Planning fallacy
The planning fallacy is a phenomenon in which predictions about how much time will be needed to complete a future task display an optimism bias and underestimate the time needed. This phenomenon sometimes occurs regardless of the individual's knowledge that past tasks of a similar nature have taken longer to complete than generally planned. The bias affects predictions only about one's own tasks; when outside observers predict task completion times, they tend to exhibit a pessimistic bias, overestimating the time needed. The planning fallacy involves estimates of task completion times more optimistic than those encountered in  similar projects in the past. The planning fallacy was first proposed by Daniel Kahneman and Amos Tversky in 1979. In 2003, Lovallo and Kahneman proposed an expanded definition as the tendency to underestimate the time, costs, and risks of future actions and at the same time overestimate the benefits of the same actions. According to this definition, the planning fallacy results in not only time overruns, but also cost overruns and benefit shortfalls. Empirical evidence. For individual tasks. In a 1994 study, 37 psychology students were asked to estimate how long it would take to finish their senior theses.  The average estimate was 33.9 days. They also estimated how long it would take "if everything went as well as it possibly could" (averaging 27.4 days) and "if everything went as poorly as it possibly could" (averaging 48.6 days). The average actual completion time was 55.5 days, with only about 30% of the students completing their thesis in the amount of time they predicted. Another study asked students to estimate when they would complete their personal academic projects. Specifically, the researchers asked for estimated times by which the students thought it was 50%, 75%, and 99% probable their personal projects  would be done. A survey of Canadian tax payers, published in 1997, found that they mailed in their tax forms about a week later than they predicted. They had no misconceptions about their past record of getting forms mailed in, but expected that they would get it done more quickly next time. This illustrates a defining feature of the planning fallacy: that people recognize that their past predictions have been over-optimistic, while insisting that their current predictions are realistic. For group tasks. Carter and colleagues conducted three studies in 2005 that demonstrate empirical support that the planning fallacy also  affects predictions concerning group tasks. This research emphasizes the importance of how temporal frames and thoughts of successful completion contribute to the planning fallacy. Additional studies. Bent Flyvbjerg and Cass Sunstein argue that Albert O. Hirschman's Hiding Hand principle is the planning fallacy writ large, and they tested the empirical validity of the principle. See also further reading below for additional studies. Methods for counteracting. Segmentation effect. The segmentation effect is defined as the time allocated for a task being significantly smaller than the sum of the time allocated to individual smaller sub-tasks of that task. In a  study performed by Forsyth in 2008, this effect was tested to determine if it could be used to reduce the planning fallacy. In three experiments, the segmentation effect was shown to be influential. However, the segmentation effect demands a great deal of cognitive resources and is not very feasible to use in everyday situations. Implementation intentions. Implementation intentions are concrete plans that accurately show how, when, and where one will act. It has been shown through various experiments that implementation intentions help people become more aware of the overall task and see all possible outcomes. Initially, this actually causes predictions  to become even more optimistic. However, it is believed that forming implementation intentions "explicitly recruits willpower" by having the person commit themselves to the completion of the task. Those that had formed implementation intentions during the experiments began work on the task sooner, experienced fewer interruptions, and later predictions had reduced optimistic bias than those who had not. It was also found that the reduction in optimistic bias was mediated by the reduction in interruptions. Reference class forecasting. Reference class forecasting predicts the outcome of a planned action based on actual outcomes in a reference class of similar actions to  that being forecast. Real-world examples. The Sydney Opera House was expected to be completed in 1963. A scaled-down version opened in 1973, a decade later. The original cost was estimated at $7 million, but its delayed completion led to a cost of $102 million. The Eurofighter Typhoon defense project took six years longer than expected, with an overrun cost of 8 billion euros. The Big Dig which undergrounded the Boston Central Artery was completed seven years later than planned, for $8.08 billion on a budget of $2.8 billion (in 1988 dollars). The Denver International  Airport opened sixteen months later than scheduled, with a total cost of $4.8 billion, over $2 billion more than expected. The Berlin Brandenburg Airport is another egregious case. After 15 years of planning, construction began in 2006, with the opening planned for October 2011. There were numerous delays. It was finally opened on October 31, 2020. The original budget was €2.83 billion; current projections are close to €10.0 billion. California High-Speed Rail is still under construction, with tens of billions of dollars in overruns expected, and connections to major cities postponed until after completion of  the rural segment.