The problem with trying to demolish Boris Johnson’s proposal to build a bridge across the Channel is that it’s a sort of category error. It’s not really about improving the communications links between Britain and France. (As Stephen points out, if you wanted to do that, you’d keep the former in the Single Market.)

What it’s actually intended for is to improve the communication links between the foreign secretary and the people who might vote in any theoretical Tory leadership contest, and on that score, given the acres of press coverage this morning, it’s been a great success.

But snarking at things made of concrete is literally what they pay me for, so let’s have at it. A bridge over the English Channel would be need to be at least 22 miles long, if you didn’t want cars to fall off into the water at one end. This is, as you can probably guess, pretty long for a bridge: about 13 times the length of the Golden Gate Bridge, say, or around 25 times that of the Humber Bridge, currently the longest in Britain.

More importantly, it’s also more than four times the length of the closest actually existing equivalent in Europe: the 5 mile Øresund Bridge which links Denmark and Sweden, has helped turn Malmo into a commuter suburb for Copenhagen, and also inspired the greatest TV crime show that there has ever been.

That said, much longer bridges are in fact technically feasible. The longest bridge in the world is a hell of a lot longer than 22 miles: the Danyang-Kunshan Grand Bridge is a 102 mile viaduct which helps carry high speed trains between Beijing and Shanghai.

That, though, runs over land, which strikes me as cheating. The longest bridge over water seems to be the 24 mile Lake Pontchartrain Causeway which bridges a lake in Louisiana. Other contenders include the Atchafalaya Basin Bridge (18 miles, also in Louisiana), the Hangzhou Bay Bridge (China; 22 miles), and the Jiaozhou Bay Bridge (also China; 16 miles, but with three arms, which has caused endless rows in bridge-measuring circles as I’m sure you can imagine).

Something you’ll notice about all these huge bridges: none of them are crossing a stretch of open sea, which has big waves and tides and so forth. That strikes me as a fairly big complicating factor when it comes to building things.

To complicate things yet further, the English Channel is one of the busiest shipping lanes in the world, with several hundred ships passing through every day. Any Channel Bridge would need to either be extraordinarily high, or, like Øresund, drop into a tunnel, to allow ships to pass.

But even then it would, by definition, mean a narrowing of the sea lanes. It’s not clear to me that this is a great way of improving international trade, as Johnson suggests. More likely we’d have ships queuing half way to Norway.

So, it’s a dumb idea. But we always knew that, didn’t we? It’s not really about infrastructure: it’s about grabbing headlines and reducing the bandwidth to talk about anything else. And by talking about it here, I’m colluding in it, so well done me.

Of course, Boris Johnson has form for this sort of nonsense. As mayor he managed to blow £46m on the Garden Bridge without a single brick ever getting laid – which, amusingly, means that the cost of the bridge per mile is literally infinite. He also gave us a cable car which, in 2015, was serving fewer passengers than 406 different London bus routes, and the New Bus For London, which isn’t much cop as a form of transport but is absolutely top class if you need somewhere to roast a turkey. (Both Garden Bridge and bus, incidentally, were designed by Thomas Heatherwick, a man who, vexingly, can still set foot in London without being immediately set upon by a mob of locals carrying barrels of tar and sacks of feathers.)

Most reminiscent of the Channel Bridge nonsense was perhaps ‘Boris Island’ – a proposal to boost London’s aviation capacity by scrapping Heathrow altogether and replacing it with a new, bigger airport on an artificial island in the Thames Estuary (where the nearby voters are helpfully much poorer). When the Airports Commission dismissed the idea of blowing £90bn on this scheme, Johnson simply refused to accept it, and did a round of interviews telling everyone that the idea wasn’t dead when it very clearly was, like the political equivalent of the parrot sketch.

It’s very clear that these schemes are less about improving the world than they are about improving Johnson’s prospects. But it’s still worth asking why he should choose grand-projets as his self-promotional tool as choice. One possibility is that it’s simply because he grew to political maturity (lol) as mayor of London, a post which has substantial power over the built environment and remarkably little over anything else. Perhaps old habits die hard.

Or perhaps his focus on infrastructure reflects a slightly Randian obsession with leaving a physical mark upon the world. Social or economic policies may be easily changed: big things made of concrete are more likely to last, so a man obsessed with power and legacy may find himself looking longingly to those.

A third, related possibility is simply that the word bridge, like the word Boris, begins with the letter B.