In message
-septem
ber.org, at 08:45:48 on Mon, 2 May 2016, Recliner
remarked:

And was it based on 1985 service levels and fares, or today's?

Fares were adjusted for inflation, but used current passenger numbers.
Back when the line was built all the passengers were going to T123,
whereas with today's figures only about half of them are. Putting some
numbers on it, Heathrow was handling 30m passengers when T4 was being
built, and latest figures (2015) have T123 handling 32.6m [T4+T5 is
40.8m].

So you need to reduce the volumes by about 10%, for the average in the
1977-1986 period?

I wasn't going to bother estimating second order quantities. For
example, I've ignored the passengers abstracted from the Piccadilly Line
by HEx.

Also, I wonder how much of that traffic was simply displaced from Hounslow
West, which previously provided the less-satisfactory Heathrow link?

That's accounted for by me using the very low figure of £2 extra fares
(at today's prices) being put in the kitty. The rest of the fare goes
towards funding Hounslow to Central London.

Obviously, with projects like this, there are other benefits
(less pollution and traffic, quicker and more predictable journeys,
etc), so even if it doesn't make a financial return, you might still do
them, and I think that's the case here and in many other urban public
transport schemes.

I was a bit surprised to discover that Atlanta's MARTA (bus and 'RER')
was only one-third funded by the fares basket.
--
Roland Perry