Is there a systematic bais in transport forecasting?

31 January 2010

Concerning the TVB News magazine programme on 2010-1-28,

新聞透視 2010-01-28 – 客從何處來? http://mytv.tvb.com/news/newsmagazine/104278#page-1

A transport department bloke maintained in the programme that the emphasis is the trend, and they did well on that, but even with that granted:

1. We all know that there is inflation. That’s a trend. That doesn’t help us very much in making our investment decisions. We need more specific numbers.
2. Why do all estimates of major (and ministers’ “pet”) projects turned out to be exaggerated by a factor of 2 or 3? Either there is a systemic error to be explained – or the programme had a bias.

Now let’s consider the following comment from an “insider”:

“wchan_9910” commented on TVB News Magazine at 2010.01.28 (四) 07:04AM:

I am a transport planner who actually work on patronage forecast / traffic forecast of some infrastructure. Forecast are done based on a series of assumptions, like economic growth, population growth, vehicle ownership, etc.. However, there is some implicit assumptions which do change over years but is assumed not to change over time, like unemployment rate by age group, work force ratio, no. of student left school by age group, retirement age, etc..

I worked in HK, UK, Europe and China. I do find the quality of HK professional is not bad at all. In some case, the work of HK professional has been made referenced by oversea player. Like travel characteristic survey which has been made referenced by at least chinese, singaporian. UK professional also adopt the methodology used by HK professional in their airport expansion projects in London in which I worked on! These methodologies has been regarded as a good way to do things in UK, albeit only on a few elements.

Now, If the forecasting work is indeed “good”, there should be overestimates and underestimates.

My question remains: Why did all the forecasts for the “major” and controversial transportation projects shown on the programme turn out to be overestimates?

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