Take a stroll down Abbot Kinney Boulevard, the high street for hipsters in Venice Beach, California, and alongside the brightly painted boutiques offering vintage T-shirts or oat-spiced lattes, you’ll find a sleek, bright red storefront with plate-glass windows that, at first glance, is easy to mistake for a car showroom.

Instead of cars, though, it sells high-end cannabis products: Huckleberry Gummies, Keef root beer, concentrates, creams and lotions, rolled joints and vaporisers, all laid out on carefully curated wooden tables. This is MedMen, part of a national chain of quality pot shops that constitute the normal now that cannabis is authorised for recreational and medical use in California and 16 other states.

Sadiq Khan, the mayor of London, may have pledged to set up an independent commission to study decriminalisation, but to anyone familiar with the largely stalled debate over cannabis in Britain, the proliferation of legal pot shops along well-trafficked thoroughfares is an exotic and improbable sight. California is far from a perfect model to follow, not least because of its ferociously complex patchwork of regulations and requirements that vary from city to city, and other states have struggled with their own problems in setting up the infrastructure of a newly legal industry. Having set off down the road toward full legalisation almost 30 years ago, though, America does have some salient lessons to offer.

First, contrary to the fears of drug alarmists, the ready availability of cannabis products has not been shown to have any significant effect on crime, addiction or traffic accident statistics. Rather, if established and regulated properly, the legal market brings a previously clandestine activity into the open and makes it safer and more accountable.

Second, there’s no virtue in half-measures. Like Britain, many US states started off attempting to restrict legalisation to medical use. But it was only after Colorado and Washington became the first states to open to legal recreational use in 2012 that any meaningful market emerged to challenge the old criminal networks.

A medical marijuana market may have been a theoretical possibility, but in practice government agencies have been reluctant to create one because many of the key decision-makers continued to feel that cannabis was – to use Boris Johnson’s term – a “harmful substance”. At the same time, doctors with no training in the drug’s uses felt no compulsion to prescribe it, which is exactly what Britain has experienced since giving the green light to medical use in late 2018.

In many ways, the UK is now where the US was 20 years ago, when the only legally recognised distributor of medical cannabis was, improbably, a lab at the University of Mississippi in the ultra-conservative deep south that proved powerless to challenge the long-established dominance of the black market.

The lab didn’t manufacture suppositories or gels or edibles. It made no effort to separate out THC, the psychoactive ingredient that gets users high, and cannabidiol, or CBD, whose benefits are said to include everything from pain relief and heart health to treatment for depression, diabetes and acne.

Rather, the lab sent out metal canisters, each containing 300 rolled joints for recipients to use as they saw fit. Many of them would unroll the joints again to pick out seeds and other extraneous material that didn’t burn smoothly, and even then they complained that the cannabis itself was of low quality.

As of the early 2000s, just four people were receiving this service – out of a population then pushing close to 300 million. Those dismal numbers are broadly in line with the findings of a report by a UK cannabis industry group last month that said NHS doctors had issued just three cannabis prescriptions in the two years since legalisation, and private-sector doctors had written just a few thousand. An estimated 1.4 million people, meanwhile, are estimated to be self-medicating with cannabis bought off the street.

Lesson number three from across the Atlantic is that the most effective way to break an impasse of this sort is through pressure from below – much as Sadiq Khan is seeking to pressure the government with his commission. Building a legal cannabis industry from scratch is a daunting prospect – much harder than it was, for example, for the United States to re-regulate the alcohol industry after the end of prohibition in the 1930s – and governments have a natural tendency to prefer the status quo, however unsatisfactory or unpopular, to plunging into the unknown.

Since the 1990s, the pressure for change in the US has come through popular referendums, known as ballot initiatives, that have set pioneering states such as California and Colorado on a collision course with the federal government. One of the peculiarities of the US system, in fact, is that while much of the country has approved legalisation of cannabis, the federal government still regards it as a controlled substance to be used only in tightly defined circumstances. That, in turn, has forced the new wave of entrepreneurs to operate in a weird legal limbo, caught between state and federal systems. Many of them are cash only, since the federal government sets banking rules, and they are unable deduct most business expenses from their federal taxes.

Still, the tide is slowly turning. Business operators are no longer at risk of being raided by federal agents, as they were in the 1990s and early 2000s, and industry lobbyists have been working steadily to relax the banking rules.

Which leads to lesson number four: that even under the best of circumstances the legalisation process is likely to be lengthy and complicated. It’s no coincidence that in California the most visible businesses are at the luxury end of the market. Red tape and high taxes have scared many other operators away, to the frustration of entrepreneurs in lower-income, African American communities that were hit hard by America’s long, punitive war on drugs and are now yearning for some modicum of compensatory justice. Instead, they see a black market continuing to thrive.

The point, though, is to start somewhere. In the early days, Washington told many states – just as Downing Street has told Khan – that campaigning for broad legalisation was a waste of time because they didn’t set drug policy. If the states had listened, US consumers would never have gained access to the sort of high-quality products and expert guidance available at MedMen and other outlets. They’d still be waiting for their canisters from the University of Mississippi – or, more likely, scoring their hits on the street.