The party theme for New Year's Eve at Casselman's, a popular nightspot in downtown Denver, was the end of Prohibition. Guests dressed up as flappers and mobsters. The Untouchables, starring Robert De Niro as Al Capone, played on TV screens above the bar. The references were all to the demise of alcohol prohibition in 1933, but the real occasion was the end of the ban on another substance. As of midnight on New Year’s Day 2014 - also known as "Green Wednesday" - recreational marijuana is legal in the state of Colorado.
In November 2012, Colorado voters passed Constitutional Amendment 64, legalising the sale, possession and use of recreational cannabis in their state. In May 2013, Governor John Hickenlooper signed a series of measures to create the world’s first fully taxed and regulated recreational marijuana market. From this month, Colorado residents over the age of 21 can purchase up to an ounce of cannabis in a variety of forms from a selection of licensed retailers, while people from outside the state can purchase a quarter ounce. Marijuana possession and sale is still illegal under federal law, but the Obama administration signalled in August that it would allow Colorado’s experiment to proceed unhindered.
So far, 136 Colorado businesses have been granted licenses to sell recreational weed, most of them in or close to Denver, where at least 17 stores were open for business today. At the 3D Cannabis Centre, which brands itself as "Denver’s discreet dispensary", a queue of at least 100 people snaked out into the snowy car-park as the doors opened at 8am. Denver native Aaron Hughes, 32, was among those waiting in line. "You can get weed tax-free [as in illegally] anywhere around here," he said, "but I wanted to be a part of this. Hopefully, we can show the federal government that there’s support for legalisation and that we can really make it work for everyone."
Many people had come from outside the state to sample Colorado's new system. Kirsten Knouse, 24, travelled from Chicago, where she is training as a teacher. Though her home state of Illinois is poised to legalise medical marijuana later this month, the drug will not be available to treat the range of conditions suffered by Ms Knouse and her husband, Tristan, who have Fibromyalgia and Ankylosing Spondylitis respectively. The couple, who are both also afflicted by seizures and mental health issues, are considering relocating to Colorado permanently. "Tristan smokes and his seizures stop." Ms Knouse said. "I have borderline personality disorder. I hear voices and things like that. But I smoke and it all goes away. Doctors have tried all sorts of things - and nothing else works."
Across town at the Medicine Man dispensary, another long queue stretched around the block in spite of the cold. Though customers waited quietly and politely, the proceedings were nonetheless being supervised by a handful of security staff from Blue Line Protection Group, a private security firm that was set up last year to cater exclusively to the legitimate marijuana industry - one of many peripheral businesses to spring up around legal weed, providing jobs in an otherwise sluggish economy.
Video: Colorado's first day of legal marijuana sales
The industry faces greater security threats than mere petty shoplifting. "You're looking at strong-arm robberies, gang activity, even potentially up to cartel level," said Blue Line spokesman Benjamin Little. "This industry is taking a lot of illegally sold marijuana off the streets - and it is hurting criminal organisations."
California was the first state to approve the sale of medical marijuana in 1996; since then, 18 other US states and the District of Columbia have done the same.
Washington State legalised recreational weed at the same time as Colorado, but sales will not begin there until the summer. As the first state to introduce a regulated recreational retail market, Colorado represents a test-bed for the rest of the US - and the world.
Supporters of Amendment 64 overcame critics of legalisation by promising vast tax revenues from the newly legitimate industry, and the Legislative Council of the Colorado General Assembly predicts that marijuana sales in the state could reach almost $580m (£350m) per year, yielding $67m in taxes to fund schools, roads and other public projects. The authorities also believe they could save as much as $60m previously spent enforcing drug laws, to put to more constructive use.
Colorado already has more than 500 medical marijuana dispensaries. The pot-scented corridor of weed-growing facilities along Interstate 70 on Denver's outskirts has been christened "The Green Zone", while a string of medical marijuana dispensaries at one end of the city’s main drag, Broadway, is known as "Broadsterdam". Only existing dispensaries were allowed to apply for the first round of recreational licenses, and the industry is deliberately taking the roll-out of recreational weed at a modest pace.
"Almost every other industry wants less regulation, less taxes," said Michael Elliott, the executive director of the state's Medical Marijuana Industry Group. "What makes our industry unique is that we've embraced regulation and supported increased taxes. We even funded the campaign to pass those taxes! For us, it's the responsible way forward."
While a "pot tourism" boom is expected - and will undoubtedly be a boon to Colorado's coffers - travellers caught carrying weed in Denver International Airport will be slapped with a $999 fine. Likewise, anyone spotted smoking on the slopes of the state’s celebrated ski resorts faces a minimum penalty of $250 from the Forest Service.
"Not many people will stand up and defend prohibition as a good policy," Mr Elliot said. "We've had a 40-year drug war, spent a trillion dollars and what do we have to show for it? Marijuana is universally available, we have the highest incarceration rate in the world, and sales have been dominated by people who use violence to sell drugs, just like Al Capone did during alcohol prohibition. But now we're creating accountable, transparent businesses."