This link seems to give a reasonably useful overview: reasonable depth without getting into the horrific fine detail of methodology:
https://www.gov.uk/government/publications/tag-guidance-for-the-senior-responsible-officer-may-2018A few observations:
1. Following the guidance should give some credibility to the idea that forecasts for different projects are
comparable; it doesn't necessarily lead to forecasts that are accurate.
2. There is some mention in the guidance of
proportionality. That is: small projects should not need the same level of analysis as large, expensive projects. I'm not sure that this principle is followed throughout the guidance.
3. The three-stage appraisal process: SOBC,OBC, and finally full business case tends to lead to a lot of re-work. Although, in theory, each stage is adding more detail, producing more accurate forecasts, it can feel like paying for the same work over and over again.
4. Forecasting demand is based on evidence, but evidence is more abundant for some types of project than for others. For example, most new stations post-Beeching have been suburban stations aimed at commuters (and others travelling into cities), so an evidence base has grown from the outcome of such stations. Evidence for completely new train services and for (say) stations serving leisure destinations is harder to find.
5. The complete appraisal process is laboriously technical, which is why there are consultants who are happy to earn a living from the work. Even so, they sometimes make mistakes, such as double-counting or omitting significant elements of cost.
6. There are basic forecasting techniques that are within the grasp of the keen amateur, needing nothing more than a spreadsheet and data - much of which is published.
7. We all love to suggest new stations and services. It would be a step forward to subject these to a bit of homebrew appraisal (because it's surprising how far/long some ideas float around without any idea existing of whether or not they could be viable).
