When Prime Minister, First Lord of the Treasury, Minister for the Civil Service, Minster for the Union and Novelty Infrastructure Tsar Boris Johnson visited Belfast last month for a Conservative Party leadership hustings, he re-iterated his previous support for a bridge between Northern Ireland and Scotland. It was “the kind of project that should be pursued by a dynamic Northern Ireland government”, as he optimistically phrased it.

It is worth re-iterating just how stupid this idea would be. Such a link would require building 30 support towers each around the same height as the Empire State Building. They would need to be built in Beaufort’s Dyke, a deep-water trench estimated to contain a million tons of explosives (the exact whereabouts of which are unknown), as well as several tons of nuclear waste. It would require building dozens of underwater skyscrapers in an underwater minefield. It would have an estimated price tag of £20 billion. It isn’t going to happen.

That said, the expense of travelling between Northern Ireland and Scotland is certainly a big issue for those travelling across the North Channel. To see how the cost of travelling between Northern Ireland and Cairnryan compared with other ferry routes, I looked at the price of travelling with a car and a driver on all of the ferry routes from or within Great Britain that carry over 100,000 passengers a year and are longer than 10km, to travel on Monday the 2nd of September 2019 (15 days from now).

Where there were multiple sailings on a route, I picked the one that was closest to 12 noon, and where all crossings were fully booked for that date I selected the first available crossing after that date. Distances between ports are estimated on a straight-line basis. If there was more than one operator on a route I picked the cheapest. Prices exclude the cost of cabins where applicable. The results are summarised at the top of the page.

On the basis of cost per kilometre, the most expensive ferry in the UK is the P&O crossing between Larne and Cairnryan, which cost £2.53 per kilometre. The second most expensive crossing is the Stena Line route between Belfast and Cairnryan, which costs £1.84 per kilometre.

On a cost per kilometre basis, these are much more expensive than crossings within Scotland of equivalent length such as between Kennacraig on the Kintyre peninsula and Islay (similar in terms of distance to the Larne – Cairnryan route) which costs £41 for a car and driver, and the route between Ullapool and Stornoway in the Isle of Lewis, which is 10km longer than the Belfast – Cairnryan route but at £62 for a car and driver costs substantially less.

In Scotland, ferries are subsidized by the Scottish government, who spent £209.7m on ferry subsidies in 2016/17. In 2016, 5.7m people were carried on subsidized services.

By contrast, 1.6m journeys were made on services between Northern Ireland and Scotland in 2015. Back-of-the-envelope analysis would suggest that it would cost around £59m per year to provide financial support to ferries across the North Channel to the same extent as the Scottish government currently provides to ferries in Scotland.

A bridge between Northern Ireland and Scotland is plainly a silly idea. However, the costs of travelling by sea across the North Channel are more expensive than equivalent routes anywhere else in Great Britain. The government subsidizing these services to the same extent as Scottish ferries would be of great assistance to communities on either side, but without the need to build dozens of underwater skyscrapers in a trench filled with explosives.

The data used for this analysis can be found here.