I have been reading the 2006 master plan for Southampton where the forecast was that passenger numbers would grow from 1.84 million in 2005 to 3.05 million by 2015. So what went wrong seeing that passenger numbers are lower now than in 2005? Was it actually ever achievable or just a fantasy report based on wrong assumptions?
Also noted that the forecast from 2015 - 2030 was to grow from 3.05 million to 6 million.
Expressflight has summed up the difficulty very neatly.
Other regional airports in the south of Britain were also wide of the mark in their master plan predictions of a decade ago, which were prompted by the then government's 2003 White Paper
The Future of Air Transport.
Cardiff Airport predicted 4.796 mppa by 2015 and 7.813 mppa by 2030. 2015 actually saw 1.158 mppa which, in fact, is over 40% down on that airport's best year, 2007.
Exeter airport predicted low and high case scenarios of 1.415-1.956 mppa by 2015 and 1.926-4.037 mppa by 2030. 2015 actually saw 0.821 mppa.
Bristol Airport predicted 8.075 mppa by 2015 and 12.476 mppa by 2030. 2015 actually saw 6.781 mppa.
All this seems to suggest that it's impossible to predict with any degree of accuracy airport growth over ten years let alone 25 years.