It’s 4/20, a.k.a. National Weed Day: the day that a heady smog rises above every capital city, and hard currencies are replaced by fistfuls of crushed Doritos. In anticipation of 4/20, states in Australia and the United States have blazed up early, by announcing their plans to (partially) legalise cannabis.

Such decisions are made at national and state level. But, while advocates used to cite data collected from cannabis convivial countries like the Netherlands and Uruguay, a move towards legalisation in many U.S. states has lead to a spate of research at city level.

So, with this in mind, what impact does legalising cannabis have on a city and its infrastructure?

Economic benefits and drawbacks

Established weed welcomers have been long been aware of the economic benefits of legalisation: in the Netherlands, tax on coffee shops alone nets the government over €400m per annum. This is despite efforts by city councils to curtail the number of people who can buy and smoke cannabis.

Since Colorado legalised cannabis in November 2012, the state capital Denver has seen a “gold rush” of tourists, investment and new residents. A recent report from the Drug Policy Alliance found that the opening of just two dispensaries in Denver created 280 jobs and an economic output of $30m in the first half of 2014. There has also been an impact on the city’s housing market, with rent prices increasing by 9.6 per cent in 2014 and real estate prices rose by 10 per cent.

That said, these numbers are only impressive if a city actually wants drugs tourists and half its workforce priced out of the housing market.

And even though the sale of cannabis has benefited the Dutch economy, in October 2011 the border-city of Maastricht started banning foreigners from buying and smoking it. City authorities declared that drugs tourism was causing major traffic problems and disrupting residents’ ability to use the city. More recently Amsterdam, has started closing coffee shops in an attempt to make its central tourist district a bit more classy (elitist) and less sketchy (fun).

Less petty crime, more serious crime

Colorado legalised cannabis in 2012. Two years later, arrests for possession were down by 95 per cent in comparison to 2010. (You can still be arrested for carrying more than one ounce at a time.)

In theory, fewer arrests means less police time spent harassing teenagers suffering from pink eye. That in turn means fewer tax dollars spent on processing (in New York City the average possession charge costs $1000-$2000); fewer non-violent, first time offenders in prison; and an economy that benefits from not having a large proportion of its potential work force behind bars.

This theory holds true for cities that have legalised cannabis in the last five years. But! There has been a slight increase in serious crime. Not enough for residents to retreat into gated communities and start hoarding Fray Bentos pies; just enough for anti-legalisation advocates to start getting twitchy.

In 2015 burglaries at Denver cannabis businesses made up 2.5 per cent of attempted robberies in the city. And local police report that the number of “marijuana related crimes” are on the up – although there’s a gaping chasm of information about how these crimes were “related” to cannabis).

It is(n’t) easy being green

By now, it’s hopefully clear to everyone that people who illegally grow cannabis are basically the Hufflepuffs of crime. But, apparently, smoking something grown in weird Barry’s asbestos-ridden attic isn’t always 100 per cent safe. Legalisation means regulation – and while there’s something rather endearing about the idea of furtive farmers taking over an old Debenhams building, the potential for large electrical fires isn’t quite as cute.

In built up areas there is a real danger that herb happy Hufflepuffs might accidentally endanger hundreds of residents. But even if a city does decide to eliminate this risk, the issue of energy consumption remains. Cannabis cultivation uses a massive amount of water and energy, something that Californian residents are starting to notice is taking a toll.

Water use by cannabis farms is already impacting some city residents’ water supply. Increased consumption will place greater pressure on politicians to consider the environmental impact of legalisation, too.