Hindsight is a wonderful thing. Economists are good at explaining market crashes, booms or recessions after the event. They are not nearly so good about telling you about them beforehand.
- Journalist David Smith, writing in the English Sunday Times, 6/8/07.

We conclude that the patronage estimates used by planners of rail infrastructure development are highly, systematically and significantly misleading (inflated). This results in large benefit shortfalls for rail projects. For road projects the problem of misleading forecasts is less severe and less one-sided than for rail. But even for roads, for half the projects the difference between actual and forecasted traffic is more than ± 20%. On this background. planners and decision makers are advised to take with a pinch of salt any traffic forecast that does not explicitly take into account the uncertainty of predicting future traffic. For rail passenger forecasts, a grain of salt may not be enough.
- Fronm an analysis of forecasts for 27 rail projects and 183 road projects by Bent Flyvbjerg, Mette Skamris Holm and Soren Buhl, in How (In)Accurate are Demand Forecasts in Public Works Projects?, Journal of the American Planning Association, spring 2005.

If the forecasters assembled for this research are representative of the profession at large, the problem of biased forecasting has little to do with venality and ambition. Rather, it relates to unmet expectations and hidden opportunities to work constructively in alternate decision-making environments.
- Conclusion of The Ethical Challenges and Professional Responses of Travel Demand Forecasts, 2003 PhD thesis by Anthony Brinkman, University of California, Berkeley. (More detail given elsewhere)

It is my belief that in transportation planning there is a need for more detailed, comprehensive and objective observation, and perhaps less theorising.
- Juri Pill, in the preface to Planning and Politics (1979)

The commanding general is well aware that the long range forecasts are no good. However, he needs them for planning purposes.
- Response of US Air Force when, during WWII, their weather forecasters found that the long range forecasts (one month ahead) were no better than numbers pulled out of a hat, and asked to be relieved of this duty.

A traffic model, which is a combination of computer software and data on traffic movements to describe traffic flow, is an idealised representation which ignores illegally-parked cars, bus-stops, incidents, irate drivers, lollipop ladies, roadworks, the marching band of the Highland Light Infantry, bad weather, malfunctioning signals and any other event likely to be encountered on a real journey.
- Stephen Druitt, in An Introduction to Microsimulation, Traffic Engineering and Control September 1998.

... the main lessons to be learned is that cost overruns of 50-100% are common and overruns above 100% are not uncommon. Traffic forecasts that are incorrect by 20-60% compared with actual development are common in large transport projects ...
- Mette Skamris & Bent Flyvbjerg, Inaccuracy of traffic forecasts and cost estimates on large transport projects

No planning information is more political - and more moral or immoral - than the traffic planner's choice of what to feed into his computer.
- Hugh Stretton, in Ideas for Australian Cities

A common feature of many environmental assessments is the doubt which submittors seek to cast on the reliability and accuracy of traffic forecasts. In the light of the importance placed on traffic volumes by VicRoads ... the Committee finds it surprising that so little attention is apparently paid by VicRoads to monitoring the outcomes of its traffic predictions.
- from Advisory Committee Report on the relocation of the Hume Freeway between Craigieburn and the Metropolitan Ring Road, 1999. VicRoads is the state road planning agency in this case, but the point is a general one.

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