying by the government planners trying to sell the public on their projects.

Similar to how those vote recounts where missing ballots in Democrat run precincts just happen to somehow be discovered and almost always end up benefiting the Democrat candidate, when random errors would be expected to balance out. What a coincidence that most government planners are also Democrats. "Inflation is actually good for you!"

Here is a summary of the average results for the transportation projects:

Type n Cost overrun N Benefits vs. projection
BRT 6 1.41 4 0.42
Rail 265 1.40 74 0.66
Tunnels 48 1.36 23 0.81
Bridges 49 1.32 26 0.96
Roads 869 1.24 532 0.96

In other words, the average rail project cost 40% more than estimated and its benefits were only 66% of what was projected. The average roadway project cost 24% more than estimated and its benefits were 96% of the projected amount.

The paper’s next task is to assess whether these results were more likely due to error or to bias. The authors point out that if errors were the cause, overruns and underruns would be about equally likely to occur. But their statistical analysis demonstrates the results are heavily skewed by what Flyvbjerg has previously identified as “optimism bias,” and by what other researchers including the late John F. Kain labeled as “deception” and the late Marty Wachs of UCLA referred to in the title of a journal article as “lying with numbers.”

Psaki-ism