Now they are talking car crashes. From Brussels comes Project Fear Mk II, a “preparedness” guide for Europe if there is no deal on Brexit. It is Brussels-speak for a terrorism red alert. It covers such things as passports, air traffic control, financial transfers, military bases, data protection, medicines licensing and all the border clutter we have spent half a century removing. Unlike the remainers’ bloodcurdling Project Fear in 2016, this is not an economic fake forecast. It is frontline reality. It is Brexit as Grand Theft Auto.

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Britain’s National Audit Office is joining in. This week it warned that, as of next March, Britons driving on the continent will need new driving permits in the event of a no-deal Brexit. There must be gigantic laybys for traffic jams at Folkestone and staff for “huge bureaucratic delays”. Airbus and Rolls-Royce are already stockpiling spares against a new tariff regime. AstraZeneca is stockpiling medicines. Theresa May is touring the Irish border, like a field-marshal surveying trenches on the Somme.

Do we laugh or cry? I am still laughing, just. The car-crash option is favoured by some leave ideologues. They are technically right that in March a no-deal UK would “crash out” of the EU and revert to World Trade Organization rules. Such anarchy has disruptive appeal to those careless of other people’s jobs, while “taking back control of borders” would meet the leavers’ prime target of stricter immigration control. But the EU allows no new deals with third-party countries, under WTO rules or whatever, until the UK is out next year. In March, ports would swiftly clog up. The movement of people and tourism would plummet. It would be chaos, and even after that “new deals with the rest of the world” could not possibly compensate.

In reality, everyone knowledgeable about Brexit agrees on what will really happen if there is no deal in March. Nothing will change. Planes will keep flying. Ferries will keep loading. Channel Tunnel officials will wave vehicles through. Orders will go out to keep moving, and await further instructions. People at the coalface of the European economy cannot afford the posturing, vanity and idiocy of the Brexit parliament this past week. They have lives to live and mouths to feed. A closed border with the EU, not least in Ireland, would be like closing the Berlin Wall after it had reopened. There would be riots. That is why crashing out would not mean hard Brexit, but rather remain in all but name. When Brexit fantasy hits practical reality, reality will win.

Hard Brexit was surely put to bed by Boris Johnson’s resignation speech in the Commons this week, a confection of negativity and verbosity. He offered no “frictionless” alternative to a customs union with the rest of Europe. The UK may be leaving the EU – for which I believe there is something to be said – but it makes no sense to erect trade barriers between an island and its neighbouring continent. Britain has spent a century moving in the opposite direction. Even in the 1950s, when it dreamed of a greater imperial market, it joined Europe’s free trade zone, forerunner of the present European Economic Area. Hard Brexit is flat-Earthism.

The failure of the House of Commons this week to vote in favour of a customs union, as opposed to May’s botched Chequers plan, was a lost opportunity. Johnson might call the plan a “fantastical Heath Robinson arrangement”, but that was because May changed it to win his support. It is his fault. She should not have bothered. As a result, the opportunity to negotiate a customs union in Brussels from a united, bipartisan base with Labour was scuppered.

The public was promised Brexit, which, as May keeps saying, means Brexit. That is happening. It was also promised frictionless trade, which means frictionless. That is achievable only under a customs union and single market.

As a result of the current shambles, the new Brexit minister, Dominic Raab, must go to Brussels to negotiate “frictionless” on the basis of Chequers, not a simple customs union. The difference is over agreeing tariff collection and regulatory alignment on trading standards. This is all but trivial. Even if the UK were to go for hard Brexit, trade with the EU would still need some such agreement, as would the much-vaunted, largely fantasy, deals with the rest of the world. Talk of Chequers as “vassalage” is ludicrous. Taking back control of trade was always making a mountain of a molehill.

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The outcome of Raab’s negotiations will be messier than were he negotiating on the basis of remaining in the EEA. But the destination has to be the same. There may be more bloodletting ahead but, come the autumn, I am sure we will all be in sight of the Norway option. Whatever may one day be agreed on migration – still Brexit’s hard core and still to be negotiated – a customs union between the nations of Europe cannot be avoided.

Prudent government should always be on guard against car crashes. It is not scaremongering to inspect the fenders and check the airbags. But a crash on Brexit will not happen, and even if it did, the outcome would not be “crashing out” of Europe but rather crashing in. The UK is going to leave the EU next spring. That is law. But no one was asked if they wanted to leave Europe’s economic community. We were promised frictionless. If Westminster’s midsummer madness does lead to a car crash, so be it. In the longer run it will make no difference. Keep laughing.

• Simon Jenkins is a Guardian columnist