WHAT is it with Boris Johnson and bridges? I thought he built red buses from wooden crates as a hobby, but it seems he has a thing about bridges and airports built from scratch in estuaries.

The dead cat of a bridge from Scotland to Ireland came back into the news recently, probably to act as a diversion from something or other he doesn’t want us to think about. But is such a thing feasible? I suppose given enough money, time and expert input such things are indeed possible – but that does not make them desirable or practical.

The longest road bridge is the Bang Na Expressway in Thailand, which is 34 miles long, Boris Johnson’s bridge would only be 22 miles. But there is a big difference – the Thailand bridge is on land and is only 30 metres high. What Johnson proposes would span the Irish sea and cut across one of our busiest sets of shipping lanes outside the English Channel.

The longest bridge to span a significant body of water is the Qingdao Haiwan Bridge which spans Jiaozhou Bay in the Shadong Province of China. It is 25 miles long in total, so it does show that perhaps the idea is not so mad after all. But crossing a bay is quite a different matter to crossing a busy sea.

There are several problems with the Scottish/Irish bridge that would need to be overcome. One is a very large cache of unexploded munitions, about one million tonnes, dumped on the seabed at the end of the Second World War, roughly where the bridge is proposed. Simply getting the foundational structures into the seabed would prove immensely dangerous should drilling disturb what are probably now unstable explosives. Even if this problem was resolved, for example by shifting the site to another location (which would increase the length of the bridge), it still doesn’t solve all the major problems the bridge would cause.

The height of the bridge and the distance between the supporting pillars would have to accommodate the ships, tankers and super-tankers that regularly use the north/south shipping lanes. You would also have to ensure that any structures could resist a tanker accidentally crashing into them, for example during dense fog or because of freak navigational errors.

Then there is the day to day weather. Currently, many bridges are closed to traffic when the wind is strong. Speed restrictions are common as is stopping high sided vehicles from travelling across the exposed lanes. The Second Severn Crossing from England to Wales is just three miles long and is often closed or partially closed during bad weather. Imagine how often such a bridge would have had to close over the past two or three months as we experience storm after storm. Cost would be another issue alongside the land needed each side of the bridge to ensure traffic could access the bridge. Cost seems to be a guessing game. In researching this column I’ve read figures from £20 billion to £100 billion. The consensus seems to be that nobody knows how much such a project would cost.

The latest entry to the crossing debate is the suggestion of a tunnel. Something we know could be achieved as we have the existing Channel Tunnel as a proof of concept. But many of these are rail tunnels rather than road. The longest road tunnel is the Laerdal tunnel in Norway at just over 15 miles – still a good couple of miles short. Tunnels will also have many major problems but would make more sense than a bridge.

Humans always want to construct the highest buildings, the longest roads, bridges, railway lines, tunnels and so forth. But achieving these massive feats of engineering often comes at a great cost financially and, sadly, in human lives when constructions go wrong.

One of the most famous bridge collapses was the Tacoma Narrows bridge in 1940. This was a suspension bridge, like the Severn crossing which due to a combination of wind speed and direction began to vibrate then move vertically. The roadway or deck of the bridge began to move vertically and rhythmically and eventually it collapsed. There was only one casualty, a cocker spaniel named Tubby who was too scared to leave his owner’s car, the last car crossing the bridge, which then crashed into the river below along with Tubby. The driver escaped luckily. Perhaps Boris Johnson with his various plans for a Garden Bridge which never materialised and an estuary airport which never got off the ground is crying out to leave a legacy. Perhaps he’s afraid that his legacy will that of a buffoon rather than an engineering genius like Brunel.