¿Imprecisión de las previsiones de los proyectos de obras públicas? Ámbito del transporte (original) (raw)
Related papers
How (In)accurate Are Demand Forecasts in Public Works Projects? The Case of Transportation
Journal of the American Planning Association, vol. 71, no. 2, pp. 131-146, 2005
This article presents results from the first statistically significant study of traffic forecasts in transportation infrastructure projects. The sample used is the largest of its kind, covering 210 projects in 14 nations worth U.S.$59 billion. The study shows with very high statistical significance that forecasters generally do a poor job of estimating the demand for transportation infrastructure projects. For 9 out of 10 rail projects, passenger forecasts are overestimated; the average overestimation is 106%. For half of all road projects, the difference between actual and forecasted traffic is more than ±20%. The result is substantial financial risks, which are typically ignored or downplayed by planners and decision makers to the detriment of social and economic welfare. Our data also show that forecasts have not become more accurate over the 30-year period studied, despite claims to the contrary by forecasters. The causes of inaccuracy in forecasts are different for rail and road projects, with political causes playing a larger role for rail than for road. The cure is transparency, accountability, and new forecasting methods. The challenge is to change the governance structures for forecasting and project development. Our article shows how planners may help achieve this.
Transport Reviews, vol. 26, no. 5, pp. 537-555, 2006
Based on a review of available data from a database on large‐scale transport infrastructure projects, this paper investigates the hypothesis that traffic forecasts for road links in Europe are geographically biased with underestimated traffic volumes in metropolitan areas and overestimated traffic volumes in remote regions. The present data do not support this hypothesis. Since previous studies have shown a strong tendency to overestimated forecasts of the number of passengers on new rail projects, it could be speculated that road planners are more skilful and/or honest than rail planners. However, during the period when the investigated projects were planned (up to the late 1980s), there were hardly any strong incentives for road planners to make biased forecasts in order to place their projects in a more flattering light. Future research might uncover whether the change from the ‘predict and provide’ paradigm to ‘predict and prevent’ occurring in some European countries in the 1990s has influenced the accuracy of road traffic forecasts in metropolitan areas.
The cost performance of transportation projects: The fallacy of the Planning Fallacy account
Transportation Research Part A-policy and Practice, 2019
Delivering transportation projects to their budgeted cost remains a challenge for many governments worldwide. An issue that has hindered progress being made to address this problem has been the availability of empirical data that reflects the changing nature of cost estimates and their difference from a project's final account. Using a homogenous dataset provided by a public sector authority in Hong Kong, we analyse the differences between the approved budget, pretender estimates, contract sum and final accounts for approximately HK$115 billion (≈US$14 billion) worth of transportation projects. We demonstrate that 47% (i.e. ≈ 5 out 10) of the projects deviate from their approved budget. In particular, when we consider the difference between the approved budget and the final contract sum, we reveal there are cases of both over and under estimating. We, therefore, question the Planning Fallacy as an appropriate explanation for describing 'how large infrastructure projects work'. The fallacy of the Planning Fallacy account revealed in this paper leads us to call upon those agencies that have actively embraced this theory to reconsider their approaches to cost estimating and risk analysis used to deliver their transportation infrastructure to ensure taxpayers are provided with better value for money.
In this paper, evidence from the literature on the inaccuracy of forecasts from transport planning models is presented and its impact on capital infrastructure planning decision-making is demonstrated. Empirical evidence suggests that forecasts used for major planning decisions internationally have been found to be rather inaccurate (when comparing forecast flows with actually realized flows after time passed). Evidence of these irregularities in the case of a major expressway infrastructure project in Southern Greece is presented, providing a typical example of a country with an inadequate freeway network. Data from the immediate impact zone of the Attiki Odos tollway in the metropolitan area of Athens are used to demonstrate that the project resulted in a considerable change in the land use patterns and density, resulting in the generation of additional traffic flows. Having demonstrated the need for rethinking how longterm traffic is forecast, suitable recommendations for promising directions are made. Dealing with uncertainty is one key aspect that can be easily incorporated to existing forecasting tools. Furthermore, the need for more detailed and areapecific models, e.g. through the integration of activity-based modeling, specific obility patterns and demands etc. are also outlined.
Cost Overruns and Demand Shortfalls in Urban Rail and Other Infrastructure
Transportation Planning and Technology, vol. 30, no. 1, pp. 9-30, 2007
Risk, including economic risk, is increasingly a concern for public policy and management. The possibility of dealing effectively with risk is hampered, however, by lack of a sound empirical basis for risk assessment and management. This article demonstrates the general point for cost and demand risks in urban rail projects. The article presents empirical evidence that allow valid economic risk assessment and management of urban rail projects, including benchmarking of individual or groups of projects. Benchmarking of the Copenhagen Metro is presented as a case in point. The approach developed is proposed as a model for other types of policies and projects in order to improve economic and financial risk assessment and management in policy and planning.
Inaccuracy in Traffic Forecasts
Transport Reviews, vol. 26, no. 1, pp. 1–24, 2006
This paper presents results from the first statistically significant study of traffic forecasts in transportation infrastructure projects. The sample used is the largest of its kind, covering 210 projects in 14 nations worth US$58 billion. The study shows with very high statistical significance that forecasters generally do a poor job of estimating the demand for transportation infrastructure projects. The result is substantial downside financial and economic risk. Forecasts have not become more accurate over the 30-year period studied. If techniques and skills for arriving at accurate demand forecasts have improved over time, as often claimed by forecasters, this does not show in the data. For nine out of ten rail projects, passenger forecasts are overestimated; average overestimation is 106%. For 72% of rail projects, forecasts are overestimated by more than two-thirds. For 50% of road projects, the difference between actual and forecasted traffic is more than ±20%; for 25% of road projects, the difference is larger than ±40%. Forecasts for roads are more accurate and more balanced than for rail, with no significant difference between the frequency of inflated versus deflated forecasts. But for both rail and road projects, the risk is substantial that demand forecasts are incorrect by a large margin. The causes of inaccuracy in forecasts are different for rail and road projects, with political causes playing a larger role for rail than for road. The cure is more accountability and reference class forecasting. Highly inaccurate traffic forecasts combined with large standard deviations translate into large financial and economic risks. But such risks are typically ignored or downplayed by planners and decision-makers, to the detriment of social and economic welfare. The paper presents the data and approach with which planners may begin valid and reliable risk assessment.
IEEE Transactions on Engineering Management
Transport projects are regularly subjected to cost misperformance. The contingency set aside to cover any increases in cost due to risk and uncertainty issues is often insufficient. We review approaches that have been used to estimate a cost contingency. We show that some approaches such as reference class forecasting, which underpins the planning fallacy theory, take a biased view to formulate a contingency. Indeed, there is a perception that the risks and uncertainties that form the parts of a cost contingency cannot be accurately assessed using heuristics. The absence of an overarching theory to support the use of heuristics has resulted in them often being downplayed in a project's investment decision-making process. This article fills this void and provides the theoretical backdrop to support the use of heuristics to formulate a cost contingency. We make a clarion call to reconcile the duality of the bias and heuristic approaches, propose a balanced framework for developing a cost contingency, and suggest the use of uplifts to derisk cost estimates is redundant. We hope our advocacy for a balanced approach will stimulate debate and question the legitimacy of uplifts to solely debias cost estimates.
Planning, Evaluation and Financing of Transport Infrastructures: Rethinking the Basics
Review of Network Economics
This paper revises some of the common views on transport infrastructure investment and proposes alternative ways to achieve a more efficient planning, evaluation and financing of transport infrastructures in a world where planners may pursue their own interests, there exist different levels of government, and budget constraints are pervasive. We focus on the need for public planning and independent economic evaluation, and the importance of deciding the pricing scheme in the planning phase. We also discuss the institutional design and its effect on investment decisions, particularly, the financing of projects under different levels of government and its perverse consequences on infrastructure capacity choices. We use as an example the development of the HSR to serve medium-distance trips in corridors where air transport is a very close substitute.
Transportation Research Part A: Policy and Practice, vol. 39, no. 6, pp. 522-530, 2005
Project promoters, forecasters, and managers sometimes object to two things in measuring inaccuracy in travel demand forecasting: (1) using the forecast made at the time of making the decision to build as the basis for measuring inaccuracy and (2) using traffic during the first year of operations as the basis for measurement. This paper presents the case against both objections. First, if one is interested in learning whether decisions about building transport infrastructure are based on reliable information, then it is exactly the traffic forecasted at the time of making the decision to build that is of interest. Second, although ideally studies should take into account so-called demand “ramp up” over a period of years, the empirical evidence and practical considerations do not support this ideal requirement, at least not for large-N studies. Finally, the paper argues that large samples of inaccuracy in travel demand forecasts are likely to be conservatively biased, i.e., accuracy in travel demand forecasts estimated from such samples would likely be higher than accuracy in travel demand forecasts in the project population. This bias must be taken into account when interpreting the results from statistical analyses of inaccuracy in travel demand forecasting.