According to Oxford professor Bent Flyvbjerg, megaprojects in the past 70 years have consistently come in over budget while delivering less than half of their promised monetary benefit. … His research on megaprojects has been cited by both backers and critics of the tunnel. Nine times out of 10, massive infrastructure jobs go over budget, he says. Tunnels on average cost 34 percent more than anticipated. No region is better at predicting costs, and estimates over the past century haven’t become more accurate, his data show. … The difference between the Seattle project and other projects was that the optimism was so pure. That was hugely deluded.” Stuck In Seattle, Bloomberg Businessweek, Apr 6, 2015.

The problem, I always found, was that when I provided what would eventually be an accurate estimate, the first reaction was “No way! It can’t cost that much and take that much time!” When I then showed them the data based upon past projects, I would get responses such as “those were flawed projects, we can’t use those to estimate our project” and “we’ll not have all the problems they had!” What do you think happened? They had similar issues. The details differed, but the costs and delays were in the same ballpark, and the quality was just barely acceptable (at least to some).

We hate it when reality intrudes upon our optimism. However, when we make the jump and accept the sticker shock of the real costs, then we’ve taken our first step towards getting realistic estimates. Once we operate with reasonable estimates based on historical performance instead of compressed schedules and minimal budgets, we start delivering projects on time with good to great quality. Once we are delivering on time and within budget, we are poised to significantly improve the cost and time needed to deliver each project. Why? Because we finally understand what it really takes to deliver a project now that we’ve stepped out of our hugely deluded past practices.

For why this works see In Project Management 9+3 Is Not Equal To 12

How well are your actual costs and schedules matching your original estimates?