This brought me to look up Norway's tunneling record & it is impressive.
There are over 900 road tunnels in Norway. The total length of the tunnels is over 750 km. Almost all of them built between 1982 & 2000. Clearly there would be substantial cost savings doing a lot of tunneling rather than just one project. IndeNorwegianina cost are extremely competitive. This goes into more detail on construction & cost
The longest road tunnels (>7 km, with opening year and length)
Lardalstunnelen, 2000, 24505 m
Gudvangatunnel, 1991, 11428 m
Folgefonntunnel, 2001, 11150 m
Korgfjelltunnelen, 2005, 8530 m
Construction costs for the tunnels which are now open are shown in Figure 2. All costs are based on year 2000 costs, according to price indexes of the Ministry of Transportation and Communication.Since there are 11 Kroner to the pound this makes tunneling costs from £3.2 million per kilometer to £10 million. Even with multilane dual carriageway & motorways we are talking about a pretty fair saving.
From 1992 to 2000, prices have increased linearly by 37 per cent. This is higher than the official price index. The reason for this is the improvement in tunnel standards, which has not been compensated for in the Ministry's price index.
Costs for planning and field work are not included for all of the tunnels. It is estimated that these costs are somewhere between NOK 2,000 & 4,000 per metre tunnel. This does not apply to the last tunnels which have been completed, where all costs are included in the survey.
The total construction costs vary from NOK 35,000 to 115,000 per metre. The Tromsasund tunnel is expensive because of its double tubes, whilst the Nordkapp tunnel is costly because of the poor rock quality in the tunnel.
The conclusions to be drawn is that subsea tunnels have become cheaper, but that rock conditions are decisive for the final price.
Useful Tunnels Projects in Scotland
Forth Crossing - I firmly believe the Forth Road Bridge can be reroped for £100 million but with traffic increases an additional tunnel would be worthwhile.
Glasgow Motorway Extention - The present above ground proposal is costed at £500 million apparrently relocting costs & because some of the ground is said to be polluted by chrome. Obviously a tunnel with bypass outlets would be far cheaper & would not cause the pollution problems opponents claim to be motivated by.
Gourock/Dunoon - Much of Argyllshire is remote from the central belt because of long lochs & roads which need to go round them. The road distance between Gourock & Dunokilometerskilometres despite facing each other across the Clyde.
Cowal Penisula/Bute - A few miles south of Dunoon. With 2 tunnels Rothesay would be about 35 miles. A pleasnt commute whereas now it takes virtually a full day including ferry.
Loch Fyne Tunnel - There are several possible crossings leading on from the Dunoon crossing which would put the Kintyre peninsula within about 60 miles of Glasgow.
Arran - Either from Ayrshire (the longer & more expensive tunnel) or from Kintyre which could tie into the roads mentioned above.
Oban Mull - Makes the place accessible to 10s of thousands of Balymory fans.
Kintyre/Jura - Another almost uninhabited island which could become a one hour drive from Glasgow.
Islay/Kintyre or Jura - Direct from Kintyre would be about 15 miles, linking to Jura would be much cheaper. Again this island has a very small population because it is, by current methods, inaccessible. Islay is know as the Queen of the Hebrides because, being the most southerly & well out into the Gulf Stream it used to be the capital of the Lordship of the Isles. When the ancient Scots kingdom & later Viking lordship communicated by sea it was very centrally located but because our transport methods are now road based it is isolated. With an area similar to the Isle of Man & & more temperate weather, because of the Gulf Stream, it could be as prosperous if it were an hour & a half drive from Glasgow.
Orkney/Mainland - This has already been proposed. It would be expensive but Orkney has an oil fund & should be prepared to put up most of the funding.
Ulster/Galloway or Kintyre - About 15 miles from Kintyre, 25 from Galloway. A Kintyre tunnel was seriously looked at last century - the technology has improved since. I assume that Ulster, which would benefit even more than Scotland would put up a proportionate share of the cost.
Isle of Man/Galloway - About 20 miles. Man could reasonably be expected to put up the bulk of the money.
Skye/Lewis - Again about 20 miles.
I don't say that all these will work & there may well be others where a tunnel would be a practical way from one glen to another. I do say that improving transport infrastructure is something where government investment almost always pays off. I can think of nothing which would so revitalise the Island communities. Check the map yourself for ideas.
Paying for it
The Executive have already talked of a Forth Bridge costing a billion & Glasgow motorway £500 million. This entire programme might well cost less. Beyond that the use of a land capture tax, whereby a proportion of the increase in value of land sales on the isles, Cowal or even Fife could be taken as payment. After that some money could be retained by local development corporations. Islay, for example has 3,000 inhabitants over 600 square miles so the land value cannot be high. It wouldn't take the building of many homes there to pay for a tunnel. There could also be a case for giving the development organisation authority comparable to that of the Manx Parliament. Home Rule did them no harm.