It's a truism that infrastructure project promoters routinely overstate benefits and understate costs. According to Bent Flyvbjerg, a professor of major program management at the University of Oxford's Said Business School, "cost overruns in the order of 50 percent in real terms are common for major infrastructure, and overruns above 100 percent are not uncommon. Demand and benefit forecasts that are wrong by 20-70 percent compared with actual development are common."
"It is not the best projects that get implemented, but the projects that look best on paper," Flyvbjerg writes. "And the projects that look best on paper are the projects with the largest cost underestimates and benefit overestimates, other things being equal."
Read more in the Bloomberg article.