Nine out of 10 transit projects worldwide come out over budget

See Why do big transit projects almost always go over budget? Excerpts:

"In fact, nine out of 10 transit projects worldwide come out over budget, and the same ratio of transit builds tend to inflate future ridership figures, according to multiple studies led by Bent Flyvbjerg, an Oxford University academic who has written extensively for over a decade about these issues.

“Transit in particular seems to have the largest overruns in the transportation sector,” Siemiatycki said.

Why?

He cited three contributing factors:

1) Transit builds are incredibly complex so there are lots of chances for costs to go over, and random issues, like water or rock, crop up during digs. Even weather in a country like Canada can affect a project’s timeline and subsequently its budget.
2) “Optimism biases”: people are prone to believe they can do it better than the last guy. So instead of transit commissions and city councils and other governments learning from past underestimations and over-budgeting these projects, they often mistakenly believe this time they can control risk and it all will be different.
3) Politicians often low-ball transit costs to gain approval in their jurisdiction and from higher levels of government. Once construction starts, these projects are rarely cancelled and taxpayers are on the hook for extra costs."