Certainly the civil servants will always come up with complex calculations about how every minute of saved journey-time adds to GDP, ditto every car-passenger-mile that is converted into a rail-passenger-mile. It's a great game for all concerned.
But at the end of the day, it's the civil engineering lobby that turns up in Downing Street and says: listen, we only exist because of the Channel Tunnel, Crossrail, the Olympics, T5, HS1, the Jubilee Line extension, the Ring Main etc etc etc to the end of time.
Yes, JM Keynes has never left Whitehall. Keynsian doctrine is the only conceivable rationale behind gigantic programmes such as the onshore and offshore wind-farm mania. It works for the civil engineers - and the consultants, and the lawyers, the financiers, the lobbyists - all good British service industries.
I wouldn't mind quite so much if the hardware content was more local. My understanding is that we have a world-class tunnelling industry in this country, so Crossrail (e.g.) is pretty good in this regard (though don't enquire too closely where the worker-ants hail from). But the wind-farms ... OK, let's not get started on that one.
So - back to the topic in hand. Severn Barrage, anyone ?