More people do die in traffic accidents during the first few months after marijuana is legalized, but then the numbers fall again, according to a new Australian study. 

The legalization of marijuana for medical use in three states and for recreational use in 10 has been a business boon and a relief to some patients, but there is also wide concern that its public health effects are poorly understood. 

Earlier this year, studies suggested that more people were dying in car accidents involving drugs than alcohol in the US. 

Others found that opioid overdose deaths fell in states like Colorado in the aftermath of marijuana legalization. 

The mixed data has public policy officials in a bit of a stalemate. 

Alongside the Australian study, a not-yet-released University of Oregon study also suggests legalization has doesn’t make much difference in the risk of death while driving intoxicated in a given state. 

Marijuana legalization has swept US states with almost unprecedented speed. 

Yet under federal law, it’s still a schedule I drug. 

Due in part to that tension and the swiftness of local legislation compared to the plodding systematic movements of public health research, it is difficult to deem legal marijuana ‘good’ or ‘bad’ for us as a nation. 

And perhaps even harder to decide how much weed is too much to drive.

We don’t have a good breathalyzer equivalent for marijuana yet, and deciding a limit for the drug is difficult due to the way it spreads in the body and how that process differs from person-to-person. 

But last year a Insurance Institute for Highway Safety (IIHS) study found a six percent surge in traffic deaths in states where the drug was legal. 

What’s more, a Governors Highway Safety Association study found that nearly 40 percent of all people who died in 2016 car crashes had marijuana in their systems. 

Despite marijuana’s probable benefits and generally good safety profile for pain and other patients, that’s pretty damning evidence against driving while high. 

But now new research suggests that the link between weed and deadly car crashes might not be as strong as it first seemed. 

A study conducted by researchers at Monash University in Melbourne, Australia, and published today in the journal Addiction found that there may be a rebound effect. 

The researchers looked at traffic deaths in nine states that had legalized recreational marijuana and nearby states to and from which people might be traveling (including over state lines to purchase marijuana). 

In the first year after legalization, they found that about one extra person died in a traffic accident per every million residents of Colorado, Washington and Oregon and the states they shared borders with – but only for a short while. 

But then the number of deaths started falling month-to-month. 

Meanwhile, the National Bureau of Economic Research is finalizing a study comparing rates of fatal accidents involving high driving by at least one party and what theoretical models of what death rates might have been if it hadn’t been legalized. 

In Colorado and Washington, where recreational marijuana was made legal in 2014 there were massive surges in fatal accidents involving high driving. 

Colorado saw a 92 percent increase in fatal wrecks involving THC, the psychoactive ingredient in marijuana, and Washington saw a 28 percent increase between 2013 and 2016. 

Nationwide, these accidents became 10 percent more common. 

Again, at first blush, the evidence looks damning. 

But, when the researchers created a model for what increases would have looked like if marijuana hadn’t be legalized, predicted driving death rates were hardly different. 

‘We find the synthetic control groups saw similar changes in marijuana-related, alcohol-related and overall traffic fatality rates despite not legalizing recreational marijuana,’ the study authors write. 

This is in part because while marijuana is known to impair reflexes and judgement and, therefore, driving, it is not thought to be as hindering as alcohol, and drinking and driving is still considered far riskier than driving while high. 

The other factor in all the seeming contradictions between empirical studies is simply time. 

Marijuana is still newly legal, and it isn’t legal in all US states yet. The Australian study’s findings suggest that the sharp increases immediately after legalization may ride on the shoulders of a surge in people buying marijuana and driving on the whole and, particularly, in people unaccustomed to driving after using marijuana. 

That is not to say that it is necessarily safe to drive while high once you’re ‘used to it,’ but rather is reflective that, right now, it’s just too soon to tell what the exact relationship between marijuana legalization and driving deaths is.  

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