TAVISH Scott makes an interesting case for spending GBP609m on a tunnel under Edinburgh Airport (Letters, November 3) for a return of GBP1.35bn, but all is not so easy.

First, there is a certain slipperiness in the phrase "transport benefits" to cover the GBP1.35bn. This does not seem to mean the more traditional profits but merely estimated benefits accruing to somebody or other. All businesses have, or can reasonably claim to have, such benefits but traditionally work from profits. All could equally claim government money to allow them to provide third-party "benefits".

Secondly, the cost-benefit ratio depends heavily on the costs being kept to - the Scottish Executive does not have a good record here.

Thirdly, and most importantly, his assessment of "benefits" is amortised over 60 years. Any accountant will explain that an investment that doesn't start paying off heavily within four years and in full within 10 is, usually, at best a marginal one and not one he could professionally recommend in normal circumstances.

This is because of interest rates. To repay 2.13 times your investment (ignoring that that we are talking of "transport benefits", not real money) implies an interest rate of 1.35per cent over 60 years. If the Scottish Executive offers me GBP609m on such terms, I could repay in pure profit let alone in job creation "benefits". I might even become a billionaire on margin, but would be willing to bear that burden. This long amortisation is reminiscent of the executive's previous argument that the parliament building was very good value if you amortise the cost over about 500 years.

I would be interested to hear of the results of his assessment of the alternative proposal put forward by a number of people, of instead building stations on the existing Glasgow and Aberdeen lines which pass within hundreds of yards of the airport and provide a moving walkway or other connection. This would be likely to cost in the hundreds of thousands of pounds rather than hundreds of millions. The Transport Ministry might be able to confirm that it would also have a comparably improved cost-benefit ratio. It would also leave GBP600m which, if used to cut business taxes, in the Irish manner, would result in several times that investment in the productive economy. It is not the grand white elephants of government policy that create a successful economy so much as the unencumbered day-to-day work of ordinary businesses.

Neil Craig, 200 Woodlands Road, Glasgow.

THE Transport Minister, Tavish Scott, has been given sufficient column inches to respond to the SNP claim, which suggests a cover-up about the economic and other perceived benefits that would ensue from the delivery of the two airport rail links.

As someone who has extensive experience in both tranport businesses, perhaps I might be given the same amount of column inches, not from a political point of view, but as one who earnestly believes the executive is taking us down this route mainly from a dogmatic point of view rather than a practical one.

The Edinburgh link, even if its costs remain at GBP609m, but are more likely to be nearer the GBP1bn mark, will never deliver GBP1.35bn in transport benefits; norwill it remove 1.7 million car journeys, per year, from Scotland's roads, especially when the best estimate of usage for the airport rail link is a figure of 20per cent, and that is over many years. The same can be said of the Glasgow link, which has a projected usage of only 4per cent, even by 2030.

These statistics have not been conjured up by myself, but are to be found in consultants' reports, prepared for the executive and other interested parties.

Those of us who objected to the Earl and Garl schemes have done so on the basis that as presently designed they will be a waste of public funds and a continuous drain on the public purse. No-one is saying that cities such as Edinburgh and Glasgow should not have rail links to their airports, but that these rail links can be provided for much less capital expenditure, and would be more likely to attract the airport user.

Mike Dooley, 52 Auchendoon Crescent, Ayr.