Managing large-scale transportation infrastructure projects is difficult due to frequent misinformation about the costs which results in large cost overruns that often threaten the overall project viability. This paper investigates the explanations for cost overruns that are given in the literature. Overall, four categories of explanations can be distinguished: technical, economic, psychological, and political. Political explanations have been seen to be the most dominant explanations for cost overruns. Agency theory is considered the most interesting for political explanations and an eclectic theory is also considered possible. Nonpolitical explanations are diverse in character, therefore a range of different theories (including rational choice theory and prospect theory), depending on the kind of explanation is considered more appropriate than one all-embracing theory.

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This paper gives an overview of good and bad practice for understanding and curbing cost overrun in large capital investment projects, with a critique of Love and Ahiaga-Dagbui (2018) as point of departure. Good practice entails: (a) Consistent definition and measurement of overrun; in contrast to mixing inconsistent baselines, price levels, etc. (b) Data collection that includes all valid and reliable data; as opposed to including idiosyncratically sampled data, data with removed outliers, non-valid data from consultancies, etc. (c) Recognition that cost overrun is systemically fat-tailed; in contrast to understanding overrun in terms of error and randomness. (d) Acknowledgment that the root cause of cost overrun is behavioral bias; in contrast to explanations in terms of scope changes, complexity, etc. (e) De-biasing cost estimates with reference class forecasting or similar methods based in behavioral science; as opposed to conventional methods of estimation, with their century-long track record of inaccuracy and systemic bias. Bad practice is characterized by violating at least one of these five points. Love and Ahiaga-Dagbui violate all five. In so doing, they produce an exceptionally useful and comprehensive catalog of the many pitfalls that exist, and must be avoided, for properly understanding and curbing cost overrun. Five Key Questions about Cost Overrun Cost overrun in large capital investment projects can be hugely damaging, incurring outsize losses on investors and tax payers, compromising chief executives and their organizations, and even leading to bankruptcy (Flyvbjerg et al. 2009, Flyvbjerg and Budzier 2011). Accordingly, cost overrun receives substantial attention in both the professional literature and popular media. Yet it is not always clear how cost overrun is defined, why it happens, and how to best 1 All authors have co-authored or authored publications based on the data, theories, and methods commented on by Love and Ahiaga-Dagbui (2018).

Full reference: Cantarelli, C. C., Molin, E. J. E., van Wee, B., and Flyvbjerg, B. 2012.Characteristics of cost overruns for Dutch transport infrastructure projects and the importance of the decision to build and project phases. Transport Policy, 22: 49-56.

Lock-in, the escalating commitment of decision-makers to an ineffective course of action, has the potential to explain the large cost overruns in large scale transportation infrastructure projects. Lock-in can occur both at the decision-making level (before the decision to build) and at the project level (after the decision to build) and can influence the extent of overruns in two ways. The first involves the "methodology" of calculating cost overruns according to the "formal decision to build". Due to lock-in, however, the "real decision to build" is made much earlier in the decision-making process and the costs estimated at that stage are often much lower than those that are estimated at a later stage in the decision-making process, thus increasing cost overruns. The second way that lock-in can affect cost overruns is through "practice". Although decisions about the project (design and implementation) need to be made, lock-in can lead to inefficient decisions that involve higher costs. Sunk costs (in terms of both time and money), the need for justification, escalating commitment, and inflexibility and the closure of alternatives are indicators of lock-in. In this paper, two case studies, of the Betuweroute and the HSL-South projects in the Netherlands, demonstrate the presence of lock-in and its influence on the extent of cost overruns at both the decision-making and project levels. This suggests that recognition of lock-in as an explanation for cost overruns significantly contributes to the understanding of the inadequate planning process of projects and allows development of more appropriate means.