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Physical real-world projects include a buffer for this, called "contingencies" or "change orders" so that if a requirement changes or they discover something like previously unknown site geology that will require changes to the foundation they can absorb it. Based on a large history of similar projects their estimates are usually pretty good but occasionally they will run over.

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Even accounting for contingencies most of civil engineering projects go way over budget.

Two elements (the first quite obvious, the second not really) seem to be particularly common in overruns:

- the bigger the project the likelier the overrun. Small road projects tend to be over estimated, complex rail projects are virtually always way underestimated, mega projects are never close to the budget.

- the lengthier the planning and pre-construction phase the likelier the overrun. This is particularly interesting because it's counter intuitive: you would expect that the more analysis is done, the more accurate the estimates, but experience tells us the truth is the very opposite.

> but occasionally they will run over

86% is more than "occasionally".

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