The point there with the deviation not improving is particularly important to him, with 70 years worth of data available how are we continuing to be so consistently low?In a worldwide study of 258 rail, bridge and road projects over 70 years, Flyvbjerg found that nine of 10 went over budget. In another analysis, he showed that the average transit system carries less than half as many riders as preconstruction forecasts predicted. ... A decade later, by collecting data from 20 nations on five continents, Flyvbjerg had produced the first statistically significant analysis to show what [an economist at the US DOT] had argued with case studies: The vast majority of public works projects go drastically over budget and aren’t as well patronized as proponents claim. He also found that modelers didn’t seem to be improving their estimates over time; the scale of overruns remained relatively constant. Rail and highway projects are often the worst boondoggles, and they form the bulk of Flyvbjerg’s research. But other researchers have found dangerous overoptimism in all kinds of megaprojects.
He calls it "strategic misrepresentation" - the crucial thing for these people is to get the project in motion, you can ask for more money later and by the time the full cost can be counted it's likely that you're out of office anyway.Few in the field question the existence of faulty cost and need estimates for public works projects. But the experts fight bitterly over the causes for the consistently inaccurate projections and how to address them. Flyvbjerg’s opinions on the subject can be distilled to a single theme: The politicians are lying to us. ... Flyvbjerg suggests that two dynamics underpin the horribly inaccurate estimates for most megaprojects: Either politicians, planners and consultants are lying outright to get the projects funded, or they’re guilty of “optimism bias,” succumbing to the psychological tendency to overestimate the likelihood of positive events. It may seem a pessimistic assessment, but Flybjerg stands by what he’s read, seen and heard.
In a sense, what he's saying isn't surprising; what is interesting is that he's got the data to back up what we would ordinarily just state as opinion or suspicion - when you see the plans for a large public project, expect to add another 50% onto that quoted cost. And his interest in public vs private projects seems to be because the public project is spending our money rather than a private corporation's money; the private developments still go off the rails and in similar patterns.