For some not very clear reason, people always bang on about “building bridges” in politics. Yet when Boris Johnson announces plans to do just that, everyone dismisses them.

To be sure, his proposed bridges have been fair targets for ridicule: there was a fatuous “garden bridge” in London suggested by Joanna Lumley, which cost £53 million (£43 million of it taxpayers’ money) before the powers that be came round to everyone else’s conclusion that it was a terrible idea and abandoned it.

Now there is the bridge from Scotland to Northern Ireland, which would almost certainly cost more than £20 billion, but won’t because it almost certainly won’t be built.

Not, however, as engineers have pointed out, because it’s impossible. The impressive bridge between Denmark and Sweden where people keep getting murdered in TV shows isn’t even one of the world’s 100 longest; there’s one in China that’s more than 100 miles long. It’s just difficult and expensive.

There’s something to be said for a bit of ambition from government – the Channel Tunnel once seemed a similarly fanciful scheme – and from the Tories’ point of view it would send an admirably Unionist message.

One suspects, though, that the Prime Minister, with his classical leanings, chiefly has in mind the title of Pontifex Maximus, supreme religious authority in ancient Rome, and a title the emperor later adopted. Even the Romans weren’t sure that the etymology really had anything to do with bridge-building, but they liked to bandy it about anyway.

The brouhaha about this particular grand projet, at this stage no more than a highly speculative notion, may be useful in drawing attention away from a more awkward question being asked by some Tories. It is, by contrast, why isn’t the government being more ambitious?

Only some of those raising this point are extreme Brexiteers. They reacted to the obstruction of their ambitions by dumping the initial aim of getting out of the EU’s political sphere while keeping much of the single market, and now want instead to be Hong Kong under Sir, or indeed St, John Cowperthwaite.

That attractive prospect has regrettably few adherents, though, even in the Conservative Party. Most activists who want bold policy announcements from the government have less ideological motives. Indeed, their calls for big ideas and urgent action are nakedly political.

The received wisdom is that any government which ends up with a hefty majority should get cracking with the stuff it really wants to do (especially if it might be unpopular) pronto. First, because if it works, the voters might see that it is working before the next election. Second, because if it’s painful, they might get over that or at least used to it, and then you can throw them a lot of sweeties just before the election, to get back in their good books.

On this view, not to get spending cuts or other large changes out of the way quickly, while you have the pull to do so, is to squander an opportunity.

So far, there seems no sign of it – whether there is going to be may be more clearly indicated by the scale of the impending reshuffle. But if there’s not much change there, it doesn’t follow that this government won’t lead to huge transformations.

It may simply be that they have decided that there is a danger in a drawn-up, mapped-out plan of the sort that the Thatcher and Blair governments went in for, because current politics make that degree of change inevitable anyway.

It’s possible, of course, that Mr Johnson doesn’t have any big ideas, or that his party can’t agree on the course to take. I think it’s more plausible that the PM feels that he has already achieved the major change by delivering Brexit (in the sense that we’ve definitely left). He has also committed himself politically on environmental matters and the infrastructure of the north of England. What’s more, he faces unknown upheavals in the international political landscape, some of which will be driven by climate policy and technology.

These issues – Brexit above all, but also new relationships with other countries, rapidly changing voters’ allegiances and priorities, the huge implications and costs of climate policy and the pace of technological change – are so significant in scale, unpredictable in effect, and have such potential for economic gains or losses, that the plans may have to be assembled in the midst of turbulent political waters. If we’re waiting for details of the exact changes Mr Johnson’s government will bring about, I suspect it’s because he is, too.