Having been frustrated in his dream of building a garden bridge over the Thames he has now proposed to build one 22 miles over the Channel, calling it “ridiculous” that Britain and France are physically linked only by a single railway line.

It is not as daft an idea as it first sounds. There is already a 34-mile combined bridge and tunnel under construction between Hong Kong and Macau, and the Straits of Dover are no less practical to cross, the water being little more than 50 feet deep at any point.

But is it worth even investigating spending billions on a cross-Channel bridge when the everyday transport which millions of us rely on daily is so poor? The answer to that, surely, is no.

In a week when the Government’s favoured contractor, Carillion, has gone bust, bringing into question several moderately ambitious road schemes, it seems somewhat rash to speculate about a project which would swallow up Britain’s road-building budget for many years ahead.

Meanwhile, towns will go without much needed bypasses. Many of our prime long distance roads, such as the A303, the main road to the West Country, will continue to have single carriageway sections.

The London Tube will remain overloaded and many of our provincial cities will struggle on without any effective public transport system.

Even our existing roads will be covered with lethal potholes which councils will refuse to fill saying they have no money.

Boris’s proposal for a Channel bridge follows a pattern which has become all too familiar over the past 20 years, whether labour or the Conservatives have been in charge. Relatively small transport improvements which would make a huge difference to ordinary people are dropped or put on the back burner.

Meanwhile, ministers dream about vanity projects such as HS2.

It is a sign of how remote our political leaders have become from the concerns of ordinary people. They see the world from their own closeted point of view. Ministers spend their lives doing long-distance day trips from Westminster yet they are spared the drudge of the daily commute thanks to taxpayer-funded accommodation near Parliament.

The best way to improve the transport system would be to build an accommodation block for MPs in Croydon, or somewhere similar.

That would concentrate their minds on our real transport problems.

Anyone who has ever opposed a government vanity project like HS2 knows what to expect. They will be attacked for being “unimaginative”.

Generalised arguments are employed to divert attention from the specific weaknesses of the project.

In the case of Boris’s proposed Channel Bridge there is a problem which doesn’t seem even to have occurred to him: demand, or rather the lack of it.

The Channel Tunnel has never handled the levels of traffic it was forecast to do.

When Mrs Thatcher and the then french President Francois Mitterand signed the treaty to build the tunnel in 1986, it was predicted that the trains would be carrying 21 million passengers a year. Yet in 2015 they carried only 10 million.

Nor has the tunnel ever reached the levels of freight that it was predicted to carry. The existing tunnel is quite able to cope with the traffic between Britain and France.

When the concession to operate Channel Tunnel services was granted to Eurotunnel, it was on condition the company would present proposals for a second Channel crossing, whether it be a bridge or drive through tunnel.

It fulfilled its promise in 1999 but it was agreed by the french and British governments that there simply wasn’t sufficient demand to justify the investment.

By then, over-optimistic traffic forecasts had put Eurotunnel in serious financial difficulties.

It had to persuade the UK and French governments to extend its concession to operate services until the 2080s.

Even then it had to undergo a financial restructuring in 2006 in order to survive.

A lot of the demand for cross-Channel traffic has been picked up by budget airlines.

As for shorter trips from Kent to Belgium and northern France, there is far more limited demand for motorists wanting to make these journeys than there is, say, from North-east to South-east London.

Yet one of Boris’s first acts as mayor of London in 2008 was to cancel the £500 million Thames Gateway bridge which would have made life so much easier for traffic trying to squeeze through the Blackwall Tunnel.

So what makes the Foreign Secretary think that investors would be interested in backing a cross-Channel bridge in open competition with an under capacity Channel Tunnel? The truth is they wouldn’t.

There are plenty of better, if less dramatic, candidates for the money available.

What about that wretched tunnel near Stonehenge which governments have been talking about for 30 years?

Or Crossrail 2, a metro system for the West Midlands? Or turning the A1 into a motorway all the way to the north?

There is no point in building a bridge across the Channel to impress foreign visitors – only for them to then get caught up on potholed roads in the eternal traffic jam that is much of England.