Boris Johnson wants to build a bridge across the Irish sea, linking Scotland to Northern Ireland. He's asked the Department for Transport to cost it, and they reckon it would be about £15 billion.

The idea has been received as a bit of a joke: a zany politician suggesting a zany policy – haha. But a "fixed link" over the Irish sea shouldn't necessarily be dismissed out of hand.

First, let me declare an interest. In January 2018 I posted a tweet semi-seriously suggesting a bridge or tunnel to link Edinburgh, Glasgow, Belfast and Dublin by high-speed rail. It went viral, got picked up by the Scottish newspaper The National as a campaign the next day, and then snowballed until Boris Johnson adopted it as a policy.

I'm of course far from the first person to suggest a crossing - something similar has been in the DUP manifesto for some time, and the idea itself is over a century old. I may however be partly responsible for the current wave of enthusiasm.

There are things to legitimately criticise about the idea: firstly, Johnson's proposed route from Stranraer to Larne, I was told about 100 times when I suggested it, is over an old WWII munitions dump, Beaufort's Dyke. Aside from being full of bombs, it is also very deep, which would pose its own construction challenges for a bridge or tunnel in itself. People have also raised other concerns, such as whether building a bridge there would interfere with fishing fleet movements.

But the fact that the idea of a fixed link across the Irish Sea is treated like a joke in the UK is not a good thing: Britain has underinvested in its infrastructure for decades, and in other countries a similar crossing would be examined as a serious proposal and probably built long ago.

Japan's islands of Honshu and Hokkaido are linked by the Seikan Tunnel, a railway tunnel that carries high-speed trains over a length longer than the channel tunnel (and indeed, longer than Scotland to Northern Ireland). Despite being sparsely populated, Norway copes with its fjords by having invested in around a dozen tunnels over five kilometres long, all meant to give more direct routes to places that would otherwise require a detour or a boat trip. Denmark and Germany are in the process of building the Fehmarn Belt Fixed Link, crossing an 11 mile stretch of the Baltic Sea, to dramatically cut travel times between the two countries. Finland and Estonia are looking seriously at linking their capitals with a 30 mile sub-sea tunnel, with the aim of bringing the two cities within commuting distance of each other.

Building bridges and tunnels to bring places closer together is not a crazy idea: it is a completely normal thing to do. Britain, of course, has the Channel Tunnel to France, which has grown into a success – with the Eurostar, car Shuttle services, and freight trains running through it. It probably wouldn't exist without the French enthusiasm for infrastructure investment, but nobody would close it now. Why not a link over the Irish Sea?

There is certainly demand to travel between Great Britain and the island of Ireland. London to Dublin is the busiest air route in Europe by some measures: it is currently plied by Ryanair and its competitors, spewing out vast amounts of CO2 shuttling people back and forth on unnecessary short-haul flights that could be done by a Eurostar-style rail service. If fighting climate change is a priority, a fixed link to replace that busy air corridor – and those from other UK cities – would be a good place to start. Around 40 per cent of Ireland's physical trade also passes overland through Britain and freight trains and lorries loaded onto shuttles could make use of the tunnel.

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Holyhead to Dublin would probably be a better place for a link than the North Channel: though further, the seabed is shallower, and it is already the route that the vast majority of surface traffic across the Irish Sea chooses to take. With a rail link like the one built for the Channel Tunnel, the train journey from Manchester and Liverpool to Dublin could be around two hours, and London to Dublin under three. Services from Scotland and other parts of the country would be viable, too: the Common Travel area means no passport checks are needed and passengers could get on and off at intermediate stops. Services could eventually continue to Belfast; a new cross-border high-speed railway could cut journey times between the two cities to about half an hour (it's a two hour drive).

The total travel time would be quicker than flying, it would be more environmentally friendly, and the journey would just be more comfortable. There would be economic benefits to making the journey easier and putting Dublin and Belfast in the same arc as England's northern cities. Plus the island of Ireland would would for the first time gain a direct rail link to the whole of Europe and Asia.