Bridge too far

Nick Dekker (Letters, January 9) asks why a Forth tunnel has been estimated at £4.5bn when Glendoe can be built for a small fraction of that. Having asked for a breakdown of this cost, I can confirm that it is made up of a remarkably round £1bn for the actual tunnel and all the rest being for "optimism bias" (no, I don't know either), "contingency fees", "inflation at 7.5% annually", access roads, unspecified "capital charges" and so on.